8/9/2019 Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA http://slidepdf.com/reader/full/performance-comparison-of-harq-with-chase-combining-and-incremental-redundancy 1/5 Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA PA1 Frenger: Stefan Parkvall, and Erik Da.hlman Ericsson Research, Ericsson Radio Systems AB, SE-164 80 Stockholm, Sweden Abstract-In this paper we compare two hybrid aut omatic repeat request (HARQ) combining strategies that currently are considered for the high speed downlink packet access (HSDPA) evolution of WCDMA. The two HARQ combin- ing schemes are Chase combining, where the retransmissions are identical copies of the original transmission, and incre- mental redundancy (IR), where the retransmissions consist of new parity bits from the channel encoder. We show in this paper that the link-level performance of a HARQ type- I system can be significantly better with 1R compared to Chase combining. The largest gains are obtained for high channel-coding rates and high modulation orders. For low modulation and coding schemes (M CSs), the link-level per- formance gains with IR are less significant. We further show that in a system that uses link adaptation we can not expect any large gains with IR as long as the link adaptation errors are reasonable small. Furthermore, we show that on fading channels there are situations when an IR system actually performs poorer than a Chase combining system. Keywords- WCDMA evolution, High Speed Downlink Packet Access (H SDP A), Chase Combining, Incremen- tal Redundancy (IR), Hybrid Automatic Repeat reQuest (HARQ). I. INTRODUCTION As a first step in the evolution of WCDMA, a new con- cept denoted high speed downlink packet access (HSDPA) is currently being developed within the 3GPP framework. Two important design targets for the HSDPA concept are to provide downlink peak data rates in the order of 8-10 Mbit/s for best effort packet based services a,nd to signifi- cantly reduce the downlink transmission delays. Some importa.nt features that are introduced in HSDPA are fast link adaptation, fast scheduling, and fast HARQ with soft combining (i.e. type-I1 [l]). A new high speed downlink shared channel (HS-DSCH) is introduced that is shared in the time domain among the active users, sim- ilar to the DSCH in WCDMA of today. Instead of fast power control, the HS-DSCH will use fast link adaptation that adapts the size of the modulation alphabet and the rate of th e chaiinel encoder t.0 the fast channel fading. Th e scheduler decides, based on e.g. t he instanta.neous channel qualities of all users, which user shall be assigned the HS- DSCH channel during the upcoming tra,nsmission time in- terval. Furthermore, if a,n error is detected by the receiver, the fast hybrid ARQ system ensures tha t the necessary re- transmission is executed quickly. Achieving low transmission delays for HSDPA is essential in order to ensure good performance also together with higher layer protocols e.g. TCP. It is important that the bandwidth-delay product of the channel is in the order of Phone: +46 0 7 7 1.52. Fax: +46 8 585 314 80. Email: pal.frengeroera ericsson.se. the TCP window-size, or else it will not be possible to fully utilize the radio link. Furthermore, for small data packets the slow-start beha.vior of TCP, and not the data bandwidth of the channel, will limit the performance unless the round trip time for TCP acknowledgments is small. Thus, in order to benefit from the increased data rates provided by HSDPA, reducing the transmission delays is a key concern [a] In the current UMTS radio access network (UTRAN) ar- chitecture the scheduling of users, selection of the transport format (including modulation and coding parameters), and the ARQ retransinissions are located in the radio network controller (RNC). Since the HSDPA fast link adaptation and fast scheduling will adapt to the fast fading of the radio channel, it is necessary to move these functionalities closer to the radio channel, i.e. to the Node-B (base station) in- stead. Also the HARQ termination point for HSDPA needs to be located in the Node-B in order to reduce the delays for retransmitted packets [3]. In this paper we will compare two different packet com- bining strategies that are considered for HSDPA. These are Chase combining, where each retransinissioii is identical to the original transmission, and incremental redundancy (IR) where each retransmission consists of new redunda.ncy bits from the channel encoder. Obviously IR have the potential of achieving better perforimmce compared to Chase com- bining. However, a HARQ system with Chase combining will have lower complexity. Th e use of IR requires some ad- ditional signaling since the retransinission numbers needs to be communicated to the receiver. Furthermore, IR re- quires larger receiver buffer size. Th e receiver buffer size increases for each IR transmission aiid it is also necessary to buffer soft bits instead of soft symbols in the mobile termi- nal (UE). Thus if IR is to be implemented for HSDPA the complexity and cost of the system will be higher. Therefor it is important to examine if there ar e any, large perfor- mance gains with IR, or if Chase combining can provide comparable performance a.t lower cost. In this paper we show that the link-level performance of a HARQ type-I1 system in some cases is significantly bet- ter for IR compared to Chase combining. The largest gains are obta,ined for high channel-coding rates a.nd high mod- ulation orders. For low modulation aiid coding schemes (r\irCSs), the link-level performance gains with IR are less significant. Furthermore, in a. system using link adaptation we can not expect any significa.iit gains with IR unless the link adaptation errors are very large. The reason for this is that Chase combining gives 3 dB additional signal energy in the first retransmission a.nd with reasonably good link 0-7803-7005-8/01/ 10.00 0 2001 IEEE 1829
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Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA
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8/9/2019 Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA