Channel Estimation for Wireless OFDM Communications

1.1 Preliminary Orthogonal frequency-division multiplexing (OFDM) communication techniques have recently received significant research attention because of their ability to maintain effective transmission and highly efficient bandwidth utilization in the presence of various channel impairments, such as severely frequency-selective channel fades caused by long multipath delay spreads and impulsive noise (Bingham, 1990; Zou & Wu, 1995). In an OFDM system, a high-rate serial information-bearing symbol stream is split into many low-rate parallel streams; each of these streams individually modulates a mutually orthogonal sub-carrier. The spectrum of an individual sub-channel overlaps with those expanded from the adjacent sub-channels. However, the OFDM sub-carriers are orthogonal as long as they are synthesized such that the frequency separation between any two adjacent sub-carriers is exactly equal to the reciprocal of an OFDM block duration. A discrete Fourier transform (DFT) operation can perfectly produce this sub-carrier arrangement and its relevant modulations (Darlington, 1970; Weinstein & Ebert, 1971). Because of the advanced technologies incorporated into integrated circuit (IC) chips and digital signal processors (DSPs), OFDM has become a practical way to implement very effective modulation techniques for various applications. As a result, OFDM technologies have recently been chosen as candidates for 4th-generation (4G) mobile communications in a variety of standards, such as IEEE 802.16 (Marks, 2008) and IEEE 802.20 (Klerer, 2005) in the United States, and international research projects, such as EU-IST-MATRICE (MATRICE, 2005) and EU-IST-4MORE (4MORE, 2005) for 4G mobile communication standardization in Europe. Regarding the history of OFDM, recall that Chang published a paper on the synthesis of band-limited signals for parallel multi-channel transmission in the 1960s (Chang, 1966). The author investigated a technique for transmitting and receiving (transceiving) parallel information through a linear band-limited channel without inter-channel interference (ICI) or inter-symbol interference (ISI). Saltzberg then conducted relevant performance evaluations and analyses (Saltzberg, 1967).