ARQ Based Half-Duplex Cooperative Diversity Protocol

Distributed antennas can be used to provide a mean to combat fading with a similar flavor as that of space diversity. This kind of reliability obtained by the creation of virtual antennas is referred to as cooperative diversity because the terminals share their resources to get the information across to the destination. Such schemes have attracted significant attention recently, and a variety of cooperation protocols have been studied and analyzed in various papers like [1] [2] [3] [4]. Recently, the authors of [5] extended the Zheng-Tse formulation [6] and characterized the three dimensional diversitymultiplexing-delay tradeoff in MIMO ARQ channels. They established that delay can be exploited as a potential source for diversity. Thus, retransmission protocol is an appealing scheme to combat fading and its performance has been recently studied in decentralized ad hoc networks [7]. Inspired by [5], we propose a new scheme for transmission in relay channel utilizing the ARQ to increase the diversity gain. We look at the tradeoff in the high snr (Signal-to-Noise Ratio) regime and point out the gain achieved by the ARQ. Moreover it is reasonable to assume the ARQ, as the receiver just needs to send back a one bit feedback to signify success or failure. Following the setup in [2], the terminals are constrained to employ half-duplex transmission, i.e. they cannot transmit and receive simultaneously. The source and the relay are allowed to transmit in the same channel using cooperative protocols not relying on orthogonal subspaces. This is in contrast to [2], where the available bandwidth is divided into orthogonal channels allocated to the transmitting terminals. In the dynamic decode and forward scheme proposed in [4] the communication is across one block of fixed length l, where l is asymptotically large. In our setting the ARQ permits the use of communication over a variable number of blocks (henceforth referred to as number of rounds) of fixed length where the