Fair and Efficient Channel Dependent Scheduling Algorithm for HSDPA System

Bader Al-Manthari', Nidal Nasser2, Hossam Hassanein'

ITelecommunications Research LaboratorySchool of Computing, Queen's University

Kingston, ON, Canada K7L 3N6{manthari, hossam}(cs.queensu.ca

Abstract

In this paper, we propose a novel medium accesscontrol packet scheduler algorithm for High SpeedDownlink Packet Access (HSDPA) to provide a priorityscheduling between users based on their instantaneouschannel conditions and their average throughputs.Simulation results show that the proposed algorithmoutperforms the maximum CIR and Proportional Fairschemes in terms of providing minimum throughputguarantees and therefore, it has a better degree offairness.

1. Introduction

The continuous success of mobile communicationsystems, as well as its consequences in terms of the needof better Quality of Service (QoS), more efficient systems,and more services has led to the development of the thirdgeneration (3G) of mobile systems or Universal MobileTelecommunications System (UMTS). UMTS promises atransmission rate of up to 2 Mbps which makes it possibleto provide a global mobility with a wide range of servicesincluding video telephony, paging, messaging, Internetaccess, and broadband data. However, it is excepted thethere will be a strong demand for multimedia applicationswhich require higher data rates above 2 Mbps in cellularsystems, especially in the downlink, where users willenjoy high-speed Internet access and broadcast services.In order to offer such broadband packet data transmissionservices, High Speed Downlink Packet Access (HSDPA)has been introduced in release 5 of UMTS. It is expectedto achieve higher performance with a peak data rate that isabout 10 Mbps five times greater than that of 3G/UMTSsystems. In HSDPA, high-speed packet transmission ispossible by code or/and time sharing a data channelamong access users. This channel is called High-SpeedDownlink Shared Channel (HS-DSCH). UMTS already

2Department of Computing & Information ScienceUniversity of Guelph

Guelph, Ontario, NIG 2W1 Canadanasser~cis.uoguelph.ca

includes a downlink shared channel but HSDPA extendsthis concept to provide significantly higher throughputand, hence, more efficient use of the radio spectrum.

A Packet scheduler (PS) controls the allocation ofchannels to users within the coverage area of the systemby deciding which user should transmit during a giventime interval. Therefore, to a large extent; the PSdetermines the overall behavior of the system. Oneimportant factor that has been added to the schedulingproblem in HSDPA is the channel conditions of the users.The scheduling algorithm in HSDPA should track theinstantaneous channel conditions of the users and selectfor transmission those who are experiencing good channelconditions in order to maximize the throughput. However,this raises the issue of fairness, as the users who arehaving bad channel conditions may not get served andthus they end up having very low average throughputs.Therefore, the design of the scheduling algorithm shouldtake into account not only the channel conditions of theusers, but also their average throughputs by giving theones who are having very low average throughputs morepriorities to increase their chance of getting served andavoid the problem of starvation. The PS for HSDPA islocated at the medium access layer, MAC-hs at the NodeB [1].

Several feasibility studies have been contributed toHSDPA such as the proportional fair scheme [2], themaximum CIR scheme [3] and channel quality indicatorscheme [4]. They are based on a priori knowledge of theradio channel condition information of each user. All theavailable radio resources are assigned to the user with thebest radio channel conditions and therefore thesealgorithms suffer from the fairness problem. In earlierstudy [5], we attempted to overcome the fairness problemby proposing two packet priority scheduling algorithms,shortest queue first and longest queue first algorithms,with out considering the instantaneous channel conditionsfor HSDPA users.

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In this paper, however, we propose a fast packetscheduling aimed at providing a minimum throughputguarantee for HSDPA users by considering theirinstantaneous channel conditions as well as their currentaverage throughputs and hence giving more priorities forthose who are having average throughputs below a certainthreshold. Namely, we propose the Fair Efficient ChannelDependent (FECD) scheduling algorithm. We evaluate itsperformance by simulating the MAC-hs packet schedulerofHSDPA.

