Multi-User Diversity without Transmitter CSI

Abstract — In this paper, we consider both the downlink and up-link of a cellular communication system operated under strict delay constraints. We further assume that, except for the one-bit ACK/NACK signal associated with the Automatic Retransmission reQuest (ARQ) protocol, no channel state information (CSI) is available at the transmitter(s). In this setting, we establish the critical role of multi-user diversity in facilitating ARQ gain, while satisfying an upper-bound on the maximum overall delay (i.e., queuing plus transmission). In order to distinguish between the different types of gains offered by multi-user diversity, we first derive our results for a symmetric multi-user channel where we consider the down-link (broadcast) and up-link (multipleaccess) scenarios separately. This reveals the interaction between multi-user diversity and ARQ gain. Then, we extend our study to the multi-user relay channel where a number of half-duplex relays are devoted to facilitate the communication. The analysis of this scenario highlights the role of multiuser diversity in allowing a large number of users to significantly benefit from a few dedicated relays. I