Distributed Integrated Sensing and Communications Systems

The continuous scaling up of the carrier frequencies and deployment of wireless communications has propelled the spectrum regulators to allow the use of the spectrum traditionally reserved for radar (sensing) applications for commercial communications systems. As a result, in recent years, integrated sensing and communications (ISAC) has emerged as a promising technology to mitigate spectral congestion and efficiently utilize radio resources. Prior research on ISAC was largely limited to colocated or centralized systems, where communications, radar, or both employed transmit/receive units placed close to each other. However, next-generation wireless networks are envisaged to deploy a decentralized resource management infrastructure and edge-device-centered network paradigms. Similarly, widely distributed radars are gaining widespread usage because they offer the advantages of spatial diversity, improved detection of stealth targets, and joint processing. To this end, we investigate the design and performance evaluation of distributed ISAC systems. We consider a general system comprising a widely distributed multiple-input multiple- output (MIMO) radar that operates in the same spectrum as a distributed MIMO communications network. A major challenge to co-design such an ISAC system is a unified performance metric for both communications and radar. We address this problem by proposing a compounded weighted sum of mutual information as an objective to obtain optimized waveforms, precoders, beamformers, and receive filters jointly for distributed sensing and communications. Our design problem also includes various practical constraints such as link budget, quality of service, and peak-to-average power ratios. We then extend our formulation to in-band full-duplex (IBFD) distributed ISAC to enable simultaneous uplink, downlink, and radar sensing transmissions. Conventional communications systems are based on either half-duplex (HD) or out-of-band full-duplex transmission for low-complexity transceiver designs leading to reduced spectral efficiency. On the other hand, IBFD enables concurrent transmission and reception in a single time/frequency channel to potentially double the attainable spectral efficiency and throughput and reduce latency. For distributed wireless systems, synchronization in time and space is a major concern. The clocks at the radar and communications transmitters are synchronized both offline and periodically. We use the feedback of the base station via pilot symbols to provide the radar receivers with the clock times of uplink user equipment. Often, distributed beamforming (DB) is employed to achieve the desired signal-to-noise and reduce power assumption by providing a coherent beamforming gain. Further, MIMO radars may also employ DB in the form of distributed coherent systems, wherein accurate phase synchronization is required to obtain coherent processing gain. In this research, we show that such synchronization techniques may also be incorporated in distributed ISAC. Further, we develop techniques for multi-target localization in a distributed ISAC. In general, echoes from multiple targets have different time-of-arrivals at different receivers. We solve this association problem for the ISAC system via a mixed-integer programming framework. Finally, we devise low-complexity design techniques, which utilize the block-coordinate descent and Barzilai-Borwein algorithms, for the aforementioned scenarios to obtain optimal design parameters for distributed ISAC simultaneously.