The rest of this paper is organized as follows.Section 2 provides a description of the system model. Theproposed algorithm is discussed in Section 3.Performance results are shown in Section 4. Finally,conclusions drawn from the paper are discussed inSection 5.

2. UMTS and HSDPA System Model

In this section, we first show the architecture of theUMTS system and its extension. We then describe themodel for HSDPA scheduler over a downlink sharedchannels in Node-B.

2.1 Architectural Design ChangesThe network architecture of UMTS as shown in

Figure I consists of three main entities [6]:

* User Equipment (UE)* UMTS Terrestrial Radio Access Network (UTRAN)* Core Network (CN)

The UE is the device that provides the user with a directaccess to the network services. The UTRAN acts as abridge between the UE and the CN. The UTRAN isdivided into individual Radio Network Systems (RNSs)where each RNS includes a Radio Network Controller(RNC) that controls one or several Node-Bs (basestations), which in turn communicate with the UserEquipments. Scheduling, selection of transport format andretransmission are handled by the RNC. However, withthe introduction of HSDPA some of the UMTS entitiesneed to be upgraded to meet the design objectives. Inparticular, Node-B protocol layers. The three mostimportant protocol layers that are implemented at Node-Bare the Radio Link Controller (RLC), the Medium AccessControl (MAC), and the Physical Layer. In HSDPA a newMAC control sub-layer (MAC-hs) is added to the Node-Bprotocol layers. The packet scheduler for HSDPA systemis located at the MAC-hs, which is a big architecturalchange compared to UMTS where the PS was located atRNC. The main reason for this is to quickly get datainformation about the instantaneous channel conditions of

the users in order to make the scheduling decisions basedon this information fast.

Figure 1: UMTS architecture

2.2 HSDPA Packet Scheduler ModelWe adapt the HSDPA packet scheduler model as in

[5]. We assume that Node-B serves N users and selectsone transmission user in a slot of some fixed timeduration. In addition, we consider only non-real-timetraffic; namely FTP service.

Upon call arrival, the RLC receives traffic in theform of IP packets from higher layers, which aresegmented into fixed size Protocol Data Units (PDUs).These PDU's are stored into the transmission queue of thecorresponding user in a first-in first-out fashion.Subsequently, the PDUs are transmitted to the appropriatemobile user according to the adopted schedulingdiscipline.

3. Fair and Efficient Channel Dependent(FECD) Algorithm

In this section, we propose a fast schedulingalgorithm for the HSDPA system which we call the FairEfficient Channel Dependent (FECD) algorithm. TheFECD aims at enhancing the average throughput for eachuser by giving more priority for those with low averagethroughputs. In this algorithm, the user with the highestpriority is selected for transmission where the priority foruser i at time t is calculated as follows:

P(t) =CQI ifS (t)>Ci CQI-W Otherwise

where CQI is the Channel Quality Indicator for user i thatrepresents the current channel condition for this user, S,(t)is the average throughput for user i up to time t, C is apredefined minimum throughput (e.g. 64 Kbps) and WC /Si(t).

The FECD algorithm prioritizes users based on theirradio channel quality. That is, users are served based ontheir channel quality condition represented by their CQlsas long as they are achieving high average throughputs.

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However, as the users average throughputs start droppingbelow C their CQIs are multiplied by a weight (W) that isinversely proportional to their throughputs (i.e. Wincreases as the users average throughputs decrease).Therefore, by introducing this weight, more priorities aregiven to the users with low average throughput, whichincreases the degree of faimess in the system.

The user's average throughput at time t is calculatedby using an exponentially smoothed filter as follows:

(1--)(t-1)+ ifi is servedin slottSi (t)= c, c

0-1--)Ss(t-]) otherwise

where tcis the time constant of the filter and is set equalto 1000 [7] and Ri(t) is the current data rate that user ican support at time t giving his current channel conditionrepresented by his CQI.

