# 5 Baseband digital modulation

# 5.1 Introduction to Chapter 5

The previous chapters have been largely concerned with so-called baseband signalling where the channel band is assumed to extend from 0 Hz upwards. In applications where contiguous bandwidth encompassing 0 Hz is not available, bandpass signalling is required. Here, the task is to centre the symbol energy at a given frequency of operation, for example, 900 MHz for a typical cellular telephone channel and 30000 Hz, (1000 nm) for an optical fibre link.

This process usually involves modulating the amplitude, frequency and/or phase of a carrier sinewave. The carrier is commonly written as cos(wct). We shall see that the choice of modulation method crucially affects the ease of implementation, the noise tolerance and occupied channel bandwidth of the resulting bandpass data modem. Below, a chart is provided showing how the bandpass spectrum is allocated to different communications services based on the carrier frequency.

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