Praise for Fundamentals of WiMAX

174 Chapter 5 • Multiple-Antenna Techniques10 1 Average SNR (dB)Average Bit Error Probability10 210 310 42 × 2 OC/MRT (no interference)1 × 4 MRC (no interference)2 × 2 OC (a strong interferer)2 × 1 MRT (no interference)1 × 2 MRC (no interference)2 × 2 Alamouti STBC(no interference)10 50 1 2 3 4 5 6 7 8 9 10Figure 5.10 Performance comparison between eigen-beamforming and diversity. MRT and MRChave the same performance for the same number of antennas.multiplexing and is discussed in the next section. In particular, Section 5.5.3 generalizes theseresults—in the absence of interference—to M data-bearing subchannels, where 1 ≤ M ≤ min( N , N ) .5.5 Spatial MultiplexingFrom a data rate standpoint, the most exciting type of MIMO communication is spatial multiplexing,which refers to breaking the incoming high rate-data stream into N tindependent datastreams, as shown in Figure 5.11. Assuming that the streams can be successfully decoded, thenominal spectral efficiency is thus increased by a factor of N t. This is certainly exciting: Itimplies that adding antenna elements can greatly increase the viability of the high data ratesdesired for wireless broadband Internet access. However, this chapter adopts a critical view ofspatial multiplexing and attempts to explain why many of the lauded recent results for MIMOwill not prove directly applicable to WiMAX. Our goal is to help WiMAX designers understandthe practical issues with MIMO and to separate viable design principles from the multitude ofpurely theoretical results that dominate much of the literature on the topic.5.5.1 Introduction to Spatial MultiplexingFirst, we summarize the classical results and widely used model for spatial multiplexing. Thestandard mathematical model for spatial multiplexing is similar to what was used for space/timecoding:y = Hx + n, (5.54)rt