3 handoff management

Handoff Management

Prof. W.-G. Teng, "Wireless & Mobile Networks", NCKU ES 2

OutlineHandoff BasicsHandoff Management

Handoff DetectionHandoff AssignmentExample: Handoff in GSM

Handoff Requirements and GoalsAdvanced Issues of Handoff Management

Handoff FailureRadio Link TransferSoft Handoff


Review:Signal Strength within Cells

The contours may not be concentric and could be distorted by atmospheric conditions and topographical contours


Handoff

A most common reason that the handoff is conducted:When the phone is moving away from the area covered by one cell and entering the area covered by another cell, the call is transferred to the second cell to avoid call terminationwhen the phone gets outside the range of the first cell

The handoff process is of major importance within any cellular networks

Failure for it to perform reliably can result in dropped calls, and this is one of the key factors that can lead to customer dissatisfaction


Basics of Handoff Management

Handover (Europe) = handoff (North America)Handoff is the process that occurs when a mobile is “handed over” from one access point to another, i.e., the access point which the mobile is using changesEither handover or handoff can be abbreviated as HO hereafter

Handoff Management(Handoff detection) How do you detect that you should handoff?(Handoff assignment) Who initiates handoff?


How HO Happens?As the MS moves away from the BS of the cell, the signal strength weakens and the radio connection of MS is changed to another adjacent cell


How HO Happens? (cont’d)By starting to look for a new BS before you need it, there is time to make a decision

X3: Start looking for a new BSX5: Time to switch

TL: Threshold for looking around

TH: Threshold for handoff







“Ping-Pong” EffectThe name of the phenomenon implies that the MS moves back and forth between the overlapped area of two adjacent cells

The solution is to allow MS continue maintain a radio link with the current BSi, until the signal strength from BSj exceeds that of BSi by some pre-specified threshold value E (shown as Xth)

Celli

Cellj


How HO Happens? (cont’d)To avoid “ping-pong” effect, the MS continues to maintain a radio link with BSi until Xth


Reality is More ComplexThree common measurements of the channel:

Word Error Indicator (WEI)Based on if the receiver is able to decode the received signal correctly

Received Signal Strength Indication (RSSI)A measure of the received signal strength

Quality Indicator (QI)Related to the signal to interference & noise ratio (S/I)

Handoff may depend more reliably on WEI of the current channel rather than RSSI

If WEI is good, then handoff is not performedHowever, it is necessary to accumulate WEI measurements over a period of time, whereas RSSI is known instantaneously


Reality is More Complex (cont’d)

To make the handoff decision accurately and quickly, it is desirable to use both WEI and RSSI

However, to use a simple policy for handoff detection can be feasible in practice

For example, GSM Specification introduces 6 basic types of causes of HO with priority order

Uplink qualityDownlink qualityUplink levelDownlink levelMS-BS distancePower budget: To improve transmission quality in the lower power level


Who Makes the HO Decision?NCHO (network controlled handoff)

The network makes the decisionUsed in CT-2 Plus and AMPS

MCHO (mobile controlled handoff)The mobile decides for itself

Forward: the mobile initiates HO and sends the request to the new APBackward: the mobile initiates HO and sends the request to the old AP

Used in DECT, PACS and Mobile IPMAHO (mobile assisted handoff)

The mobile provides data for the network to make the decisionUsed in GSM and IS-95 CDMA


NCHONetwork Controlled Handoff

The BS monitors the signal strength and quality from the BSThe network uses multiple (current and surrounding) BSs to supervise the quality of all current connections by making measurements of RSSIThe MSC makes the decision when and where to effect the handoff

Drawback: Heavy network signaling traffic and limited radio resources at BSs prevent frequent measurements of neighboring links

Long handoff time: up to 10 sec. or more


MCHOMobile Controlled Handoff

The mobile decides for itself by monitoring signal strength and quality from the current and candidate BSs

When it finds a better candidate, it initiates a handoff

In MCHO, most of the work is done by the mobile (as it knows who it can hear, how well it can hear them, and can even consider its battery level, etc)Handoff time

DECT: 100~500 ms.PACS: 20~50 ms.


MAHOMobile Assisted Handoff

The mobile provides data which the network uses to make the decisionEssentially, it is a variant of NCHO but uses the mobile to help reduce the handoff time

Example: In GSM, the MS transmits measurements twice a second ⇒ GSM handoff time ~ 1 sec.Note in both NCHO and MAHO, if the network cannot tell the mobile about the new channel / time slot / ... to use before the link quality has decayed too far, then the call may be terminated


Handoff in GSMA handoff is decided by the network, not the MS

BSS criteriaReceived signal levelChannel qualityDistance between MS and BTS

Network operation criteriaCurrent traffic load of the cellOngoing maintenance work

Link control for handoff preparationPeriodically, the MS checks the downlink signal strength of its current BS and the neighboring BSsUplink signal quality is monitored by the network sideMS sends measurement report to its current BS and this report is to be evaluated for handoff decisions by the network side


Handoff in GSM (cont’d)Handoffs include several procedures: measurements, decision and destination selection


Handoff in GSM (cont’d)Use measurement results from the MS and BS

To identify possible other BSs as targets for handoversTo determine the optimal moment to execute the HO

Objective: to keep (#HO per cell change) smallIdeally, (#HO per cell change)=1In reality, radio conditions are often not very stable when an MS leaves the BS => (#HO per cell change)=1.5~5







Handoff RequirementsRequirements from a handoff scheme are:

Latency: the time required to effect the handoff should be appropriate for the rate of mobility of the mobile terminal, as well as the nature of data transferredScalability: the handoff procedure should support handoffs within the same cell, between different base stations in the same or in different networksMinimal drop-off and fast recoveryQuality of service should be maintained or re-negotiatied after the handoff is completedMinimal additional signaling


Handoff GoalsMinimal impact on traffic

Making a handoff at the “right” time

Tolerance / adaptation for congestion and capacityThe new and old cells may have different levels of utilization, available bandwidth, ...

