Parameter Optimization for Amplify-and-Forward Relaying with Imperfect Channel Estimation

Cooperative diversity is a promising technology for future wireless networks. In this paper, we consider a cooperative communication system operating in an amplify-and-forward (AF) mode with an imperfectly-known relay fading channel. It is assumed that a pilot symbol assisted modulation (PSAM) scheme with linear minimum mean square estimator (LMMSE) is used for the channel estimation. A simple and easy-to-evaluate asymptotical upper bound (AUB) of the symbol-error-rate (SER) is derived for uncoded AF cooperative systems with quadrature amplitude modulation (QAM) constellations. Based on the AUB, we propose a criterion for the choice of parameters in the PSAM scheme, i.e., the pilot spacing and the Wiener filter length. We also formulate an optimum power allocation problem for the considered system. The optimum power allocation can be found by means of a gradient search over a continuous range. Numerical simulations are presented to verify the correctness of the theoretical results and the benefits of the parameter optimization.