A new generation of Rayleigh fade compensated pi /4-QPSK coherent modems

A novel fade estimation and compensation method based on the pilot symbol insertion (PSI) concept is proposed and analyzed for coherent pi /4-QPSK (quadrature phase shift keying) modems. The estimated fade characteristics are used to compensate the random phase modulation caused by the Doppler spread in a fast Rayleigh fading channel so that the signals can be demodulated coherently. The theoretical performance of the fade compensated coherent modem in a frequency-selective fast Rayleigh fading channel corrupted by additive white Gaussian noise and cochannel interference is evaluated. The results show that the fade compensated coherent demodulation achieves the same performance as noncoherent demodulation in a slow fading channel. The main advantage of the proposed technique is that it removes the error floor caused by Doppler spread in a fast fading channel.<>