Chapter 1Joint Diversity and Beamforming forDownlink Communications1




1.1. Introduction

Mobile communication systems must be able to cope with fading and multi-user interference. Since the introduction of GSM until today, these problems have been considered from different angles and several approaches have been proposed to mitigate impairments.

Digital processing of the signal coming from an array of antennas (called smart antenna techniques) [JAK 74, RAP 01, YAC 93] has played a very important role in the progress achieved in this area so far. Among the novel techniques in this area, we can cite beamforming, diversity and MIMO (multiple input multiple output) techniques as the most successful. Smart antenna techniques can be applied either at the base station or at the mobile, and on the downlink or uplink. For technological and economical reasons, it is often more advantageous to only have an array of antennas at the base station, and a single antenna at the mobile. For the sake of simplicity, but without loss of generality, in this chapter we will consider the downlink of a mobile system with an antenna array at the base station and a single antenna at the mobile.

The main goal of beamforming is to increase the signal-to-noise ratio (SNR) at the desired mobile and to reduce the interference generated toward other mobiles present in the system. This is done by directing the radiated signal towards the receiver. The chosen direction does not necessarily match the geographical ...

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