RF Interference Mitigation for Dual S-Band Links on a 6U CubeSat: Preliminary Hardware Validation

This paper presents the RF communication system architecture and preliminary hardware validation for the ALIGN (Autonomous Laser-based Inter-satellite Gigabit Network) mission, comprising a pair of 6U CubeSats equipped with FOCUS (Free-space Optical Communications Unit for Space), an experimental payload demonstrating high-data-rate optical inter-satellite links in Low Earth Orbit (LEO).
Each platform features co-located S-band Radio Ground-to-Satellite Link (RGSL) and Radio Inter-satellite Link (RISL) subsystems operating within the same frequency band (2025–2110~MHz). The compact CubeSat form factor and SWaP-C constraints impose significant limitations on the achievable isolation between the two RF links, making co-site RFI management a critical design challenge. A multi-layered mitigation strategy is implemented, combining Time Division Multiplexing (TDM) at the protocol level with hardware-level isolation strategies including asymmetric transceiver duplex configuration, 55~MHz frequency separation, orthogonal antenna placement providing 15~cm physical separation and 90~$^circ$ angular isolation, and polarization diversity between the RHCP RGSL and LP RISL links between antennas. The end-to-end hardware feasibility test was conducted using Flight Model (FM) modules in a certified anechoic chamber under far-field conditions and across the full operational power range of both links, constituting a valid first-step validation approach within the equipment and budget constraints typical of university CubeSat projects. The results confirmed successful preliminary bilateral RISL handshake and the absence of co-site RFI at the RGSL across all tested configurations, validating the combined isolation strategy as an effective and practical approach within the SWaP-C constraints of the platform. These findings provide an initial validated design framework for establishing multi-link RF systems of constellation of CubeSats within the same frequency band.