Praise for Fundamentals of WiMAX

98 Chapter 3 • The Challenge of Broadband Wireless Channels(a)RayleighDistribution(b)RayleighDistributionFigure 3.16 (a) The shape of the Doppler power spectrum ρ t( ∆f) determines the correlation envelopeof the channel in time. (b) Similarly, the shape of the multipath intensity profile A τ( ∆τ)determinesthe correlation pattern of the channel frequency response.usually causes much more damaging interference than frequency dispersion does, since adjacentsymbols are smeared together.3.5.2.4 Multidimensional CorrelationIn order to present the concepts as clearly as possible, we have thus far treated time, frequency,and spatial correlations separately. In reality, signals are correlated in all three domains.A broadband wireless data system with mobility and multiple antennas is an example of asystem in which all three types of fading will play a significant role. The concept of doubly selective(in time and frequency) fading channels [25] has received recent attention for OFDM. Thecombination of these two types of correlation is important because in the context of OFDM, theyappear to compete with each other. On one hand, a highly frequency-selective channel—resultingfrom a long multipath channel as in a wide area wireless broadband network—requires a largenumber of potentially closely spaced subcarriers to effectively combat the intersymbol interferenceand small coherence bandwidth. On the other hand, a highly mobile channel with a largeDoppler causes the channel to fluctuate over the resulting long symbol period, which degrades thesubcarrier orthogonality. In the frequency domain, the Doppler frequency shift can cause significantinter carrier interference as the carriers become more closely spaced. Although the mobilityand multipath delay spread must reach fairly severe levels before this doubly selective effectbecomes significant, this problem facing mobile WiMAX systems does not have a comparable