Paving the Path for High Data Rates by GERAN

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Paving the Path for High Data Rates by GERANEvolution EDGE2 with Dual-Carrier

K. Ivanov, C. F. Ball, R. MullnerRadio Access Division

Nokia Siemens NetworksMunich, Germany

[email protected]

Abstract- The introduction of the upcoming GERAN evolutionfeature package in current GSMIEDGE deployments offersoperators significant boost in network capacity and mobile datausers UMTSIHSPA like high speed packet data services alongwith competitive latency. Intelligent radio resource managementsupports novel dual-carrier capable mobile stations by dynamicconfiguration of GPRS/EDGE packet data channels (PDCHs) onmultiple non-BCCH carriers. In addition the currentlystandardized EDGE2 1 level B (EDGE2-B) concept providesenhanced PDCH data rates up to 118.4 kbps pe r timeslot. In thispaper system level simulation results for the end-to-endperformance of GERAN over TCPIIP are presented assumingconventional 4 timeslots up to future po tential 14 timeslotscapable EDGE and EDGE2-B mobiles showing up to 800 11600kbps p ea k data rates . FTP-application throughput has beeninvestigated with respect to both download tile size andimportant TCP set tings such as e.g, receiver window size. TheGERAN dual-carrier performance has been evaluated for EDGEand EDGE2-B both under ideal radio conditions and in regularhexagonal cellula r deployments depend ing on system load,exemplifying FT P 500 kByte download with 8 timeslots capablemobiles. At medium system load EDGE2-B compared to EDGEreveals about 100% capac ity gain and more than 60% gain inmean user throughput.

Keywords- GERAN Evolution; EDGE; EGPRS; EGPRS2;TCPIIP; dual-carrier;

I. INTRODUCTION

GERAN (GSM EDGE Radio Access Network) is today'sbackbone of mobile communications with almost 3 billionsubscribers providing worldwide access and roaming for voiceand packet data services. Current deployments of GPRS andespecially the recent wide-spread rollout of EDGE in existingGSM networks have opened the door for worldwide mobileInternet services [I]. Commercially available mobile stations(MS) with a downlink multi-slot capability of 4 - 5 timeslots(TS) provide typical application peak data rates of up to 75 - 90kbps with GPRS, and 225 - 275 kbps with EDGE, respectively[2].

The goal of the upcoming GERAN evolution presentlyunder 3GPP standardization is to significantly increasecapacity and spectrum efficiency along with a boost of userthroughput at a very competitive and significantly reduced

1 By convention in this paper the term EDGE refers to EGPRS, and theterm EDGE2 refers to EGPRS2.

978-1-4244-2644-7/08/$25.00 2008 IEEE

H. WinklerProgram and System Engineering

SiemensAG

Vienna, [email protected]

overall latency [3]. GERAN evolution relies on the EDGE2concept, a comprehensive feature package including theintroduction of higher order modulation schemes (such asQPSK, 16-QAM and 32-QAM) along with increased symbolrate (1.2 times the normal GSM symbol rate), mobile stationreceive diversity (MSRD), advanced turbo coding in downlink,reduced latency by improved interleaving schemes (LATRED)and fast Ack/Nack reporting (FANR) [4], [5]. A detailed studyof the EDGE2 uplink performance is found in [6]. EDGE2 incombination with the downlink dual-carrier approach, inparticular, will break through the currently immanent 4 - 5 TSMS limit, opening the possibility to offer enhanced 2 to 3 timeshigher data rate compared to conventional single carrierEDGE.

As a consequence, on the network side both an intelligentradio resource management (RRM) as well as efficient radiolink quality control (LQC) strategy have to be implemented fordynamically handling MS dual-carrier allocations on BCCHand non-BCCH transceivers (TRX) with multiple reuseplanning (MRP) characterized by variable radio conditions [7].

