time/frequency-domain mobile communication. Channel coding implies that characteristics. each bit of information to be transmitted is spread over several, often very many, code bits. If these coded bits are 4.4.7 Frequency diversity with then, via modulation symbols, mapped to a set of OFDM subcarriers that are well distributed over the overall OFDM: Importance of channel transmission bandwidth of the OFDM signal, as illus- coding trated in Figure 4.4-14, each information bit will expe- rience frequency diversity in case of transmission over As discussed in Section 4.3.3 in the previous chapter, a radio channel that is frequency selective over the a radio channel is always subject to some degree of transmission bandwidth, despite the fact that the frequency selectivity, implying that the channel quality subcarriers, and thus also the code bits, will not experi- will vary in the frequency domain. In case of a single ence any frequency diversity. Distributing the code bits wideband carrier, such as a WCDMA carrier, each in the frequency domain, as illustrated in Figure 4.4-14, modulation symbol is transmitted over the entire signal is sometimes referred to as frequency interleaving. This is bandwidth. Thus, in case of the transmission of a single similar to the use of time-domain interleaving to benefit wideband carrier over a highly frequency-selective from channel coding in case of fading that varies in time. channel (see Figure 4.4-13a), each modulation symbol Thus, in contrast to the transmission of a single will be transmitted both over frequency bands with rel- wideband carrier, channel coding (combined with atively good quality (relatively high signal strength) and frequency interleaving) is an essential component in frequency bands with low quality (low signal strength). order for OFDM transmission to be able to benefit from Such transmission of information over multiple fre- frequency diversity on a frequency-selective channel. As quency bands with different instantaneous channel channel coding is typicall