Handover in Mobile WiMAX Networks

376 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 12, NO. 3, THIRD QUARTER 2010

Handover in Mobile WiMAX Networks:The State of Art and Research Issues

Sayan Kumar Ray, Student Member, IEEE, Krzysztof Pawlikowski, Senior Member, IEEE,and Harsha Sirisena, Senior Member, IEEE

Abstract—The next-generation Wireless Metropolitan AreaNetworks, using the Worldwide Interoperability for MicrowaveAccess (WiMAX) as the core technology based on the IEEE802.16 family of standards, is evolving as a Fourth-Generation(4G) technology. With the recent introduction of mobility man-agement frameworks in the IEEE 802.16e standard, WiMAX isnow in competition with the existing and forthcoming generationsof wireless technologies for providing ubiquitous computingsolutions. However, the success of a good mobility frameworklargely depends on the capability of performing fast and seamlesshandovers irrespective of the deployed architectural scenario.Now that the IEEE has defined the Mobile WiMAX (IEEE802.16e) MAC-layer handover management framework, the Net-work Working Group (NWG) of the WiMAX Forum is workingon the development of the upper layers. However, the path tocommercialization of a full-fledged WiMAX mobility frameworkis full of research challenges. This article focuses on potentialhandover-related research issues in the existing and futureWiMAX mobility framework. A survey of these issues in theMAC, Network and Cross-Layer scenarios is presented alongwith discussion of the different solutions to those challenges. Acomparative study of the proposed solutions, coupled with someinsights to the relevant issues, is also included.

Index Terms—Mobile WiMAX, IEEE 802.16e, Handover,MAC-layer, IP-layer, Cross-layer, Issues.

I. INTRODUCTION

THE ONGOING global boom in the number of users ofthe global Internet has led to the development of different

fixed and mobile broadband technologies providing supportfor high speed streaming multimedia, customized personalizedservices, ubiquitous coverage and unhampered QoS. Thoughthe existing Wireless Local Area Network (WLAN) and thirdgeneration (3G) technologies have been successfully provid-ing broadband access for the last several years, they havetheir specific drawbacks, inhibiting their full-fledged growth.WLANs suffer from short range and restricted scalability. Onthe other hand, the 3G systems have such constraints as lowbandwidth and high infrastructural expenses. The culminationof the recent IEEE 802.16-based WiMAX family of standards(IEEE 802.16a, 16d and 16e) for Wireless Metropolitan AreaNetworks has filled this gap between the LAN and WAN

Manuscript received 9 August 2008; revised 28 May 2009.Sayan K. Ray and K. Pawlikowski are with the Department of

Computer Science and Software Engineering, University of Canter-bury, Christchurch, New Zealand, e-mail: ([email protected],[email protected]).H. Sirisena is with the Department of Electrical and Computer En-

gineering, University of Canterbury, Christchurch, New Zealand, e-mail:([email protected]).Digital Object Identifier 10.1109/SURV.2010.032210.00064

technologies. Devised as a truly broadband access solution,the WiMAX technology offers promising features in terms ofhigh bandwidth, extended coverage area and low cost. Thishas led to its rapid rise as one of the most popular last milebroadband access technologies and as a likely component infuture 4G networks. While the OFDM-based IEEE 802.16d[1] technology (commonly termed Mobile WiMAX) providesfixed broadband access from anywhere within a metropolitanarea network, the new mobile air interfaces specified inthe IEEE 802.16e [2] (commonly termed mobile WiMAX)has successfully addressed the requirements for higher datarates and efficient spectral efficiencies in provisioning full-fledged mobile broadband access. An IEEE 802.16e-basedBase Station (BS) can support both fixed and mobile broad-band wireless access.

Similar to the different cellular and broadband technologies,global mobility related research in WiMAX is mostly focusedon two main areas of concern: location management andhandover management. In the former, the underlying networktechnology tracks and maintains the exact whereabouts ofwireless terminals when they are powered-on, powered-off oreven on the move. On the other hand, the latter deals withthe active transfer of wireless terminals from the control of aBS in one cell to the control of another BS in a different cell.Handovers can be broadly classified into two different typesdepending on the underlying technology: horizontal handoversand vertical handovers. Horizontal handovers are homoge-neous intra-network inter-cellular, while the vertical onesare heterogeneous inter-network inter-cellular. For example,handovers between multiple WiMAX networks are horizontal,whereas those between WiMAX and 3G or WLAN networksare vertical. In this paper, we focus on homogeneous handovermanagement. Mobility aspects in WiMAX are specified asan individual Mobility Agent (MA) layer, above the MAC(link) layer, with some network layer signaling to develop acomplete solution. The existing WiMAX mobility structuredefines three types of link layer handover procedures in ahomogeneous environment. Of these, Hard Handover (HHO)is the default handover mechanism and two soft handovermechanisms, Macro-Diversity Handover (MDHO) and FastBase Station Switching (FBSS), are the optional procedures.The standard specifies a highly flexible and scalable layer2 (MAC-layer) handover policy, allowing handovers to beinitiated and optimized by the mobile station (MS), the BSor the backbone network. Facilities are provided to supportall types of probable handover activities like intra- and inter-

1553-877X/10/$25.00 c© 2010 IEEE

RAY et al.: HANDOVER IN MOBILE WIMAX NETWORKS: THE STATE OF ART AND RESEARCH ISSUES 377

cell, intra- and inter-sector, inter-layer, as well as intra- andinter-system.The existing WiMAX handover mechanisms suffer from

certain drawbacks, particularly related to wastage of channelresources, handover latencies and loss of data. According to[3]-[4], WiMAX is envisioned to support low-latency seamlesshandovers of much less than 100 ms and almost zero packetloss, with an MS speed of 120 km/h or more during the han-dover activity. The global telecommunication sector is quitepositive that WiMAX technology has the potential to achievethis performance. However, several mobility and handoverrelated research issues must be resolved before the potential ofWiMAX is realized. Every step in the technological advance-ment of WiMAX from the standardization of its network layermobility architecture to devising an universally accepted cross-layer handover management (CLHM) framework, presentsconsiderable challenges. The already standardized MAC-layermobility and handover framework nevertheless raises certainresearch issues. Furthermore, in addition to internal chal-lenges, WiMAX also faces competition from technologiessuch as 3GPP Long Term Evolution (LTE) [5]. The differenthandover related WiMAX research issues need to be resolved,both to allow WiMAX to fulfil its potential and to ensure thatit sees more widespread adoption.The aim of this paper, to the best of our knowledge the

first of its kind, is to give an overview of these potentialissues along with the different proposed and probable researchsolutions, starting right from the advent of IEEE 802.16etechnology until today, thus identifying the research directionsrelated to the existing and future WiMAX homogeneoushandover scenarios. In this article, we focus on the MobileWiMAX technology and will use the acronym ’MWiMAX’instead of Mobile WiMAX in the rest of the paper. A listof the different acronyms used in the paper is provided inAppendix A.The rest of the article is organized as follows. In section

II, we briefly recapitulate the different handover techniques inMWiMAX and present a comparative study of the advantagesof the different handover procedures. This is followed insection III by a brief discussion about the different potentialdeployment architectures of the MWiMAX technology andtheir relevancy. Section IV presents a brief comparative studybetween MWiMAX and LTE technologies. The MWiMAXlink layer, network layer and cross-layer (layer 2+3) homo-geneous handover issues, with insights to the proposed andpossible solutions to each of them, are then categorized anddiscussed in detail in section V. Conclusions are drawn insection VI.

II. MWIMAX HANDOVER SCENARIO

The IEEE 802.16 standardization group has defined threetypes of approaches towards handover for the 802.16e tech-nology [2] depicted in Figure 1. While HHO is the defaulthandover procedure, FBSS and MDHO are the optional types.In MWiMAX, a handover initiation decision by a wirelessterminal or BS is dependent on the Received Signal Strengths(RSS) from the current serving BS (SBS) and the neighbouringBSs (NBS). The MS and the SBS jointly decide on when

to initiate a handover activity. Whenever the RSS from theSBS drops below a certain threshold, which might hamper anongoing communication session, the MS goes for a handoverwith one of the chosen NBSs, called the target BS (TBS).The HHO [Figure 1(a)], is a Break-Before-Make (BBM)

procedure, in which the MS breaks its communication withthe SBS before getting connected with the TBS. Thus, theMS experiences a communication gap between its terminationfrom the previously connected BS and the reconnection to thenew targeted BS. On the other hand, both MDHO [Figure 1(b)]and FBSS [Figure 1(c)] are of the Make-Before-Break (MBB)type (soft handover), where the MS starts communicating withthe new BS before terminating its service with the previousBS. Clearly, these latter two types of handover procedure donot experience any gaps in the ongoing communication andthe MS remains connected to multiple BSs simultaneously.Although the different handover techniques in IEEE 802.16ehave been designed from the layer 2 handover perspective,both FBSS and MDHO, which are seamless and fast in nature,can provide support for even higher-layer handovers. The nextsub-sections briefly describe the three handover procedures.

A. Hard Handover

The entire process of HHO in IEEE 802.16e is broadlydivided into Network Topology Acquisition Phase (NTAP)and the Actual Handover phase (AHOP). Detailed explanationof the entire procedure can be found in [2].

Network Topology Acquisition Phase: During the NTAP, theMS and serving BS (SBS), together with the help of thebackhaul network, gather information about the underlyingnetwork topology before the actual handover decision ismade. This is done to identify lists of potential NBSs, out ofwhich one particular TBS may be chosen for the handoveractivity. Figure 2 shows the message sequence chart for theprocedure. The major tasks involved in this phase are brieflyas follows:

• BS advertises the Network Topology: Using MOB NBR-ADV (Mobile Neighbour Advertisement) message, theSBS periodically broadcasts information about the stateof the NBSs, preparing for potential handover activities.The SBS keeps on gathering these channel informationof the NBSs with the help of the backbone network.

• Scanning of advertised neighbouring BSs by MS: TheMS scans the advertised BSs within specific time frames,to select suitable candidate BSs for the handover. Alist of potential candidate TBSs is thus maintained.This procedure is carried out with the help of ScanningInterval Allocation request and response messages(MOB SCN-REQ and MOB SCN-RSP), respectively,sent by the MS and the SBS. In the end, ScanningResult Report (MOB SCN-REP) summarizes all thescanning activities.

• Ranging and Optional Association Activities: Thescanning is followed by contention/non-contention


WiMAX Backbone Router

WiMAXBS

MS

WiMAXBS

(a) HHO

Diversity Set(DS)

ABS

DS BSDS BS

DS BS

NBSNBS MS

MS Monitoring the Signal Strenghthsto Update the DS

MS Transmission of UL and DL Traffic simultaneously to all active BSs in the DS

(b) MDHO

NBS NBS

DS BS

DS BSDS BS

ABS

MS

MS Monitoring the Signal Strenghths to Update the DS

MS Monitoring Signal Strenghths to update the Anchor BS

MS Transmission of UL and DL Traffic

Diversity Set(DS)

(c) FBSS

Fig. 1. MWiMAX Handover Procedures (a) HHO (b) MDHO [6] (c) FBSS [6]

ranging activities through which the MS gathers furtherinformation about the PHY channel related to theselected TBSs. Ranging Request (RNG REQ) andRanging Response (RNG RSP) messages are used forthis purpose. Ranging may be followed by optionalassociation activities through which the MS getsassociated with the potential target BS candidates.Association Result Reports (MOB ASC-REP) are usedfor this purpose.

Actual Handover Phase: During the AHOP (Figure 3), theMS switches location from the SBS to the selected TBS. The

major tasks involved are briefly described as follows:

• Deciding on the TBS: Here the MS chooses the final TBSto handover to, out of the multiple TBSs selected fromthe scanning activities. The decision or initialization ofa handover process may arise at the MS, the SBS orat the network associated. If the decision arises at theMS, it communicates the MOB MSHO-REQ messagecontaining the list of selected TBSs to the SBS and theSBS replies back with the MOB BSHO-RSP message.On the other hand, if the decision arises at the SBS, theMOB BSHO-REQ message is used. However, handover


M S SBS TBS1 TBS2

NBS Adver t isement

MOB_NBR-ADV

Negot iates wi th the Potent ia l TBSs

Ranging Request

Network TopologyAdver t isement

Al locat ion Request

MOB_SCN-REQ

Allocat ion Grant

MOB_SCN-RSP

S c a n n i n g T i m e S l o t A l l o c a t i o n

Scanning of Potential TBSs

PHY Channel Information of TBS 1

Content ion Resolut ion of TBS 1

RNG_REQ

D L S y n c & ( O p t i o n a l ) A s s o c i a t i o nw i t h P o t e n t i a l T B S 1

Ranging Response

RNG_RSP

Ranging Request

PHY Channel Information of TBS 2

Content ion Resolut ion of TBS 2

RNG_REQ

D L S y n c & ( O p t i o n a l ) A s s o c i a t i o n w i t h P o t e n t i a l T B S 2

Ranging Response

RNG_RSP

Association ResultReport (Opt ional )

MOB_ASC-REP

Scanning Result Report

MOB_SCN-REP

Scanning Ranging & (Optional) Associat ion

Fig. 2. NTAP Message Sequence Charts

decision and initiation messages from the MS are alwaysgiven preference.

• Initiating the Handover: Depending on theabovementioned messages, once a particular TBSis selected from the list of the suitable candidate TBSs,the MS informs the current SBS about the beginning ofthe HO activity by sending a MOB HO-IND (MobileHandover Indication) message. It is at this point that theMS terminates its connection with the current SBS.

• TBS synchronization and Ranging Process: Appropriatesynchronization and ranging activities take place onceagain with the TBS, to resume DL/UL retransmissions.

• Authorization and Registration Phases: Lengthy autho-rization and registration processes of the MS with theTBS follow next. It marks the onset of the network re-entry phase of this MS, after which it becomes fullyfunctional with the new SBS.

