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

Abstract

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.