A Fully Reconfigurable RF Vector Modulator based Wideband Phase Shifter for NextG Beamforming Phased Array in Satellite Communications (SATCOM)

Technical Summary: A Fully Reconfigurable RF Vector Modulator-based Wideband Phase Shifter for NextG Beamforming Phased Array in Satellite Communications

Problem Statement
The rapid deployment of non-geostationary satellite constellations (e.g., Starlink, Kuiper) necessitates advanced beamforming phased arrays capable of electronic beam-steering to maximize spectral efficiency and system capacity. While conventional phased arrays are typically optimized for single-band operations (such as Ku-, K-, or Ka-bands), the emergence of 6G NextG systems introduces a significant challenge: the requirement for wideband operation spanning from 7.125 GHz to 24.25 GHz (FR-3) and beyond.

Existing phase shifter technologies face specific limitations in this wideband context. Switched-line and loaded-line phase shifters offer moderate tuning ranges and coarse resolution. True Time Delay (TTD) modules, while effective at preventing beam-squinting, suffer from high insertion loss, discrete delay resolution, and complex calibration requirements. Traditional In-phase and Quadrature (I-Q) vector modulators provide continuous phase control but rely on quadrature all-pass filtering (QAF) networks or hybrid couplers to generate 90° phase shifts. These conventional components exhibit finite bandwidth, frequency-dependent amplitude/phase imbalances, and excessive losses, which degrade performance across the broad S-band to Ku-band spectrum required for modern Satellite Communications (SATCOM).

Methodology
To address these constraints, the authors propose a fully reconfigurable RF Vector Modulator (RFVM)-based phase shifter utilizing a novel transversal multi-branch Finite Impulse Response (FIR) network topology. The design diverges from conventional I-Q modulation by avoiding the QAF network entirely.

The core methodology involves the following steps:

1. Transversal FIR Topology: The RF input signal is divided into multiple branches (transversal sections). Each branch applies individual gain weightings and specific time delays. The signals are then re-summed to synthesize the output.
2. Dual-Route Signal Propagation: Unlike single-chain FIR representations, the proposed RFVM employs a dual-route architecture (Route 1 and Route 2). The input signal is split into two complementary propagating routes that traverse the same set of weighted delay branches before being recombined.
3. Generalized Phase Control: The system introduces a generalized transfer function defined by two control parameters: a continuous phase-tuning factor ($\alpha$) and a discrete angular-polarity selector ($\gamma$). By manipulating these parameters, the system achieves continuous 360° phase shifting across four quadrants.
4. Coefficient Synthesis: The branch-gain distribution follows an odd-harmonic coefficient law inspired by discrete Fourier series expansion. The effective coefficients are derived from the Fourier projection of the target magnitude response, allowing the synthesis of the desired phase state while preserving magnitude response.
5. Circuit Implementation: The hardware realization consists of three functional modules: signal division (using wideband baluns and Wilkinson Power Dividers), signal modulation (using Variable Gain Amplifiers and SPDT switches to route signals through specific delay paths), and signal re-combination.

Key Results and Performance
The paper presents theoretical analysis and derived frequency responses for a 5-branch (N=2) RFVM implementation:

• Wideband Coverage: The design targets the frequency range from S-band up to Ku-band (2 GHz to 18 GHz).
• Phase Shifting Capability: The theoretical analysis demonstrates the capability for full 360° phase shifting, divided into four selectable quadrants (Q1: 0°–90°, Q2: 90°–180°, Q3: -180°–-90°, Q4: -90°–0°).
• Magnitude Preservation: Within the effective bandwidth, the theoretical model indicates the design maintains magnitude response. For a center frequency of 20 GHz with an instantaneous bandwidth (IBW) smaller than 500 MHz (typical for SATCOM), the maximum in-band magnitude deviation ($\Delta\delta_{mag}$) is limited to 1.18 dB, and the phase deviation ($\Delta\delta_{ph}$) is limited to 3.9°.
• Scalability: The analysis indicates that increasing the number of tap pairs (N) reduces the in-band RMS deviation and ringing effects (Gibbs phenomenon), approaching an ideal square wave response with wider effective bandwidth.

Significance and Claims
The authors claim that this work offers a novel approach to wideband phase shifting that relaxes the frequency constraints inherent in conventional quadrature generation methods. By utilizing a transversal FIR network with differential signal conversion, the proposed RFVM avoids the bandwidth limitations and losses associated with QAF networks and hybrid couplers.

The paper positions this design as a critical enabler for NextG (6G) SATCOM systems, specifically addressing the need for agile, continuous beam-steering across wide spectrum bands without the performance degradation caused by amplitude imbalance and phase errors in traditional I-Q modulators. The authors conclude that the proposed topology provides a flexible solution for real-time wideband RF signal processing in phased array systems. Future work is identified as the integration of this RFVM into a wideband beamforming transceiver link for over-the-air (OTA) system-level evaluations.