ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING IN MULTIPATH FADING CHANNELS

Abstract

The Multi-Carrier Modulation (MCM) technique, commonly referred to as the Orthogonal Frequency Division Multiplexing (OFDM), is a very useful digital modulation scheme for terrestrial digital audio and video broadcasting. In this dissertation, the OFDM system has been classified by coherent OFDM and noncoherent OFDM, and has been thoroughly studied in terms of the bit error rate (BER) and symbol error rate (SER) performance in frequency-selective Rayleigh and Rician fading. In OFDM, a serial data stream is divided into several streams of much lower bit rate. These substreams are transmitted in parallel by modulating a number of orthogonal subcarriers. This transmission process can be accomplished by performing inverse fast Fourier transform (IFFT) on the parallelized data at the transmitter and performing the inverse process at the receiver. The main feature of OFDM system is the avoidance of intersymbol interference (ISI), which is normally introduced by frequency selective multipath fading in a high data rate system in a mobile radio environment.

Expressions for evaluating the SER performance of the coherent OFDM system in frequency-selective Rayleigh fading and the BER performance of the noncoherent OFDM system in frequency-selective Rician fading have been derived. In both cases, diversity reception and the addition of the cyclic prefix are used to improve the error performance.

To exactly evaluate the error performance of coherent OFDM system, M-ary phase shift keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) have been studied in Rayleigh fading with three different diversity receptions: maximum ratio combining (MRC) with independent and identical (i.i.d) channels, MRC with dissimilar channels and the selection combining (SC). By using the exact SER expressions derived for MQAM, the OFDM-MQAM system has been studied in Rayleigh fading and the interchannel interference (ICI) which causes the loss of orthogonality among subcarriers is mainly considered as the reason lo degrade the error performance. The cyclic prefix which is assumed longer than the maximum delay spread is used to mitigate the frequency selectivity of the channel. MRC with i.i.d channels is proven to perform much better than MRC with dissimilar channels and SC, and certainly should be used in the OFDM-MQAM system.

For noncoherent OFDM system, the OFDM-MDPSK system is studied in frequency-selective Rician fading with MRC diversity reception for three delay profiles: one-sided exponential, uniform and double spike delay profiles. Both slow and fast fading condition are considered. As expected, the larger the order of diversity reception and the stronger the LOS component in the received Rician faded signal, the better the system BER performance. The optimal number of subcarriers can be obtained for a given normalised root mean square (rms) delay spread and a fixed normalised maximum Doppler shift. In addition, with the increase of the Doppler shift for a given delay spread, the optimal number of subcarriers decreases while with the increase of the delay spread.