The Effect of Channel State Information on Optimum Energy Allocation and Energy Efficiency of Cooperative Wireless Transmission Systems

This paper considers the problem of how to efficiently allocate transmission energy in a wireless communication system with two delay-constrained cooperating sources and one destination. The sources in the system relay each other´s transmissions via an orthogonal amplify-and-forward protocol. The channels are assumed to be flat fading and the sources are each required to satisfy an outage probability constraint. The analysis focuses on optimum energy allocation and energy efficiency for two distinctly different scenarios: (i) the sources have access to partial channel state information (the instantaneous channel amplitudes) and (ii) the sources have access to only the channel statistics. Numerical examples are presented for independent Rayleigh fading channels demonstrating that partial channel state information significantly improves the energy efficiency of cooperative transmission. Our results also show that, while cooperative transmission tends to have better energy efficiency than direct (noncooperative) transmission, opportunistic direct transmission with partial channel state information is often more energy efficient than cooperative transmission without knowledge of the channel state.