3. Performance Evaluation

In this section, we evaluate the performance of ourproposed scheduling algorithm by means of dynamicsimulation with the help of Network Simulator ns-2 andits Enhanced UMTS Radio Access Network Extensions(EURANE) [8]. Before proceeding, we first describe thesimulation model and the channel model. We then showsimulation results.

3.1 Simulation ModelWe simulated a one-cell case and, for simplicity, we

did not consider handoff. The Node B is located at thecenter of the cell Therefore, only one Node-B is involvedin allocating the radio resources. User are connected tothe Node B on the downlink by High speed PhysicalDownlink Shared Channel (HS-PDSCH) which is theactual physical channel for HSDPA and on the uplink byHigh Speed Physical Dedicated Control Channel HS-PDCCH which used to send the users' current estimatesof their channel conditions to the Node B. The Node B isconnected to RNC by a duplex link of 622 Mbpsbandwidth and 15 ms delay. The RNC is connected to theServing GPRS Support Node (SGSN) by a duplex linkwith 622 Mbps and 15 ms delay. The SGSN is connectedthe Gateway GPRS Support Node (GGSN) by a duplexlink of 622 Mbps bandwidth and 10 ms delay (the SGSNand GGSN are part of the Core Network and are used tosupport packet-switched services). The Core Network isconnected to the Internet by a duplex link of 100 Mbpsbandwidth and 10 ms delay. There is an FPT server whichis connected to the Internet by a duplex link of 100 Mbpsand 35 ms delay. Each user sends a request for one FTPfile and then dies after the download is complete. The size

of each FTP file is 0.5 MB. Call arrivals are modeled as aPoisson process with a mean value of 1 s.

At initialization, N users are uniformly distributed inthe cell. Every mobile user moves inside the cell with aconstant speed of 3Km/hr, which is the recommendedvalue for Pedestrian A environment by the 3rd GenerationPartnership Project (3GPP). The simulation time step isone time frame, which is 2 ms and the simulation time, is100 s.

3.2 Channel ModelThe channel model describes how much the radio

signal attenuates on its way from the Node B to the userand therefore it describes how the channel condition ofthe user changes with time depending on the environmentof the user and his speed. In our Simulation, thepropagation model consists of five parts: distance loss,shadowing, multi-path fading, intra-cell interference, andinter-cell interference. In EURANE, each one of theseparts is considered independent and is expressed in dB.The path loss is calculated as follows:

L(d) = 137.4 + 10 61oglo(d)where d is the distance from the UE to the Node B inKilometers and / is the path loss exponent and is equal to3.52. Shadowing is modeled through a lognormaldistribution and a correlation distance with a mean valueof 0 dB. The multi-path fading corresponds to 3GPPchannel models for Pedestrian A environments. The intra-cell and inter-cell interference are assumed to be constantsand are set equal to 30 and -70 dBm respectively. Then atthe user side, the Signal-to-Noise Ratio (SNR) (the signalstrength relative to background noise) is extracted fromthe received signal from the Node B to determine howstrong the signal is according to the following formula:

SNR=Pn 4tala logo(I& 4ntra Lm>tail a Iler)10 10

where Pa is the transmitted code power in dBm, '-'ial isthe sum of the path loss, shadowing, and multipath fadingin dB, and Iintra and 'inter are the inter and intra cellinterference respectively in dBm. The SNR is thenmapped to Channel Quality Indicator (CQI) that is used todetermine the rate at which the user can support from theNode B according to the following equation [9]:

0O SNR516

CQI= SNR +16.621 -16 < SNR < 14[1.02 J30 14<SNR

The HSDPA specification comes with tables thatdetermine the data rates for each combination of CQI andchannel codes used. These tables are used in oursimulation and can be found in [9].