B. Macro Diversity Handover and Fast Base Station Switch-ing

In the case of the optional handover approaches, MDHOand FBSS, the MS simultaneously communicates using theair interfaces of multiple BSs, i.e. the MS is connected tomultiple BSs at a time, unlike the HHO procedure in whichthe MS remains connected to single BS at any instant. Boththe MDHO and the FBSS use the concepts of Diversity Set(DS) and Anchor BS (ABS). Each MS has a DS.At any time, depending on the signal strengths, the DS

includes the most active NBSs that could be involved ina handover. The ABS is chosen as the one with the mostpowerful signal strength (the most active BS). In case of theMDHO, each MS simultaneously communicates with all theBSs in its DS. However, in FBSS, the MS communicatesonly with the ABS during the downlink (DL) and uplink(UL) activities. So, signal strengths of neighbouring BSs arecontinuously monitored by each MS for efficient updating ofits DS and ABS. The important concepts in the MDHO andFBSS approaches are:


M S SBS TBS1 TBS2

MS HO In i t ia t ion Request

MOB_MSHO-REQ

SBS HO Ini t iat ion Response

MOB_BSHO-RSP (Recom. TBSs)

MS HO Indicat ion

MOB_HO-IND(Selected TBS_ID: 1)

Pre-HO Noti f icat ion to TBS 1 and TBS 2

Noti f icat ion Response (Al located Ranging Slots)

Confirm TBSs

Retain or Release Pre-Al located Resources (e .g . Ranging Slots)

S B S C o n n e c t i o n T e r m i n a t e d( B r e a k - B e f o r e - M a k e )

TBS Synchronizat ion (UL and DL Parameters)

Content ion Resolut ion

Ranging Request

RNG_REQ

Ranging Response

RNG_RSP

H O R a n g i n g

Would be SBS Backbone Network

Request for MS Info.

Response Request for MS Info.

Response

Basic Capabi l i t ies Negotiat ion

MS Author izat ion & Authent icat ion

TEK Exchanges

Registrat ion

N e t w o r k E n t r y P r o c e s s O p t i m i z a t i o n

Terminates MS’s Context

IP Connect iv i ty Establ ishment

Normal Operat ionNormal Act iv i t ies

Context Terminat ion

HO Decision & In i t ia l izat ion

TBS Sync & Ranging

Network (Re)Ent ryPhase

Fig. 3. AHOP Message Sequence Charts

• Diversity Set Updating: Update of the DS at any timedepends on two different thresholds, the H Add thresholdand the H Delete threshold, contained in the DownlinkChannel Descriptors (DCD) that are broadcasted by theBSs. Based on a given MS’s scanning of the BSs, thoseactive BSs in its current DS with long-term CINR lowerthan the H Delete Threshold value are deleted from thecurrent DS and new active BSs with long-term CINRmore than the H Add Threshold value are inserted inthe current DS.

• Updating and Selecting the new ABS: Update andselection of the new ABS for the modified DS is doneby its MS and the BSs based on the signal strength

measurements performed. For doing this, 802.16e useseither the traditional MAC Management mechanism orthe Fast ABS Selection Feedback mechanism [2].

• Handover Occurrence: In both the MDHO and the FBSSmechanisms, a handover occurs when a new BS, havinga more powerful signal strength than the serving BS,moves into the Active Set when it is updated. In the caseof MDHO, during the handover, the MS simultaneouslytransmits or receives unicast messages and traffic frommultiple BSs included in the DS. On the other hand,in FBSS, the normal handover procedure is not invokedwhile the MS switches BSs from the current ABS to thenewly selected target ABS. The MS and the current ABS


jointly do the selection of the target ABS [7]. During theBS switching, the MS remains connected to the currentand the target ABSs.

C. Comparative Advantages of the Handover Techniques inMWiMAX

1) HHO: The HHO mechanism in IEEE 802.16e is verysimilar to that used in Beyond 3G (B3G) technologies likeEV-DO [8] and HSDPA [9]-[10]. However, unlike its cellularcompetitors, the HHO scheme in 802.16e is highly bandwidthefficient, fast, smooth and nearly glitch-free. This NetworkOptimized HHO mechanism [11] has the potential to minimizehandover overheads and achieve a Layer-2 handover delay ofless than 50 ms in the case of high-speed full mobility. This isthe simplest MWiMAX handover technique ensuring efficientsupport for the provisioning of different high-speed real-timeapplications without significant interruptions and degradationsof QoS. As in any other HHO technique, in MWiMAX too,an MS assumes that any new target BS always has adequateresources available to accommodate it, thus reducing thechances of call drops and delays.The seamless nature of the HHO procedure in a typical

MWiMAX sectorized deployment scenario facilitates losslessinter-frequency handover between sectors having differentcarrier frequencies but a fixed frequency reuse pattern [12].The MWiMAX PHY and MAC layers provide support fordynamic and correct measurements of UL and DL signalstrengths of the NBSs by the MS and the SBS, as well asefficient support for broadcast-related features. This helpsto lower resource wastages and handover delays. However,the real advantage of MWiMAX’s HHO scheme is the lowdeployment cost of the HHO, requiring very few spaced apartBSs.

2) MDHO And FBSS: The MWiMAX HHO model is notvery attractive for handling voice-centric applications withhigh-speed mobility users. On the other hand, the two optionalhandover procedures MDHO and FBSS are designed to allowfull seamless mobility at much higher speeds (up to 120kmph). With design features allowing very low (less than1%) packet loss, very fast switching and low handover latency(less than 50 ms), these two inter-sector handover techniqueshave all the potential to support high-speed real-time voice-centric applications like VoIP. Of course, to achieve this, thedeployment cost would be considerably greater than for theHHO model, as a larger number of MWiMAX BSs would berequired within a specified area.In a MWiMAX scenario, both MDHO and FBSS models

have the capability to further reduce the handover delaysand save more resources, as these two techniques do notrequire invocations of explicit HO signaling messages [2]when switching ABSs within the current AS. Moreover, theirnetwork re-entry procedures need not be performed every timewhen switching between anchor BSs. Further, unlike HHO,both MDHO and FBSS have the advantage of performinghandovers within sectors having the same carrier frequency,due to their employing the universal reuse concept [12].However, between the two, owing to the provision of better

TABLE IBRIEF COMPARISON OF THE MWIMAX HANDOVER TECHNIQUES

Parameters Hard Handover FBSS MDHO

Latency High Medium Low

Complexity Low Medium High

Reliability Low Medium High

Packet Loss High Low Low

Cost Low Medium High

Support for De-lay Sensitive Ap-plications

Low High High

Speed Low Medium High

Link Quality Low Medium High

support for handling delay-sensitive applications, FBSS is thepreferred handover option in such cases.Both the macro-diversity handover schemes used in

MWiMAX are designed to provide better performance withrespect to multi-access interference, flexibility and cover-age, than their CDMA competitors do. Application of bothOFDMA fully used sub-channelization (FUSC) and partiallyused sub-channelization (PUSC) techniques in MWiMAX [12]macro-diversity handover mechanisms has improved the rangeand cell coverage. Much research activity in this area isbeing carried out globally by organizations like Intel, Nortel,Alvarion and others, with the aim of further improving thecoverage, particularly at the cell edges. Another advantage ofMWiMAX MDHO and FBSS is the ability of these techniquesto enhance the ultimate system capacity. Depending on the un-derlying radio-link conditions, an MS can dynamically activateand deactivate these when required, to prevent unnecessarywastage of radio resources [13].Finally, it can be concluded that, though both MDHO

and FBSS offer significantly better handover performance incomparison to HHO, there is still a long way to go beforeadequate support measures for these two techniques can bedeveloped and deployed in MWiMAX networks. Accuratelysharing the same carrier frequency among the multiple BSsin the AS, perfectly synchronizing the active set BSs andhandling the increased deployment expenses, seem to be themajor challenges so far. Table I provides a brief comparison ofthe three handover techniques with respect to the MWiMAXHO scenario.

III. MWIMAX DEPLOYMENT ARCHITECTURES

Currently the Network Working Group (NWG) in theMWiMAX forum is working on the implementation of afull-fledged MWiMAX mobility architecture supporting bothhomogeneous and heterogeneous mobility. However, devisinga successful mobility and handover management frameworkdepends much on the choice of suitable network deploymentarchitecture. While a hierarchical or centralized architectureof 3G networks is suitable for supporting high-speed usermobility, it suffers from high latency and high cost [4]. On theother hand, low latency flat architectures, as in recent Wi-Finetworks, do not really support high-speed mobility. Althoughnothing has been decided yet, this alternative is apparentlymore suitable for Layer 3 implementation (which is yet to


BS 1

BS N

Connect ivi ty Serv ice Network

ASN-GW(RR Control)(HO Control )

BS 1

BS N

ASN-GW(RR Control)(HO Control )

R6

R6

R6

R6

R8

R8

Access Service Network 1


R4R8 HA, AAAServer e tc .

I n t e r n e t

R3

R3

R1

R1

M S

M S

R2

ASN-GW - Access Service Network Gateway

AAA - Authent icat ion, Authorizat ion and Account ing

RR - Radio Resource

HO - Handover

BS - Base Stat ion

MS - Mobi le Stat ion

HA - Home Agent

(a) Centralized

Connect ivi ty Serv ice Network

ASN-GW

R6

R6

R8



R4HA, AAA

Server e tc .

I n t e r n e t

R3

R3

R1

R1

M S

R2



RR - Radio Resource

HO - Handover

BS - Base Stat ion


BS (RR Control)(HO Control )

BS (RR Control)(HO Control )

HA - Home Agent

ASN-GWM S

(b) Flat

BS N(RR Control)(HO Control )

Connect iv i ty Serv ice Network

ASN-GW

R6

R6

R6

R6

R8

R8



R4R8 HA, AAAServer e tc .

I n t e r n e t

R3

R3

R1

R1

M S

M S

R2



RR - Radio Resource

HO - Handover

BS - Base Stat ion


ASN-GW

BS 1(RR Control)(HO Control )

BS 1(RR Control)(HO Control )

BS N(RR Control)(HO Control ) HA - Home Agent

(c) Hybrid

Fig. 4. MWiMAX Deployment Architectures: (a) Centralised [12] (b) Flat [12] (c) Hybrid


be standardized), as the different MIPv6 functionalities canbe implemented without detailed consideration of the facetsof the underlying technological implementations. The NWGis currently deciding on the best Layer 3 implementationprotocol deployment architecture to meet all the above objec-tives. A brief discussion on the different potential MWiMAXdeployment architectures is presented here in order to help thereader understand how the different layers and the issues arerelated to these architectures.Figure 4 shows three probable MWiMAX deployment

architectures consisting of multiple subnets with individualcharacteristics. In Figure 4(a), which shows a MWiMAX cen-tralized architecture, a subnet consists of one Access NetworkGateway (ASN GW) and multiple BSs under its control. TheASN GW has centralized control of the subnet. The IP-layerfunctionalities are also located in the individual ASN GWs,which efficiently support seamless handover along with lowlatency micro and macro-mobility activities. In contrast, Fig-ure 4(b) shows the flat architecture, an alternative deploymentscenario. In this case, a subnet consists of exactly one BS andone ASN GW. The IP-layer functionalities are located in theindividual BSs. The architecture supports macro-mobility andhandover with optional session anchoring capabilities [4]. Athird option may be the hybrid architecture (Figure 4(c)) inwhich different BSs control the handover and radio resourceactivities. In this context, we will explain ASN-anchored mo-bility and CSN-anchored mobility, respectively, with respectto Layer 2 and Layer 3 handovers in MWiMAX for a betterunderstanding of different situations in those layers.

IV. MWIMAX AND LTE: A BRIEF COMPARATIVE STUDYOF MOBILITY AND HANDOVER ASPECTS

Before long, telecommunication network operators world-wide will have a tough time in deciding on which accesstechnology to choose for their consumers. MWiMAX couldface strong challenges from some of the near-future technolo-gies like 802.11n [14], 802.20 [15], iBurst [16] and LTE [5].IEEE 802.11n is standardized by now. However, the recentamendments made in the draft are apparently facing someproblems with the Commonwealth Scientific and IndustrialResearch Organization (CSIRO) of Australia, which owns afew of the patents used in the draft [14]. This might detersome vendors from taking up the technology. The IEEE 802.20standard, originally harnessed within the 16e working groupand optimized for long-range wireless broadband mobility ofdata, has much in common with 802.16e. Therefore, it isunlikely that vendors already planning to push the 802.16etechnology would again be interested in adopting the 802.20in future. Moving to iBurst, a High-Capacity Spatial DivisionMultiple Access-based technology developed by ArrayCommand heavily backed by a leading manufacturer, Kyocera, offersfull-mobility handovers but at a higher cost than MWiMAX.Lastly, the LTE, which is expected to hit the market soon, isforecasted by analysts as the 3GPP’s response to MWiMAX,in order to be in the forefront of the wireless communicationmarket. So, 3GPP LTE can be considered to be the strongestpotential competitor to MWiMAX technology. Below wepresent a brief comparison of the mobility and handoveraspects of LTE and MWiMAX.

IP CORE NETWORK

UE

UE

UE

UE

eNB

eNBeNB

eNB

MME/SAE-GW

MME/SAE-GW

S1S1 S1

S1

S1

S1

S1S1

X 2

X 2

X 2

X 2

X 2

X 2

Fig. 5. LTE Architecture

The main drivers of the LTE technology are better coverage,higher throughput, increased capacity and weaker latencyrequirements. The LTE architecture shown in Figure 5 consistsof BSs called eNBs, which are interconnected by the X2 links.The eNBs are connected to the Mobility Management Entity(MME)/SAE Gateway by the S1 links. Unlike in MWiMAX,the eNBs can directly communicate with each other andmake intra-LTE handover decisions independently. Also, LTEis aimed at providing full mobility at speeds of 350 - 500km/h and global roaming. Macro-diversity soft handovers arenot supported by LTE. Table II compares the mobility andhandover-related features of these two technologies.

It shows that 3GPP is projecting LTE as being morepowerful than the existing versions of MWiMAX. Of course,LTE will face a strong challenge from the future 802.16m[17] version of MWiMAX, which will be standardized verysoon [18]. The major drawback of LTE in comparison toMWiMAX is its delayed commercialisation, which is plannedfor 2011 in the earliest. However, global telecommunicationanalysts are optimistic in predicting that the two technologieswill converge rather than become competitive. This is because,being increasingly based on a similar set of telecommunicationtechnologies, both have the capabilities to deliver highermobility, greater bandwidth, larger range and flexibility withhandover options. While the 802.16m version is adoptingmany features of LTE, the latter will also use solutionssimilar to those of the existing and future mobility versionsof MWiMAX. Hence, it is expected that both technologieswould have increasing overlap in future markets. Convergencewould occur not only in handheld multi-mode user devices andin laptops, but also in providing seamless session handovercapabilities between the two while roaming (both being IP-based). Moreover, somewhat similar architectures would makeit easier to provide seamless support for IPTV, VoIP and otherSession Initiation Protocol-based services even while roaming.


