Distributed TAS/MRC and TAS/SC Schemes for Fixed-Gain AF Systems with Multiantenna Relay: Outage Performance

Although transmit-antenna selection (TAS) is an alternative low-cost strategy to capture the advantages of multi-antenna systems, its application may require a large amount of feedback transmissions. Owing to this fact and in order to reduce such requirement, herein we analyze the outage performance of two distributed TAS (DAS) schemes. One scheme employs maximal-ratio combining (DAS/MRC) at the destination, whereas the other employs selection combining (DAS/SC). We consider a dual-hop fixed-gain amplify-and-forward relaying network, equipped with multi-antenna source/relay nodes and a single-antenna destination. A lower bound expression is derived for the outage probability of each investigated scheme. Importantly, the derived bounds prove to be very tight approximations to the exact outage performance. Also, capitalizing on a strikingly interesting property of Stirling numbers of the second kind, we provide closed-form asymptotic expressions for the obtained bounds. Our results show that the diversity order of the proposed distributed schemes is identical to that of their optimal centralized counterparts, namely N_t + min(N_rr,N_rt), with N_t, N_rr, and N_rt denoting the number of transmit antennas at the source, the number of receive antennas at the relay, and the number of transmit antennas at the relay, respectively. In addition, as the relay approaches the destination, the outage performance of the proposed schemes approaches the optimal one.