7.1 INTRODUCTION

In both analog and digital microwave radio systems, it is essential that demodulated baseband signals have an adequate signal-to-noise ratio, S/N, and that the system is reliable, that is, has high availability. Availability is affected by fading of the received signal and equipment failure or may be reduced by interference on one or more hops.

In an analog system, a reduction in received signal level (RSL) results in a proportional increase in noise. The system is designed to meet the noise performance requirement. Space diversity can provide some protection against fading, whereas frequency diversity provides protection against both fading and equipment failure.

The digital system is quite different. The final judgment of the performance quality is the number of bits received in error in a particular time period that is, how large the probability of error or the bit error rate (BER) is. The error rate and noise in a voice channel are independent of the RSL until a low carrier-to-noise ratio (CNR) exists where the BER changes rapidly with small changes in RSL followed by loss of synchronization. For example, a BER of 10 ^?4 at a CNR of 20 dB, for instance, may define the minimum acceptable performance and the minimum RSL under faded conditions. For signal levels a few decibels above this, the error rate can be considered to be negligible and the system to be error-free most of the time.

A second important difference between analog and digital...