Performance of adaptive transmit power control in π/4 DQPSK mobile radio systems in flat Rayleigh fading channels

We theoretically study and analyze adaptive transmit power control schemes for π/4 shifted differentially encoded QPSK based mobile radio transmissions with both coherent and differential detection under flat Rayleigh fading conditions. By assuming the power control to be fast enough to compensate for Rayleigh fading, we develop theoretical general expressions for average relative transmitted power and average bit error rate (BER) for different levels of power control. The analytical results indicate the average transmit power for a given BER of 10^-3 can be reduced to a maximum of 7~9 dB relative to the conventional constant transmit power scheme. The approximate conditional BER expression, assumed for differential detection, is found to be very close representation of the exact BER expression available in the literature