Performance of Optimum Switching Adaptive -QAM for Amplify-and-Forward Relays

Optimization of the switching thresholds for constant-power adaptive five-mode M -ary quadrature amplitude modulation (M -QAM) transmission with an amplify-and-forward (AF) relay network is developed. The optimization criterion is the maximization of spectral efficiency subject to an average bit-error-rate (BER) constraint. This approach results in a constant-BER variable-rate M -QAM AF relay system, which requires feedback of log2(N) bits for N modes. The performance analysis is based on an upper bound on the total effective SNR. Expressions are derived for the outage probability, the achievable spectral efficiency, and the error-rate performance for the AF cooperative system over both independent identically distributed (i.i.d.) and non-i.i.d. Rayleigh fading environments. The tightness of the upper bound is validated by Monte Carlo simulation. Adaptive five-mode M -QAM with optimum switching levels is shown to offer performance gains of 2–2.5 dB compared with fixed switching in terms of the transmit SNR to achieve specific spectral efficiency. Furthermore, the spectral efficiency of adaptive five-mode M -QAM with optimized switching comes within ∼6 dB of the theoretical Shannon channel capacity. However, this performance gain, which is obtained by employing adaptive M -QAM under cooperative diversity, comes at the cost of increased system complexity that is incurred due to the additional complexity of transmitter and receiver design.