In this study simultaneous allocation of 4 downlink TS onsingle TRX and up to 14 downlink TS on two TRX has beenassumed. Preserving the present EDGE coding schemes thefocus is set on the end-to-end performance under ideal radioconditions (single cell, not coverage limited scenario) as wellas in a real network interference limited environment. Inaddition the new EDGE2-B concept has been investigatedunder the same conditions to evaluate the resultingperformance gain in terms of user throughput and networkcapacity.

The effects of TCP/IP as today's dominant transport layerprotocol over Internet on the application throughput in wirelessnetworks have been thoroughly investigated [7], [8], [9].Valuable recommendations concerning the setting of the TCPreceiver window size on the client side have been given.Furthermore the dependency of the application throughput onthe FTP download file size has been derived. FTP applicationthroughput results under varying system load are presented forslow moving MS in cellular hexagonal deployments withrelaxed frequency reuse.

The paper is structured as follows. Section II gives anoverview of the GERAN Evolution architecture and thenetwork simulation model including the novel GERAN dualcarrier approach. In Section III simulation results for idealradio conditions have been presented. Target throughput

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GERAN for DL packet switched data transmission along withthe RLC maximum data rate per TS.

Coding ModulationRL C Data Rate

Standard Family pe r TimeslotScheme Scheme[kbps]]CS-l 8.0

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EDGE2 DBS-8 A(Aa) 16-QAM 59.2 (54.4)LevelB DBS-9 B 16-QAM 67.2

DBS-I0 A (A ) 32-QAM 88.8(81.6)DBS-ll A a 32-QAM 108.8DBS-12 A 32-QAM 118.4

figures for 4 TS MS up to 14 TS MS have been deriveddepending on TCP receiver window size and FTP downloadfile size. Section IV deals with system level simulation resultsin regular hexagonal GERAN deployments depending onpacket data load. EDGE and EDGE2-B dual-carrier allocationwith 8 TS on two non-BCCH carriers in relaxed 4x3 frequencyreuse has been evaluated. The main conclusions are drawn inSection V.

II. NETWORKSIMULATION MODEL INCLUDINGGERANDUAL-CARRIERARCHITECTURE

The network simulation model shown in Fig. 1 includes allGERAN network elements considering latency, queuing,transmission delay and all relevant call processing features.The following layers of the protocol stack have beenimplemented [4], [5].

1) The physical layer covers GPRS, EDGE and EDGE2-Blink adaptation (LA) as well as incremental redundancy (IR).The physical link is modeled by block erasure rate (BLER) vs.carrier-to-interferer-ratio (CIR) mapping obtained from linklevel simulations performed for TV3 (no FH) in case of EDGEand TV3 (ideal FH) in case of EDGE2-B. A decent MSreceiver performance has been assumed excluding advancedfeatures like MS receive diversity, and single / dual antennainterference cancellation (SAIC / DAlC). Table I gives anoverview of the modulation and coding schemes available in

TABLE!. GPRS, EDGE AND EDGE2 RLCUSER DATA RATES

a. Family A with padding



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2) Radio Link C o n t r o l / M e d i u m Access Control(RLC/MAC) layer: the selective ARQ protocol for RLC hasbeen completely implemented. For EDGE the round trip time(RTT) on RLC level (Le. signaling delay from the MS to thepacket control unit (PCU) and vice versa) was adjusted to 100ms, the Relative Reserved Block Period (RRBP) to 40 ms [10].For EDGE2 both RTT and RRBP have been reduced, to 80 msand 20 ms, respectively.

3) Logical Link Control (LLC) layer: the mobile specificLLC flow control function in the SGSN operates on estimatedMS throughput and memory congestion state. The SGSN flowcontrol is a token leaky bucket algorithm and receives flowcontrol commands from the PCU.

