# 8 Multi-user digital modulation techniques

## 8.1 Introduction to Chapter 8

For the majority of data communications that take place, there is a requirement for several users to share a common channel resource at the same time. This resource could be the high speed optical fibre links between continents, the frequency spectrum in a cellular telephone system, or the twisted pair 'ethernet' cable in the office. For multiple users to be able to share a common resource in a managed and effective way requires some form of access protocol that defines when or how the sharing is to take place and the means by which messages from individual users are to be identified upon receipt. This sharing process has come to be known as multiplexing in wired communication systems, and multiple access in wireless digital communications. |

Three classes of multi-user access techniques will be considered in this chapter: techniques where individual users are identified by assigning different frequency slots, techniques where users are given different time slots, and techniques where users are given the same time and general frequency slots, and are identified by different codes. We have already touched upon time-based multiplexing techniques when looking at packet transmission in the section on the fundamentals of data networks and protocols in Chapter 1.

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