OFDM-Based Broadband Wireless Networks: Design and Optimization

Chapter 2

OFDM Fundamentals

Despite being a nearly 50-year-old concept, it is only in the last decade that OFDM becomes the modem of choice in wireless applications. One of the biggest advantages of an OFDM modem is the ability to convert dispersive broadband channels into parallel narrowband subchannels, thus significantly simplifying equalization at the receiver end. Another intrinsic feature of OFDM is its flexibility in allocating power and rate optimally among narrowband sub-carriers. This ability is particularly important for broadband wireless where multipath channels are “frequency-selective” (due to cancellation of primary and echoed signals). From a theoretical standpoint, OFDM was known to closely approximate the “water-filling” solutions of information theory that are capacity achieving. Some early work of Weinstein and Ebert [7] and Hirosaki [8] based on an FFT implementation of OFDM achieved both complexity and decoded bit count that was comparable to single-carrier counterparts. OFDM potential came to fruition in the designs of discrete multi-tone systems (DMT) for xDSL/ADSL applications, IEEE 802.11.a wireless LAN, digital broadcasting systems DAB-T/DVB-T, the recent 802.16 broadband wireless access. A highlight of the wireless OFDM landscape is depicted in Figure 2.1.

This chapter describes the underlying principles of OFDM modem. To better appreciate the capability of OFDM in combating channel impairments, we will first devote the space to characterize the wireless ...