4) RRM: the radio resource management includes acomprehensive functionality for dynamic and fixed allocationof radio and Abis resources to voice and data services [11]. Forpacket data services, several strategies of the temporary blockflow (TBF) allocation onto PDCHs as well as RLC schedulers(cyclic polling, fairly weighted, and QoS-based) have beenimplemented and can be chosen accordingly. Intra-cellhandover and periodical GPRS/EDGE TS downgrade andupgrade procedures are used to improve throughput. PDCHscan be configured arbitrarily on BCCH and/or non-BCCHcarriers in different reuse patterns. Shared (on demand) PDCHsmight suffer from voice service soft preemption. The noveldual-carrier approach allows dynamic configuration of themobile's PIXH allocation simultaneously on two TRX. As anexample a mobile might utilize in downlink 12 PDCHs(4 PDCHs on TRX-2 and 8 PDCHs on TRX-3) as well as 2PDCHs in uplink on TRX-2.

5) The network layer comprises the transmission of IPpackets as well as routing functionality.

6) The transport layer offers both User Datagram Protocol(UDP) as well as TCP (Reno). Specific features o f TCP havesevere impact on the overall performance of wireless dataservices. Thus the model covers for example the choice of themaximum TCP segment size (MSS), advertising window sizeof the receiver/client (AWND), congestion window

management at the sender/server and TCP slow start. The TCProundtrip time is continuously measured and filtered to updatethe retransmission timeout (RTO). RTO expiry causes TCPretransmissions and a new slow start. In addition the effects ofduplicate acknowledgments (DUPACKs) combined with fastrecovery and fast retransmit are part of the model. Hence thecomplete TCP Reno implementation of the transport layer hasbeen incorporated in the simulator.

7) The application layer consists of a variety of trafficmodels for WAP, HTTP, email, FTP, SMS, MMS andstreaming services. Because of the open architecture of thesimulator, new traffic models or traces of real sessions caneasily be imported [12], [13]. User's behavior is modeled byprobability distributions of the number and size of downloadsper Internet session and reading times between separatedownloads. Nevertheless the network simulation resultspresented in this paper are exclusively performed for the FTPdownload service with deterministic file volume of 500 kByte.Fig. 1 shows the network elements and interfaces included inthe simulation model as well as the path through the network

for an IP packet (from the server to the client) on a downloadrequest. When a mobile leaves the idle state, a packet dataprotocol (PDP) context is generated at the SGSN. The mobilemakes an access to the GPRSIEDGE network and submits adownloadrequest via the mobile network and the Internet to aserver. The server divides the requested data volume into TCPsegments, adds a TCP/IP header and sends them as IP packetsvia a router to the SGSN. Furthermore the server initializes theTCP flow control parameters, e.g. to perform the slow start.

The SGSN creates LLC frames out of the IP packets andtransmits them over the Gb interface to the PCU, if apermission has been obtained from the leaky bucket flowcontrol, otherwise the LLC frames are queued. Packet queuingon the Gb interface due to congestion is considered.

Meanwhile the PCU allocates radio resources (PDCHs) andthe necessary bandwidth on the dynamic Abis interface.

The LLC frames wait within a queue in the PCU for beingsegmented into RLC blocks. The RLC blocks are scheduledand transmitted over the air interface to the mobile. The PCUpolls the mobile for a bitmap to indicate the correctly anderroneously received RLC blocks. The latter are retransmitted.During the TBF lifetime the PCU performs a periodic TSupgrade/downgrade and LA.

As soon as the MS has correctly received all the RLCblocks belonging to the same LLC frame, it reassembles theLLC frame and sends it to the connected Laptop/PC client. Inthe client the corresponding IP packet and hence the TCPsegment is reassembled. On the receipt of a TCP segment withthe expected sequence number, the client sends anacknowledgement to the server. Delayed acknowledgement isconsidered.For segments out of sequence, the TCP layer of theclient transmits DUPACKs. Depending on the state o f the TCPparameters the server invokes on receipt of the TCPacknowledgements and their sequence numbers the appropriateTCP algorithm, e.g. flow control and congestion windowmanagement, RTO handler and retransmission management, aswell as fast recovery/retransmissions.

As soon as the client has received all TCP segments of theapplication data volume, the network resources of the packetcall are released. The client/user might send additionaldownload requests after a certain idle period. Otherwise theGPRS/EDGE session is finished and the PDP context is deletedin the SGSN.