EfficiencyThe handoff should result in improved efficiency (in terms of traffic, energy consumption, reduced interference, …)The handoff process should try to minimize the resources it consumes

Improve availabilityHandoff should result in using a BS which provides better bandwidth, lower cost, lower delay, low delay variance, ...


Handoff Goals (cont’d)The mobile should be able to use the maximum set of BSs (which may involve changing spreading code, modulation, coding, or changing to a different radio module) to achieve a better system optima

Fast handoffOptimal BTS selectionReduced signaling overheadIncreased speech qualityAvoidance of far-away-cell effectConsideration of adjacent-cell capacityAvoidance of ping-pong effectReduced handover failure







Handoff FailureWhat happens if there are insufficient resources in the new BS?

Existing calls cannot be handed over ⇒ forced terminationsNew calls are blocked

Goals of channel assignmentTo achieve high spectrum utilizationTo maintain a given service qualityTo use a simple algorithmTo require a minimum number of database lookups

Unfortunately, it is hard to do all of these at once!


Channel AssignmentSeveral schemes are introduced to reduce the number of forced terminations, at the cost of increased blocking or decreased efficiency, including:Nonprioritized scheme (NPS)

A handoff call is treated the same as a new callReserved channel scheme (RCS)

To reserve some resources for handoffsQueuing priority scheme (QPS)

To exploit the handoff area (i.e., cell overlap) to queue up mobiles waiting for handoff

Subrating scheme (SRS)To downgrade an existing call in the new cell and to share resources with the call being handed over (e.g., by changing a full-rate to a half-rate CODEC)


Radio Link TransferWhen performing handoff, it is sometimes necessary to transfer radio link among system componentsIn the GSM network, several handoff types can be enumerated:

Intra-cell HOInter-cell or inter-BS HOInter-BSC HOInter-MSC HOIntersystem HO between two PCS networks (?!)


Preview:Part of the GSM Architecture

BTS: base transceiver stationBSC: base station controllerMSC: mobile switching center


Different HO Types in the GSM Network


Intra-cell & Inter-cell HOsIntra-cell HO

For administrative reasons or because of channel quality (channel-selective interferences)

Inter-cell HOBecause of weak signal field strength and bad channel qualityAlso for administrative reasons (e.g., for traffic load balancing)


Flows of HO within One MSCWhen both cells are connected to the same MSC, the mobile can signal that it is going to change cells (by providing measurements), then the MSC can set up the correct resources deliver traffic in the new cell

This often involves setting up a bridge to copy traffic to both the old and new channels


Inter-MSC HOWhen the two cells are connected to different MSCs, the situation is more complex


Inter-MSC HO (cont’d)Anchor-based approach

Anchor MSC


What Happens If the Mobile Moves Again?


What Happens If the Mobile Moves Again? (cont’d)

Note that the call always goes via the so-called anchor MSC (in this case MSC1) to relay MSCs

This is because the phone attached to the PSTN knows nothing about mobility and the originating exchange thinks the call is still in existence

Without path minimization, the chain of chunks between MSCs could continue to grow as long as the call lasts and the mobile keeps moving to new MSCs

With voice calls, the call duration is generally rather limited,but with data communication it could continue for a very long time


Hard Handoff vs. Soft HandoffHard handoff

Break-before-makeThe mobile connects only to a single BS at a time

Soft handoffMake-before-breakThe mobile receives from & transmits to multiple BSssimultaneouslyThe signal of the best of all connected channels is utilizedGenerally used in CDMA systems


Hard Handoff

X

X


Soft Handoff

X


Softer HandoffSofter handoff: In soft handoff, the network and the mobile combine all the signals to produce a clearer copy of the signal in both the up and down linksRe

ceiv

ed P

ower

Distance

Total at the MSCell ACell B


WCDMA HandoffsSofter handoff is more advantageous to soft handoff while soft handoff is more advantageous to hard handoff. However, not all handoffs in CDMA systems can be performed as softer handoffsExample: In WCDMA systems, we have the following scenarios:

Softer handoffThe handoff is within one cell between different sectors

Soft handoffThe handoff is between different BSs

Hard handoffThe handoff is between different frequencies or between WCDMA and GSM


Soft Handoff in Multiple FormsSome CDMA systems use very precise link level timing to enable the signals from multiple BSs to arrive additively at the mobile, thus leading to a physically stronger signal


Advantages of Soft Handoff

Advantages of utilizing soft handoff include:Reduces number of call dropsIncreases the overall capacityMobile transmit power is reducedVoice quality near the cell boundaries are improved

Technology

3 handoff management