Feedback protocols for multiuser diversity based multicast scheduling in wireless networks

Multiuser diversity (MUD) underlies much of the recent work on both, unicast and multicast scheduling design in wireless networks. In that context, a large channel variation among users is not a drawback. In the case of unicast, the user with the best quality channel can access the medium while the other users can wait for a better quality channel. In the case of multicast, the sender transmitting to a multicast group can also exploit the channel variation among users. Instead of transmitting at a data rate decodable by the worst user, the sender transmits in each transmission to the best ¿ users until all users are served. ¿ is a user selection ratio. MUD based scheduling algorithms need the sender to query receivers for their channel conditions. In a previous work, we proposed and analysed different feedback protocols in the case of MUD based unicast [4], [5]. In this paper we propose, analyse and compare new feedback protocols for multiuser diversity based multicasting. We investigate the guard time for which a sender has to wait to get information about channel conditions of multicast users. Corresponding expressions for the guard time and the capacity are presented. The numerical analysis of these expressions show that our proposed feedback approaches present a lower guard time than the Full Feedback approach. We showed also that the guard time has no impact on the optimal value of ¿ which is equal to 1 with the decrease in the number of multicast users and the increase in the CNR.