III. SIMULATION RESULTS FOR IDEAL RADIO CONDITIONS

The GSMIEDGE standard specifies nine modulation andcoding schemes MCS-l .. . MCS-9 utilizing both GMSK and 8PSK and providing RLC data rates of up to 59.2 kbps perPDCH. The GERAN evolution concept EGPRS2-B supportinghigher order modulation and coding schemes with highersymbolrate and turbo coding allows for 118.4 kbps per PDCH,i.e, twice higher than that provided by legacy EDGE. A BLERbased LA algorithm selects the most appropriate MCS/DBSaccording to the radio conditions optimizing the overallthroughput [14], [15]. Hence under ideal radio conditions (zeroBLER) the highest MCS-9/DBS-12 with EDGE/EDGE2-B willbe selected all the time. Furthermore the support of theextended UL TBF feature has been assumed.



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Fig. 2a shows the simulated FTP 2 MByte end-to-end meanapplication throughput for a single TS MS (one PDCHallocated) up to 14 TS MS depending on the TCP receiverwindow size ranging from 16 kByte (Windows XP default) upto 48 kByte. Application throughput means that upper layereffects such as TCP slow start as well as overhead includingTCP/IP and LLC headers have been considered, the latterreducing the peak data rates by upto 5%.

The Windows XP default TCP window size of 16kByte isabsolutely sufficient for 1 up to 4 TS allocations. Targetthroughput of 56 kbps for single TS, 112 kbps for 2 TS, 169kbps for 3 TS and 225 kbps for 4 TS MShas been achieved.

Obviously 16 kByte receiver window size is absolutelyinsufficient for the dual-carrier approach and has to be properlyadjusted. A throughput degradation of roughly 10% fromapproximately 340 kbps down to 300 kbps is clearly visible for6 TS MS. A TCP receiver window size of24 kByte is requiredfor 6 and 8 TS MS to obtain peak throughput of 340 and 450kbps, respectively. A lOTS MS needs a TCP window size of32 kByte to achieve data rates of 550 kbps. A potentialthroughput of650 kbps 1750 kbps to become feasible with 12 I

14 TS MS a TCP receiver window size of at least 40 kByte isrecommended.

The results shown in Fig. 2b for EDGE2-B reveal that theTCP receiver window size has to be adjusted to 64 kByte to

File Size [kByte]

I.....MSCapabHity4 TS . . . . STS ~ 8 T S. . . . . 10TS . . . . 12TS ~ 1 4Tsl

Figure 3b. EDGE2-B DBS-12 FTP application throughput for 4 TS up to 14TS MS depending on file size with 64 kByte TCP receiver window size.(30% EDGE2 latency improvements assumed in addition).

support data rates o f up to 1.45 Mbps achievable with 14 TSMS on an EDGE2-B dual-carrier. It is worth noticing that for acertain application throughput the TCP receiver window sizerequired with EDGE2-B is significantly less than that requiredwith EDGE due to the latency reduction features to beintroduced with EDGE2-B significantly improving the TCP/IPround trip time (ping reduction from currently 160 ms down toless than 100 ms expected).

The impact of FTP download file size on the achievablepeak application throughput for different MS TS capabilitiesranging from 4 TS up to 14 TS is illustrated in Fig. 3a forEDGE and in Fig. 3b for EDGE2-B respectively. The file sizehas been varied from rather small 10kByte to a quite large oneof 10 MByte. For a small file size no major difference inthroughput has been observed with different MS multi-slotcapabilities. Due to the adverse TCP slow start effect the userdata rate is heavily degraded down to approximately 100 kbpsfor both EDGE and EDGE2-B. With increasing file size theapplication throughput grows very rapidly up to a certainsaturation level depending on the MS multi-slot capability, e.g.225 kbps for a 4 TS MS in EDGE and 450 kbps in EDGE2-B.Apparently the higher the MS multi-slot class the larger is thefile size required for throughput saturation, e.g. a file size of 1MByte is sufficient to obtain the target throughput of 225 I 450



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Figure 5. PDCH utilization vs. system load on 2 non-BCCH carriers in 4x3frequency reuse (4 reserved PDCH per carrier).

