Coherent uplink and downlink communications in RF-photonic distributed MIMO systems for 6G mmWave
Date
2025
Authors
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Publisher
University of Delaware
Abstract
This thesis presents multi-user beamspace processing techniques for both uplink and downlink in a distributed multiple-input multiple-output (MIMO) RF-photonic system operating at millimeter wave (mmWave) frequencies. For the uplink, a novel Beamspace Interference Rejection Combining (BIRC) algorithm is proposed to address the limitations of conventional interference rejection combining (IRC) techniques in dense electromagnetic (EM) environments. Traditional element-space IRC suffers performance degradation as the array size increases and the element-wise signal-to-noise ratio (SNR) during channel state information (CSI) estimation decreases. The BIRC algorithm overcomes these challenges by performing both CSI estimation and interference suppression in a reduced-dimensional beamspace, derived through the spatial Fourier transform and power-based beam selection, and implemented in the analog-photonic domain. This approach improves the robustness of CSI and reduces the dependence on high-speed RF chains. Simulations using the NYUSIM channel model demonstrate that BIRC outperforms its element-space counterpart, with performance gains growing with array size. ☐ For the downlink, the thesis introduces an RF-photonic MIMO architecture that enables coherent beamforming across widely distributed antennas, supporting wideband, multi-user, and multi-beam transmission. A centralized optical local oscillator (LO) is distributed to maintain phase and frequency coherence across antennas separated by tens of kilometers. Mathematical formulations establish the foundations of IQ modulation and the implementation of beamforming weights in the optical domain using phase and amplitude control. The experimental implementation and validation of various precoding schemes show strong alignment with simulation results, confirming the feasibility of coherent downlink transmission using distributed antennas connected via long-distance fronthaul. In addition, the beam squint effect is analyzed in simulation, demonstrating noticeable performance degradation when a single-frequency precoder is applied to wideband signals. ☐ A circularly symmetric cell-free (CF) MIMO system model is introduced to formulate and evaluate the system-level performance of a cell-free architecture, where multiple access points (APs) are distributed along a circular perimeter to serve users located within a concentric region of interest. A sum capacity metric is used for performance validation and is shown to closely follow the beamforming-based sum rate. Further simulation analysis demonstrates the cumulative distribution function (CDF) of the sum capacity while varying the inter-AP angle. The results demonstrate that performance improves with increased inter-AP angle due to enhanced spatial diversity. To validate this relationship, the best- and worst-performing user equipment (UE) combinations from Monte Carlo simulations are identified and visualized. Additionally, a beam selection algorithm based on volume maximization is introduced for the RF-photonic cell-free MIMO system. The proposed algorithm is compared against several existing algorithms, including the Interference-Aware Beam Selection (IA-BS) algorithm, brute-force search, maximum-power-based selection, and full-beamspace (i.e., fully digital) methods.
Description
Keywords
Beamspace processing, Cell-free, Channel state information, Millimeter wave, Time-division duplexing