Coherently distributed RF antenna arrays using photonic links
Date
2025
Authors
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Publisher
University of Delaware
Abstract
This thesis presents a coherently distributed multiple-input multiple-output (MIMO) radio-frequency (RF)-photonic system operating at millimeter wave (mmWave) frequencies. Over long distances with negligible propagation loss, this system uses radio-frequency-over-fiber (RFoF), or RF-photonic links, to implement distributed antenna system that preserves both the spatial and temporal coherence of RF signals. It utilizes the principles of photonic signal generation and the distribution of RF signals using a centralized optical local oscillator (OLO) to maintain phase and frequency coherence across antennas separated by tens of kilometers. Based on the proposed RF-photonic system architecture, this thesis presents the development and simulation of various antenna configurations, including single-array architectures, unconstrained distributions of subarrays and individual elements. These configurations are analyzed as proof-of-concept demonstrations to validate the precoding strategies, including zero forcing (ZF) and maximal ratio transmission (MRT), and their performance is evaluated using a simulation precoder design algorithm to establish the feasibility of the overall system design. ☐ This RF-photonic system also includes a photonic processing unit (PPU) that contains a tunable optical paired source (TOPS), photonic feed network that enables a complex weighting matrix, which enables phase and amplitude control in the optical domain to apply beamforming weights, and finally high-power photodiodes (PDs) in the remote RF frontend. The system is used to implement optimized complex weights, on distributed antenna elements, to realize “flat,” i.e., lying in the plane of propagation, holographic field profiles. In order to implement the practical performance of the holographic field profiles in the phase-synchronized distributed beamforming systems, hardware and software development are carried out. This distributed beamforming system includes the implementation of the transmitter (Tx) module, including both the software framework developed in LabVIEW and hardware enhancements achieved through the integration of supplementary test equipment for improved data acquisition and system monitoring. ☐ Various calibration techniques are used to obtain far-field radiation patterns and to maintain phase coherence across multiple distributed antennas. Experimental validation of a one-dimensional indoor system is used to demonstrate the aforementioned “in-plane” holograms, along with their comparison to simulation results. The experimental implementation and validation of various precoding schemes are utilized to implement the long range distribution of RF signals without being limited by cable loss. The proposed system preserves coherence even with differential fiber lengths to the different remote antennas. This system can be expanded to support multiple users by increasing the number of transmit antennas and incorporating data modulation on the coherent carriers. With these advancements, full system-level performance evaluation including data rate, spectral efficiency and bit error rate is planned for future work.
Description
Keywords
Distributed antenna, Optical local oscillator, Radio-frequency, Zero forcing, Photonic processing unit