An analysis of future π/4 shifted DQPSK cellular mobile communication systems impaired by ISI and modulator nonlinearity and imbalance

A simulation study of the performance of differentially coherent differential quadrature phase-shift keying (DQPSK) receivers in the presence of several modulator impairments, typical of digital cellular mobile cummunication systems, is presented. The bit error rate (BER) is determined in terms of the RMS of the error vector magnitude in the presence of ISI and modulator nonlinearity and imbalance. The current standards allow a maximum RMS of 0.125. The BER with RMS=0.125 is obtained for different modulator impairments. It is shown that the BER is a function of the RMS of the error vector magnitude and the type of modulator nonlinearity involved. An adaptive amplitude and phase adjustment technique to compensate for the above system impairments is introduced. It is demonstrated that such an adjustment technique significantly improves the overall system performance