TABLE IIMOBILITY AND HANDOVER-RELATED COMPARISON BETWEEN MWIMAX AND LTE

Parameters MWiMAX LTE

HO Types Supported Both HHO and SHO No MDHO (SHO)

Mobility Limited and Nomadic Mobility (up to 120 km/h):802.16e; Full Mobility (350 km/h): 802.16m

Full Mobility (350 - 500 km/h)

Network Architecture Centralized, Flat, Hybrid, IP-based; BS + ASN-GW Very Flat, IP-based; eNB + MME/SAE GW

Services Packet Data and VoIP Packet Data and VoIP (more efficient for VoIP opti-mization)

Access Technology SOFDMA in UL and DL (for 802.16e) DL: OFDMA; UL: SC-FDMA

Expected HO Latency 35-50 ms: 802.16e; < 30 ms: 802.16m < 50 ms

Backwards Compatibility None Still Full 3GPP Interoperability

Roaming Supported MWiMAX - MWiMAX (i.e. local / regional) Full Global Roaming

Cell Radius (during mo-bility)

2-7 km 5 km

HO Decisions Depends On MS and SBS On eNB

V. RESEARCH ISSUES IN MWIMAX HANDOVERSCENARIOS

Any new technology faces many technological and non-technological hurdles and challenges at its early stages andbroadband MWiMAX is no exception. Despite significantresearch activity worldwide, universally accepted efficientMWiMAX location and handover management frameworksare yet to be developed. This is in contrast to the cellular-based technologies that have got many years of experiencein providing mobility support to users. Though the IEEEgroup dealing with the MWiMAX family of standards hascome up with HHO, MDHO and FBSS techniques to dealwith all types of applications, these procedures are not freefrom their own technical drawbacks. Figure 6 gives a conciseoverview of some of the probable layer 2 (L2), layer 3 (L3)and cross-layer (L2+L3) research hurdles that may hinder thesuccessful design and implementation of a globally acceptedMWiMAX handover management framework. In this section,these highlighted issues will be discussed in detail.

A. MWiMAX Layer 2 Handover Issues

In this case as shown in Figure 4(a), the BSs supportonly PHY and MAC-layer functionalities and any intra-subnethandovers (e.g. from BS1 to BSN within ASN1) are carriedout using MAC-layer mobility management functionalitiesonly. Such situations arise in the case of ASN-anchored mo-bility (intra-ASN mobility) where an MS’s movement insidea subnet is controlled by the particular ASN-GW of thatsubnet. The individual ASN and MSs generally control allhandovers in these cases, with support from the different BSsin the subnet. In case of layer 2 handovers, no change in theMS IP (network) layer configuration takes place. MWiMAXhandover procedures, irrespective of the layered handoverarchitecture, suffer from a huge range of issues, like resourcewastage, high latency, unwanted packet losses, call dropsand ping-pong activity, to name a few. Therefore, for eachlayer, new ideas have been proposed to deal with these andrelated problems. This section discusses the various MAClayer handover problems encountered by the HHO, FBSS andMDHO techniques in MWiMAX.

1) HHO Technique: Although HHO is the mandated mostbandwidth-efficient handover technique in MWiMAX, yetsuch handover activities are crippled by serious problemslike excessive scanning activity in a somewhat non-optimizedscanning interval before finalizing a TBS and prolongedinter-handover connection gaps. Though these issues are stilldrawing major research attention, as discussed below, severalother important issues, such as unwanted network re-entryactivities during the handover owing to ping-pong effects,IP connectivity delay during the network re-entry phase,and optimization of handover-based load distribution, haveyet to be investigated in much depth. Apart from these, thesubsections below also discuss less important HHO issuesin a MWiMAX environment like efficiently exploiting boththe UL and DL signals of the SBS and MS before initiatinga handover activity and means of avoiding the wastageof unused ranging slots during pre-handover situation. Asummary of these issues is provided in Table III to give thereader an overview of the different aspects discussed beforegoing into the details.

a) Excessive Scanning and Association Activities: Oneof the primary advantages of MWiMAX handover techniquesis the provision of both layer 2 (L2) broadcast and scanningconcepts during the NTAP by which the MS can receivechannel signal strength information of its NBSs. The MS canscan some of the NBSs as potential TBS candidates. However,the HO technique does not clearly say anything regardingthe number of NBSs that a MS may need to scan beforeultimately deciding on a TBS. This may result in redundantscanning of NBSs [19] leading to unnecessary wastage ofchannel resources and degrading the overall performance.Moreover, along with scanning, synchronization, ranging andassociation activities are also performed one after another(i.e. not simultaneously) during the NTAP. Hence, redundantscanning, followed by prolonged synchronisation, ranging,and association activities proportional to the number of NBSsscanned, increases the overall handover delay. Also, whileexcessive scanning of the NBSs may affect the schedulerperformance of the SBS particularly for the delay sensitive


Fig. 6. MWiMAX Handover Research Issues

downlink traffic, unnecessary contention-based ranging resultsin unwanted consumption of the contention slots therebyaffecting the overall throughput [28].

Potential Research Solutions: A number of measureshave been proposed to simplify scanning related proceduresduring the topology acquisition phase, to minimize theoverall delay and enhance the system performance. Theauthors of [19]-[20] have proposed unique network topologyacquisition schemes to identify the potential TBS beforeperforming any type of scanning-related activities. In [19],the authors argued that, from the MOB NBR-ADV messages,the MS can acquire the preamble-based mean Carrier toInterference-plus-Noise Ratio (CINR) along with the arrivaltime difference of the downlink signal (relative to the SBS)of the individual NBSs. From that, it can select the TBSto be the one having the largest mean CINR and smallestarrival time difference. Then, the MS performs ranging,synchronization and association activities only with that TBS.Though this scheme reduces the handover delay by skippingunnecessary scanning, it considers neither the MS’s directionof motion nor the current load of the selected BS. This mightlead to unwanted ping-pong activity as well as call drops.

In [20], it is proposed to predict the potential TBS prior toany scanning activity based on the different parameters likeMS’s movement direction, average time differences betweenprevious handovers, position and distance of NBSs withrespect to the SBS and load of the different NBSs. Thisscheme not only reduces the scanning-oriented overheads butalso proves to be energy-efficient as the ranging procedure(which consumes lots of energy) is only limited to theparticular predicted TBS. Another idea discussed in [29] isto modify the MOB NBR-ADV broadcast message, whichcontains static channel-related information on the NBSs, toprovide link quality parameters-oriented dynamic informationon the BSs. This would decrease the need for scanning as theMS can gather more handover decision related informationfrom broadcast messages themselves. Elimination of NBSs asTBS candidates, prior to scanning, depending on QoS, activeservice flows and bandwidth requirements of the MS, is alsoa good solution for avoiding unwanted scanning activities[30]-[31]. However, in spite of all these proposals, there is stilla need to come up with universally accepted ideas on dealingwith unwanted delays and wastage of channel resourcesowing to excessive scanning, ranging and association relatedactivities during MWiMAX handover operations. Standard


TABLE IIISUMMARY OF THE PROBABLE MAC-LAYER HHO-RELATED ISSUES IN MWIMAX

Issues Effects Proposed Research Directions

Excessive Scanning andAssociation Activities

Redundant NBS scanning, ranging and associationactivities may lead to unnecessary L2 handover delayand resource wastages.

Based on parameters like MS’s trajectory of motionand previous HO intervals along with link qualityinformation [19]-[20] of the NBSs, an MS can selectthe potential TBS before the scanning operations.

Optimizing Scanning In-terval

Temporary suspension of data exchange between theMS and the SBS during scanning interval degradesthe overall handover performance.

In a multi-MS MWiMAX environment, NBSs canexchange configuration parameters to figure out theideal scanning interval required [21].

Efficient Exploitation ofDL and UL Signals

QoS may be hampered if both downlink and uplinkparameters are not considered during handover initi-ation and execution.

Combination of effective measurements of MS’suplink signal strengths and SBS’s downlink signalstrengths at the handover region enhances the han-dover performance [22].

Wastage of Ranging Slots The non-retained ranging slots of the other candidateBSs, allocated during the scanning phase, add up tothe handover resource wastage after the MS selectsthe particular TBS [19].

Selection of the TBS prior to the handover pre-registration phase [19]-[20] can debar other candi-date BSs from allocating ranging slots.

Prolonged Handover Con-nection Disruption Time(CDT)

Inter-handover connection gap degrades QoS owingto service disruptions.

New MAC management message [23] can enable theMS to receive traffic immediately after the handover.Also, MS can perform the new network entry processduring its idle period to receive traffic continuously[24].

Network Re-Entry Activ-ity due to Ping Pong Ef-fects

Unnecessary network re-entry procedures owing toping-pong effects cause delays and call disruptions.

The SBS notifies the MS about the time duration thatthe traffic for MS will remain buffered in the SBS[25]. This avoids network re-entry procedures.

IP Connectivity Delayduring Network Re-entry

MS needs to know more clearly during or beforethe network re-entry activity whether a switch in theIP connectivity is required after the HO. Otherwiseunnecessary connectivity activities only enhance theoverall delay.

If the TBS can know of the MS’s previous AR andthe IP address, it can help in reacquiring the MS’sIP connectivity context [26]

Optimising Handover-based Load Distribution

Evenly balancing the traffic loads and evenly dis-tributing available resources over different BSs in anarea is important in MWiMAX. Solving this issuewould not only enable better QoS but would alsoweaken call disruptions and call blockings.

Both BS-initiated directed handovers and MS-initiated rescue handovers are conducted in parallelto offer better load balancing scheme enabling satis-factory QoS and much fewer ping-pong effects [27]

methods for performing the CINR measurements are alsodesirable.

b) Optimizing Scanning Interval: In the MWiMAXHHO scenario, scanning of multiple channels is an inevitableactivity for discovering the NBS that is most suitable to be thepotential TBS. Hence, though it is difficult to avoid scanningprocess completely, one can try to keep it within limits, asdiscussed previously. During scanning, MWiMAX handovermechanisms temporarily pause the uplink and downlink ofdata transfer between the MS and the SBS. These scanningintervals are allocated by the SBS dynamically on gettingscanning interval allocation requests from the MS. However,frequent temporary suspension of data exchange lowersthe system throughput, and adds more delay to the overallhandover process. Also, QoS requirements may get disruptedowing to this. Moreover, during scanning intervals, all datameant for the MS are buffered at the SBS, what leads towastage of channel resources. Hence, it is desirable to devisetechniques of effective estimation and minimisation of boththe frequency and the time interval needed for scanning.Required also are the methodologies to carrying out scanningand data exchange concurrently.

Potential Research Solutions: It should be noted that, asthe QoS might get hampered in case of both long and shortscanning intervals, optimization of scanning intervals is animportant issue. An efficient Adaptive Channel Scanningalgorithm in a multi-MS oriented MWiMAX environment,relying on the exchange of configuration parameters betweenthe NBSs in order to find out the required scanning timefor a MS, is proposed in [21]. Along with optimization ofthe allocated scanning intervals for all MSs, the scheme alsomaintains the QoS of the application traffic in the system.However, utilization of unlimited channel buffers, in orderto make the packet loss almost negligible, complicates theproblem of channel resource wastage. Another proposal, forminimizing the influence of scanning intervals by concurrentscanning and data transmission by the MS is discussed in[19]. This fast synchronization and association model usesthe unique IDs of the SBS and the NBSs (unique BSIDs), todistinguish between the UL/DL messages of the SBS and theNBSs. As the MS can clearly identify and separate the SBS’sdata exchange messages from the NBSs’ synchronization andassociation messages, it can communicate with both of them atthe same time, with the ranging slots appropriately adjustedby the SBS to minimize the chances of collisions. This


scheme, however, neither considers a multi-MS environmentnor considers an environment where the different NBSsand the SBS might not be controlled by the same serviceprovider network [21]. An MS’s sleep mode option [2] alsoprovides an interesting mechanism for the MS to performscanning without hampering the communication with the SBS.

c) Efficient Exploitation of DL and UL Signals:MWiMAX promises to deliver streaming multimediaapplications in the form of voice and data. However, theQoS of data and voice services might not be the same andtheir requirements may vary for UL and DL transmissions.This would degrade the system performance. Hence, toprovide effective and stable QoS for all types of applications,it is advantageous to consider both UL and DL signalparameters while initiating and executing handover. This isparticularly important for delay-sensitive voice and data-oriented applications in MWiMAX.

Potential Research Solutions: In a mobility scenario, theUL and DL signals of an MS and the SBS respectivelyare not strictly correlated with respect to distance betweenthem. From an user’s perspective, though, it seems that,as the distance between an MS and its SBS changes, theMS’s UL signal strength measured at the SBS and the SBS’sDL signal strength measured at the MS also changes ina correlated fashion, this is not true always. DL and ULsignals are considered jointly in [22], to propose a hardhandover scheme based on the MS’s UL signal strengthsand the SBS’s DL signal strengths measured at the SBSand the MS, respectively. A handover process is triggeredonce the two signal strengths fall below some pre-determinedthresholds. This scheme assumes that an MS does not needto perform unnecessary monitoring and scanning of theNBSs’ signal strengths in the non-handover region in a cellbefore a handover is initiated. Unwanted delays as wellas ping-pong and outage probabilities are thereby reducedsignificantly. Though much work has not been done yeton utilizing both downlink and uplink signals to direct andinitiate a MWiMAX handover, in comparison to the downlinksignal-based schemes, this choice may have the potentialto provide better QoS, reduced scanning requirements andimproved overall system throughput. Clearly, it demandsfurther research.

d) Wastage of Ranging Slots: MWiMAX supportshandovers initiated by either the MS, or the SBS, or even theunderlying network. In the case of MS-initiated handovers,when the suitability of the potential candidate NBSs selectedby the MS during the NTAP is accepted, the individual BSsallocate ranging slots for the MS, which then selects the newTBS and retains only the ranging slots provided by that BS.The other unused ranging slots add up to the list of resourcesbeing wasted during the entire handover process.

