The paper examines the problem of cancellation of direct signal, multipath and clutter echoes in passive bistatic radar (PBR). This problem is exacerbated as the transmitted waveform is not under control of the radar designer and the sidelobes of the ambiguity function can mask targets including those displaced in either (or both) range and Doppler from the disturbance. A novel multistage approach is developed for disturbance cancellation and target detection based on projections of the received signal in a subspace orthogonal to both the disturbance and previously detected targets. The resulting algorithm is shown to be effective against typical simulated scenarios with a limited number of stages, and a version with computational savings is also introduced. Finally its effectiveness is demonstrated with the application to real data acquired with an experimental VHF PBR system.

A Multistage Processing Algorithm for Disturbance Removal and Target Detection in Passive Bistatic Radar

This paper provides a detailed overview of the Digital Video Broadcasting Terrestrial (DVB-T) signal structure and the implications for passive radar systems that use these signals as illuminators of opportunity. In particular, we analyze the ambiguity function and explain its delay and Doppler properties in terms of the underlying structure of the DVB-T signal. Of particular concern for radar range-Doppler processing are ambiguities consistent in range and Doppler with targets of interest. In this paper we adopt a mismatched filtering approach for range-Doppler processing. We also recognize that while the structure of the DVB-T signal introduces ambiguities, the structure can also be exploited to better estimate the transmitted signal and channel, as well as any mismatch between transmitter and receiver (e.g., clock offsets). This study presents a scheme for pre-processing both the reference and surveillance signals obtained by the passive radar to mitigate the effects of the ambiguities and the clutter in range-Doppler processing. The effectiveness of our proposed scheme in enhancing target detection is demonstrated using real-world data from an (Australian) 8k-mode DVB-T system. A 29 dB reduction in residual ambiguity levels over existing techniques is observed, and a 36 dB reduction over standard matched filtering; with only a 1 dB reduction in the zero-delay, zero-Doppler peak.

DVB-T Passive Radar Signal Processing

This paper addresses target detection in passive multiple-input multiple-output (MIMO) radar networks comprised of non-cooperative transmitters and multichannel receivers. A generalized likelihood ratio test is derived, and approximate test statistic distributions are presented for both hypotheses under common scenario conditions. Analysis and simulation results show that this detector outperforms other passive MIMO radar detectors because it exploits more correlations within the measurement data. This detector is also compared against related detectors for active MIMO radar and passive source localization sensor networks. These comparisons reveal that passive MIMO radar detection performance varies between active MIMO radar and passive source localization detection performance as a function of direct-path signal quality. Therefore, passive MIMO radar unifies active MIMO radar and passive source localization sensor networks in a common theoretical framework.

Detection in Passive MIMO Radar Networks

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.

WiFi-Based Passive Bistatic Radar: Data Processing Schemes and Experimental Results

We investigate the use of WiFi Received Signal Strength Information (RSSI) at a mobile phone for the recognition of situations, activities and gestures. In particular, we propose a device-free and passive activity recognition system that does not require any device carried by the user and uses ambient signals. We discuss challenges and lessons learned for the design of such a system on a mobile phone and propose appropriate features to extract activity characteristics from RSSI. We demonstrate the feasibility of recognising activities, gestures and environmental situations from RSSI obtained by a mobile phone. The case studies were conducted over a period of about two months in which about 12 hours of continuous RSSI data was sampled, in two countries and with 11 participants in total. Results demonstrate the potential to utilise RSSI for the extension of the environmental perception of a mobile device as well as for the interaction with touch-free gestures. The system achieves an accuracy of 0.51 while distinguishing as many as 11 gestures and can reach 0.72 on average for four more disparate ones.

The telepathic phone: Frictionless activity recognition from WiFi-RSSI

In this paper the practical feasibility of a WiFi transmissions based passive bistatic radar (PBR) is analyzed. The required data processing steps are described there including the adopted techniques for: (i) the control of the signal Ambiguity Function usually yielding a high sidelobe level and (ii) the removal of the undesired signal contributions which strongly limit the useful dynamic range. The performance of the conceived system is evaluated with reference to typical signals broadcasted by a IEEE 802.11 access point exploiting an OFDM modulation. The achievable results are presented against a real data set collected by an experimental setup. This allowed us to preliminarily demonstrate the potentialities of a WiFi-based PBR for local area surveillance applications.

Experimental results for OFDM WiFi-based passive bistatic radar

After summarizing the problems related with the variability of the characteristics of the waveforms received by PBR, especially when operating in the FM radio band, the concept of the MF integration has been introduced, aiming at (i) increasing the global SNR and (ii) making the detection performance robust w.r.t. the program content of the individual radio stations.

Multifrequency integration in FM radio-based passive bistatic radar. Part I: Target detection

Due to its high power levels provided, and its wide coverage, FM radio could be a good opportunity transmitter for passive coherent location (PCL) radar systems. In this paper we study the effectiveness of FM signals as radar waveforms by means of simulated and real data analysis. To this purpose, an emulation of FM radio transmitter is presented, whose outputs are compared with real FM data collected by an experimental digital receiver. In this way, we also achieve a reliable instrument to optimize target detection performance by a successive adaptive signal processing. To complete the analysis of opportunity waveforms, the signals' self-ambiguity functions and spectra are evaluated, so it is possible to improve the knowledge of how to select the most appropriate FM channel. Since emulated data differ from real data in means of the transmission channel, a statistical analysis of the real channel is presented.

Analysis and Emulation of FM Radio Signals for Passive Radar

The concept of MF operation introduced in [1] has been extended to the target DoA estimation stage, aiming at the same time to (i) increase the angular localization accuracy and (ii) make the DoA estimation performance robust w.r.t. the time-varying characteristics of the waveforms received by PBR, especially when operating in the FM radio band.

Carlo Bongioanni

Papers

WiFi-Based Passive Bistatic Radar: Data Processing Schemes and Experimental Results

Experimental results for OFDM WiFi-based passive bistatic radar

Multifrequency integration in FM radio-based passive bistatic radar. Part I: Target detection

Analysis and Emulation of FM Radio Signals for Passive Radar

Multifrequency integration in FM radio-based passive bistatic radar. Part II: Direction of arrival estimation