Conventional HARQ Research Articles

Rate selection for wireless multicast system adopting incremental redundancy type hybrid-ARQ

We propose a rate selection method in wireless multicast systems where an incremental redundancy type hybrid automatic repeat request (HARQ-IR) scheme is employed under Rayleigh block-fading channels. We aim at maximizing the long-term average transmission rate (LATR) while the maximum number of retransmissions is limited to L — 1 and the probability that at least one user fails to decode a message within L (re)transmissions is limited to e. We consider two user-distribution scenarios: 1) Receivers are located at the same distance from the source and 2) receivers are uniformly distributed in a circular cell. For the former scenario, we derive two closed-form approximations for the LATR. Among the two, the simpler one is expressed as $\mu - {{Q^{-1}(1-0.5^{1/U})\sigma}\over{\sqrt L}}$ for a large L value where U, μ, σ, Q denote the number of receivers, the ergodic capacity, and the deviation of the mutual information for one HARQ round, and the complementary Gaussian cumulative distribution function, respectively. We also analyze the second scenario using point mass approximation. We show that the HARQ-IR scheme outperforms the conventional fixed-length coding and Chase-combining type HARQ schemes in wireless multicast network for both scenarios.

Rate Adaptation for Cooperative HARQ

We analyze the average throughput and the outage probability for relay-based incremental redundancy HARQ transmission over block fading channel. We focus on the effects of having an error-free multi-bit feedback channel instead of the single-bit ACK/NACK feedback used in conventional HARQ. This multi-bit feedback message provides the cooperating nodes with outdated channel state information (CSI) so that they can adapt their transmission rate. We discuss a network with M relay nodes and assume adaptive transmission rate for all the cooperating nodes. We describe the adaptation problem as a Markov Decision Process (MDP) and employ the Dynamic Programming (DP) for optimization. The numerical results obtained in the case of one relay transmission, indicate that significant throughput gains can be obtained when compared to non-adaptive, i.e, fixed-rate HARQ. Moreover, we study the performance limits of the system model by finding an upper bound of the throughput for the cooperative HARQ channel which is applicable to both adaptive and non-adaptive models. Finally, we analyze the discretization issues and conclude that only a small number of feedback bits is required by the adaptive system to outperform the conventional single-bit HARQ.