their allocated time slots, this method can cope with broadcast systems. Generally TDMA is more effi- cient than FDMA for the same conditions of operation. ABCDEFABCDEF ABCDEF Since there are no collisions and retransmissions within TDMA (as for FDMA) the offered load equals the throughput, if the retrans- Frame 1 Frame 2 Frame n missions due to transmission errors are ignored. Also, once the time slots have been allocated to users, the performance of any user is unaffected by the loading of other users. Each user experiences a Figure 13.20 The frame structure within TDMA delay determined by the frame period (Tp), the propagation delay within the medium, the transmission time of an average message, and any queueing delays it may have. Assuming that the channel Because the users always occupy the same position within a frame capacity C is equally divided between N users (i.e. each user has full the receiver knows where to look to collect its data. It is essential to use of the channel for 1/N of the time or has an average capacity of ensure, however, that some form of synchronisation system is used C/N), the normalised transfer delay is given by Equation 13.30. Non-contention systems 13/15 Delay D=1 + (13.30) Figure 13.23 shows typical average delay curves for TDMA. As expected the delay increases as the throughput increases, due to queuing for access to the user's time slots, and also increases as the number of users is increased, since this results in further division of the communication channel and an increase in the frame repetition period, T F 10000 Average traffic per user Figure 13.25 Effect of average traffic per user on N - 500 average delay within a fixed assignment multiple access system N = 100 13.7.1.3 Code division multiple access N = 10 The most common application of CDMA is spread spectrum multiple access (SSMA) and the two terms are often used synony- mously (Schilling, 1990; Schilling, 1991). CDMA allows the transmission from various users to overlap in frequency and time (unlike FDMA and TDMA, 