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The integrated sensing and communication (ISAC), in which the sensing and communication share the same frequency band and hardware, has emerged as a key technology in future wireless systems due to two main reasons. First, many important application scenarios in fifth generation (5G) and beyond, such as autonomous vehicles, Wi-Fi sensing and extended reality, requires both high-performance sensing and wireless communications. Second, with millimeter wave and massive multiple-input multiple-output (MIMO) technologies widely employed in 5G and beyond, the future communication signals tend to have high-resolution in both time and angular domain, opening up the possibility for ISAC. As such, ISAC has attracted tremendous research interest and attentions in both academia and industry. Early works on ISAC have been focused on the design, analysis and optimization of practical ISAC technologies for various ISAC systems. While this line of works are necessary, it is equally important to study the fundamental limits of ISAC in order to understand the gap between the current state-of-the-art technologies and the performance limits, and provide useful insights and guidance for the development of better ISAC technologies that can approach the performance limits. In this paper, we aim to provide a comprehensive survey for the current research progress on the fundamental limits of ISAC. Particularly, we first propose a systematic classification method for both traditional radio sensing (such as radar sensing and wireless localization) and ISAC so that they can be naturally incorporated into a unified framework. Then we summarize the major performance metrics and bounds used in sensing, communications and ISAC, respectively. After that, we present the current research progresses on fundamental limits of each class Manuscript

The practical feasibility of a WiFi transmissions based passive bistatic radar (PBR) is analyzed here. The required data processing steps are described including the adopted techniques for 1) the control of the signal autocorrelation function (ACF) usually yielding a high sidelobe level, and 2) the removal of the undesired signal contributions which strongly limit the useful dynamic range. The performance of the proposed techniques is firstly evaluated against simulated data generated according to the IEEE 802.11 Standards. Moreover the results are presented against a real data set collected by an experimental setup when using the conventional dual (reference and surveillance) channels PBR receiving scheme. This allows us to demonstrate the potentialities of a WiFi-based PBR for local area surveillance applications, where vehicles and people can be detected and tracked. Based on the digital nature of the exploited signals of opportunity, the attractive possibility is also investigated of avoiding the use of a dedicated receiving channel for the reference signal, by synthesizing it from the surveillance channel. This approach is shown to yield comparable performance with respect to the conventional PBR approach while yielding a remarkable saving in terms of system complexity

In this paper the exploitation of different linear polarimetric channels is considered as a way to mitigate the effect of interfering sources and to improve the target detection capability in a FM radio-based passive radar. The signals emitted on same/adjacent frequency channels by potential transmitters (other than the illuminator of opportunity) located in the surveillance area are shown to significantly affect the performance of the considered system in terms of both disturbance cancellation and target detection. Moreover, it is verified that the achievable performance is dependent on the adopted polarization at both the reference and surveillance channel since employing vertically or horizontally polarized antennas might yield very different results. Therefore effective approaches are introduced to jointly exploit the diversity of information conveyed by different polarizations at both the reference and surveillance channels. The results are shown against a real data set collected by an experimental multi-channel passive radar prototype developed and fielded at the DIET Dept. of the University of Rome “La Sapienza” (Italy). A significant performance improvement is achieved with the proposed processing schemes that always provide better results than the standard processing of the signals collected at the best polarimetric reference and surveillance channels. Besides the expected improvement due to non-coherent integration of target echoes received on multiple channels, the proposed multi-polarimetric operation is shown to yield a remarkable improvement when operating in a severe interference scenario so that it can be regarded as practical solution for robust and effective PCL

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In this paper an advanced version of the extensive cancellation algorithm (ECA) is proposed for robust disturbance cancellation and target detection in passive radar. Firstly some specific limitations of previous ECA versions are identified when dealing with a highly time-varying disturbance scenario in the presence of slowly moving targets. Specifically, the need to rapidly adapt the filter coefficients is shown to yield undesired effects on low Doppler target echoes, along with the expected partial cancellation. Therefore a sliding version of the ECA is presented which operates on partially overlapped signal batches. The proposed modification to the original ECA is shown to appropriately counteract the limitations above by taking advantage of a smooth estimate of the filter coefficients. An efficient implementation is also discussed to limit the corresponding computational load. The benefits of the proposed approach are demonstrated against real data sets accounting for quite different passive radar applications

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In this study the authors investigate the two-dimensional target localisation capabilities of a passive radar system based on WiFi transmissions. It is well known that the most straightforward way to achieve the target position estimation in the horizontal plane with a passive radar exploits the measurements either of a single bistatic range plus a direction of arrival (DoA) or of two bistatic ranges collected by two separate receivers. However, for a practical application it is interesting to clearly define which one of the two approaches provides the passive radar target localisation with a higher accuracy and whether combining both multiple bistatic range plus DoA measurements provides a further advantage. A multistatic configuration is considered which allows to collect a set of range/Doppler/angle measurements for a given target. Different target localisation strategies are devised and compared, based on subsets of the available measurements with the aim of understanding the localisation accuracies achievable using the different combinations of measurements. Experimental results are shown based on a passive radar prototype developed and fielded at the DIET Department - Sapienza University of Rome. This will contribute to demonstrate the fruitful application of the passive radar concept for short range surveillance