Praise for Fundamentals of WiMAX

C H A P T E R 4Orthogonal FrequencyDivision MultiplexingOrthogonal frequency division multiplexing (OFDM) is a multicarrier modulation techniquethat has recently found wide adoption in a widespread variety of high-data-rate communicationsystems, including digital subscriber lines, wireless LANs (802.11a/g/n), digital video broadcasting,and now WiMAX and other emerging wireless broadband systems such as the proprietaryFlash-OFDM developed by Flarion (now QUALCOMM), and 3G LTE and fourth generation cellularsystems. OFDM’s popularity for high-data-rate applications stems primarily from its efficientand flexible management of intersymbol interference (ISI) in highly dispersive channels.As emphasized in Chapter 3, as the channel delay spread τ becomes an increasingly largemultiple of the symbol time T s , the ISI becomes very severe. By definition, a high-data-rate systemwill generally have τ ≫Ts , since the number of symbols sent per second is high. In a nonlineof sight (NLOS) system, such as WiMAX, which must transmit over moderate to long distances,the delay spread will also frequently be large. In short, wireless broadband systems of alltypes will suffer from severe ISI and hence will require transmitter and/or receiver techniquesthat overcome the ISI. Although the 802.16 standards include single-carrier modulation techniques,the vast majority of, if not all, 802.16-compliant systems will use the OFDM modes,which have also been selected as the preferred modes by the WiMAX Forum.To develop an understanding of how to use OFDM in a wireless broadband system,this chapter:• Explains the elegance of multicarrier modulation and how it works in theory• Emphasizes a practical understanding of OFDM system design, covering such key conceptsas the cyclic prefix, frequency equalization, and synchronization 11. Channel estimation for OFDM is covered in Chapter 5 in the context of MIMO-OFDM.113