Effect of channel estimation error on M-QAM BER performance in Rayleigh fading

We determine the bit-error rate (BER) of multilevel quadrature amplitude mo dulation (M-QAM) in flat Rayleigh fading with imperfect channel estimates. Despite its high spectral efficiency, M-QAM is not commonly used over fadin g channels because of the channel amplitude and phase variation. Since the decision regions of the demodulator depend on the channel fading, estimatio n error of the channel variation can severely degrade the demodulator perfo rmance. Among the various fading estimation techniques, pilot symbol assist ed modulation (PSAM) proves to be an effective choice. We first characteriz e the distribution of the amplitude and phase estimates using PSAM. We then use this distribution to obtain the BER of M-QAM as a function of the PSAM and channel parameters. By using a change of variables, our exact BER expr ession has a particularly simple form that involves just a few finite-range integrals. This approach can be used to compute the BER for any value of M . We compute the BER for 16-QAM and 64-QAM numerically and verify our analy tical results by computer simulation. We show that for these modulations, a mplitude estimation error leads to a 1-dB degradation in average signal-to- noise ratio and combined amplitude-phase estimation error leads to 2.5-dB d egradation for the parameters we consider.