al thousand, with the subcar- rier spacing ranging from several hundred kHz down to a few kHz. What subcarrier spacing to use depends on for k1#k2 (4.4.2) what types of environments the system is to operate in, including such aspects as the maximum expected radio- Thus basic OFDM transmission can be seen as the channel frequency selectivity (maximum expected time modulation of a set of orthogonal functions Qk(t), where dispersion) and the maximum expected rate of channel variations (maximum expected Doppler spread). Once the subcarrier spacing has been selected, the number of = otherwise (4.4.3) subcarriers can be decided based on the assumed overall transmission bandwidth, taking into account acceptable The 'physical resource' in case of OFDM transmission is out-of-band emission, etc. The selection of OFDM often illustrated as a time-frequency grid according to subcarrier spacing and number of subcarriers is discussed Figure 4.4-4 where each 'column' corresponds to one in somewhat more detail in Section 4.4.8. OFDM symbol and each 'row' corresponds to one As an example, for 3GPP LTE the basic subcarrier OFDM subcarrier. spacing equals 15 kHz. On the other hand, the number of subcarriers depends on the transmission bandwidth, with in the order of 600 subcarriers in case of operation in a 4.4.2 OFDM demodulation 10 MHz spectrum allocation and correspondingly fewer/ more subcarriers in case of smaller/larger overall trans- Figure 4.4-5 illustrates the basic principle of OFDM mission bandwidths. demodulation consisting of a bank of correlators, one for The term Orthogonal Frequency Division Multiplex is each subcarrier. Taking into account the orthogonality due to the fact that two modulated OFDM subcarriers between subcarriers according to (4.4.2), it is clear that, xk (t) and Xk2 (t) are mutually orthogonal over the time in the ideal case, two OFDM subcarriers do not cause any interval i.e. interference to each other after demodulation. Note that Time (OFDM symbol number) Figure 4.4-4 OFDM time-frequency grid. OFD