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Title: Designing Orthogonal Chirp Division Multiplexing Waveforms for Next Generation Wireless Communications
Committee:
Dr. Xiaoli Ma, ECE, Chair, Advisor
Dr. John Barry, ECE
Dr. Gordon Stuber, ECE
Dr. Mary Ann Weitnauer, ECE
Dr. Yao Xie, ISyE
Abstract: The objective of this dissertation is to analyze the performance of orthogonal chirp division multiplexing (OCDM) as a waveform for wireless communications, identify issues to its adoption in future networks and propose solutions to these problems. OCDM is an extension of the chirp spread spectrum (CSS) as it employs linear chirps to carry data. However, in contrast with CSS, OCDM can achieve the Nyquist signaling rate and therefore, it has the same spectral efficiency as orthogonal frequency division multiplexing (OFDM). The first part of this dissertation analyzes the performance of OCDM affected by typical wireless channel impairments such as multipath fading, carrier frequency offset (CFO) and interference. Results show that OCDM shows better performance in multipath fading channels without channel coding than OFDM and is more robust to interference than both single carrier block transmissions and OFDM. This makes it an attractive alternative to OFDM in future networks. On the other hand, the analysis highlights some key issues which stem directly from the fact that it spreads data over the time and frequency domains. First, it suffers from inter-symbol interference (ISI) in frequency-selective channels and hence does not lend itself easily to uplink multiuser transmissions and pilot-based channel estimation. Second, there is no way of digitally introducing guard bands without altering hardware sampling rates. Third, OCDM has a very high peak-to-average power ratio (PAPR). Solutions are subsequently proposed for each of the identified issues. A complex precoding technique is first proposed to enable full diversity transmissions and orthogonal multiuser transmissions in the uplink. A low complexity OCDM transmitter is proposed which is capable of digitally controlling the OCDM spectrum, thus making it compatible with digital filtering techniques that were originally proposed for OFDM. Several pilot multiplexing techniques are proposed which allow for pilot and data symbols to exist in the same block without causing interference. Finally, machine learning based techniques are proposed to lower the PAPR of both OFDM and OCDM without the need of additional signaling, and without needing to make any changes at the receiver.
