BIT ERROR RATE PERFORMANCE OF PI/4-DQPSK IN A FREQUENCY-SELECTIVE FAST RAYLEIGH FADING CHANNEL

The bit error rate (BER) performance of pi/4-DQPSK modems in cellular mobile communication systems is derived and analyzed. The system is modeled as a frequency-selective fast Rayleigh fading channel corrupted by additive white Gaussian noise (AWGN) and co-channel interference (CCI). The probability density function of the phase difference between two consecutive symbols of M-ary DPSK signals is first derived. In M-ary DPSK systems, the information is completely contained in this phase difference. For pi/4-DQPSK, the BER is derived in a closed form and calculated directly. Our numerical results show that for the 24 kBd (48 kb/s) pi/4-DQPSK operated at carrier frequency 850 MHz and C/I < 20 dB, the BER will be dominated by CCI if the vehicular speed is below 100 mi/h. In our derivation, frequency-selective fading is modeled by two independent Rayleigh signal paths. Only one co-channel is assumed in our derivation. However, the approach used is readily applicable to multipath, multi-CCI, and ACI systems. The results obtained are also shown to be valid for discriminator detection of M-ary DPSK signals.