Potential Research Solutions: Such wastage of unwantedresources can be avoided if the SBS can select the new TBSbefore the allocation of ranging slots, as proposed in schemes[19]-[20]. So, once selected, only that TBS may allocate

ranging slots, debarring the other NBSs from unnecessarilyallocating such slots as well.

e) Prolonged Handover Connection Disruption Time(CDT): Being a break before make technique, the HHOconcept in MWiMAX suffers from a lengthy “inter-handover”CDT that could lead to unwanted hazards like packet losses,call disruptions or even call drops, while on the move. Thisoccurs in the actual handover phase, when an MS terminatesthe connection with the SBS and tries to set-up connectionswith the selected TBS. While a CDT in the range of 200 ms isacceptable for real-time streaming media traffic [32], anythingmore than that is disruptive [33]. In MWiMAX, data, voiceand multimedia contents are intermixed and each requiresdifferent mechanisms for its transmission, particularly duringhandover. So, such a lengthy CDT may cause serious servicedisruptions in case of real-time high-speed delay-sensitivevoice and streaming multimedia applications in MWiMAXnetworks.

Potential Research Solutions: To counter the above drawbacks,considerable research has been conducted over the last fewyears to minimize the inter-handover service interval time.The IEEE MWiMAX group has incorporated the MDHOand FBSS techniques, which are ideal for delay sensitiveapplications like VoIP. However, as these two techniques arevery complicated and can increase deployment costs, researchactivities have been carried out to further reduce the QoSrelated hazards to real-time services caused by the CDT.Sik Choi et. al. [23] have proposed a link-layer fast han-

dover scheme for MWiMAX HHO scenario that significantlyreduces the probabilities of packet loss and transmission delayduring handover. This scheme introduces Fast DL MAP IEMAC management message, which enables an MS to receivedownlink traffic just after the downlink synchronization withthe TBS, even before the completion of the uplink synchro-nization phase. A similar idea, called Passport Handover, isdiscussed in [33] where an MS could resume the DL re-transmissions with the TBS before the completion of theauthorization procedures, by using the CIDs of the previousSBS. Though both these mechanisms managed to achieve animprovement of the overall handover performance, they didnot consider potential possibilities of unsuccessful authoriza-tion activities while switching domains. This is fixed in [34],in which, having predicted the TBS by considering criteria likeMS’s movement trajectory, NBSs’ locations and MS’s averageinter-handover gap, the SBS passes on MS’s authorizationparameters to the TBS over the backhaul network. Also, theranging results are stored by both the TBS and the MS fora certain period of time until they are re-used during theconnectivity disruption stage. Hence, this omits the need for asecond ranging activity and the MS could thus switch domainsvery quickly without having to worry about authorizationactivities, which have already been done during the pre-handover stage. However, there is still scope for research onthese aspects, to see how smoothly the lengthy authorizationapproach could be done prior to the actual handover phasewith or without the help of the backhaul network. This isbecause transferring the stored authorization messages from


the SBS to the TBS may increase the overall load in thebackhaul network.Another interesting idea proposed in [35] deals with an MS

maintaining simultaneous network connectivity with the SBSand the TBS. In this case, it is assumed that the coverageareas of the two BSs overlap so that the MS gets sufficienttime to complete the network re-entry process at the targetnetwork, before it loses connectivity with the SBS. This maybe a possible scenario in the case of MWiMAX networks dueto the large coverage areas of the BSs. However, this schemerequires further study to investigate such feasibility factors asduration of overlap, effects of blind spots at the overlappedregions and the cost. MS’s idle periods could also play animportant factor in this issue as suggested in [24]. As statedthere, if the MS performs the network re-entry signaling withthe TBS during the idle mode of the MS, it would allow theMS to continue data exchange simultaneously with the SBSleading to a very low latency HO procedure. However, thisidea requires the BSs to be synchronized, and this might bea problem in the case of HHO. Therefore, it still remains aresearch challenge to devise suitable frameworks for dealingwith the CDT issue in MWiMAX HHO.

f) Network Re-Entry Activity due to Ping-Pong Effects:In MWiMAX HHO, when an MS wants to get connectedto a new BS, it has to complete the entire network re-entryprocedure comprising of the series of security and connectionre-establishment processes. This takes a long time. In asituation where in the middle of an ongoing communication,an MS, that is performing network re-entry procedures witha TBS, wants to come back to the previous SBS due tochange in signal strengths, it leads to further delays if theentire re-entry procedure needed to be performed again forthe old SBS. Handover overheads caused by unnecessary re-entry procedures resulting from such ping-pong effects maydegrade the overall system performance.

Potential Research Solutions: What is really needed aremechanisms to make the previous SBS able to differentiateping-pong re-entries from new re-entries, so that the overall re-entry phase for the previous one could be shortened. Researchcarried out on this problem resulted in a mechanism in whichthe TBS, upon learning about the ping-pong effect, informsthe previous SBS about the MS’s reverting back to it [36].This will help the previous SBS to identify the return of theMS as an effect of ping-pong and not as an altogether newnetwork entry. So, not only will it provide non-contentiousranging opportunities to the returning MS, but will also resumethe communication quickly, provided the SBS has retainedthe MS’s connection information. However, this scheme willnot work if the SBS has not retained the state information ofthe MS. In that case, however, the allocated ranging slots forthe returning BS will be wasted. So, a more effective methodis proposed in [25] in which, prior to a handover, the SBSintimates to the MS about how long the MS’s connectioninformation would be retained. During the ping-pong effect,if the MS knows that the SBS is still retaining the previousconnection information, it can act accordingly to quickly

resume the previous communication with the SBS. Also, inthe case of a dropped call during handover, the TBS can usethe connection information retained by the SBS regarding theMS and can very quickly perform the call recovery procedure.However, there is no suitable explanation for such a scenariowhen an MS, due to the ping-pong effect, has to come back tothe SBS in spite of knowing that the SBS is not retaining theprevious connection information any longer. Further researchis needed to deal with such situations arising from the ping-pong effect. Minimization of handover overheads, reductionof resource wastages and early recovery of any call dropsare the important factors which should be kept in mind whileformulating such solutions.

g) IP Connectivity Delay during Network Re-entry:During a MWiMAX HO process, if an MS moves to a TBSunder the same access router within the same subnet, then theHO does not incur any change in the MS’s IP connectivityscenario. MS’s IP connectivity context with reference to thenew SBS remains the same as with the old SBS. However,this is not the case if the TBS falls under a different subnetaltogether. In that case, the MS has to go for the lengthyprocedure of IP connectivity acquisition during the re-entryphase to complete the HO process. In the current scenario,it is clearly a challenging issues of how an MS actuallydetermines whether a change in the IP connectivity contextis at all required as part of an ongoing HO activity. If achange is not required then it would save significant amountof HO-related latency as the MS would not go for that at all.In the current MWiMAX standard, a HO optimization flagin the MOB NBR-ADV message [2] indicates whether anIP subnet switch is required during a HO activity. However,this is not a very fruitful detection mechanism as it incursadministrative overhead.

Potential Research Solutions: In order to get rid ofsuch delays, MSs need to figure out, beforehand, if theTBS falls under a different subnet altogether. If yes, thenonly it has to initiate the lengthy IP context acquisitionprocedure during the network re-entry phase, else not. Asolution to this problem is proposed in [26]. Depending onthe information provided by an MS, a TBS could reacquirethe MS’s IP connectivity context, thereby minimizing theoverall delay. During a HO activity, the MS needs toprovide the TBS information regarding its last IP addressand Fully Qualified Domain Name (FQDN) of its last AR[26]. Based on these information, the TBS instructs the MSwhether or not it can retain the previous IP connectivitycontexts. Devoid of any administrative overheads, the solutionclaims to be independent of any MWiMAX RAN architecture.

h) Optimizing Handover-based Load Distribution: In amobile communication environment, the QoS experienced byMSs can degrade significantly owing to increased traffic loadin a cell. Problem like unbalanced traffic load distribution[37] between different adjacent cells can force the trafficload in a particular cell to exceed the ultimate capacity ofthat cell. With the overlapping nature of the cells, unevenlydistributed resource utilizations among the different adjacentBSs incur additional cost and hamper the service quality.


Therefore, evenly balancing the loads and evenly distributingthe different available resources within a cluster of BSs isa relevant and interesting research issue. This is a problemin the MWiMAX scenario as well. Though the MWiMAXForum has supported a Radio Resource Management (RRM)framework for efficient load balancing and resource utilization[38] with the help of BS-initiated directed handovers [27],the specification provides only a framework and lacks anydetailed implementation concepts and algorithms [39]. Thus,it is an open research issue.

Potential Research Solutions: Here, MWiMAX researchhas been mostly focussed on designing and implementing anefficient algorithm for evenly distributing MSs, which resideon the overlapping areas of the adjacent cells, among adjacentBSs. Another idea, which has not been advanced much yet, isto gather the resources to areas where majority of the trafficis located [39]. The MWiMAX Forum has looked at theformer idea. In the BS-initiated directed handover scheme, thecongested SBS forces the MS to handover to a non-congestedTBS. This BS-controlled and initiated HO scheme offersgood QoS in comparison to traditional MS-initiated rescueHO schemes, in which the load balancing logic resides inthe MSs and the MS handovers to a less congested TBSwhenever the signal strength drops below a threshold.An efficient load balancing scheme is proposed in [27]

in which directed and rescue HO mechanisms are conductedin parallel. The scheme uses Spare Capacity Reports (SCR)[38] broadcasted by the different BSs in an area to let theirpeers know of their loads. Depending on such reports, theBSs classify their loading states as underloaded, balanced oroverloaded. Directed HO to a TBS occurs in the case ofunderloaded conditions, whereas rescue HO takes place ifthe TBS is in balanced or overloaded states. This schemeoffers satisfactory QoS and much reduced ping-pong activities.Additionally, one could consider different prioritization meansby which the MSs can be handed over to the TBS. They couldtake into account e.g. traffic priority and channel conditions[27].Another proposal made in [40] considers an MS-initiated

rescue HO mechanism, in which handovers between thedifferent frequency assignments (FA) (MWiMAX assignsmultiple FAs to the different operators) take place. Asopposed to the standard MWiMAX HO scenario, where notarget FAs are indicated, this scheme not only introduces theconcept of target FAs but also offers seamless HO from thecrowded serving FA to the non-crowded target FAs. Despitesuch research attempts, considerable work is still neededbefore choosing the BS-initiated directed HO scheme overthe traditional MS-initiated rescue scheme.

2) MDHO And FBSS: Similar to the HHO, these softhandover techniques for supporting inter-sector handoversalso suffer from few drawbacks. As discussed previously,while the drawbacks of the NTAP also apply to thesehandover techniques, both MDHO and FBSS suffer fromperformance hindrance challenges, specifically with theaccuracy of updates of the active sets during the actualhandover phase. Not much work has been done for dealing

with these important issues and as such, they are openfor future research contributions. These challenges arehighlighted in Table IV and a detailed discussion is presentedin the next sections.

a) Ping-Pong Effects while Updating the AS: In MDHOand FBSS, depending on the signal strengths of the BSs, anMS always maintains an AS of NBSs, comprising of theNBSs with the most powerful signal strength at that particularinstance of time. The AS also contains the serving or anchorBS (ABS). The other NBSs remain in the set of probablecandidate BSs (candidate set) for the active set. The MSalways monitors these BSs to update the AS, depending ona threshold value. However, specific discussions are requiredto determine the acceptable threshold value at any particularinstance, to avoid unnecessary updating of the AS.

Potential Research Solutions: The difference betweenthe new threshold value and the existing value should belarge enough to trigger the requirements for AS updatingas there are always possibilities that due to a very lowthreshold value difference, NBSs from the candidate set maymove in and out of the AS unnecessarily. Such enhancedping-pong activities would not only make the AS updatesmeaningless, but also hike the resource consumption inregard to the required signaling [41], degrading the overallperformance. So efficient methods of determining the rightthreshold values to update the AS are required to reduce suchperformance-hampering activities.

b) Inaccurate AS Updating based on the BSs’ SignalStrengths: The FBSS and MDHO rely on the signal strengthof the NBSs as the sole basis for updating the AS. Theytake into account neither the path followed by the MS, northe mobility of the MS. Relying only on signal strengthsdoes not always result in optimum performance, especiallyin regard to channel and resource wastages. This is because,in such cases, it can be concluded that the AS, at anyparticular instance, may get populated by such NBSs withwhich the MS will not perform a handover activity at thenear future. Though the signal strengths of such NBSs maybe strong enough to be included in the AS, they mightnot fall into the MS’s movement trajectory. Automaticallysuch BSs would drop out of the AS after some time,when the MS moves further away from them, resulting infrequent and unnecessary updating of the AS. Thus, in termsof channel usage, inclusion of such NBSs is a complete waste.

Potential Research Solutions: Inclusion of unnecessaryNBSs in the AS can be avoided if, along with the signalstrengths, the MS also considers its direction of motion forchoosing the AS constituents. The handover performanceenhancing technique “Predictive Base Station Switching“, forselecting and updating the current SBS and the AS at anyinstant, was proposed in [42]. This technique considers notonly the signal strengths of BSs but also the current directionand speed of the MS, to make a selection decision fromamong the NBSs. The scheme also predicts the probablefuture behaviour of the MS while making a decision. It isthus imperative that future MWiMAX handover research on


TABLE IVSUMMARY OF THE PROBABLE MAC-LAYER FBSS AND MDHO-RELATED ISSUES IN MWIMAX


Ping Pong Effects whileUpdating the AS

Non-significant difference between new and existingthreshold values may cause unnecessary update ofthe AS enhancing ping pong effects.

Accurately analysing threshold values [41] reducesunnecessary updating of ASs.

In-accurate AS Updatingbased on BSs’ SignalStrengths

Channel resources may be wasted owing to inclusionof unnecessary BSs in the AS depending only onBS’s signal strengths.

AS upgrading process may also consider the MS’sdirection of motion [42] along with the BS’s signalstrengths.

In-accurate AS Updatingbased on AbsoluteThreshold Values

Absolute threshold values may not be the best param-eters to upgrade the AS in real-life situations whereload of cells changes dynamically.

