Wireless Communications

12Multiple-Access Channels

Abstract

The primary thought behind various multiple-access (MA) schemes in current use is rooted in the paradigm of orthogonality. Collision of two arbitrary symbol waveforms is unavoidable if they have components in the same one-dimensional space. To allow multiple users to share the same channel without interference with each other, the dimension of the space defined by the channel must be extended, so that each user exclusively occupies a mutually orthogonal subspace. The principle behind various MA schemes is to seek appropriate basis functions for multiple users, and this can be done in the time domain, the frequency domain, or their combination. Mathematically, the space defined by a channel remains the same, irrespective of the choice of its basis functions. Physically, however, different choices have different system implications.

12.1 Introduction

Modern communication systems often support multiple users, thereby requiring a multiple-access (MA) technique to enable the users to share the same physical channel. MA technology is at the core of modern wireless communications. The choice of appropriate MA schemes is often the issue of debates for each generation of cellular systems.

When multiple users share the same channel resource for transmission, signals from different users can interfere with each other, resulting in inter-user interference (IUI), or more often called multiple-access interference (MAI). This interference is a dominating ...