Algorithms for Communications Systems and their Applications, 2nd Edition

Chapter 10Spread‐spectrum systems

The term spread‐spectrum systems [1–7] was coined to indicate communication systems in which the bandwidth of the signal obtained by a standard modulation method (see Chapter 6) is spread by a certain factor before transmission over the channel, and then despread, by the same factor, at the receiver. The operations of spreading and despreading are the inverse of each other, i.e. for an ideal and noiseless channel the received signal after despreading is equivalent to the transmitted signal before spreading. For transmission over an ideal additive white Gaussian noise (AWGN) channel, these operations do not offer any improvement with respect to a system that does not use spread‐spectrum techniques. However, the practical applications of spread‐spectrum systems are numerous, for example in multiple‐access systems, narrowband interference rejection, and transmission over channels with fading, as discussed in Section 10.2.

10.1 Spread‐spectrum techniques

We consider the two most common spread‐spectrum techniques: direct sequence (DS) and frequency hopping (FH).

10.1.1 Direct sequence systems

The baseband equivalent model of a DS system is illustrated in Figure 10.1. We consider the possibility that users in a multiple‐access system simultaneously transmit, using the same frequency band, by code division multiple access (CDMA) (see Appendix 18.B ...