Optimization of link adaptation and HARQ schemes for multicast in high speed cellular networks

This paper targets multicast transmission where the same data is destined to many users simultaneously. Although multicast allows bandwidth saving, it prevents a precise link adaptation over radio links. Indeed, users are subject to the same bitrate despite their different and variable radio conditions. Neither operators nor 3 GPP standard offer solutions to support link adaptation in a multicast scenario. In this context, we propose different solutions. We compare conservative and aggressive schemes by computing the resulting throughput performance. For this purpose, we propose a model to compute the average number of retransmissions. We also study the mapping between the reported SNR and the packet sizes in HSDPA systems. We show that the existing mapping offers the best performance for unicast but cuts down the throughput in a multicast scenario. Then, we propose a convenient mapping for multicast, namely shifted mapping. Despite the better precision of this solution, the resulting gain remains marginal.