Downlink performance of multiuser multi-relay cooperative networks: A low-complexity relay-destination selection scheme

This paper investigates the outage performance of multiuser multi-relay cooperative networks in which a source node communicates with one out of L destinations through the help of one out of N decode-and-forward (DF) relays. A low-complexity relay-destination selection scheme is proposed, which first selects the best destination node based on the channel quality of the direct links and then selects the best relay that yields the best path from the source to the selected destination. In our analysis, the source node may be equipped with either a single antenna or M multiple antennas. A closed-form expression for the outage probability is derived and validated by means of Monte Carlo simulations. An asymptotic analysis is carried out and reveals that, for the single-antenna source case, the diversity order reduces to L+N, whereas for the multiple-antenna source case, the diversity order equals to ML + N. Assuming two network topologies, a comparison between our scheme and the optimal joint relay-destination selection scheme is provided and it reveals that the proposed selection scheme achieves a very close outage performance to that of the joint selection scheme while reduces considerably the system complexity (in terms of channel estimations, implementation issues, and simulation time). The effects of the number of relay and destination nodes on the system performance as well as its influence on the best relay position are examined through representative numerical plots. It is noticed a tradeoff concerns to the system performance and spectral efficiency when multiple antennas are employed. More specifically, in one hand, by increasing the number of antennas at the source node, a better outage behavior is obtained. On the other hand, the system loses in terms of spectral efficiency.