Relay Selection in AF Cooperative Systems: An Overview

In cooperative networks, mobile users do not only have the role of a data source or destination but also act as relays to help transmission from others. However, in a wireless network, there might be tens or even hundreds of mobile users that can play the role of relays for a given transmission. Relay selection strategies are, therefore, paramount to ensure successful deployment of cooperative techniques for next-generation wireless networks. In this article, we present an overview of some advances on the topic of relay selection in amplify-and-forward (AF) cooperative systems. We first consider an AF opportunistic relaying (OR) with fixed-rate transmission to illustrate the advantages of relay selection. Then we consider cross-layer discrete-rate adaptive modulation techniques. We highlight that it is possible to achieve the theoretical capacity scaling and diversity order while providing a given quality of service to users respecting a target average bit error rate (BER) requirement. We also show that by optimally choosing the switching thresholds for the adaptive modulation strategy, it is possible to significantly increase the network spectral efficiency. Finally, since relay selection depends on the channel state information feedback, we present results quantifying the impact of outdated channel information on the performance of vehicular AF OR systems.