Cooperative Transmissions in 5G Large-scale Relay Systems: How to Keep a Balance Between Performance and Complexity?

A well-known dilemma in large-scale cooperative systems is that the use of more relays can boost reception reliability but consume extra bandwidth resources due to the coordination among the relays. In this paper, the tradeoff between performance and complexity is studied in large-scale decode-and-forward (DF) relay systems. First, the cumulative distribution function (CDF) of the sum of the K -largest order statistics of N independent exponential random variables is developed. By using this result, the impact of different cooperation strategies to exploit relays on the tradeoff between system performance and complexity can be characterized. In particular, explicit expressions for the outage probability and the expected number of the used relays employed can be obtained for these strategies, and facilitate asymptotic studies of the addressed large-scale cooperative network. Then the performance of relay cooperation is also evaluated with the presence of channel estimation error. Finally, numerical results are provided to verify the accuracy of the developed theoretic results, and also demonstrate the performance and the complexity achieved by different relay cooperation strategies.