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

Abstract

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 3GPP 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.

An efficient HARQ scheme for applications in multicast communication systems

“…In this case, a single source node, namely the access point (AP), transmits the same communication flow to the set of m nodes forming the MG. Each link from the AP to a generic node is modeled as an independent lossy channel. In such scenario, the simplest retransmission scheme consists in retransmitting a new set of m copies of a same packet till all the nodes belonging to the MG receive that packet without errors . However, this solution is not efficient because it cannot take into account that some nodes could have successfully received the informative packet during previous transmission attempts.…”

“…On the other hand, it is well known in the literature [1][2][3][4][5][6][7][8] that the communication delay and the transmission energy associated to reliable multicast communications decrease as the number of users increases. In particular, the mean time required to successfully transmit an information packet to all the members of a multicast group (MG) increases with the MG size; hence, the overall energy increasing related to the reliable transmission of an information packet increases as well.…”

“…On the other hand, several papers proposed the use of HARQ schemes in multicast communication systems [1,8,[12][13][14]. In particular, Wang et al [15] proposed several HARQ strategies.…”