Praise for Fundamentals of WiMAX

182 Chapter 5 • Multiple-Antenna Techniques1 1n 11x 1y 1MMn MMx My MFigure 5.15 Spatial subchannels resulting from linear precoding and postcodingClearly, all these assumptions will be at least somewhat compromised in a cellular MIMOdeployment. In many cases, they will be completely wrong. We now discuss how to addressthese important issues in a real system, such as WiMAX.5.6.1 MultipathBecause WiMAX systems are expected to have moderate to high bandwidths over nonnegligibletransmission distances, the multipath in WiMAX is expected to be substantial, as discussedextensively in Chapters 3 and 4. Therefore, the flat-fading assumption appears to beunreasonable. However, OFDM can be introduced to convert a frequency-selective fading channelto L parallel flat-fading channels, as discussed in Chapter 4. If OFDM with sufficient subcarriersis combined with MIMO, the result is L parallel MIMO systems, and hence the model ofEquation (5.54) is again reasonable. For this reason, OFDM and MIMO are a natural pair, andthe first commercial MIMO system used OFDM in order to combat intersymbol interference[49]. MIMO-OFDM has been widely researched in recent years [5, 57]. Since WiMAX is basedon OFDM, using the flat-fading model for MIMO is reasonable.5.6.2 Uncorrelated AntennasIt is much more difficult to analyze MIMO systems with correlated antennas, so it is typicallyassumed that the spatial modes are uncorrelated and hence independent, assuming Gaussian andidentically distributed. For a single user, identically distributed channels—that is, equal averagepower—are a reasonable assumption, since the antennas are colocated, but in general, the channelswill be spatially correlated. On the other hand, if the antennas are considered to be at differentMSs, the antennas will likely be uncorrelated, but the average power will be widely varying.Considering the case of a single-user MIMO channel, the main two causes of channel correlationare (1) insufficient spacing of the antenna elements, and (2) insufficient scattering in thechannel. The first problem of insufficient spacing is prevalent when the platform is small, as isexpected for MSs. Insufficient scattering is a frequent problem when the channel is approxi-