his is demonstrated in Figure 10.25. It follows that each subcarrier will carry a data symbol, which is then transformed into the time domain and transmitted as shown in Figure 10.26. Each symbol may carry one or more bits of data as a multiple PSK or multiple QAM signal constellation utilised. The constellation chosen and therefore the number of bits a symbol may carry is found from the capacity of the channel as determined from the subcarrier bandwidth and the received SINR. Power control and channel equalisation may adjust the received SINR level to ensure an overall channel achieves maximum capacity as effects of channel variations are minimised. The receiver operation is the reverse of that of transmitter and is shown in Figure 10.27. Amplitude Frequency Figure 10.25 Three orthogonal waveforms individually power controlled in the frequency domain Frequency domain Time domain Symbols Digital to Transmit input analogue Carrier frequency Figure 10.26 An IDFT OFDM transmitter Terrestrial Broadband Wireless Telecommunications Frequency domain Time domain Symbols Analogue to Filter Receive Output digital Carrier frequency Figure 10.27 A DFT OFDM receiver Time slot 1 Time slot 2 Mobile MS1 Time slot 3 Mobile MS2 Time slot 4 Frequency Note PC MS3 Note PC MS6 Base station PDA MS5 Mobile MS4 Figure 10.28 Resource allocation in an OFDMA system Orthogonal Frequency Division Multiple Access Multiple users need to access the same network in a mobile telecommunications system. In OFDM-based systems, multiple users may send and receive data to/from a base station using different orthogonal frequency carriers, and at different times. This access mode is known as Orthogonal Frequency Division Multiple Access (OFDMA). Here each OFDM subcarrier is assigned to one user at a time, over which the subscriber data are transmitted. Subcarriers are used on a TDMA basis, that is, multiple users may use the same subcarrier on a time-slot by time-slot basis. This is illustrated in Figure 10.28. Furthermore, the tele- communications resou