Figure 4b. Application throughput gain ofEDGE2-B vs. EDGE.

kbps for 4 TS MS, however, 5 MByte are required for 8 TS MSto achieve 450 / 900 kbps and 10 MByte for 12 TS MS at 680 /1360 kbps with EDGE / EDGE2-B respectively.

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IV. SYSTEM LEVEL SIMULATION RESULTS FOR REGULARHEXAGONAL CELL DEPLOYMENT

System level simulations have been performed for a dualcarrier deployment scenario assuming 8 TS MS and FTPdownload service with a constant download volume of 500kByte (not a full buffer!) in a regular hexagonal cellsinterference limited network with a 4/4/4 configuration (i.e, a3-sector site with four TRX per sector) and 700 m cell radius.In each cell (sector) 8 reserved PDCHs have been configuredon two non-BCCH TRXs planned in a relaxed 4x3 frequencyreuse. LA has been enabled in both scenarios EDGE andEDGE2-B, while IR has been enabled only in EDGE.

Fig. 4a depicts the mean application throughput along withthe 10th and 90th user percentiles for varying system loadmeasured in terms of mean user busy hour (BH) data rate. Inthe investigated scenarios an offered load of e.g. 500 bps

translates to 18.6 kbps per TS.It shall be pointed out that the EDGE dual-carrier mean

user throughput of 350 kbps to 360 kbps achieved with 8 TSMS under very low load conditions (up to 100 bps offered

load) is as high as in 3G-UMTS Re1.99 networks. 10% of theEDGE users enjoy the top data rates of roughly 400 kbps and900/0 of the subscribers achieve data rates higher than 250 kbps.As the offered load increases the perceived user throughputgradually decreases due to increased interference level in thenetwork and resource sharing between users. An excellentmean user throughput of 200 to 250 kbps has been obtained atmedium load (400 to 600 bps), and even in a fully loadedsystem (800 bps) mean user data rates well above 100 kbps arefeasible. Further increase of the data load drives the EDGEnetwork into congestion. The worst 10% of the users getpractically out of service (less than 32 kbps).

EDGE2-B outperforms EDGE in terms of both userthroughput and capacity over the entire system load range. Thegain in user throughput achieved by the introduction of thehigher order modulation and coding schemes with turbo codingDBS-5 through DBS-12, increased 1.2 symbol rate as well asthe latency reduction features in GERAN evolution EDGE2has been evaluated as a function of the offered system load(Fig. 4b). Obviously the gain in peak data rates of roughlyfactor 2 is independent of the load. The gain in both mean userthroughput and that of the best 10% users varies inthe range of60% to 90% for an offered load up to 700 bps. At system loadbeyond 800 bps the EDGE scenario runs into congestioncausing the exponential gain growth. For an offered load higherthan 650 bps (24 kbps/TS) EDGE2-B provides more thandoubled throughput for the worst 10% users.

Furthermore the improvement in throughput performancereduces the effective load in the network since the sojourn timeof each EDGE2-B user gets shorter. This improves the capacityof the system. Fig. 5 clearly indicates the reduced PDCHutilization with EDGE2-B as the offered load increases. Atmedium to high offered load (300 to 700 bps) the PDCHutilization measured in the EDGE scenario has been reducedby nearly 30% in the EDGE2-B scenario. The sparedresourcesalong with the enhanced link level performance and latencyreduction features in EDGE2-B translate to a roughly 100%capacity gain as indicated in Fig. 4a. While the EDGE scenarioruns into an overload situation at 900 to 1000 bps offered loadthe PDCH-utilization of 70% to 80% observed in the EDGE2B scenario still allows for excellent mean user throughput ofabout 200 kbps.



