y used in most cases of mobile quality is also referred to as frequency diversity. communication this is not a very serious drawback, es- On the other hand, in case of OFDM transmission pecially taking into account that a significant part of the each modulation symbol is mainly confined to a relatively available frequency diversity can be captured already narrow bandwidth. Thus, in case of OFDM transmission with a relatively high code rate. Single wideband carrier OFDM signal Subcarrier experiencing very bad channel quality Figure 4.4-13 Transmission of single wideband carrier and OFDM transmission over a frequency-selective channel. CHAPTER 4.4 OFDM transmission Information bit Channel coding Channel Frequency Coded bits coding interleaving modulation Frequency interleaving (mapping to subcarriers) Figure 4.4-14 Channel coding in combination with frequency-domain interleaving to provide frequency diversity in case of OFDM transmission. 4.4.8 Selection of basic OFDM as a consequence. Figure 4.4-15 illustrates the subcarrier parameters signal-to-interference ratio due to inter-subcarrier interference between two neighbor subcarriers, as a function of the normalized Doppler spread. When If OFDM is to be used as the transmission scheme in considering Figure 4.4-15, it should be had in mind that a mobile-communication system, the following basic a subcarrier will be subject to interference from multiple OFDM parameters need to be decided on: subcarriers on both sides, 3 that is the overall inter- The subcarrier spacing Af. subcarrier interference from all subcarriers will be higher The number of subcarriers N which, together with than what is illustrated in Figure 4.4-15. the subcarrier spacing, determines the overall trans- In practice, the amount of inter-subcarrier mission bandwidth of the OFDM signal. interference that can be accepted very much depends on The cyclic-prefix length TCP. Together with the the service to be provided and to what extent the subcarrier spacing of = 1/T, the cyclic-prefix length received