where one of these parameters are used for separation) but segregation between users is obtained by using different codes which are matched between Throughput (S) corresponding senders and receivers. (This coding clearly occupies bandwidth.) SSMA is the most common form of CDMA. Two techniques are used, frequency hopping and phase coding. In phase coded SSMA Figure 13.23 Averaged normalised delay within TDMA the phase modulation of the carrier is changed in a coded pattern, whilst in frequency hopping the transmission frequency is varied according to the coded pattern. Figure 13.26 shows a simplified The dependency of delay on number of user is shown more generally in Figure 13.24, so that this is an important parameter in the choice of TDMA systems. These perform best when there is a User A User C User A User D relatively small number of users, each sending a large amount of traffic. Figure 13.25 shows the reduction of average delay as the average traffic per user is increased. User B User A User D User C User C User B User C User B User D User D User B User A Figure 13.26 Simplified view of user assignment within CDMA (frequency hopping) representation of CDMA using frequency hopping. The trans- Number of users missions are shown synchronised to each other, although in practice no such synchronisation is needed between users. As seen from Figure 13.26 each user 'hops' around between frequency bands, Figure 13.24 Effect of number of users on average delay these varying from one time slot to another. within a fixed assignment multiple access system CDMA has several advantages. It has high security against jamm- ing, since no one frequency is used, and for this reason it is used in 13/16 Multiple access techniques military systems. In addition it can co-exist with other systems since priority system in which some users are polled more often than separation is via coding, and it is resilient to fading. If fading occurs others in any period. in a given frequency band then for FDMA all users within this band The time