# 6 Multi-level digital modulation

# 6.1 Introduction

#### Thus far, we have only considered bandpass data modulation schemes using binary signalling, and with only one property (amplitude, frequency or phase) of the carrier wave being altered. We have also seen that for binary ASK, FSK or PSK, the minimum bandwidth required for transmission is at least twice the minimum bandwidth required to pass the baseband binary data stream. In order to improve on the bandwidth efficiency of bandpass data transmission, we can of course increase the number of symbol states used (except for the case of FSK, where increasing the number of frequencies can increase the occupied bandwidth). As a general rule, however, we know that as the number of symbol states is increased, the tolerance to noise is reduced. We shall see that there are two exceptions to this rule, QPSK and orthogonal M-ary FSK. |

#### M-ary bandpass modulation formats, particularly M-ary Quadrature Amplitude Modulation, are widely used in both wired and wireless digital communications links.