Relative threshold values can upgrade the ASs moreaccurately [43].

related issues pays more attention to devising significantpotential NBS selection techniques, taking into accountthe MS’s direction of motion along with the NBSs’ signalstrengths. This will reduce unnecessary resource wastageand will result in a better system performance. However,the means of accurately estimating the speed of the MS andits direction of motion need to be formulated, especiallyduring full vehicular mobility. Along with MS’s movementtrajectory, QoS requirements of the MS are also an issue.To provide the best network performance, the AS should beupdated with those NBSs that meet the QoS and bandwidthrequirements of the MS.

c) Inaccurate AS Updating based on Absolute ThresholdValues: In the MDHO and the FBSS, the MS updates theAS based on the absolute H ADD and H DELETE thresholdvalues contained in the DCDs broadcasted by the BSs. Atany instant, all the NBSs in the AS having CINR value lessthan H DELETE threshold are removed from set and those,from the candidate set (CS), with CINR values more thanH ADD threshold are added to the AS. However, in reality,with the load of a cell changing at every moment, relativethreshold values instead of absolute values seem to be morerealistic for accurate updating of the AS.

Potential Research Solutions: A similar technique basedon the relative threshold values was discussed in [43]. Inthis scheme, an NBS from the CS is transferred to the ASprovided Neighbour BS CINR − ABS CINR < H ADDthreshold and a BS from the AS is transferred to the CSprovided Active BS CINR − ABS CINR > H DELETEthreshold. Though this method provides a more accurate wayof active set updating, it is more complicated to implement.Therefore, in the current day scenario, with a substantialincrease in the number of mobile users each day, it is anuphill task to formulate suitable means of correctly choosingthe threshold values at any particular instant of time in orderto correctly update the AS.

B. MWiMAX Layer 3 Handover Issues

In MWiMAX technology, the network architecture from IP-layer onwards is still undefined and non-standardized. TheIEEE MWiMAX group, after specifying the L2 HO-relatedover-the-air messaging and procedures, has left further designsand standardization of the architecture to the NWG of the

MWiMAX Forum, which is currently developing the L3-related network messaging and further HO procedures ontop of the L2-base. Though several research activities aregoing on worldwide on designing a MWiMAX L3 onwardsmobility management framework, there is still a long way togo before something acceptable can be devised. A reference tothe MWiMAX CSN-anchored mobility (inter-ASN mobility)is required here. A handover in such a macromobility scenariooccurs when an MS moves from the current SBS in the currentsubnet to a different BS in a different subnet controlled by adifferent ASN-GW. Therefore, the IP-layer (L3) configurationof an MS changes as a result of such a handover. Unlike theASN-anchored scenario, in this case the mobility managementand the handover aspects engage both the ASN and CSNentities and are generally network-initiated [44]. Referringto Figure 7, whenever a terminal performs an inter-subnethandover (e.g. from BS1/BS2 under ASN-GW1 to BS3 underASN-GW2), it results to an IP-layer (L3) handover. It isrelated to re-configuration and reestablishment of new IP-connectivity. On the other hand, in Figure 4(b), every changeof BS automatically implies a change in the subnet and thusa change in the IP-connectivity of a terminal. Hence, in suchan architecture, the handovers always involve an alteration tothe MS IP-layer configurations.

However, though the NWG of the MWiMAX Forumhas embraced the different IETF protocols and provideda MWiMAX Network Reference Model (RFM) [28],[45]as a framework to develop the ASN and CSN-anchoredmobility schemes, the technical solutions to the differenthandover-related issues discussed in this paper are left openfor research on standard-compliant acceptable schemes andimplementations. In this context, it should be noted that incomparison to the MAC-layer handover issues, those in theIP layer have not attracted much research attention yet andas a new technology, with non-standardized network andupper layer architecture, MWiMAX faces a plethora of issuesrelated to IP handover, starting right from large L3 handoverlatencies to suitable choices of handover protocols. Theseissues should not only be dealt individually but also alongwith L2 issues to get the optimum results. For example, inorder to reduce the overall MWiMAX handover latency, itis required to reduce both the L3 handover latency and theL2 handover latency, in order to get the maximum reductionin latency. Hence, to do this, schemes should be devisednot only to reduce the L3 handover latency separately butalso to tackle it jointly with the L2 handover latency. The


ASN-GW1 ASN-GW2

W i M A XBS1

AAA

CSN

WiMAX CORE NETWORK

INTERNET

ASN-ANCHORED MOBILITY

CSN-ANCHORED MOBILITY

M S M S

W i M A XBS2

W i M A XBS3

Fig. 7. ASN and CSN-Anchored Mobility in MWiMAX

L3 handover schemes in case of CSN-anchored mobility arelargely based on either MIPv4 or MIPv6, as in WLAN, butas MIP is not very suitable for providing acceptable handoverperformance for the different time-sensitive applications,hence both the research community and the NWG areconsidering alternative means to tackle the L3 issues fordesigning effective MWiMAX inter ASN as well as ASN-CSN HO procedures in MWiMAX [46]. A summary (TableV) is followed by detailed discussion on some of these issues,see subsections below.

d) Large L3 Handover Latency: Similar to WLANand other cellular technologies, in MWiMAX too, duringinter-subnet mobility, the overall handover latency is thesum of the handover latencies in the MAC and IP-layers.Compared to the L2 handover latency, the latency in the L3handover scenario is larger, as it comprises of the delaysincurred in the discovery of the new point of attachment, theestablishment of the new CoA in the new subnet, and the MSnotifying its new location to the HA and other correspondents[52]. In the case of MWiMAX, which promises to providenon-disruptive QoS even for delay-sensitive high-speedstreaming multimedia applications, large L3 handover latencymay lead to unwanted communication disruption.

Potential Research Solutions: In view of the above-discussedideas, reduction of the MWiMAX L3 handover latency iscurrently gaining attention. As, in an IP-layer handover, theBSs involved always reside in different IP-subnets, and theterminals need to perform such L2 handovers along with newIP configurations in order to maintain connectivity [53]. Thus,

an L3 handover is always preceded by a well-establishedL2 connection. Issues on how indication of an ongoing L2handover process could help an early L3 handover initiationby the MS are discussed in [47]. Such an approach reducesthe L3 handover latency as the MS does not need to wait forthe Mobile IP (MIP) router advertisement procedure, whichtakes a longer time. However, in the case of MWiMAXenvironments, this scheme needs to pay further attention tosuch practical issues as proposing acceptable L2 triggeringmethodologies indicating a probable or an ongoing link layerhandover activity, along with suitable timings for the L2triggering. In MWiMAX, L2 handover triggers can originateat the MS or at the BS or even at the backbone network.In a scheme proposed in [48], anticipating a potential L2handover activity, the SBS sends a pre-handover notificationmessage to the corresponding access router. This helps thenetwork layer to initiate an early L3 handover procedure,thereby reducing the handover latency. However, selectingthe type of L2 triggers, whether predictive or event-based,is still an open issue. Predictive triggers, though giving anearly indication of a probable change in the system state,sometimes lead to false alarms as discussed in [54], and canbe hazardous for L3. Event-based triggering is devoid ofsuch problems but the advantage of early trigger initiationis absent in such cases. So, deciding upon the ideal choiceand timings of L2 triggers in MWiMAX networks in orderto reduce L3 handover latency is an open problem.

e) MAC State Migration Problem: MWiMAX HHOdoes not typically support MAC state transmission from thesource to the destination networks. Therefore, all MAC PDUsat the source network that remain non-transmitted duringthe handover are discarded and new PDUs are constructedat the target network from the received IP packets after thehandover is completed. However, there is always a highprobability that some of the untransmitted MAC PDUs maynot be recoverable [12], and resetting the MAC state in thetarget network can only be done by retransmissions, withthe help of the higher layers, like transport or application.However, this will cause serious delays unwanted for real-time delay-sensitive applications.

Potential Research Solutions: In order, to counter thisproblem, it may be possible that the serving network buffersall the IP packets [12] transmitted to the MS, such that incase of lost PDUs, the corresponding IP packets from thebuffer can be aptly tunnelled to the target network over thebackbone. The target MAC can accordingly reset the MACPDUs from those. Though the buffer size required in thiscase is large, the handover delay would be much less.

f) Interworking with MIPv6: In MWiMAX, a majorissue is supporting efficient IP mobility, particularly incase of inter-subnet movement of MSs. In order to provideunhampered and reliable QoS the IP connections should becontinuously and ably maintained across the changing routers.MIPv6 supports such global IP mobility in an efficient andscalable way. In a MIP-supported mobility environment, anMS can maintain its home address throughout its movement.


TABLE VSUMMARY OF THE PROBABLE IP-LAYER HANDOVER ISSUES IN MWIMAX


Large L3 Handover La-tency

Delay incurred in performing the different L3 han-dover steps is large. This affects the overall handoverperformance.

Timely indication of organised L2 triggers [47]-[48]can lead to early initiation of L3 handover activities.

MAC State MigrationProblem

Non-transmitted MAC state frames during HHO maybe lost and the delay incurred in retransmitting themmay degrade the system performance.

Serving network can buffer the IP packets meant forthe MS to reset the lost MAC frames from thosestored packets [12].

Interworking with MIPv6 Using MIP mobility concepts over non-standardizedMWiMAX upper-layer framework may lead to chal-lenges related with maintaining fast handovers, longsignalling and handover delays and failed data con-nectivity.

MIPv6-based fast and advanced handover schemesover MWiMAX are proposed in the forms ofFMIPv6 [49], HMIPv6 [50] and PMIPv6 [51].

When under a foreign router, the MS registers a newconfigured care-of-address (CoA) with a home networkrouter, which thus acts as the MS’s Home Agent (HA). TheHA tunnels all packets for the MS to its current location,based on its home address and the CoA. Figure 8 shows apotential Mobile IP Architecture in a MWiMAX environment.However, the large latencies occurring in MWIMAX handovercannot be reduced by MIPv6 alone, because MIPv6 mostlyserves as a location and path-management protocol [53]rather than a handover management protocol. It suffers fromdrawbacks like long handover latencies in case of new CoAconfiguration and MS’s location registration with the HA.Also, duplicated address detection (DAD) and long tunnelingdelays resulting from tunneling all packets for an MS throughits HA are the major issues here [55].

Potential Research Solutions: To counter all such MIPrelated drawbacks, the IPv6 Forum has collaborated with theMWiMAX NWG for discussing the MIP-related problems inMWiMAX mobility and handover scenario with the goal ofpromoting smooth MIP connectivity over MWiMAX. Apartfrom the basic MIP related mobility problems discussedabove, the collaboration has formulated other challenges[56]-[57] related to IPv4 or IPv6 adoption over the MWiMAXnetworks. During an inter-subnet handover, a MWiMAX-enabled MS, immediately after entering a foreign network,fails to maintain further data connectivity. This is because IPconnectivity in MIPv6 is re-established only after completionof the handover. The MS thus lacks any broadcasting or anyother communication facilities for IPv6 packet exchanges,which could have facilitated the detection of appropriaterouters or other nodes in the foreign network. Anotherserious IPv6-related problem in MWiMAX networks is theapplication of fast handovers over Mobile IPv6 links insuch networks. As identified in [49], such fast handovertechniques enables an MS to quickly detect its movementto a new subnet link and thus the MS can immediatelystart packet exchanges from that new link. Such handoverstherefore significantly reduce the overall L3 handover latency.However, to effectively carry out such HO techniques,role of L2 triggers is important, which MIPv6 truly lacks.As pointed out in [56], MWiMAX MSs do not have thefacility for multicasting of IPv6 packets after performingnetwork re-entry during a handover activity. Owing to this,immediately after entering a new network, a MWiMAX MS

I n t e r n e t

M S

WBS WBS

WBSWBS

WiMAX Subnets

CN

WBS WBS

WBSWBS

WBS WBS

WiMAX Subnets

HA

FAFA

M S

WiMAX Domain 1

WiMAX Domain 2

WiMAX Domain 3

AccessRouter 1

AccessRouter 3

AccessRouter 2

Fig. 8. Mobile IP Architecture in a MWiMAX Scenario

has no capability whatsoever for data connectivity and suffersfrom drawbacks like address resolution, router discovery andDAD [56]. These problems are more relevant in MWiMAXcentralized deployment architectures where the BSs maynot have any MIP functionalities loaded in them. Hence,the underlying architecture between the MWiMAX BSs androuters will control the MIP adoption methodologies forMWiMAX [58]. In this context, mechanisms like FMIPv6,HMIPv6 and PMIPv6, which are also gaining importancein the context to similar L3 issues in WLAN environments,are discussed to deal with the MIPv6-related problems inMWiMAX handovers.

g) Fast Handover for Mobile IPv6 (FMIPv6) inMWiMAX Mobility Scenario: FMIPv6 [49] takes care of thelatency factors in MIPv6 arising out of address configurationand movement detection procedures in MIPv6. It providesa seamless HO solution based on the IPv6 address spaceand efficient use of L2 triggers. L2 triggers enable anFMIPv6-enabled MS to quickly detect its movement to a newsubnet. FMIPv6 helps the MS to achieve its CoA even beforethe initiation of handover. It occurs in two possible scenarios:the predictive and reactive modes, respectively, depending


on whether the L3-HO occurs after setting-up a tunnelbetween old and new ARs or not. A detailed explanationof the procedure is given in [49]. However, a considerableperformance degrading connection disruption interval stillexists between the MS being disconnected from the oldAR and reconnected to the new AR. Also, in the case of aMWiMAX HO, if FMIPv6 is occurring in a reactive mode,it leads to increased latency and packet losses owing to theabsence of the tunnel.

