| Orthogonal frequency division multiplexing (OFDM) is a popular modulation technique for wireless digital communications. It provides a relatively straightforward way to accommodate high data rate links over harsh wireless channels characterized by severe multipath fading. OFDM has two primary drawbacks, however. The first is a high sensitivity to time variations in the channel caused by Doppler, carrier frequency offsets, and phase noise. The second, and the focus of this thesis, is that the OFDM waveform has high amplitude fluctuations, a drawback known as the peak-to-average power ratio (PAPR) problem. The high PAPR makes OFDM sensitive to nonlinear distortion caused by the transmitter's power amplifier (PA). Without sufficient power backoff, the system suffers from spectral broadening, intermodulation distortion, and, consequently, performance degradation. High levels of backoff reduce the efficiency of the PA. For mobile battery-powered devices this is a particularly detrimental problem due to limited power resources.; A new PAPR mitigation technique is presented. In constant envelope OFDM (CE-OFDM), the high PAPR OFDM signal is transformed to a constant envelope 0 dB PAPR waveform by way of angle modulation. The constant envelope signal can be efficiently amplified with nonlinear power amplifiers thus achieving greater power efficiency. In this thesis, the fundamental aspects of the CE-OFDM modulation are studied, including the signal spectrum, the signal space, optimum performance, and the performance of a practical phase demodulator receiver. Performance is evaluated over a wide range of multipath fading channel models. It is shown that CE-OFDM outperforms conventional OFDM when taking into account the effects of the power amplifier.; This work was done at UCSD's Center for Wireless Communication, under the "Mobile OFDM Communications" project (CoRe research grant 00-10071). |
