Date of Award

8-25-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Applied Science

First Advisor

Seshadri Mohan

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is a popular multi-carrier modulation technique used in wireless communication systems due to its robustness against multipath fading. A major drawback inherited in OFDM is the non-constant envelope with high peak-to-average power ratio (PAPR). High PAPR drives the transmitter's power amplifier into saturation and causes nonlinear distortions. One way to avoid this is the use of companding to reduce the dynamic range of the time-domain OFDM signal. Usually, the PAPR is reduced at the cost of some degradation in bit error rate (BER) performance. A new scheme that utilizes traditional companding functions with side information is proposed to reduce the PAPR in OFDM wireless communication systems without impairing their BER performance. A sufficient mild condition for the superiority of the proposed scheme in terms of BER performance is derived for a general nonlinear monotonically increasing companding function and tested for a set of functions to determine specific conditions for each. A condition is derived for the companding parameters to ensure that the average power of the OFDM signal is maintained unchanged after companding. This condition satisfies the first condition to ensure the superiority of the proposed scheme over the traditional companding schemes. It is shown that, due to the nature of the auto-regressive (AR) model, the side information used in the proposed scheme is robust to channel noise and can reliably generate good corrective vectors even when a few bits are received in error. Complexity analysis of the proposed scheme is quantified by the number of real multiplications, real additions and memory locations required for the implementation of the proposed scheme. It is shown that the additional complexity is comparable to that of a FFT block and less than that of most PAPR reduction techniques. The reduction of the quantization error of the analog-to-digital converter at the receiver as a side advantage is also examined and it is shown that a gain of 3-6dB in signal-to-noise ratio (SNR) can be obtained due to companding. Computer simulations have been conducted to verify that the proposed scheme has significantly lower BER as compared to conventional companding schemes. It is shown that gains of a few orders of magnitude in BER performance are obtained at different SNR without impairing the PAPR reduction performance. Simulations are conducted for a 1024-subcarriers, OFDM-based, WiMAX standard in the downlink partial use subcarrier (DL-PUSC) mode.

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