Variable-rate adaptive modulation with optimum switching thresholds for cooperative systems with relay selection

In this paper, the performance of variable-rate adaptive modulation schemes in the amplify-and-forward cooperative systems with relay selection is analyzed over Rayleigh fading channels. We consider constant power and discrete-rate adaptive multi-level modulation techniques. The switching levels required for discrete-rate adaptive modulation have been determined for two schemes, namely fixed switching levels and optimum switching levels, both respecting a target bit error rate requirements, where in the later scheme, the switching levels are optimally determined in a way that the average spectral efficiency of the system is maximized. Two M-ary modulation schemes, namely quadrature amplitude modulation and phase shift keying, are considered. Closed-form expressions are derived for three performance metrics, namely average spectral efficiency, outage probability, and average bit error rate, for two cases: independent and identically distributed fading relay links and independent and non-identically distributed links. It is shown that, compared with using fixed switching levels, employing optimum switching levels provides a slight improvement in the spectral efficiency and moderate improvements in the signal-to-noise ratio gain and in the outage probability of the system. It is also shown that compared with the independent and identically distributed links, independent and non-identically distributed relay links yield a slight increase in the signal-to-noise ratio gain and a slight decrease in the diversity order of the system. Copyright © 2011 John Wiley & Sons, Ltd.