Praise for Fundamentals of WiMAX

1.7 Technical Challenges for Broadband Wireless 25with ISI but at high data rates requires too much processing power. OFDM has become the solutionof choice for mitigating ISI in broadband systems, including WiMAX, and is covered inChapter 4 in detail.Frequency dispersion due to motion: The relative motion between the transmitter and thereceiver causes carrier frequency dispersion called Doppler spread. Doppler spread is directlyrelated to vehicle speed and carrier frequency. For broadband systems, Doppler spread typicallyleads to loss of signal-to-noise ratio (SNR) and can make carrier recovery and synchronizationmore difficult. Doppler spread is of particular concern for OFDM systems, since it can corruptthe orthogonality of the OFDM subcarriers.Noise: Additive white Gaussian noise (AWGN) is the most basic impairment present in anycommunication channel. Since the amount of thermal noise picked up by a receiver is proportionalto the bandwidth, the noise floor seen by broadband receivers is much higher than thoseseen by traditional narrowband systems. The higher noise floor, along with the larger pathloss,reduces the coverage range of broadband systems.Interference: Limitations in the amount of available spectrum dictate that users share theavailable bandwidth. This sharing can cause signals from different users to interfere with oneanother. In capacity-driven networks, interference typically poses a larger impairment than noiseand hence needs to be addressed.Each of these impairments should be well understood and taken into consideration whiledesigning broadband wireless systems. In Chapter 3, we present a more rigorous characterizationof the radio channel, which is essential to the development of effective solutions for broadbandwireless.1.7.2 Spectrum ScarcityThe second challenge to broadband wireless comes from the scarcity of radio-spectrumresources. As discussed in Section 1.5, regulatory bodies around the world have allocated only alimited amount of spectrum for commercial use. The need to accommodate an ever-increasingnumber of users and offering bandwidth-rich applications using a limited spectrum challengesthe system designer to continuously search for solutions that use the spectrum more efficiently.Spectral-efficiency considerations impact many aspects of broadband wireless system design.The most fundamental tool used to achieve higher system-wide spectral efficiency is theconcept of a cellular architecture, whereby instead of using a single high-powered transmitter tocover a large geographic area, several lower-power transmitters that each cover a smaller area,called a cell, are used. The cells themselves are often subdivided into a few sectors through theuse of directional antennas. Typically, a small group of cells or sectors form a cluster, and theavailable frequency spectrum is divided among the cells or sectors in a cluster and allocatedintelligently to minimize interference to one another. The pattern of frequency allocation withina cluster is then repeated throughout the desired service area and is termed frequency reuse.For higher capacity and spectral efficiency, frequency reuse must be maximized. Increasingreuse, however, leads to a larger potential for interference. Therefore, to facilitate tighter reuse, the