Praise for Fundamentals of WiMAX

3.4 The Broadband Wireless Channel: Fading 89Table 3.3 Summary of Broadband Fading Parameters, with Rules of ThumbQuantity If “Large”? If “Small”? WiMAX Design ImpactDelay spread,τIf τ ≫ T , frequencyselectiveIf τ ≪ T , frequencyflatThe larger the delay spread relative tothe symbol time, the more severe theISI.Coherence bandwidth,B1If ≪ T ,Bccfrequency flatProvides a guideline to subcarrier1If ≫ T ,width B and hence numbersc≈ Bc/10Bcof subcarriers needed in OFDM:frequency selectiveL ≥ 10 B/B c.Doppler spread,As f becomes non-negligible,fcυ If f , fast fading If , slow fadingfsubcarrier orthogonality is compromised.D=υ ≫ ccf υ ≤ cD/ BscccCoherence time,If T , slow fading If , fast fadingT ≫ cTT ≤ T ccT csmall necessitates frequent channelestimation and limits the OFDMsymbol duration but provides greatertime diversity.Angular spread,NLOS channel, lots ofdiversityEffectively LOS channel,not much diversityMultiantenna array design, beamformingversus diversity.Coherencedistance,θ RMSD cTEffectively LOS channel,not much diversityNLOS channel, lots ofdiversityDetermines antenna spacing.c≈ 1 .fD(3.35)This makes intuitive sense: If the transmitter and the receiver are moving fast relative to eachother and hence the Doppler is large, the channel will change much more quickly than if thetransmitter and the receiver are stationary.Table 3.4 gives some typical values for the Doppler spread and the associated channelcoherence time for two candidate WiMAX frequency bands. This table demonstrates one of thereasons that mobility places extra constraints on the system design. At high frequency andmobility, the channel changes completely around 500 times per second, placing a large burdenon channel-estimation algorithms and making the assumption of accurate transmitter channelknowledge questionable. Subsequent chapters (especially 5–7) discuss why accurate channelknowledge is important in WiMAX. Additionally, the large Doppler at high mobility and frequencycan also degrade the OFDM subcarrier orthogonality, as discussed in Chapter 4.