The affecting factors in resource optimization for cooperative communications: A case study

Cooperative networks provide enhanced system performance by exploiting spatial diversity in a distributed manner. Optimum resource allocation can help improve the performance of cooperative networks and increase the efficiency of resource usage. In the literature, various system performance and optimization results have been reported for different systems and with different optimization metrics. However, there lacks a unifying framework delineating the effects of different factors on resource optimization and the resultant benefit. In this paper, we investigate the relative effects of optimization metric (error rate versus outage probability), modulation type (coherent versus differential) and relaying protocol (amplify-and-forward (AF) versus decode-and-forward (DF)). To facilitate such a case study, we provide a comprehensive set of system performance for four commonly adopted cooperative systems: coherent amplify-and-forward (CAF), coherent decode-and-forward (CDF), differential amplify-and-forward (DAF), and differential decode-and-forward (DDF). A resource optimization problem that minimizes the total transmit energy is formulated. Since energy optimization has been intensively studied in the literature, location optimization will be investigated. The analyses and simulations suggest that: i) The error rate and outage probability metrics yield similar optimization results for AF relaying systems; ii) The relaying protocol determines the optimization results while the modulation type has no effect; and iii) The difference between different relaying protocols diminishes when the number of relays increases.