Praise for Fundamentals of WiMAX

130 Chapter 4 • Orthogonal Frequency Division MultiplexingSNR Degradation in dB1010.10.01SNR = 20 dBSNR = 10 dBS NR = 0 dB_____ Fading Channel-- -- -- AWGN Channel0.0010 0.010.02 0.03 0.04 0.05Relative Frequency Offset, δFigure 4.11 SNR loss as a function of the frequency offset δ, relative to the subcarrier spacing.4.4.3 Obtaining Synchronization in WiMAXThe preceding two sections discussed the consequences of imperfect time and frequency synchronization.Many synchronization algorithms have been developed in the literature; a partiallist includes [6, 16, 28, 40, 43]. Generally, the methods can be categorized as based on eitherpilot symbol or blind—cyclic prefix.In the first category, known pilot symbols are transmitted. Since the receiver knows whatwas transmitted, attaining quick and accurate time and frequency synchronization is easy, but atthe cost of surrendering some throughput. In the WiMAX downlink, the preamble consists of aknown OFDM symbol that can be used to attain initial synchronization. In the WiMAX uplink,the periodic ranging (described in Chapter 8) can be used to synchronize. Since WiMAX is anOFDM-based system with many competing users, each user implements the synchronization atthe mobile. This requires the base station to communicate the frequency offset to the MS.Blind means that pilot symbols are not available to the receiver, so in the second category,the receiver must do the best it can without explicitly being able to determine the effect of thechannel. In the absence of pilot symbols, the cyclic prefix, which contains redundancy, can alsobe used to attain time and frequency synchronization [40]. This technique is effective when thenumber of subcarriers is large or when the offsets are estimated over a number of consecutivesymbols. The principal benefit of CP-based methods is that pilot symbols are not needed, so thedata rate can nominally be increased. In WiMAX, accurate synchronization, and especiallychannel estimation, are considered important enough to warrant the use of pilot symbols, so theblind techniques are not usually used for synchronization. They could be used to track the chan-