Introduction

The previous chapter shows that there are benefits to be gained when M-ary (M = 4) signaling methods are used rather than straightforward binary signaling. In general, M-ary communication is used when one needs to design a communication system that is bandwidth efficient. It is based on the observation that as the time duration of a signal, Ts, increases, the bandwidth requirement decreases. See Examples 2.11, 2.16, and Problem 2.38, which illustrate this. Typically, unlike QPSK and its variations in the previous chapter, the gain in bandwidth is accomplished at the expense of error performance. M-ary modulation is also a natural choice when the source is inherently M-ary, for example, the transmission of the English alphabet or when error control coding is used.

However, even when the source is inherently M-ary, the usual scenario is that the M messages are mapped to a sequence of bits, e.g., the ASCII code used for text. Therefore, even in these situations the final source output is binary and from the perspective of the modulator looks like a binary source. The typical application of M-ary modulation is one where a binary source has its bit stream blocked into groups of λ bits. The number of different bit patterns is 2λ, which means M = 2λ, where each bit pattern is mapped (modulated) into a distinct signal.