Performance Comparison of HARQ with Chase Combining and Incremental Redundancy for HSDPA

Abstract-In this paper we compare two hybrid automatic repeat request (HARQ) combining strategies that currently are considered for the high speed downlink packet access (HSDPA) evolution of WCDMA. The two HARQ combining schemes are Chase combining, where the retransmissions are identical copies of the original transmission, and incremental 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-I1 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 (MCSs), the link-level performance 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 (HSDPA), Chase Combining, Incremental Redundancy (IR), Hybrid Automatic Repeat reQuest (HARQ). I . INTRODUCTION As a first step in the evolution of WCDMA, a new concept 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][9][10] Mbit/s for best effort packet based services a,nd to significantly 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, similar 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 the chaiinel encoder t.0 the fast channel fading. The scheduler decides, based on e.g. the instanta.neous channel qualities of all users, which user shall be assigned the HS-DSCH channel during the upcoming tra,nsmission time interval. Furthermore, if a,n error is detected by the receiver, the fast hybrid ARQ system ensures that the necessary retransmission 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 the T C P window-size, or else it will not be possible t o 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 T C P 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) architecture 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) instead. 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 combining 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 combining. However, a HARQ system with Chase combining will have lower complexity. The use of IR requires some additional signaling since the retransinission numbers needs to be communicated to the receiver. Furthermore, IR requires larger receiver buffer size. The receiver buffer size increases for each I R transmission aiid it is also necessary to buffer soft bits instead of soft symbols in the mobile terminal (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 are any, large performance 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 better for IR compared to Chase combining. The largest gains are obta,ined for high channel-coding rates a.nd high modulation 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 I R 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 lin

Relay Switching Aided Turbo Coded Hybrid-ARQ for Correlated Fading Channel

Hybrid-Automatic-Repeat-reQuest (HARQ) has become an indispensable technique in reliable communications systems. However, its performance is inevitably affected by the channel’s fading correlation. In this paper, we proposed a novel relay-switching aided HARQ scheme in order to mitigate the detrimental effects of correlated fading without unduly increasing the system’s complexity and delay. Our results show that the proposed relay-switching regime operates efficiently in correlated channels, hence significantly reduces the error floor of turbo-coded HARQ. Additionally, a HARQ scheme using Segment Selective Repeat (SSR) is incorporated in the relay-switching scheme for achieving further improvements. Quantitatively, the proposed relay-switching aided turbo-coded HARQ scheme using SSR may achieve an approximately 2 dB gain, compared to the conventional amplify-and-forward aided turbo coded HARQ arrangement using Chase Combining. Index Terms - Relay switching, correlated fading channel, Hybrid-ARQ, turbo codes, chase combining, incremental redundancy, selective segment repeat

System Level Design Considerations for HSUPA User Equipment

Publication in the conference proceedings of EUSIPCO, Florence, Italy, 200

Joint Scheduling and ARQ for MU-MIMO Downlink in the Presence of Inter-Cell Interference

User scheduling and multiuser multi-antenna (MU-MIMO) transmission are at the core of high rate data-oriented downlink schemes of the next-generation of cellular systems (e.g., LTE-Advanced). Scheduling selects groups of users according to their channels vector directions and SINR levels. However, when scheduling is applied independently in each cell, the inter-cell interference (ICI) power at each user receiver is not known in advance since it changes at each new scheduling slot depending on the scheduling decisions of all interfering base stations. In order to cope with this uncertainty, we consider the joint operation of scheduling, MU-MIMO beamforming and Automatic Repeat reQuest (ARQ). We develop a game-theoretic framework for this problem and build on stochastic optimization techniques in order to find optimal scheduling and ARQ schemes. Particularizing our framework to the case of "outage service rates", we obtain a scheme based on adaptive variable-rate coding at the physical layer, combined with ARQ at the Logical Link Control (ARQ-LLC). Then, we present a novel scheme based on incremental redundancy Hybrid ARQ (HARQ) that is able to achieve a throughput performance arbitrarily close to the "genie-aided service rates", with no need for a genie that provides non-causally the ICI power levels. The novel HARQ scheme is both easier to implement and superior in performance with respect to the conventional combination of adaptive variable-rate coding and ARQ-LLC.Comment: Submitted to IEEE Transactions on Communications, v2: small correction

Backlog and Delay Reasoning in HARQ Systems

Recently, hybrid-automatic-repeat-request (HARQ) systems have been favored in particular state-of-the-art communications systems since they provide the practicality of error detections and corrections aligned with repeat-requests when needed at receivers. The queueing characteristics of these systems have taken considerable focus since the current technology demands data transmissions with a minimum delay provisioning. In this paper, we investigate the effects of physical layer characteristics on data link layer performance in a general class of HARQ systems. Constructing a state transition model that combines queue activity at a transmitter and decoding efficiency at a receiver, we identify the probability of clearing the queue at the transmitter and the packet-loss probability at the receiver. We determine the effective capacity that yields the maximum feasible data arrival rate at the queue under quality-of-service constraints. In addition, we put forward non-asymptotic backlog and delay bounds. Finally, regarding three different HARQ protocols, namely Type-I HARQ, HARQ-chase combining (HARQ-CC) and HARQ-incremental redundancy (HARQ-IR), we show the superiority of HARQ-IR in delay robustness over the others. However, we further observe that the performance gap between HARQ-CC and HARQ-IR is quite negligible in certain cases. The novelty of our paper is a general cross-layer analysis of these systems, considering encoding/decoding in the physical layer and delay aspects in the data-link layer