Praise for Fundamentals of WiMAX

5.1 The Benefits of Spatial Diversity 15310 −110 −2Bit Error Probability (BEP)10 0 SNR (dB)N r= 110 −310 −410 −5N r= 4N r= 210 −610 −7- - - - - - Array Gain______ Diversity Gain10 −80 2 4 6 8 10 12 14 16 18 20Figure 5.2 BEP trends for N r=[1 2 4] . Here, the BEP (0 dB) is normalized to 1 for each technique.Statistical diversity has a dramatic impact on BEP, whereas the impact from the array gainis only incremental.BEP to 1 for γ =1. The trend is clear: Sufficient spacing for the antennas is critical for increasingthe system reliability.5.1.3 Increased Data RateDiversity techniques are very effective at averaging out fades in the channel and thus increasingthe system reliability. Receive-diversity techniques also increase the average received SNR atbest linearly, owing to the array gain. The Shannon capacity formula gives the maximum achievabledata rate of a single communication link in additive white Gaussian noise (AWGN) asC = Blog(1 + γ),2where C is the capacity, or maximum error-free data rate; B is the bandwidth of the channel; andγ is again the SNR (or SINR). Owing to advances in coding, and with sufficient diversity, itmay be possible to approach the Shannon limit in some wireless channels.Since antenna diversity increases the SNR linearly, diversity techniques increase the capacityonly logarithmically with respect to the number of antennas. In other words, the data ratebenefit rapidly diminishes as antennas are added. However, it can be noted that when the SNR islow, the capacity increase is close to linear with SNR, since log(1 + x)≈ x for small x. Hence inlow-SNR channels, diversity techniques increase the capacity about linearly, but the overallthroughput is generally still poor owing to the low SNR.In order to get a more substantial data rate increase at higher SNRs, the multiantenna channelcan instead be used to send multiple independent streams. Spatial multiplexing has the(5.9)