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3.3 Simulation ResultsIn this section, we compare the performance of our

algorithm (denoted by FECD) with the maximum CIR(denoted by Max CIR) and Proportional Fair (denoted byPF) schemes. We use C = 128 kbps for all the simulationexperiments. We compared the algorithms in terms of thecell throughput, distribution of users' average throughputs,and the user satisfaction in terms of providing a minimumaverage throughput guarantees.

Figure 2 shows the performance of the threealgorithms in terms of the cell throughput. As the figureshows, the maximum CIR achieved the highest cellthroughput (2.1 Mbps) because it serves only those withthe best channel conditions. The performance ofFECD ispretty close to PF, 1.36, and 1.56 Mbps respectively. Thereason that FECD achieves lower throughput than PF is itgives more priority for those who are getting loweraverage throughput than 128 Kbps.

Figure 3 depicts the Cumulative DistributionFunction (CDF) of the users average throughputs. Thesteeper the CDF curve is, the fairer the algorithm becausethat means the users' average throughputs are distributedover a small interval (i.e. users get relatively equalaverage throughputs to each others). The FECD has thesteepest slope since it prevents users from achieving lowaverage throughputs by increasing their priority by W.The maximum CIR has the flattest slope since it is unfairalgorithm. Only those with the best channel conditions areserved at the expense of ignoring the rest. This results inunfair distribution ofthe users average throughputs.

User satisfaction is shown in Figure 4. A user issatisfied if his average throughput is greater than or equalto 128 Kbps. As wee can see the FECD algorithmoutperforms the maximum CIR and Proportional Fairscheme because it increases the chance of those with lowthroughput of getting served by multiplying theirpriorities by a weight that is increases as their thedifference between the minimum throughput and theirthroughout increases.

4. Conclusions

HSDPA has been introduced in the new UMTSstandard to provide high data rate multimedia services inwireless cellular networks. In this paper, a fast packetscheduling algorithm, Minimum Throughput Guarantee(FECD), has been proposed for HSDPA. The algorithmprovides a priority scheduling between different users andprovides a fair access to the radio network resourcesamong them. Simulation results show that the FECDalgorithm surpasses the maximum CIR and ProportionalFair scheme in term of the degree of fairness and usersatisfaction.

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References[1] 3GPP TS 25.308, "High Speed Downlink Packet Access

(HSDPA); Overall Description", Release 5, March 2003.[2] A. Jalali, R. Padovani and R. Pankaj, "Data Throughput ofCDMA-

HDR a High Efficiency-high Date Rate Personal CommunicationWireless System", Proc. of the IEEE VTC, May 2000, pp. 1854-1858.

[3] S. Borst, "User-level Performance of Channel-aware SchedulingAlgorithms in Wireless Data Networks," Proc. of the IEEEINFOCOM, vol. 1, March 2003, pp.321-331.

[4] M. Kazmi and N. Wiberg, "Scheduling Algorithms for HS-DSCHin a WCDMA Mixed Traffic Scenario", Proc. ofthe IEEE PIMRC,Beijing, China, September2003, pp. 1485-1489.

[5] Nidal Nasser, Bader Al-Manthari and Hossam Hassanein, "APerformance Comparison of Class-based Scheduling Algorithms inFuture UMTS Access", Proc. Of the IEEE IPCCC, Phoenix,Arizona, April 2005, pp. 437-441.

[6] W. Permdl, "Scheduling Algorithms for UMTS FDD Downlink",Ph.D.Thesis, Vienna University ofTechnology, July 2001.

[7] Y. Ofuji, S. Abeta, and M. Sawahashi " Comparison of PacketScheduling Algorithms Focusing on User Throughput in HighSpeed Downlink Packet Access", IEICE Transaction onCommunications, vol. E86-B, No.1, Jan 2003, pp. 132-139

[8] Enhanced UMTS Radio Access Network Extensions for NS2,hft1://www.ti-wmc.nl/eurane/. July 2005.

[9] 3GPP TS25.214, "Physical Layer Procedures", Release 5, version5.5.0, June 2003.

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