Orthogonal chirp-frequency-division multiplexing

Abstract

This doctoral dissertation proposes and investigates orthogonal chirp-frequency-division multiplexing (OCFDM), a novel multicarrier waveform designed to address the flexibility limitations of orthogonal chirp-division multiplexing (OCDM) while maintaining its robustness against interference. Based on the proposed discrete modular chirp transform (DMCT), OCFDM enables flexible partitioning of the frequency band into subbands (voices) filled with orthogonal chirps, effectively combining the frequency-division capabilities of orthogonal frequency-division multiplexing (OFDM) and orthogonal time-frequency-division multiplexing (OTFDM) with the chirp spread spectrum (CSS) characteristics of OCDM. Numerical analyses in single-user scenarios show that OCFDM offers a tunable trade-off between data rate and symbol-error rate (SER) performance depending on the selected tile geometry. Furthermore, the technique achieves a higher asymptotic data rate than OCDM and can further enhance throughput by exploiting chirp correlations within voices. In the context of multiuser communications, the dissertation introduces orthogonal chirp-frequency-division multiple access (OCFDMA). Leveraging the block-circulant property of the DMCT, OCFDMA implements orthogonal multiple access (OMA) via frequency-division multiple access (FDMA), assigning distinct subchannels to different users. The study derives optimal pilot sequences for minimum-variance-unbiased (MVU) and minimum-mean-square-error (MMSE) estimators, providing a robust framework for channel estimation. Simulation results show that OCFDMA outperforms OFDMA, OCDMA, and DFTS-OFDMA in terms of bit-error rate (BER) when using uniform geometry. Finally, to address the challenges of high-mobility environments, the generalized OCFDM (GOCFDM) is proposed by incorporating a chirp-rate parameter into the transform. This novel technique is robust against Doppler-induced interference in doubly-dispersive channels, outperforming conventional OCFDM and achieving performance comparable to state-of-the-art chirp-based techniques such as orthogonal chirp-division multiplexing (GOCDM) and affine frequency-division multiplexing (AFDM).