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Multichannel passive radar systems : signal processing techniques and design strategies
Full text linkPassive Coherent Location (PCL) system is one of the most rapidly developing technology in the radar field and significant progress has been made in recent years. However, some aspects that are not under the control of the radar designer still prevent PCL technology from reaching its point of maturity. The goal of this thesis is to address this issue by resorting to the exploitation of information diversity conveyed by multiple receiving channels.
The main novelties that this research has led to can be identified in two main areas, one concerning the exploitation of polarization diversity for target detection, the other regarding the exploitation of spatial and frequency diversity for target localization purposes. Although the ideas underlying the main achievements reported in this work arise from the need to overcome PCL systems limitations, the research performed to reach this goal has led to achievements that have a broad scope of application, not limited to passive radar systems, which increases the scientific value of this work.
In the first part of this thesis, we deal with the problem of target detection in coherent radar systems exploiting polarimetric diversity. We build upon the demonstrated benefits that a suitable use of signals collected by differently polarized antennas can lead to, and we present a new polarimetric adaptive target detection scheme. To this aim, we resort to a parametric approach and we model the disturbance affecting the data as a multi-channel AR process. First, we show the effectiveness of the proposed strategy via an extensive simulated analysis, then we carry out a performance assessment under spectral model mismatch conditions, and finally we demonstrate its validity against real data, collected by both active and passive radar systems.
In the second part of this thesis, we address the problem of target direction of arrival (DoA) estimation in systems that jointly exploit spatial and frequency diversity. We derive a reliable performance characterization of a multi-carrier maximum likelihood DoA estimator in the threshold region. The capability of predicting the performance of the considered estimator in low signal-to-noise ratio scenarios is a powerful tool that can also be used to select the best performing receiving system layout, given a number of constraints. The experimental results obtained using configurations selected according to this criterion show that a suitable exploitation of spatial and frequency diversity allows to both extend the angular sector where the target DoA can be unambiguously estimated and improve the estimation accuracy
Polarimetric detection scheme for passive radar based on a 2D auto-regressive disturbance model
Full text linkSuitable strategies to exploit polarimetric diversity have been proved to be able to enhance the target detection capability of passive radar (or passive coherent location - PCL) systems. In this work, the authors describe a novel polarimetric adaptive detection scheme, based on a two-dimensional autoregressive model for the disturbance. The effectiveness of the proposed strategy is shown against experimental data collected by means of a FM radio based multi-channel PCL prototype. The obtained results are also compared with alternative polarimetric detection schemes in order to provide a first insight into the benefits of the newly proposed solutio
Experimental results of polarimetric detection schemes for DVB-T-based passive radar
Full text linkThis study investigates the potential advantages conveyed by the exploitation of polarimetric diversity in Passive Coherent Location (PCL) system exploiting Digital Video Broadcasting - Terrestrial (DVB-T) signals. To this purpose, different polarimetric detection schemes are considered that jointly exploit the signals collected at different receiving channels characterised by differently polarised antennas. A performance analysis has been carried out using experimental data collected by a passive radar system developed by Fraunhofer, namely PARASOL. This PCL system employs two orthogonal linearly polarised receiving channels (horizontal and vertical). The results obtained against cooperative targets demonstrate that a Polarimetric Generalised Likelihood Ratio Test (P-GLRT) detection scheme provides a remarkable improvement in terms of target detection capability as compared to the single-polarisation operation and the non-coherent integration of the results obtained at the available polarimetric channels
Comparing reference‐free WiFi radar sensing approaches for monitoring people and drones
Full text linkThe use of WiFi signals for sensing purposes has attracted a lot of interest from both the radar and communications communities and several techniques have been explored. In the attempt of meeting the requirements for small sensor size, compactness, and easy deployment, the authors consider reference‐free approaches, namely approaches that do not require a good copy of the transmitted waveform to be available at the radar receiver. To this end, the authors first resort to a passive radar‐based processing scheme that only exploits the invariant a priori known initial portion of the physical layer protocol data unit, that is, the PHY Preamble, and its limitations in practical applications is investigated. Specifically, the authors show that, with this approach, an accurate time, phase, and frequency synchronization is essential and a possible strategy is investigated. As an alternative solution the authors consider a forward scatter radar‐based approach where only the amplitude modulation of the received signal is exploited to detect the presence of a moving target thus avoiding the need to know the transmitted signal. For the first time, the authors comparatively investigate advantages and drawbacks of the two reference‐free
approaches and present practical strategies to handle the limitations observed. The results are reported for experimental tests with people and drones using WiFi transmissions in the 2.4 and 5 GHz band
Human and drone surveillance via RpF-based WiFi passive radar. Experimental validation
No full textThis work deals with the short-range monitoring of small radar cross section targets using commercial WiFi transmitters as source of opportunity. Proper modifications to the conventional WiFi-based passive radar signal processing scheme are presented, based on the use of the Reciprocal Filter (RpF) approach for range compression, that (i) simplify the overall scheme (ii) make the processing robust to current and future IEEE 802.11 standards that might be characterized by mixed modulation transmissions (iii) result in a better sidelobe control (iv) enable an easier and cost-effective clutter cancellation strategy. Then, we introduce an appropriate solution to tackle the signal-to-noise ratio loss introduced by the presented strategy. The effectiveness of the resulting processing scheme is tested on experimental data, collected in both the 2.4 and 5 GHz WiFi bands, against both humans and a small drone
Outlier Rejection Approach for Direction of Arrival Estimation in Low SNR Conditions
No full textThis paper deals with the problem of Direction of Arrival (DoA) estimation through a Non-Uniform Linear Array (NULA) in low signal-to-noise ratio (SNR) conditions. By exploiting the spatial diversity offered by the different subarrays of a given NULA configuration, we propose a simple strategy that aims at identifying and discarding DoA estimates that are recognized to be outliers caused by the generally high level of the employed array beampattern sidelobes. The effectiveness of the proposed algorithm is shown on both simulated and experimental data for a drone surveillance application in the Wi-Fi band
Threshold region performance of multi-carrier maximum likelihood direction of arrival estimator
Full text linkThis paper addresses performance characterization of a direction of arrival (DoA) estimator in the low signal-to-noise-ratio (SNR) region. The case of a sensor array simultaneously collecting signals emitted at multiple carrier frequencies by a single source is considered. A maximum likelihood (ML) approach is used as a reference method for DoA estimation and its accuracy is characterized in terms of mean square error (MSE). It is well known that, for SNR values included in the so-called threshold region, the DoA estimation accuracy decreases rapidly, due to the presence of outliers. This effect can be possibly mitigated when multiple frequency channels are jointly exploited. However, the capability to predict this performance degradation is fundamental either for assessing the robustness of an existing sensor or for supporting its design. Therefore, the scope of this paper is to introduce appropriate approximations to the MSE of a multi-frequency ML DoA estimator in order to provide a reliable characterization of its performance in the threshold region. Two models for the source signals are considered and separately discussed, namely the deterministic (or conditional) and stochastic (or unconditional). An extensive simulated analysis is reported to prove the tightness of the approximations and to characterize the benefits steming from the exploitation of signals emitted at multiple carrier
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