Effects of Doppler spread on adaptive orthogonal frequency division multiplexing channel capacity employing diversity in multipath environments

This paper derives the capacity of a fading channel with orthogonal frequency division multiplexing transmission employing diversity techniques and adaptive policies in Rayleigh fading environments. The effects of Doppler shift fd on channel capacity are analysed. The main contribution of this work is to model intercarrier interference as a function of fd and symbol duration Ts. Two diversity techniques are considered (i) maximal ratio combining (MRC) and (ii) selective combining (SC). Three adaptive policies are combined with MRC and SC to estimate the following channel capacity: (i) optimal rate adaptive; (ii) optimal power and rate adaptive; and (iii) channel inversion with fixed rate adaptive. Closed-form expressions and bounds on channel capacity employing different diversity techniques are derived. Simulation results are given.