Potential Research Solutions: In order to reduce thedrawbacks and to efficiently support FMIPv6 over theMWiMAX technology, a fast FMIPv6 scheme has beenproposed in [59] to facilitate MWiMAX inter-subnethandovers. Capable of operating in both predictive andreactive modes, this method uses four L2/L3 handovertriggers [59] to reduce the L3 handover latencies. Eachof these triggers is introduced during individual phases ofaccess router discovery, handover preparation, handoverexecution and handover completion, respectively. However,the scheme lacks an effective blending between the L2and L3 handover management messages and thus resultsin limited improvement of the overall performance. A newL3-HO trigger message, HO FASTHI, transmitted by theselected TBSs to the ARs, is proposed in [60]. It containsinformation about MS’s CoA and the previous AR. Usingthese information, prior to the probable HO activity, the ARsof the TBSs establishes HO tunnel with the previous ARto carry out the entire activity in a predictive mode. Thisenhances the overall HO performance. However, effectivelysetting-up the HO tunnels to make the HO run in a predictivefashion and using new L2-triggers during the FMIPv6-MWiMAX HO activity are some of the issues requiringfurther research.

h) Hierarchical MIPv6 (HMIPv6) in MWiMAX MobilityScenario: Like FMIPv6, HMIPv6 is also an improvedsolution for MIPv6 operating in both micro and macro-mobility modes. It reduces the amount of signaling overheadbetween the MS, its correspondent node (CN) and the HA.HMIPv6 supports a special network entity called the MobilityAnchor Point (MAP), which is basically a router or a groupof routers. During mobility, MAP acts as an HA to the MSand channels all traffic to the MS through the CoA. Thus,within a domain, a binding always remains between the MAPand the MSs. A detailed description of the scheme can befound in [50].

Potential Research Solutions: In the HMIPv6 handoverprocedure in MWiMAX, an L3 handover is initiated onlyafter the completion of the L2 handover. The overallperformance would be much better in terms of latency andpacket loss, if the L3 and L2 processes occur in parallel,utilizing timely L2 handover indications as discussed in[61]. However, as the HMIPv6 is only a localized or anintra-domain solution, there is scope for research on issuesrelated to effectively handling MWiMAX MSs when movingoutside the domain. Efficient interworking between HMIPv6and MIPv6 in a MWiMAX scenario requires more research.

Another advancement of the HMIPv6 protocol is proposedin [62]. This fast handover mechanism based on HMIPv6over MWiMAX is termed as Partner-Assisted HMIPv6. Herewith the help of another static subscriber station, called thepartner station (PS), an MS could detect the presence of aneighbour BS early enough, when it feels the necessity of ahandover. It might happen that an MS requiring a handoveris not within the coverage area of any neighbour BSs. At thispoint, with the help of a PS, within the coverage area of aneighbouring BS, the MS could detect the existence of theBS early enough to initiate a handover activity. It is assumedthat every PS in this context has relaying abilities and acts asa relay agent between an MS and the neighbouring BSs. ThePS helps an MS to perform L3 handover early enough beforethe MS actually reaches the TBS. This scheme gives betterresults than HMIPv6 in the MWiMAX scenario in terms oflatency and packet loss. However, finding suitable PSs inthe neighbouring MAP domains and within the neighbouringsubnet to perform pre-handover operations is an uphill taskin this scheme.

i) Proxy MIPv6 (PMIPv6) in MWIMAX MobilityScenario: A very recent proposal based on MIPv6 is thePMIPv6 mechanism, which is a network-based mobilityscheme [51]. PMIPv6 provides network-based mobilitymanagement support to MSs within a localized domain and isrecently becoming prevalent in the WLAN environments aswell. PMIPv6 introduces a new functional entity, the ProxyMobile Agent (PMA), a kind of MIPv4 foreign agent locatedon the AR. The PMA acts as a relay node between the HAand the MS. The MS does not participate in any sort ofmobility related signaling activities, as they are performed bythe PMA instead, on behalf of the MS. A detailed discussionabout PMIPv6 can be found in [51].

Potential Research Solutions: The NWG of the MWiMAXForum has identified PMIPv6 as a mechanism aligned withthe architectural direction of MWiMAX and thus as apotential solution to the MWiMAX MIPv6-related problems.An advantage of PMIPv6 in this context is that it canalso be useful in scenarios where the MWiMAX operatorsmight have an interest in host-based MIPv6 solutions, inorder to maintain some hosts in a network-based manner.Hence, a common infrastructure can be maintained bothfor host-based and network-based mobility. Other benefitsin terms of optimized HO performance offered by PMIPv6in the MWiMAX mobility scenario are moderate HOlatency, enhanced location privacy and low HO-relatedsignaling overheads [63]. However, PMIPv6-based mobilityis preferred in cases where the mobility is restricted within adomain. Room for further research exists with the applicationof PMIPv6 over MWiMAX network, specially combiningPMIPv6 with FMIPv6 [63] and HMIPv6 to further reducethe handover latency with the help of link layer triggers.In this context, Table VI provides a brief comparison of

MIPv6 and its different advancements with respect to thehandover techniques. Detailed discussion on these can befound in [64],[65] and [66]. Further research on designingimproved L3 handover frameworks is needed. Special con-


TABLE VIBRIEF COMPARISON OF L3 HO SCHEMES

Parameters MIPv6 HMIPv6 FMIPv6 PMIPv6

Complexity Medium High High Medium

Latency High High Low Medium

Scalability Medium Medium Medium Low

Packets Loss High Medium Medium Medium

Mobility Host-based

Host-based

Host-based

Network-based

Signalling Over-heads

High Medium High Medium

sideration should be given to issues like effective signalingmanagement, IP stack implementations across the MWiMAX-enabled BSs and MSs, and standardization of the design ofMWiMAX convergence sub-layers facilitating fast and losslesstransportation of IP packets. Also MIP-based handover fordelay-sensitive real-time traffic in MWiMAX needs specialconsideration. Ongoing MIPv6-related research activities areexpected to reduce the MIP handover related drawbacks and,hopefully, would be effective for both MWiMAX horizontalinter-subnet handovers as well as vertical handovers.

C. MWiMAX CROSS-LAYER (L2 + L3) HANDOVER ISSUES

Research contributions to the MWiMAX handoverframework to date have been mostly focussed on the linklayer aspects. Of late, the MWiMAX NWG, along withthe IPv6 forum and IETF, have initiated work on thenetwork and upper layer implementation facets, to proposea universally accepted MWiMAX macro-mobility andhandover framework. However, it is difficult to use thesesingle-layer-based solutions to provide a promising mobilityand handover support framework. The performance of sucha framework will depend on the integrated performance ofthe individual layers, specifically the link and the networklayers. Hence, optimization of MWiMAX seamless handoverperformance will largely depend on how effectively thelink-layer (L2) and the network layer (L3) HO methodologiescan be integrated without causing significant breaks in the IPconnectivity between the two handovers. In comparison tothe L3 challenges, significant research has been reported onsuch MWiMAX cross-layer issues, as explicit and impreciselower layer HO triggers to the upper layer, see Table VII.However, nothing concrete has been accepted yet. On theother hand, equally important issues like seamless integrationof L2 and L3 handover management messages and two-wayhandover information flows have not been much exploredyet. The following sub-categories present different potentialcross-layer handover research issues in Mobile MWiMAX.

j) Explicit HO Notifications to Upper Layers: MWiMAXmobility and HO-related research activities should concentrateon proposing HO generic dynamic event services [53], whichare triggered in time from the PHY or MAC layers andreported to the upper layers. This would result in betterHO performance resulting from reduced delays and resourcewastages, in comparison to situations where HO decisionsare solely based on the L3 indications. For example, a fast

handover process in a MIPv6 environment, improving theperformance of the overall handover procedure, is discussedin [49]. However, in this case, the IP layer handover proceduregets initiated only after the completion of the L2-handoverprocess. Thus, it increases the total handover delay, which isthe sum of handover delays in both the layers.

Potential Research Solutions: L2 HO event services[69] indicate a probable L2-event marking an upcomingchange in the L2-point of attachment of an MS within aparticular subnet. In the case of MWiMAX, such indicationsmay either be solicited or unsolicited MAC messagesdirectly from the MAC layer, or they might be derived fromother MAC management messages. Effective usage of suchL2 triggers are proposed in [59], based on fast seamlessinter-domain handover mechanisms in MWiMAX by timelyexploiting the L2 handover indicators. Multiple L2 triggerslike New Link Detected (reports detection of a new link),Link Handover Impend (a L2 handover is to occur soon)and Link Up (Link layer handover completed) are introducedin the different stages of the overall handover procedure.However, the scheme lacks an effective blending between theL2 and L3 handover management messages and thus resultsin limited improvement of the overall performance. So, alongwith generation of effective L2 event triggers, what is reallyneeded is a meaningful correlated overlay of the MWiMAXL2 and L3 layers. This would also enable L3 to effectivelyand successfully derive any inexplicit L2 handover indicationon the fly.

k) Imprecise L2 Triggers: Timely generation of aneffective L2-handover trigger is a big challenge. In regardto a MWiMAX L3-handover scenario, if a L2-trigger is notgenerated well in advance, then it would not be possible toachieve the expected boost in the overall performance. Ideally,an L2-handover trigger should be generated much before theonset of a L2-handover event, so that there remains sufficienttime for the Layer 3 to predict a probable handover activityand act accordingly. The effect of concurrent processing ofthe L2 and L3 handover mechanisms, which is an importanttechnique to maintain a stable QoS for delay-sensitiveapplications, would be large, provided the L2 notificationsare communicated in time.

Potential Research Solutions: Timely initiation of a L2-handover trigger is well recognized as a difficult problem andhence has drawn considerable attention. As discussed in [70],an untimely generation of L2-handover trigger changes theFMIPv6 handover mode from a predictive one to a reactiveone, causing a significant degradation in the entire handoverperformance. However, there is much scope for furtherresearch on this issue, particularly as the standardization ofthe MWiMAX layer-3 architecture is still an open issue.The L2 handover trigger in the form of predicted signalstrength (RSSI) values, tracked periodically by an MS, hasbeen introduced in [54]. Timely generation of such triggersalways initiates the MWiMAX L3-handover activity in apredictive manner well in advance, thus minimizing packetlosses. However, this scheme also suffers from unwanted


TABLE VIISUMMARY OF THE PROBABLE CROSS-LAYER HANDOVER ISSUES IN MWIMAX


Explicit HO Notificationsto Upper Layers

Lack of HO generic suitable dynamic event triggersfrom MWiMAX PHY/MAC layers to the IP-layerdegrades HO performance as in that case the L3 HOgets initiated after the completion of the L2 HO.

Explicit L2 to L3 event triggers during the variousstages of the overall MWiMAX HO activity areproposed in [59] for enhancing the performance.

Imprecise L2 Triggers Untimely generation of L2-triggers hampers themaximum boost in the HO performance. In addition,false L2-triggers degrade performance.

MSs can send the L2-HO trigger early enough to theupper layers in the form of predicted RSSI values[54].

Seamless Integration ofL2 and L3 Mobility Man-agement Messages

Merely overlaying the MWiMAX L2 and L2 HOprocedures without any effective correlation betweenthem increases the overall latency.

Removal of related HO management messages fromboth the MWiMAX L2 and L3 HO proceduresand coincidental processing of both the proceduresenhances the overall performance [67]-[68].

Two-Way Cross-LayerHandover InformationFlow

Dynamic collaboration of the HO procedures of dif-ferent layers with diverse functionalities is a difficulttask.

Multiple event and command services to improve theFMIPv6 HO support over the MWIMAX MAC [53].

MIP signaling overheads owing to false L2 handover alarms.Generation of false or untimely L2-handover alarms can bea big problem in case of Mobile MWiMAX networks withmoderate cell sizes. In the case of high-speed mobility, itmaximizes the chance of ping-pong activities. Hence, inorder to achieve maximum gain in performance, triggersshould be genuine and generated only when the MS or theBS becomes sure of a probable/ongoing handover activity.However, finding the ideal L2 notification time, which wouldmaximize the gain, is itself tricky and specific to the IP-layertechnologies used.

l) Seamless Integration of L2 and L3 MobilityManagement Messages: Efficiently integrating the MWiMAXL2 and L3 MAC management messages poses significantchallenge, particularly owing to the non-standardized IP-layerof MWiMAX architecture. Recently, a couple of host-based IP-layer localised mobility management techniques,like FMIPv6 [49] and HMIPv6 [50], have been proposedand have drawn significant research attention from theMWiMAX community, focussing on tentative seamlessmerging techniques between the IP-layer protocols and theMWiMAX MAC layer mobility techniques.

Potential Research Solutions: Seamless integration ofMWiMAX L2 and L3 mobility management messagesrequires an effective correlation between the messages ofboth the layers, rather than simply overlaying the L2 andL3 handover procedures as can be seen in [59]. Mereoverlaying of the layers may hamper the improvement of thehandover performance, as it would cause increased delaysin processing more handover control messages. On theother hand, effectively correlating the mobility managementmessages of the MAC and the IP-layer can reduce thenumber of related messages in both the layers, and reducesthe overall handover delay. The schemes proposed in [67]-[68] discuss effective integration scenarios between FMIPv6in the IP-layer and 802.16e MAC-layer mobility managementtechniques. Both the schemes propose integrated cross-layerdesign approaches, based on seamless combination of the L2and L3 handover management messages. Removal of relatedmanagement messages from both layers and coincidental

processing of both the layers have led to an improvementof the overall handover delay and reduction of resourcewastages. Despite such research advancements, designing anuniversally accepted IP and MAC-layer integrated MWiMAXhandover framework has still a long way to go because ofsuch unresolved issues like choosing the best managementprotocol for the MWiMAX IP-layer in terms of scalability,complexity and implementation cost, effectively identifyingand removing the related cross-layer mobility controlmessages, and keeping the QoS unhampered in a cross-layerscenario. The trade-off between improved latency, highercomplexity and cost should also be taken into account.

m) Two-Way Cross-Layer Handover Information Flow:The usefulness of the two-way (back and forth) cross-layerinformation flow model during a handover activity hasbeen identified in [71]. To get the maximum improvementin the overall handover performance along with the lowerlayer triggers (event services), explicit notification ofevents from the upper layers to the lower layers (commandservices) are also required [72]-[73]. In comparison to thedifferent MWiMAX cross-layering handover approachesdiscussed before, which use a single-way signaling techniqueparticularly from the MAC-layer to the IP-layer, a two-waysignaling scheme not only helps to achieve fast handoverbut also is useful in terms of resource utilisation. However,designing such a two-way signaling scheme is complicatedbecause it requires the two different layers, with differentfunctionalities and performing different tasks, to collaboratedynamically, which is undoubtedly a steep challenge.

