Bit error rate (BER) results are presented for π/4 differential quadrature phase-shift keying (DQPSK) with Nyquist pulse shaping in flat and frequency-selective fading channels. BER results are shown for a wide range of multipath delays, receiver speeds and power ratios (C/D) between the main ray and the delayed ray in a two ray channel model. Co-channel interference effects are also considered. In addition, the instantaneous BER for π/4 DQPSK in various fading channels is simulated. The simulation is first performed off line, and then bit error patterns are replayed in real time to allow subjective evaluation of link quality between a source and sink. This method is being used to provide real time bit error simulation of US Digital Cellular data channels using simple computer hardware.