11.1 Introduction

As shown by Fig. 10.17, the probability of bit error ( P _e) in a digital transmission can be reduced by increasing [ E _b/ N ₀], but there are practical limits to this approach. Equation (10.24) shows that for a given bit rate R _b, [ E _b/ N ₀] is directly proportional to [ C/ N ₀]. An increase in [ C/ N ₀] can be achieved by increasing transmitted power and/or reducing the system noise temperature (to reduce N ₀). Both these measures are limited by cost and, in the case of the onboard satellite equipment, size. In practical terms, a probability of bit error ( P _e of Eq. 10.18) of about 10 ^?4, which is satisfactory for voice transmissions, can be achieved with off the-shelf equipment. For lower P _e values such as required for some data, error control coding must be used. Error control performs two functions, error detection and error correction. Most codes can perform both functions, but not necessarily together. In general, a code is capable of detecting more errors than it can correct. Where error detection only is employed, the receiver can request a repeat transmission (a technique referred to as automatic repeat request, or ARQ). This is only of limited use in satellite communications because of the long transmission delay time associated with geostationary satellites, and of course radio and TV broadcast is...