Decision feedback-based demodulation and adaptiveequalization of DDPSK signals

Abstract

Differentially encoded phase shift keying (DPSK) and double differentially encoded phase shift keying (DDPSK) that allow noncoherent demodulation have attracted a great interest in a variety of applications, because of their immunity to the phase and frequency uncertainty. However, this robustness is achieved at the cost of performance degradation caused by noisy reference signals. In addition, under the channel environment of intersymbol interference (ISI) there are some difficulties in equalizing DPSK and DDPSK signals recovered by differentially coherent (noncoherent) detection scheme. This is because of the nonlinearity of the differential detector. Motivated by the limitations we designed simple decision feedback-based demodulators for detecting DDPSK signals in AWGN channel as well as decision feedback demodulation-based adaptive equalizers for recovering DPSK and DDPSK signals in multipath fading channels under the condition of relatively large random frequency offsets. First, in order to overcome the performance degradation caused by the noisy reference signals we extended the decision feedback demodulation scheme used for DPSK systems to the case of DDPSK systems, which resulted in a cascade of two decision feedback detectors for DPSK signals. The developed demodulators are non-recursive/recursive decision feedback-based inphase-quadrature (IQ) and autocorrelation demodulators. We also derived their approximate bit error probability. It was shown through numerical analysis that the performance of the proposed demodulators was comparable to that of coherent demodulator; and their implementation was considerably simple. Next, we proposed to combine linear feedback equalization and decision feedback demodulation to yield a DFE-like equalizer implemented before the differential detector of DPSK and DDPSK signals. By modifying the equalizer outputs based on decision feedback demodulation scheme and normalizing the result before feeding back into its feed...