Praise for Fundamentals of WiMAX

110 Chapter 3 • The Challenge of Broadband Wireless Channelswhich is a hybrid of MLSD and decision-feedback equalization [7] and reduced-state sequence estimation(RSSE) [9] are reasonable suboptimal approximations for MLSD in practical scenarios [12].3.6.5 The Multicarrier ConceptThe philosophy of multicarrier modulation is that rather than fighting the time-dispersive ISIchannel, why not use its diversity? For this, a large number of subcarriers (L) are used in parallel,so that the symbol time for each goes from T → LT. In other words, rather than sending asingle signal with data rate R and bandwidth B, why not send L signals at the same time, eachhaving bandwidth BL / and data rate RL / ? In this way, if BL / ≪ B c, each signal will undergoapproximately flat fading, and the time dispersion for each signal will be negligible. As long asthe number of subcarriers L is large enough, the condition BL / ≪ B ccan be met. This elegantidea is the basic principle of orthogonal frequency division multiplexing (OFDM). In the nextchapter, we take a close look at this increasingly popular modulation technique, discussing itstheoretical basis and implementation challenges.3.7 Summary and ConclusionsIn this chapter, we attempted to understand and characterize the challenging and multifacetedbroadband wireless channel.• The average value of the channel power can be modeled based simply on the distancebetween the transmitter and the receiver, the carrier frequency, and the pathloss exponent.• The large-scale perturbations from this average channel can be characterized as lognormalshadowing.• Cellular systems must contend with severe interference from neighboring cells; this interferencecan be reduced through sectoring and frequency-reuse patterns.• The small-scale channel effects are known collectively as fading. Broadband wirelesschannels have autocorrelation functions that tell us a lot about their behavior.• Realistic models for time, frequency, and spatial correlation can be developed from popularstatistical channel models, such as Rayleigh, Ricean, and Nakagami.• A number of diversity-achieving techniques are available for both narrowband and broadbandfading.3.8 Bibliography[1] J. G. Andrews. Interference cancellation for cellular systems: A contemporary overview. IEEE WirelessCommunications Magazine, 12(2):19–29, April 2005.[2] S. Catreux, P. Driessen, and L. Greenstein. Attainable throughput of an interference-limited multipleinputmultiple-output (MIMO) cellular system. IEEE Transactions on Communications, 49(8):1307–1311, August 2001.