Digital Communications with Emphasis on Data Modems

6AMPLITUDE SHIFT KEYING MODULATION, DEMODULATION, AND PERFORMANCE

6.1 INTRODUCTION

This chapter discusses communication waveforms involving various forms of amplitude shift keying (ASK). As used here ASK is a general term that applies to the modulation of a carrier signal with discrete amplitudes that uniquely identify a symbol of binary data or bits. When the amplitude keying is expressed in terms of α_m = 2m: m = 0, 1, and applied to one quadrature component of the carrier frequency, the modulation is referred to as on–off keying (OOK) and when α_m = −1 + 2m: m = 0, 1, it is referred to as antipodal ASK or, when applied to a radio frequency (RF) carrier, OOK is the same as binary phase shift keying (BPSK). The more general case of ASK applied to one quadrature component occurs when α_m = −(M − 1) + 2m: m = 0, 1, …, M − 1 and this is referred to as pulse amplitude modulation (PAM) or, more specifically, M‐ary PAM. When PAM is applied to each quadrature component, the modulation is referred to as quadrature amplitude modulation (QAM) or M‐ary QAM. Based on this description, it is evident that BPSK is a subset of PAM modulation and quadrature phase shift keying (QPSK) is a subset of QAM modulation. QAM necessarily involves a phase‐modulated carrier with the resulting signal rest‐points at the intersection of the quadrature amplitude terms α_Im and α_Qm, where I and Q are the inphase and quadrature carrier components. The term rest‐points refer to the ideal noise‐free optimum signal ...