Potential Research Solutions: A cross-layering designapproach for improving the FMIPv6 handover support over theMWiMAX MAC layer technology has been proposed in [53].Using the back and forth signaling flow model, this schemeintroduces three different triggers from L2 to L3, namely,NEW CANDIDATE BS FOUND, LINK GOING DOWNand LINK UP, along with LINK SWITCH, a hint from L3 toL2, at the different stages of the handover activity. It is shownthat this approach results in fast handover activity both in thepredictive and reactive modes of handover. Another similarapproach employing a two-way information flow modelproviding solutions for both intra-MAP (micro mobility) and


TABLE VIIIEVENT AND COMMAND TRIGGERS FOR MWIMAX CROSS-LAYER HO

Type ofServices

Name of Services / Proposed w.r.t From To Description of Services

Event NEW LINK DETECTED /NEW CANDIDATE BS FOUND.

Proposed w.r.t: FMIPv6

L2 L3 Reports the L3 about the detection of a new BS for apotential HO activity.

Purpose: To learn about the Access Router (AR) associatedwith the newly detected BS.

Event LINK HANDOVER IMPEND /LINK GOING DOWN.


L2 L3 Reports the L3 about imminent execution of an HO activity.

Purpose: To indicate the L3 to get prepared for a likely HOprocedure.

Event LINK UP.


L2 L3 Reports the L3 that the link-layer connection establishmentwith the new BS is accomplished successfully.

Purpose: Enables the L3 to check whether it has reallymoved to the predicted target network.

Event RSSI / SNR values.

Not incorporated yet.

L2 L3 A low value reports the L3 that a HO is expected within acertain Δt time.

Purpose: Enables the L3 to start the L3-HO procedureearly enough even before the L2-HO.

Event HO-NOTIF.

Proposed w.r.t: FMIPv6.

L3 HO-INITIATE.

Proposed w.r.t: HMIPv6.

L2 L3 Reports the L3 of an impending HO activity and containsthe information of the SBS recommended TBSs and MACaddress of MS.

Purpose: Enables the initiation of the L3-HO.

Event L3 Buffer-INITIATE.

Proposed w.r.t: HMIPv6

L2 L3 During an intra-domain HO, MAP reports the MS’s LCoAto the new AR after receiving the local binding update fromthe old AR.

Purpose: Enables the new AR to initiate buffering forthe MS during the impending HO phase.

Event L2 HO-COMPLETE.

Proposed w.r.t: HMIPv6

L2 L3 TBS reports the completion of the L2-HO activity to thenew AR.

Purpose: Initiates the completion of an L3 HO andchannelization of all buffered packets for the MS from thenew AR.

Event LINK LOST.

Not incorporated yet.

L2 L3 Reports the TBS of a ping-pong effect.

Purpose: Initiates the AR associated with the new TBS toflush back all the buffered data to the old SBS.

Event HO-FASTHI.


L2 L3 A fast tunnelling message, through which, the selected TBSstransmit the NCoA of the MS to the associated ARs.

Purpose: Initiates the associated ARs to validate theNCoA of the MS and sets up a tunnel with the PAR if theNCoA is valid.

Command LINK SWITCH.


L3 L2 Forces an MS to perform an L3 switch from under thecurrent SBS to the TBS.

Purpose: The L3 asks the L2 to transmit the MOB HO-INDcommand asap.

inter-MAP (macro mobility) handovers in HMIPv6-basedsystems is proposed in [74]. A detailed description of thedifferent event and command services, as outcomes of recentMWIMAX cross-layer handover research activities, is givenin Table VIII. However, as limited attention has been focusedon a two-way notification approach, so there is significantscope for research in this area. Specifically, emphasis

should be given on designing explicit approaches (commandservices) by which the upper layers (IP layer in this case)can timely notify (or hint) the MWiMAX lower layers (MACand PHY) about the processing of the application data basedon MAC layer control messages. However, this could havean enhanced effect on the handover performance only if thelayers are interleaved seamlessly.


As evident from the above discussion, there is still along way to go before an universally accepted performance-optimized MWiMAX CLHM framework could be formulated.It should noticeably minimize the handover overheads likedelays, connection drops and packet losses, both in case ofMWiMAX intra- and inter-technology handover scenarios.Along with HMIPv6 and FMIPv6, PMIPv6 also requires moreattention as a potential IP-layer technology in the MWIMAXcross-layer handover domain. On the other front, reasonslike non-standardization of MWiMAX upper layers and theMWiMAX NWG not being sure about the potential architec-tural deployment scenario (hierarchical, flat or hybrid), hinderthe devising of an ideal MWiMAX CLHM.

VI. CONCLUSION

Efficient support of seamless handover management activityis an important requirement for communication technologiesthat are intended to be universally accepted in next-generationcommunication systems. Although MWiMAX has a numberof attractive features, its handover framework is not free fromdrawbacks and has attracted significant research attention. Thispaper has identified the diversified MAC layer and potentialnetwork layer handover issues in MWiMAX, and also hashighlighted those cross-layer (L2+L3) challenges that demandmore attention. Out of these, the MAC-layer HHO issuesrelated to the reduction and optimization of scanning activitiesand inter-handover CDT are still considered to be wide open,as the MWiMAX Forum has not reached a definite conclusionregarding whether and how to modify the existing standardto incorporate the changes suggested to date. On the otherhand, the issues identified in Table III, on optimization of thenetwork re-entry activities, load distributions, and those on theMDHO and FBSS presented in Table IV, have not yet attractedmuch research attention. Moving up the ladder, cross-layerchallenges have gained more attention than the solely Layer3 ones. This could be because the overall macro-mobilityhandover performance depends jointly on the performance ofthe L2 and L3 handovers rather than only on the L3 han-dover. However, even then, cross-layer issues, like seamlessintegration of L2 and L3 handover management messagesand efficient bidirectional flow of these messages, have beenless visited than the other issues and need more attentionin order to devise a good MWiMAX CLHM framework. Astandardization of the MWiMAX L3 framework would helpto achieve this. Attention should be given to choosing thebest option for MIPv6 (e.g. FMIPv6, HMIPv6 or PMIPv6),to provide seamless handover performance for high-speedreal-time multimedia applications. Although, it appears thatPMIPv6 is the most promising option, being the mechanismclosely aligned with the architectural direction of MWiMAX,since nothing has been decided yet, this is still a wide openarea for investigation.

ACKNOWLEDGMENT

The authors would like to thank Dr. Allan McInnes, Depart-ment of Electrical and Computer Engineering, University ofCanterbury for stimulating discussions and helpful comments,and the reviewers for suggestions, which improved the contentand style of the paper.

APPENDIX ALIST OF ACRONYMS USED IN THE PAPER

Acronyms Full FormABS Anchor Base StationAHOP Actual Handover PhaseAR Access RouterAS Active SetASN Access Service NetworkASN GW Access Network GatewayBS Base StationBSID Base Station IdentificationB3G Beyond 3GCDMA Code Division Multiple AccessCDT Connection Disruption TimeCID Connection IdentificationCINR Carrier to Interference + Noise RatioCLHM Cross-Layer Handover ManagementCoA Care-of-AddressCN Correspondent NodeCS Candidate SetCSN Connectivity Services NetworkDAD Duplicate Address DetectionDCD Downlink Channel DescriptorDL DownlinkDL MAP IE Downlink MAP Information ElementDS Diversity SeteNB E-UTRAN (UMTS Terrestrial Radio

Access Networks) Node BEV-DO Evolution-Data OptimizedFA Frequency AssignmentsFBSS Fast Base Station SwitchingFMIPv6 Fast Handovers for MIPv6FUSC Fully Used Sub-Channelization4G Fourth-GenerationHA Home AgentHHO Hard HandoverHMIPv6 Hierarchical MIPv6HO HandoverHSDPA High-Speed Downlink Packet AccessIEEE Institute of Electrical and Electronics

EngineersIETF Internet Engineering Task ForceL2 Layer 2L3 Layer 3LTE Long Term EvolutionMA Mobility AgentMAP Mobility Anchor PointMBB Make-Before-BreakMDHO Macro-Diversity HandoverMIP Mobile IPMIPv6 Mobile IP version 6MIPv4 Mobile IP version 4MME Mobility Management EntityMOB ASC-REP Association Result ReportMOB HO-IND Mobile Handover IndicationMOB MSHO-REQ Mobile Station Handover RequestMOB BSHO-REQ Base Station Handover RequestMOB BSHO-RSP Base Station Handover ResponseMOB NBR-ADV Mobile Neighbour AdvertisementMOB SCN-REQ Scanning Interval Allocation RequestMOB SCN-RSP Scanning Interval Allocation ResponseMOB SCN-REP Scanning Result ReportMS Mobile StationMWiMAX Mobile Wireless Interoperability for

Microwave Access (Mobile WiMAX)NBS Neighbouring Base StationNTAP Network Topology Acquisition PhaseNWG Network Working GroupOFDM Orthogonal Frequency-Division Multi-

plexing


OFDMA Orthogonal Frequency-Division Multi-ple Access

PDU Protocol Data UnitPMA Proxy Mobile AgentPMIPv6 Proxy MIPv6PS Partner StationPUSC Partially Used Sub-ChannelizationQoS Quality of ServiceRAN Radio Access networkRNG REQ Ranging RequestRNG RSP Ranging ResponseRRM Radio Resource ManagementRSSI Received Signal Strength IndicationSAE System Architecture EvolutionSBS Serving Base StationSCR Spare Capacity ReportSHO Soft HandoverTBS Target Base Station3G Third-GenerationUCD Uplink Channel DescriptorUL UplinkUMTS Universal Mobile Telecommunications

SystemVoIP Voice-Over-Internet ProtocolWLAN Wireless Local Area NetworksWiMAX Wireless Interoperability for

Microwave Access

REFERENCES

[1] IEEE 802.16-2004: IEEE Standard for Local and Metropolitan AreaNetworks-Part 16: Air Interface for Fixed Broadband Wireless AccessSystems.

[2] IEEE 802.16e-2005: IEEE Standard for Local and Metropolitan AreaNetworks-Part 16: Air Interface for Fixed and Mobile BroadbandWireless Access Systems.

[3] Ed Agis et. al. Global, Interoperable Broadband Wireless Networks:Extending WiMAX Technology to Mobility. Intel Technology Journal,8(3):173-187, 2004.

[4] R. Q. Hu et al. On the Evolution of Handoff Management and NetworkArchitecture in WiMAX. In Proc. IEEE Mobile WiMAX Symposium,pages 144-149, Florida, USA, 25-29 March 2007.

[5] Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN) V7.3.0. 3rd Generation Partnership Project, March 2006 (TR25.913). http://www.3gpp.org

[6] Z. Becvar and J. Zelenka. Handovers in Mobile WiMAX. Research inTelecommunication Technology, 1:147-150, 2006, ISBN 80-214-3243-8.

[7] Mobile WiMAX-Part I: A Technical Overview andPerformance Evaluation. White Paper, WiMAX Forum,August 2006. http://www.wimaxforum.org/news/downloads/Mobile WiMAX Part1 Overview and Performance.pdf

[8] CDMA 2000 High Rate Packet Data Air Interface Specifica-tion. 3GPP2 C.S0024-A, Standard, ver. 2.0, 27 October 2000.http://www.3gpp2.org/public html/specs/C.S0024 v2.0.pdf

[9] Physical Layer Aspects of UTRA High Speed DownlinkPacket Access. 3GPP TR 25.848, 3GPP Specification Series.http://www.3gpp.org/ftp/Specs/html-info/25848.htm

[10] Technical Specification Group Radio Access Network: Physical layer-General description. 3GPP TR 25.201, 3GPP Specification Series.http://www.3gpp.org/ftp/Specs/html-info/25201.htm

[11] Mobile WiMAX-Part II: A Comparative Analysis. White Paper, WiMAXForum, May 2006. http://www.wimaxforum.org/news/downloads/ Mo-bile WiMAX Part2 Comparative Analysis.pdf

[12] S. Das et al. System Aspects and Handover Management for IEEE802.16e. Bell Labs Technical Journal, 11(1):123-142, 2006.

[13] B. Gage et al. WiMAX: Untethering the Internet User. Nortel TechnicalJournal, 2:31-38, July 2005.

[14] A. R. Gana. Wireless Communication System for Land Seismic Op-erations: A Feasibility Study. Master of Science Thesis, NorwegianUniversity of Science and Technology, July 2008.

[15] Mobile Broadband Wireless Access (MBWA). IEEE 802.20 WorkingGroup Permanent Documents, http://www.ieee802.org/20/index.html.

[16] iBurst Technical Profile. Kyocera Global Site, White Paper.http://global.kyocera.com/prdct/telecom/office/iburst/technicaloverview.pdf

[17] IEEE 802.16 Task Group m (TGm). http://wirelessman.org/tgm

[18] F. Wang et al. Mobile WiMAX Systems: Performance and Evaluation.IEEE Commun. Mag., 46(10):41-49, October 2008.

[19] D. H. Lee, K. Kyamakya and J. P. Umondi. Fast Handover Algorithmfor IEEE 802.16e Broadband Wireless Access System. In Proc. 1st

International Symposium on Wireless Pervasive Computing, Phuket,Thailand, 16-18 January 2006.

[20] Sayan K. Ray et al. Hybrid Predictive Base Station (HPBS) Selec-tion Procedure in IEEE 802.16e-Based WMAN. In Proc. AustralasianTelecommunication Networks and Applications Conference (ATNAC),pages 93-98, Christchurch, New Zealand, 2-5 December 2007.

[21] R. Rouil and N. Golmie. Adaptive Channel Scanning for IEEE 802.16e.In Proc. IEEE Military Communications Conference (MILCOM), pages1-6, Washington D.C, USA, 23-25 October 2006.

[22] S. Cho et al. Hard Handoff Scheme Exploiting Uplink and DownlinkSignals in IEEE 802.16e Systems. In Proc. IEEE Vehicular TechnologyConference (VTC), vol. 3, pages 1236-1240, Melbourne, Australia,Spring 2006.

[23] S. Choi et al. Fast handover scheme for Real-Time Downlink Services inIEEE 802.16e BWA System. In Proc. Vehicular Technology Conference(VTC), vol. 3, pages 2028-2032, Stockholm, Sweden, Spring 2005.

