10.1 Introduction

Orthogonal frequency-division multiplexing (OFDM) is a bandwidth-efficient signaling scheme for wideband digital communications. The main difference between traditional frequency-division multiplexing (FDM) and OFDM is that in OFDM, the spectra of the individual carriers overlap. Nevertheless, the OFDM carriers exhibit orthogonality on a symbol interval if they are spaced in frequency exactly at the reciprocal of the symbol interval, which can be accomplished by using the discrete Fourier transform (DFT). With the development of modern digital signal processing technology, OFDM has become practical to implement and has been proposed as an efficient modulation scheme for applications ranging from modems, digital audio broadcast, to next -generation high-speed wireless data communications. For example, as discussed in Chapter 1, the high-speed wireless LAN standard IEEE 802.11a is based on OFDM.

One of the principal advantages of OFDM is that it effectively converts a frequency-selective fading channel into a set of parallel flat-fading channels. Both the intersymbol interference (ISI) and intercarrier interference (ICI) can be eliminated completely by inserting between symbols a small time interval known as a guard interval . The length of the guard interval is made equal to or greater than the delay spread of the channel. If the symbol signal waveform is extended periodically into the guard interval (cyclic prefix), orthogonality of the carrier is maintained over the symbol period, thus eliminating ICI. Also, ISI is eliminated because successive symbols do not overlap due to the guard interval. Hence at the receiver there is no need to perform channel equalization, and the complexity of the receiver is quite low.

In this chapter we discuss receiver design for OFDM systems signaling through unknown frequency-selective fading channels. In particular, we focus on the design of turbo receivers in a number of OFDM systems, including an OFDM system with frequency offset, a space-time block coded OFDM system, and a space-time coded OFDM system based on low-density parity-check (LDPC) codes. This chapter is organized as follows . In Section 10.2 we introduce the OFDM communication system. In Section 10.3 we present an MCMC-based blind turbo receiver for OFDM systems with unknown frequency offset and frequency-selective fading. In Section 10.4 we discuss a pilot-symbol-aided turbo receiver for space-time block coded OFDM systems. In Section 10.5 we present an LDPC-based space-time coded OFDM system and the corresponding turbo receiver structure.

The following is a list of the algorithms appearing in this chapter.

• Algorithm 10.1: MCMC-based OFDM blind demodulator in the presence of frequency offset and frequency-selective fading

• Algorithm 10.2: Metropolis “Hasting sampling of frequency offset

• Algorithm 10.3: Gibbs sampling of frequency offset

• Algorithm 10.4: LDPC decoding algorithm