Praise for Fundamentals of WiMAX

3.6 Mitigation of Fading 109user must use several codes simultaneously, which generally results in self-interference.Although this self-interference can be corrected with an equalizer (see Section 3.6.4), thislargely defeats the purpose of using spread spectrum to help with intersymbol interference.In short, spread-spectrum is not a natural choice for wireless broadband networks, since bydefinition, the data rate of a spread-spectrum system is less than its bandwidth. The same trendhas been observed in wireless LANs: Early wireless LANs (802.11 and 802.11b) were spreadspectrum 13 and had relatively low spectral efficiency; later wireless LANs (802.11a and802.11g) used OFDM for multipath suppression and achieved much higher data rates in thesame bandwidth.3.6.4 EqualizationEqualizers are the most logical alternative for ISI suppression to OFDM, since they don’t requireadditional antennas or bandwidth and have moderate complexity. Equalizers are implemented atthe receiver and attempt to reverse the distortion introduced by the channel. Generally, equalizersare broken into two classes: linear and decision directed (nonlinear).A linear equalizer simply runs the received signal through a filter that roughly models theinverse of the channel. The problem with this approach is that it inverts not only the channel butalso the received noise. This noise enhancement can severely degrade the receiver performance,especially in a wireless channel with deep frequency fades. Linear receivers are relatively simpleto implement but achieve poor performance in a time-varying and severe-ISI channel.A nonlinear equalizer uses previous symbol decisions made by the receiver to cancel outtheir subsequent interference and so are often called decision-feedback equalizers (DFEs).Recall that the problem with multipath is that many separate paths are received at different timeoffsets, so prior symbols cause interference with later symbols. If the receiver knows the priorsymbols, it can subtract out their interference. One problem with this approach is that it is commonto make mistakes about what the prior symbols were, especially at low SNR, which causeserror propagation. Also, nonlinear equalizers pay for their improved performance relative to linearreceivers with sophisticated training and increased computational complexity.Maximum-likelihood sequence detection (MLSD) is the optimum method of suppressing ISI buthas complexity that scales like OM ( v ),where M is the constellation size and v is the channel delay.Therefore, MLSD is generally impractical on channels with a relatively long delay spread or highdata rate but is often used in some low-data-rate outdoor systems, such as GSM. For a high-data-ratebroadband wireless channel, MLSD is not expected to be practical in the foreseeable future, althoughsuboptimal approximations, such as delayed-decision-feedback sequence estimation (DDFSE),13. Note that the definition of spread spectrum is somewhat loose. The FCC has labeled even the 11Mbpsin 20MHz 802.11b system as “spread spectrum,” but this is generally inconsistent with its historicaldefinition that the bandwidth be much larger than the data rate. See, for example, [26, 31, 33] and thereferences therein.