Digital Communications with Emphasis on Data Modems

4PHASE SHIFT KEYING (PSK) MODULATION, DEMODULATION, AND PERFORMANCE

4.1 INTRODUCTION

The various forms of phase shift keying (PSK) waveform modulations are discussed in this chapter with an emphasis on binary PSK (BPSK) and quadrature PSK (QPSK) corresponding to M = 2 and 4 respectively. A significant feature of multiphase PSK (MPSK) waveforms is that they result in a constant envelope waveform that provides for the efficient use of transmitter power amplifiers. Furthermore, they are relatively simple to implement and provide performance robustness with severe channel conditions like: hard limiting, fading, impulse noise, and Doppler rate. The simplest implementations use rect(t/T) time‐domain symbol shaping resulting in a sinc(fT) spectrum. A major drawback with the sinc(fT) spectrum is the poor spectral containment leading to more severe intersymbol interference (ISI) with channel filtering and a greater sensitivity to adjacent channel interference (ACI). The QPSK waveform and its variant offset QPSK (OQPSK)¹ are unique, in that, the bit‐error performance, in terms of the energy‐per‐bit, is identical to that of BPSK. The OQPSK modulation is characterized by the quadrature‐phase channel, or rail, being shifted in time by the bit duration relative to the in‐phase channel. This offsetting of the quadrature channels provides: robustness to spectral re‐growth in a hard‐limited ...