OFDM Communications over Time-Varying Channels

Orthogonal frequency-division multiplexing (OFDM), also known as multicarrier modulation (Bingham, 1990; Cimini Jr, 1985; Keller & Hanzo, 2000; Le Floch, Alard, & Berrou, 1995; Sari, Karam, & Jeanclaude, 1995; Wang & Giannakis, 2000; Zou & Wu, 1995), relies on the concept of parallel data transmission in the frequency domain and mainly owes its success to the easy equalization for linear time-invariant (LTI) frequency-selective channels. In OFDM systems, the data symbol stream is split into L parallel flows, which are transmitted on equispaced frequencies called subcarriers, each one characterized by a transmission rate that is 1/L times lower than the original data rate. This is obtained by splitting the original data stream into multiple blocks, which are transmitted in consecutive time intervals, where each symbol of a block is associated to a specific subcarrier. This frequency-domain multiplexing can be efficiently performed by means of fast Fourier transform algorithms. Due to the use of orthogonal (equispaced) subcarriers, OFDM systems with LTI frequency-selective channels avoid the so-called intercarrier interference (ICI) among the data symbols of the same OFDM block. Differently from conventional frequency-division multiplexing, a frequency overlapping among the spectra associated to different substreams is permitted, resulting in a significant reduction of the bandwidth requirements. Moreover, for LTI frequency-selective channels, the absence of ICI allows an easy channel equalization, which can be performed on a per-subcarrier basis by means of scalar divisions. The intersymbol interference (ISI)1 among data symbols of different OFDM blocks, induced by multipath propagation, is avoided by a suitable cyclic extension of each OFDM block, usually referred to as cyclic prefix (CP) (Sari et al., 1995; Wang & Giannakis, 2000; Zou & Wu, 1995). However, when the channel experiences a nonnegligible time variation, each subcarrier undergoes a Doppler spreading effect that destroys the subcarrier orthogonality, producing significant ICI (Robertson & Kaiser, 1999; Russell & Stüber, 1995; Stantchev & Fettweis, 2000). Dually to the ISI in single-carrier systems, the ICI power reduces the signal-to-interference-plus-noise ratio (SINR) and, when left uncompensated, impairs the performance of OFDM systems. A simple method that reduces the ICI is the shortening of the OFDM block duration. This way the channel becomes (almost) constant