Special Studies Project Report (M.Eng.) - Asian Institute of Technology, 2002

A research study submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Advanced Technologies

Orthogonal Frequency Division Multiplexing (OFDM) has been considered as a promising candidate for high data rate transmissions in a frequency selective radio channels due to its robustness against multipath fading and impulse noise. However, the performance of OFDM system may be degraded when inter-symbol interference (ISi) channels have spectral nulls or close to or on fast Fourier transform (FFT) grid. In addition, when ISi channels have many taps, the data rate overhead due to the inse1iion of the cyclic prefix is high. There are two schemes to improve performance of OFDM system over multipath fading channel and to reduce the cyclic prefix, called precoded and vector OFDM (POFDM, VOFDM). In this research study, we will address these schemes. The precoding scheme is simply to inse1i one or more zeroes between each two sets of K consecutive information symbols, although it can be generalized to general form. This precoding scheme may be able to remove the spectral nulls of an ISi channel without knowing the ISi channel. When compared to coded OFDM (COFDM) with the same code rate KIM, the cyclic prefix data rate overhead in POFDM system is reduced M times, and performance improvement is 2-3 dB in a fading channel. Based on the idea of precoder, VOFDM system has been proposed, in which we linearly precode (combine) symbols before they are multiplexed. The design rules for achieving maximum diversity order are provided. When compared to OFDM system, the VOFDM reduces the cyclic prefix K times and offers 4-5 dB gain over a fading channel. From the principle of the promising candidate for high data rate transmission in multi-access environment, OFDM-CDMA, the simulation of combination of POFDM and VOFDM with CDMA are carried out in A WGN and multipath fading channels. The performances of VOFDM-CDMA and POFDM-CDMA offer 4-6 and 2-3 dB gain compared to the conventional OFDM-CDMA for the fading channels, respectively. It is due to inherent advantage of both techniques POFDM, VOFDM and CDMA.