Optimal Power Adaptation for Cooperative AF Relaying With Channel Side Information

In this paper, we develop optimal power adaptation schemes by means of power amplification coefficients at the relay for half-duplex single-relay amplify-and-forward (AF) cooperative systems with channel side information (CSI) available at the relay. We consider both the orthogonal AF (OAF) and the nonorthogonal AF (NAF) transmission protocols. In particular, assuming that the relay has full knowledge of the channel gains, optimal power adaptation schemes under a long-term average power constraint are established in closed form using the mutual information (MI) and pairwise error probability (PEP) criteria with Gaussian inputs at the source. The proposed solutions can be understood as multidimensional (multi-D) distributed water filling in time and space. Numerical results show that the proposed power amplification methods provide a significant improvement over conventional schemes using either channel distribution information (CDI) or channel inversion (CI). The gain is observed with regard to the MI with Gaussian inputs and the bit-error-rate (BER) performance using finite constellations for both coded and uncoded systems.