Praise for Fundamentals of WiMAX

4.1 Multicarrier Modulation 115on each substream much greater than the delay spread of the channel or, equivalently, to makethe substream bandwidth less than the channel-coherence bandwidth. This ensures that the substreamswill not experience significant ISI.A simple illustration of a multicarrier transmitter and receiver is given in Figure 4.1,Figure 4.2, and Figure 4.3. Essentially, a high data rate signal of rate R bps and with a passbandbandwidth B is broken into L parallel substreams, each with rate RL / and passbandbandwidth BL / . After passing through the channel H( f), the received signal would appear asshown in Figure 4.3, where we have assumed for simplicity that the pulse shaping allows aperfect spectral shaping so that there is no subcarrier overlap. 2 As long as the number of subcarriersis sufficiently large to allow the subcarrier bandwidth to be much less than the coherencebandwidth, that is, B/L≪B c , it can be ensured that each subcarrier experiencesapproximately flat fading. The mutually orthogonal signals can then be individually detected,as shown in Figure 4.2.Hence, the multicarrier technique has an interesting interpretation in both the time and frequencydomains. In the time domain, the symbol duration on each subcarrier has increased toT = LT s, so letting L grow larger ensures that the symbol duration exceeds the channel-delayspread, T ≫ τ, which is a requirement for ISI-free communication. In the frequency domain,the subcarriers have bandwidth B/L≪B c , which ensures flat fading, the frequency-domainequivalent to ISI-free communication.cos(2 f c)R bpsS/PR/L bpsR/L bps...R/L bpsXXcos(2 f c+ f)X+x(t)cos(2 f c+(L–1) f)Figure 4.1 A basic multicarrier transmitter: A high-rate stream of R bps is broken into L parallelstreams, each with rate R/L and then multiplied by a different carrier frequency.2. In practice, there would be some roll-off factor of β, so the actual consumed bandwidth of such asystem would be (1 + β )B. As we will see, however, OFDM avoids this inefficiency by using acyclic prefix.