 signal is corrupted due to noise and other im- determines the overall OFDM symbol time T = pairments. As an example, on the cell border of large cells TCP + Tu or, equivalently, the OFDM symbol rate. the signal-to-noise/interference ratio will be relatively low, with relatively low achievable data rates as a conse- quence. A small amount of additional inter-subcarrier 4.4.8.1 OFDM subcarrier spacing interference, for example, due to Doppler spread, may then be more or less negligible. At the same time, in high There are two factors that constrain the selection of the signal-to-noise/interference scenarios, for example, in OFDM subcarrier spacing: small cells with low traffic or close to the base station, The OFDM subcarrier spacing should be as small as where high data rates are to be provided, the same possible (Tu as large as possible) to minimize the amount of inter-subcarrier interference may have a much relative cyclic-prefix overhead TCP/(T + TCP), see more negative impact. further Section 4.4.8.3. It should also be noted that, in addition to Doppler A too small subcarrier spacing increases the sensitiv- spread, inter-subcarrier interference will also be due to ity of the OFDM transmission to Doppler spread and different transmitter and receiver inaccuracies, such as different kinds of frequency inaccuracies. frequency errors and phase noise. A requirement for the OFDM subcarrier orthogonal- ity (4.4.2) to hold at the receiver side, i.e. after the 4.4.8.2 Number of subcarriers transmitted signal has propagated over the radio channel, is that the instantaneous channel does not vary noticeably Once the subcarrier spacing has been selected based on during the demodulator correlation interval Tu (see environment, expected Doppler spread and time Figure 4.4-5). In case of such channel variations, e.g. due dispersion, etc., the number of subcarriers can be very high Doppler spread, the orthogonality between determined based on the amount of spectrum available subcarriers will be lost with inter-subcarrier interf