In practice, FDMA is often used in combination with TDMA or CDMA to increase capacity on a single channel in an FDMA system. As shown in Figure 4-15, FDMA/TDMA divides the available bandwidth into channels and then divides each channel into time slots that are allocated to individual users. FDMA/TDMA is used by GSM cellular phones, with eight time slots available in each 200 kHz radio channel. Orthogonal Frequency Division Multiplexing Orthogonal frequency division multiplexing (OFDM) is a variant of frequency division multiplexing (FDM), in which a number of discrete subcarrier frequencies are transmitted within a band with frequencies chosen to ensure minimum interference between adjacent subcarriers. This is achieved by controlling the spectral width of the individual subcarriers (also called tones) SO that the frequencies of subcarriers coincide with minima in the spectra of adjacent subcarriers, as shown in Figure 4-16. Chapter Four Frequency User 1 User 2 User 3 User 4 TDMA time slot allocation within each FDMA channel User in User n+ 1 User n+2 User n+3 Time slot; Ts Figure 4-15: FDMA/TDMA Multiple Access System as Used in GSM Cellular Phones Frequency Figure 4-16: Orthogonality of OFDM Subcarriers in the Frequency Domain In the time domain, the orthogonality of OFDM tones means that the number of subcarrier cycles within the symbol transmission period is an integer, as illustrated in Figure 4-17. This condition can be expressed as: Ts = Ni / V or Vi=ni/Ts where Ts is the symbol transmission period and Vi is the frequency of the ith subcarrier. The subcarriers are therefore evenly spaced in frequency, with separation equal to the reciprocal of the symbol period. Radio Communication Basics Figure 4-17: Orthogonality of OFDM Subcarriers in the Time Domain There are a number of ways in which the multiple subcarriers of OFDM can be used: OFDM can be used as a multiple access technique (OFDMA), by assigning single subcarriers or groups of subcarriers to individual users according to their bandwidth needs. A ser