Praise for Fundamentals of WiMAX

376 Chapter 11 • Link-Level Performance of WiMAX7.06.05.0QPSK R1/2QPSK R3/416 QAM R1/216 QAM R3/464 QAM R2/364 QAM R3/464 QAM Constrained Capacity64 QAM Constrained Capacity (3dB Shift)3 dB ShiftNormalized Throughput (bps/Hz)4.03.02.01.00.0Figure 11.5 WiMAX spectral efficiency0 2 4 6 8 10 12 14 16 18 20SNR (dB)channel-estimation algorithms, we also provide some results for a receiver that is assumed to haveperfect channel knowledge, a scenario often referred to as the perfect CSI model.Figures 11.7–11.10 show the BER as a function of SNR at the receiver for the Ped A and Bchannels. As discussed in Section 12.1 in greater detail, the Ped B channel has a larger delayspread and hence a smaller coherence bandwidth than the Ped A channel. Since in a fading channel,the BER is dominated by the occurrence of the deep fades, a small coherence bandwidthimplies that when the signal fades in one part of the spectrum, it very likely can be retrieved fromanother part of spectrum. This frequency-selective fading of the Ped B channel allows for a formof signal diversity commonly referred to as frequency diversity. A comparison of the BER for anygiven modulation and coding scheme (MCS) shows that the frequency diversity of the Ped Bchannel results in a considerably lower error rate, especially for the PUSC subcarrier permutation6 mode, since it is designed to take advantage of frequency diversity.Figures 11.7–11.10 also highlight the benefit of the band AMC subcarrier permutation atpedestrian speeds. In the case of band AMC operations, we assume that the receiver provides thechannel-quality feedback, using the CQICH channel once every 5msec frame. Since the bestsubchannel can be allocated to the MS based on the feedback, the performance of band AMC issignificantly better than PUSC, particularly in Ped A channel where the PUSC subcarrier per-6. PUSC and other subcarrier permutation schemes are discussed in Section 8.6.