Signal compensation schemes over multipath fading channels for OFDM communication systems

Abstract

Orthogonal frequency division multiplexing (OFDM) is a kind of multi-carrier modulation (MCM) schemes. OFDM allows overlapping of adjacent frequency bins called subchannels or subcarriers with orthogonality between them. It results in high spectral efficiency and the symbol period of the OFDM modulated signal is considerably long compared to single carrier schemes. In multipath fading channels, the performance of any communication system becomes better with the increased symbol period. However, with the increased symbol period, the transmission rate may decreases. To avoid this unfavorable phenomenon, the complicated equalizers are required in single carrier modulation system to compensate the distortion induced by the multipath fading channels. The OFDM communication systems can increase the symbol period without any reduction in transmission rate and adoption of complicated equalizers. Thus the performance of the OFDM system has been considered better compared to the single carrier communication systems in the multipath fading environments. In spite of the advantages over the single carrier schemes, the previously proposed OFDM schemes for multipath fading channels revealed several problems in compensating channel distortion. The previous schemes compensate the channel distortion in time domain and result in noise enhancement and irreducible error floor. In this dissertation work, we find new compensation schemes for the OFDM signal distortion resulting from the multipath fading channels. At first, we evaluate the OFDM signal compensation schemes for frequency nonselective fading channels. In those channels, we find that the characteristics of the signal distortion take the form of the frequency domain inter-symbol interference (ISI). Thus we propose the frequency domain equalizers to compensate the distortion. As the frequency domain equalizers, we propose the frequency domain linear equalizer (FDLE) and the frequency domain decision feedback equalizer (FDDF...