Spectrum and Energy Efficient Relaying Algorithms for Selective AF-OFDM Systems

This paper proposes a selective AF-OFDM (Amplify-and-Forward, Orthogonal Frequency Division Multiplexing) relaying protocol, in which selective relaying policy is adequately utilized to improve the spectrum efficiency of cooperative multi-carrier transmission, and addresses the power minimization problem subject to a target quality of service (QoS) constraint for corresponding battery-operated systems. By transforming the original power minimization issue into a two-stage problem, i.e., a power loading subproblem and a selective relaying policy decision subproblem, an energy efficient power control (PC) technique is proposed. Specifically, the binary selective relaying policies are determined for each subcarrier based upon channel state information (CSI), while power loading can be mathematically solved by employing standard Lagrange techniques, leading to analytical solutions for individual transmit powers at the source and the relay. In order to further improve the energy efficiency, an independent \emph{min-min} relay selection policy is investigated by making use of available diversity of the multiple-relay channel. Numerical results illustrate the superiority of the proposed selective AF-OFDM relaying protocol and validate the efficiency of our proposed algorithms. It is shown that with PC, the proposed selective AF-OFDM relaying can significantly reduce the system power consumption, and the \emph{min-min} relay selection policy is more appropriate than the conventional \emph{max-min} strategy for our transmission protocol.