Coherent Radio-over-Fiber Links with Increased Spectral Efficiency

Abstract

This thesis advances radio-over-fiber (RoF) links by jointly improving spectral use, phase-noise tolerance, and hardware simplicity. We develop and experimentally validate three complementary architectures, each paired with a tailored digital signal-processing (DSP) algorithm.

First, we propose a cascaded intensity/phase-modulation (IM/PM) coherent RoF architecture in which four independent microwave vector signals are power-multiplexed in pairs, modulated on an optical carrier, and then polarization-multiplexed with an unmodulated optical carrier from the same laser used for modulation. After coherent detection, the DSP algorithm recovers the IM/PM signals, demultiplexes the power-multiplexed pairs, and mitigates polarization rotation, unstable offset frequency, and phase noise, achieving low error-vector magnitude (EVM) at modest received optical powers. This structure leverages both intensity and phase together with power-multiplexing to use the optical spectrum more efficiently and address limitations of prior RoF systems.

Second, we introduce a transmitter based on a dual-drive Mach–Zehnder modulator (DD-MZM) for coherent RoF links that phase-modulates two power-multiplexed signal pairs on a single optical carrier via two RF ports of the modulator. A joint DSP algorithm cancels shared phase noise and unstable offset frequency, enabling independent recovery of digital signals on multi-GHz RF carriers and delivering up to 4× per-carrier spectral efficiency using only one optical wavelength and polarization.

Third, to reduce cost and power by avoiding an external local oscillator, we design a self-coherent, direct-detection RoF link that employs a single-sideband (SSB) pilot as an in-band reference for optical-phase retrieval. The corresponding DSP algorithm cancels AWG-induced phase noise, enabling the simultaneous demodulation of two RF signals. Experiments show constellations with low EVM at sub-milliwatt received powers across representative symbol rates, indicating robustness to dispersion and front-end impairments such as phase noise and in-phase and quadrature (I/Q) imbalances.

Collectively, these results show that power-multiplexing combined with a tailored DSP algorithm yields higher per-carrier spectral efficiency and improved phase-noise tolerance while simplifying hardware, offering a scalable path to compact, low-cost RoF front ends and clear opportunities for future RoF systems.