[24] Y. Saifullah and A. Reid. Low Latency Handover. IEEE 802.16 Broad-band Wireless Access Working Group Project, IEEE C802.16g-05/18r0,29 April 2005. http://wirelessman.org/netman/contrib/C80216g-05 018r0.pdf

[25] H. Kang, C. Koo and J. Son. Resource Retain Time for Handoveror Ping Pong Call Recovery. IEEE 802.16 Broadband Wireless Ac-cess Working Group Project, IEEE C802.16e-04/55r2, 17 May 2004.http://wirelessman.org/tge/contrib/C80216e-04 55r2.pdf

[26] B. Meandzija and P. Iyer. Minimizing IP Connectivity Delay dur-ing Network Re-Entry. IEEE 802.16 Broadband Wireless AccessWorking Group Project, IEEE C802.16e-04/151r1, 25 June 2004.http://wirelessman.org/tge/contrib/C80216e-04 151.pdf

[27] T. Casey, N. Veselinovic and R. Jantti. Base Station Controlled LoadBalancing with Handovers in Mobile WiMAX. In Proc. IEEE 19th

International Symposium on Personal, Indoor and Mobile Radio Com-munications (PIMRC), pages 1-5, Cannes, France, 15-18 September2008.

[28] WiMAX Forum Network Architecture-Stage 2: Architecture Tenets,Reference Model and Reference Points-Release 1, Version 1.2. WiMAXForum Network Working Group, WiMAX Forum, January 2008.

[29] P. Barber, Revision of Handover Mechanism for MobilityEnhancement. IEEE 802.16 Broadband Wireless Access WorkingGroup Project, IEEE C802.16e-03/57, 30 October 2003.http://wirelessman.org/tge/contrib/C80216e-03 57.pdf

[30] H-G. Choi, J. Jeong and H. Choo. CTBS: Cost-Effective Target BSSelection Scheme in IEEE 802.16e Networks. In Proc. AustralasianTelecommunication Networks and Applications Conference (ATNAC),pages 99-103, Christchurch, New Zealand, 2-5 December 2007.

[31] H. Fattah and H. Alnuweiri. A New Handover Mechanism for IEEE802.16e Wireless Networks. In Proc. International Wireless Communi-cations and Mobile Computing Conference (IWCMC), pages 661-665,Crete Island, Greece, 6-8 August 2008.

[32] N. Banerjee, K. Basu and S. K. Das. Handoff Delay Analysis in SIP-based Mobility Management in Wireless Networks. In Proc. Interna-tional Parallel and Distributed Processing Symposium (IPDPS), pages224-231, Nice, France, 22-26 April 2003.

[33] W. Jiao, P. Jiang and Y. Ma. Fast Handover Scheme for Real-Time Ap-plications in Mobile WiMAX. In Proc. IEEE International Conferenceon Communications (ICC), pages 6038-6042, Glasgow, Scotland, 24-28June 2007.

[34] S. K. Ray, K. Pawlikowski and H. Sirisena, A Fast MAC-Layer Han-dover for an IEEE 802.16e-Based WMAN. In Proc. 3rd InternationalConference on Access Networks (Accessnets), Las Vegas, USA, 15-17October 2008.

[35] H-A (Paul) Lin. Handoff for Multi-interfaced 802 MobileDevices. IEEE P802 Handoff ECSG, Document, May 2003.www.ieee802.org/21/archived docs/Documents/Submissions/Handoff for Multi interfaced MN.pdf

[36] H. Kang et al. Ping Pong Call Resuming Procedure duringHO. IEEE 802.16 Broadband Wireless Access WorkingGroup Project, IEEE 802.16e/03-26r1, 11 March 2004.http://www.ieee802.org/16/tge/contrib/C80216e-04 26r1.pdf

[37] H. Velayos, V. Aleo and G. Karlsson. Load Balancing in OverlappingWireless LAN Cells. In Proc. IEEE International Conference on Com-munications (ICC), vol. 7, pages 3833-3836, Paris, France, 20-24 June2004.

[38] WiMAX Forum Network Architecture-Stage 3: Detailed Protocols and


Procedures-Release 1.1.2. WiMAX Forum Network Working Group,WiMAX Forum, 11 January 2008.

[39] T. Casey. Base Station Controlled Load Balancing with Handoversin Mobile WiMAX. Master of Science Thesis, Helsinki University ofTechnology, 10 January 2008.

[40] S. H. Lee and Y. Han. A Novel Inter-FA Handover Scheme for LoadBalancing in IEEE 802.16e System. In Proc. IEEE Vehicular TechnologyConference (VTC), pages 763-767, Dublin, Ireland, Spring, 22-25 April2007.

[41] K. Raivio. Analysis of Soft Handover Measurements in 3G Network.In Proc. 9th ACM International Symposium on Modeling Analysis andSimulation of Wireless and Mobile Systems (MSWiM), pages 330-337,Terromolinos, Spain, 2-6 October 2006.

[42] O. C. Ozdural. Performance-Improving Techniques for Wireless Sys-tems. PhD Thesis 2007, Oregon State University.

[43] A. Ulvan. Using the Relative Thresholds in HandoverProcedure. IEEE 802.16 Broadband Wireless Access WorkingGroup Project, IEEE C802.16j-07/086, 8 January 2006.http://wirelessman.org/relay/contrib/C80216j-07 086.pdf

[44] J. Pinola and K. Pentikousis. Mobile WiMAX. The Internet ProtocolJournal, Cisco Systems, 11(2):19-35, June 2008.

[45] K. Etemad. Overview of Mobile WiMAX Technology and Evolution.IEEE Commun. Mag., 46(10):31-40, October 2008.

[46] P. Iyer et. al. All-IP Network Architecture for Mobile WiMAX. In Proc.IEEE Mobile WiMAX Symposium, pages 54-59, Florida, USA, 25-29March 2007.

[47] C-T. Chou and K.G. Shin. An Enhanced Inter-Access Point Protocol forUniform Intra and Inter Subnet Handoffs. IEEE Trans. Mobile Comput.,4(4):321-334, July-August 2005.

[48] Y. H. Choi, Y. U. Chung and H. Lee. Early HandoverTrigger. IEEE 802.16 Broadband Wireless Access WorkingGroup Project, IEEE C802.16j-07/150, 8 January 2007.http://wirelessman.org/relay/contrib/C80216j-07 150.pdf

[49] R. Koodli. Mobile IPv6 Fast Handovers. MIPSHOP Working Group,Internet-Draft, draft-ietf-mipshop-fmipv6-rfc4068bis-07.txt, 17 April2008. http://tools.ietf.org/html/draft-ietf-mipshop-fmipv6-rfc4068bis-07

[50] H. Soliman et. al. Hierarchical Mobile IPv6 Mobility Management(HMIPv6). RFC 4140, 2005.

[51] S. Gundavelli et. al. Proxy Mobile IPv6. NETLMM WG, Internet-Draft, draft-sgundave-mip6-proxymip6-01, 5 January 2007.http://tools.ietf.org/html/draft-sgundave-mip6-proxymip6-01

[52] N. Montavont and T. Noel. Handover Management for Mobile Nodesin IPv6 Networks. IEEE Commun. Mag., 40(8):38-43, August 2002.

[53] Y-H. Han et. al. A Cross-Layering Design for IPv6 Fast HandoverSupport in an IEEE 802.16e Wireless MAN. IEEE Network, 21(6):54-62, November-December 2006.

[54] Y-H. Choi et. al. Cross-Layer Handover Optimization Using LinearRegression Model. In Proc. International Conference on InformationNetworking (ICOIN), pages 1-4, Busan, Korea, 23-25 January 2008.

[55] C-K. Chang. A Mobile-IP based Mobility System for WirelessMetropolitan Area Networks. In Proc. International Conference Work-shops on Parallel Processing (ICPPW), pages 429-435, Oslo, Norway,14-17 June 2005.

[56] J. Jee et al. 16ng Problem Statement. Network Working Group, Internet-Draft, draft-jee-16ng-problem-statement-02.txt, 16 October 2005.http://bgp.potaroo.net/ietf/all-ids/draft-jee-16ng-problem-statement-02.txt

[57] J. Jee et al. Mobile IPv4 Fast Handovers for 802.16e networks. NetworkWorking Group, Internet-Draft, draft-jee-mip4-fh80216e-00.txt, 11 Oc-tober 2005. http://www.potaroo.net/ietf/idref/draft-jee-mip4-fh80216e

[58] M-K. Shin, J-M. Moon and Y-H. Han. Scenarios and Considerations ofIPv6 in IEEE 802.16 Networks. draft-shin-ipv6-ieee802.16-02, February2006.

[59] H. J. Jang et. al. Mobile IPv6 Fast Handovers over IEEE 802.16e Net-works. MIPSHOP Working Group, Internet-Draft, draft-ietf-mipshop-fh80216e-07.txt, 10 March 2008. http://tools.ietf.org/html/draft-ietf-mipshop-fh80216e-07

[60] J. Kim, J. Jeong and H. Choo. An Efficient Handover Scheme with Pre-Configured Tunneling in IEEE 802.16e Systems. In Proc. AustralasianTelecommunication Networks and Applications Conference (ATNAC),pages 408-413, Christchurch, New Zealand, 2-5 Deember 2007.

[61] D-G. Kim, H-J. Shin and D-R. Shin. A Network-based HandoverScheme for Hierarchical Mobile IPv6 over IEEE 802.16e. In Proc.10th International Conference on Advanced Communication Technology(ICACT), vol. 1, pages 468-472, Gangwon-Do, South Korea, 17-20 Feb.2008.

[62] Y-S. Chen et al. A Cross-Layer Partner-Assisted Handoff Scheme forHierarchical Mobile IPv6 in IEEE 802.16e Systems. In Proc. IEEE

Wireless Communications and Networking Conference (WCNC), pages2669-2674, Las Vegas, USA, 31 March-03 April 2008.

[63] J. Lei and X. Fu. Evaluating the Benefits of Introducing PMIPv6for Localized Mobility Management. In Proc. International WirelessCommunications and Mobile Computing Conference (IWCMC), pages74-80, Crete Island, Greece, 6-8 August 2008.

[64] X. P-Costa, M. T-Moreno and H. Hartenstein. A Performance Com-parison of Mobile IPv6, Hierarchical Mobile IPv6, Fast Handoversfor Mobile IPv6 and their Combination. ACM SIGMOBILE MobileComputing and Communications Review, 7(4):5-19, October 2003.

[65] J. Lei and X. Fu. Evaluating the Benefits of Introducing PMIPv6for Localized Mobility Management. Technical Report, University ofGottingen, Germany, June 2007.

[66] Z. Zhang et al. Performance Comparison of Mobile IPv6 and Its Exten-sions. In Proc. International Conference on Wireless Communications,Networking, and Mobile Computing (WiCom), pages 1805-1808, 21-25September 2007.

[67] Y-W. Chen and F-Y. Hsieh. A Cross Layer Design for Handoff in802.16e Network with IPv6 Mobility. In Proc. IEEE Wireless Commu-nications and Networking Conference (WCNC), pages 3844-3849, HongKong, 11-15 March 2007.

[68] J. Park, D-H. Kwon and Y-J. Suh. An Integrated Handover Schemefor Fast Mobile IPv6 Over IEEE 802.16e Systems. In Proc. VehicularTechnology Conference (VTC), pages 1-5, Montreal, Canada, Fall, 25-28September 2006.

[69] L-A. Larzon, U. Bodin and O. Schelen. Hints and Notifications. In Proc.IEEE Wireless Communications and Networking Conference (WCNC),vol. 2, pages 635-641, Florida, USA, 17-21 March 2002.

[70] J. Kempf, J. Wood and G. Fu. Fast Mobile IPv6 Handover Packet LossPerformance: Measurement for Emulated Real Time Traffic. In Proc.IEEE Wireless Communications and Networking Conference (WCNC),vol. 2, pages 1230-1235, Louisiana, USA, 16-20 March 2003.

[71] V. Srivastava and M. Motani. Cross-Layer Design: A Survey and theRoad Ahead. IEEE Commun. Mag., 43(12):112-119, December 2005.

[72] F. Foukalas, V. Gazis and N. Alonistioti. Cross-Layer Design ProposalsFor Wireless Mobile Networks: A Survey and Taxonomy. IEEE Com-mun. Surveys Tutorials, 10(1):70-85, 1st Quarter 2008.

[73] M. Shariat, A. U. Quddus, S. A. Ghorashi and R. Tafazolli. Schedullingas an Important Cross-Layer Operation for Emerging Broadband Wire-less Systems. IEEE Commun. Surveys Tutorials , 11(2):74-86, 2ndQuarter 2009.

[74] Y. Zhang, F. Liu and X. Wang. A Cross-Layer Fast Handover Mech-anism for IEEE 802.16e Networks with HMIPv6 Mobility. In Proc.Workshop on Power Electronics and Intelligent Transportation System(PEITS), pages 3-7, Guangzhou, China, 4-5 August 2008.

Sayan Kumar Ray has received the Bachelor of Engineering and Masterof Technology degrees in Computer Science and Engineering in 1999 and2002, respectively, from Gulbarga University, India and University of Calcutta,India. He is currently a Ph.D student in the Department of ComputerScience and Software Engineering at the University of Canterbury, NewZealand. His research interests include performance analysis, mobility andQoS management of high-speed mobile broadband wireless networks.

Krzysztof Pawlikowski is a Professor of Computer Science at the Universityof Canterbury, in Christchurch, New Zealand. He received a Ph.D degreein Computer Engineering from Gdansk University of Technology, Poland,and worked at that University until February 1983. The author of over160 journal and conference papers, and four books, Prof. Pawlikowski hasgiven invited lectures at over 80 universities and research institutes in Asia,Australia, Europe and North America. He was the Humboldt Research Fellow(Germany) in 1983-84 and 1999, and a Visiting Professor at universitiesin Austria, Australia, Italy, Germany and the USA. His research interestsinclude performance modelling of multimedia telecommunication networks,teletraffic modelling, methodologies of discrete-event computer simulationand distributed processing.

Harsha Sirisena has a B.Sc.(Eng) (Honors) degree from the University ofCeylon and a Ph.D. degree from the University of Cambridge. He is a Profes-sor of Electrical and Computer Engineering at the University of Canterbury,and has held visiting appointments at Australian National University, LundUniversity, National University of Singapore, University of Minnesota andUniversity of Western Australia. He has research interests in Next GenerationNetworks, including Quality of Service, Mobility Management, Resiliencyand Security. He is a Senior Member of the IEEE and a Member of the IET.