5G-Compliant Integrated Sensing and Communication at Sub-6 GHz: Experiments with SDRs and OpenAirInterface

Integrated Sensing and Communication (ISAC) is attracting significant interest, with numerous studies highlighting its potential in 5G and beyond. Unfortunately, due to the inherent difficulty in integrating ISAC into non-proprietary hardware and software platforms, experimental works remain few and mainly focused on the use of 5G NR-like waveforms (often within simulated environments), lacking a 5G-compliant protocol stack enabling an end-to-end network connectivity. In this work, we present an experimental testbed for monostatic ISAC based on Software Defined Radios (SDRs) and OpenAirInterface (OAI), an open-source 5G framework adhering to the 3GPP standard. In addition to a description of the main implementation details, we present a fully digital signal processing pipeline which utilizes downlink slots of a 5G OFDM-based communication in the sub-6 GHz n77 band to enable ISAC capabilities at the 5G Node B. In the first phase, processing steps include temporal alignment of the TX and RX I/Q samples, compensating for the processing delays introduced by the OAI framework and SDR platforms. Then, after dealing with the lack of TX-RX isolation and associated Self-Interference (SI) through fully digital mitigation techniques, the digitized signals are processed using either a Matched Filter or Channel Estimation-like scheme. Experimental evaluations conducted in a cluttered indoor environment under controlled target conditions show promising capabilities in estimating both range and velocity of targets, also highlighting notable trade-offs arising from the presence of side-lobes in the range-Doppler maps, linked to residual SI and use of 5G communication signals.