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The cumulative distribution functions (CDF) of theapplication throughput at low (100 bps), medium (500 bps) andhigh system load (800 bps) are presented in Fig. 6.

The distributions clearly demonstrate the optimumexploitation of the good radio conditions (good CIR) on thenon-BCCH carriers in 4x3 reuse. Especially at low loadEDGE2-B users can overwhelmingly profit from the excellentCIR which is typically higher than 25 dB in 90% of the cel larea. While the application throughput for about 70% of the

EDGE users is l imited to the peak value of 380 kbps (c f Fig.4a), 50% of the EDGE2-B users enjoy 240 kbps higherthroughput (higher than 620 kbps). The best 10th EDGE2-Busers perceive almost double data rates (higher than 720 kbps).Furthermore, it is worth mentioning that the user throughputperception with EDGE2-B at high system load is at least asgood as with EDGE at medium load revealing the significantlyimproved spectral efficiency of EDGE2-B. In addition the best10th EDGE2-B users achieve data rates higher than 500 kbps.

v. CONCLUSIONS

The novel dual-carr ier approach as well as the currentlystandardized EDGE2-B GERAN evolution feature packagehave been investigated by means of system level simulations

under both ideal and realistic radio conditions. Preserving themodulation and coding schemes currently used with EDGE adual-carrier implementat ion on two non-BCCH carriersplanned in 4x3 frequency reuse demonstrates a substant ialperformance gain over today's single carrier approach. A 3GUMTS Re1.99 like mean user throughput can be obtained inEDGE dual-carrier networks.

Mean user throughput of 360 kbps has been measured forFTP 500 kByte download with 8 TS capable MS at low systemload. TCP receiver window size and download file size have amuch stronger impact on the end-to-end performance ofEDGE2-B compared to that o f EDGE. As an example, a TCPreceiver window size of 24 kByte is required for 8 TS MS toachieve peak data rate of approximately 450 kbps in EDGEwhile the receiver window size has to be adjusted to 40 kBytefor an EDGE2-B MS with 8 TS to support a peak data rate of900 kbps. Fo llowing this recommendation and assumingsufficiently large download file size target throughput of up to800 I 1600 kbps could be achieved under ideal radio conditions(single cell not coverage limited scenario) with the dual-carrierapproach using EDGE I EDGE2-B capable MS with 14TS.

In a cellular interference limited deployment (4x3frequency reuse) at medium system load EDGE2-Boutperforms EDGE providing more than 60% gain in mean

user throughput and an increase in network capacity of aboutfactor 2.

GERAN evolution including dual-carrier and EDGE2-B isa promising method for enhancing GERAN packet data servicetowards UMTS/HSDPAlHSUPA such that in near futuresubscribers can enjoy seamlessly high data rates in multi RATmobile networks.

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[3] 3GPP TSG GERAN, "Feasibil ity study for evolved GSM/EDGE radioaccess network (GERAN)", 3GPP TR 45.912, Ver. 7.2.0, available atwww.3goo.org.

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[12] C.F. Ball, C. Masseroni, R. Trivisonno, "Mu lt i RAB-Based andMultimedia Services over GERAN Mobile Networks", In Proc. IEEEVTC Fall, Dallas, 2005.

[13] C.F. Ball, C. Masseroni, R. Trivisonno, "Introducing 3G likeConversational Services in GERAN Packet Data Networks", In Proc.IEEE VTC Spring, Stockholm, 2005.

[14] C.F. Ball, K. Ivanov, P. Stockl, C. Masseroni, S. Parolari, R. Trivisonno,"Link Quality Control Benefits from a Combined IncrementalRedundancy and Link Adaptation in EDGE Networks", In Proc. IEEEVTC Spring, Milan, 2004.

[15] C.F. Ball, K. Ivanov, L. Bugl, P. Stockl, "Optimizing GPRSIEDGE Endto-End Performance by Link Adaptation and RLC ProtocolEnhancements", IEEE Global Mobile Congress - GMC, Shanghai, 2004.

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Paving the Path for High Data Rates by GERAN