A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering. School of Advanced Technologies

Thesis (M.Eng.) - Asian Institute of Technology, 1999

Orthogonal Frequency Division Multiplexing (OFDM) has been considered as a promising candidate to achieve high rate data transmission in a mobile environment. OFDM shows favorable properties such as robustness to channel fading and intersymbol interference (ISI) and is more immune to impulse noise. The explosive growth and demand for new services such as wireless multimedia services, wireless internet access and wireless video conferencing suggest a very promising future for high data rate cellular communication systems based on OFDM. However, there are two major obstacles in using OFDM in transmission systems. OFDM signal exhibits a very high Peak to Average Power Ratio (P APR) and it is ve1y sensitive to carrier frequency offsets. In this thesis, we address these limitations. We analyze peak windowing method to reduce the Peak to Average Power Ratio of an OFDM signal and then propose a method, pre-amplification, to be used with peak windowing to reduce the P APR further. The pre-amplification increases the average value of the OFDM signal while peak windowing removes the peaks. Thus, the OFDM signal is compressed to reduce its dynamic range. The effect on bit error rate performance is investigated in A WGN channels and multipath slow fading channels. The effect of out of band radiation due to peak . windowing and pre-amplification is also considered. Pre-amplification with peak windowing reduces P APR further with increased system complexity and loss in SNR. Simulation results show that additional loss in SNR due to pre-amplification is much in multipath fading channel comparing with A WGN channel while peak windowing causes almost same additional loss in SNR in A WGN and multipath fading channel. The performance degradation of an OFDM system due to frequency offset is analyzed theoretically and then a method to reduce sensitivity to frequency offset errors is proposed. Transmitting same modulated symbols on the adjacent subca1Tier is proposed in self ICI cancellation scheme to remove intercarrier interference. Here, we propose to transmit same modulated symbols on k th and (N-k-1) th subcarriers. This is an effective way to remove intercarrier interference and it exhibits a diversity effect in multipath fading channel. Performance of the proposed scheme is compared with self ICI cancellation scheme by simulation and theoretically. Comparison of proposed scheme with half rate convolutinal coded scheme is also given to realize the diversity capability of the proposed method. Proposed method gives better bit error rate performance comparing with self ICI cancellation scheme for small values of frequency offset. Thus, it is more suitable for mobile environment having small Doppler shift. However, both schemes are wasting half of the bandwidth by transmitting same symbol twice.