Cooperative ARQ: A Medium Access Control (MAC) Layer Perspective

Cooperative Automatic Retransmission reQuest (C-ARQ) schemes have become a very active research topic over the last years. C-ARQ schemes constitute a practical way of executing cooperation in wireless networks with already existing equipment. C-ARQ schemes exploit feedback from the receiver, i.e. cooperation is only executed when needed, and thus are sometimes referred to as cooperation on-demand cooperative schemes. In short, the idea of C-ARQ is to exploit the fact that, due to the broadcast nature of the wireless channel, any transmission can be received by any of the stations in the transmission range of the transmitter. What has been traditionally considered as interference, is exploited in C-ARQ schemes to attain spatial diversity. Upon a transmission error, a retransmission can be requested from any (or some) of the stations which overheard the original transmission, which can act as spontaneous helpers (or relays). The result is that the destination of a packet can receive different copies of the same information arriving via statistically independent transmission paths, i.e., space diversity. C-ARQ schemes have been already studied in the literature from a theoretical point of view and there is no doubt that, under some conditions, they can dramatically boost the performance of wireless communications compared to traditional ARQ, where retransmissions are performed only from the source. However, involving a number of users in a communication link requires coordination. To this end, efficient Medium Access Control (MAC) protocols are necessary to get the maximum efficiency of the communications. In this chapter we emphasize the important role of the MAC layer in this context of C-ARQ. Along the chapter, we first review in Section 2 the motivation and operation of C-ARQ schemes into detail. We go through the parameters that affect the performance of these schemes and we point out the role of the MAC layer. Taking into account the specific requirements of the MAC layer in this kind of schemes, we present in Section 3 a novel highperformance MAC protocol specifically tailored for this purpose. Computer-based simulations are presented to evaluate the performance of the protocol. Finally, Section 4 concludes the chapter. 12