Constant Envelope OFDM Phase Modulation - Dr. James R. Zeidler

ABSTRACT OF THE DISSERTATIONConstant Envelope OFDM Phase ModulationbySteve C. ThompsonDoctor of Philosophy in Electrical Engineering (Communications Theory andSystems)University of California San Diego, 2005Professor James R. Zeidler, ChairProfessor John G. Proakis, Co-ChairOrthogonal frequency division multiplexing (OFDM) is a popular modulation techniquefor wireless digital communications. It provides a relatively straightforward way to accommodatehigh data rate links over harsh wireless channels characterized by severemultipath fading. OFDM has two primary drawbacks, however. The first is a high sensitivityto 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 waveformhas high amplitude fluctuations, a drawback known as the peak-to-average power ratio(PAPR) problem. The high PAPR makes OFDM sensitive to nonlinear distortion causedby the transmitter’s power amplifier (PA). Without sufficient power backoff, the systemsuffers from spectral broadening, intermodulation distortion, and, consequently, performancedegradation. High levels of backoff reduce the efficiency of the PA. For mobilebattery-powered devices this is a particularly detrimental problem due to limited powerresources.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 PAPRwaveform by way of angle modulation. The constant envelope signal can be efficientlyamplified with nonlinear power amplifiers thus achieving greater power efficiency. Inxvi