Antenna Theory And Design

Recently, ambient backscatter communication has been introduced as a cutting-edge technology which enables smart devices to communicate by utilizing ambient radio frequency (RF) signals without requiring active RF transmission. This technology is especially effective in addressing communication and energy efficiency problems for low-power communications systems such as sensor networks, and thus it is expected to realize numerous Internet-of-Things applications. Therefore, this paper aims to provide a contemporary and comprehensive literature review on fundamentals, applications, challenges, and research efforts/progress of ambient backscatter communications. In particular, we first present fundamentals of backscatter communications and briefly review bistatic backscatter communications systems. Then, the general architecture, advantages, and solutions to address existing issues and limitations of ambient backscatter communications systems are discussed. Additionally, emerging applications of ambient backscatter communications are highlighted. Finally, we outline some open issues and future research directions.

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Ambient Backscatter Communications: A Contemporary Survey

Wireless mediums, such as RF, optical, or acoustical, provide finite resources for the purposes of remote sensing (such as radar) and data communications. Often, these two functions are at odds with one another and compete for these resources. Applications for wireless technology are growing rapidly, and RF convergence is already presenting itself as a requirement for both users as consumer and military system requirements evolve. The broad solution space to this complex problem encompasses cooperation or codesigning of systems with both sensing and communications functions. By jointly considering the systems during the design phase, rather than perpetuating a notion of mutual interference, both system’s performance can be improved. We provide a point of departure for future researchers that will be required to solve this problem by presenting the applications, topologies, levels of system integration, the current state of the art, and outlines of future information-centric systems.

Survey of RF Communications and Sensing Convergence Research

In this paper, we investigate the feasibility of uncooperatively and covertly detecting people moving behind walls using passive bistatic WiFi radar at standoff distances. A series of experiments was conducted which involved personnel targets moving inside a building within the coverage area of a WiFi access point. These targets were monitored from outside the building using a 2.4-GHz passive multistatic receiver, and the data were processed offline to yield range and Doppler information. The results presented show the first through-the-wall (TTW) detections of moving personnel using passive WiFi radar. The measured Doppler shifts agree with those predicted by bistatic theory. Further analysis of the data revealed that the system is limited by the signal-to-interference ratio (SIR), and not the signal-to-noise ratio. We have also shown that a new interference suppression technique based on the CLEAN algorithm can improve the SIR by approximately 19 dB. These encouraging initial findings demonstrate the potential for using passive WiFi radar as a low-cost TTW detection sensor with widespread applicability.

Through-the-Wall Sensing of Personnel Using Passive Bistatic WiFi Radar at Standoff Distances

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 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

The history of passive radar dates back to the early days of radar in 1935 when the Daventry experiment was conducted in the UK. It continues in WW II with the German Klein Heidelberg passive radar and receives new interest today, as passive covert radar (PCR) systems like Silent Sentry and Homeland Alerter 100 are ready for operation. The future of PCR will strongly depend on the availability of transmitters of opportunity such as FM-radio and digital broadcast networks.

Passive radar from history to future

We present a system characterisation of a passive bistatic radar (PBR). The system under investigation exploits dasiailluminators of opportunitypsila, which in this case are commercial, non-cooperative, VHF FM broadcast transmissions. The PBR under investigation demonstrates the detection of large passenger-jet aircraft in the airspace over greater London. FM based PBRs such as the type examined here have been shown to be effective at remote sensing of auroral turbulence, density irregularities in the E-region and F-region of the ionosphere and meteor trails. Detection of aircraft is an important step in attempting to remotely sense outer-atmospheric phenomena as aircraft scattered power is of a comparable magnitude and similar Doppler frequency to the natural phenomena described. This paper also analyses the merits of FM broadcast transmissions for use as radar signals. A system characterisation with performance predictions is presented. The results show that target detections have been achieved to ranges in excess of 70 km (bistatic range). Multiple broadcast channels from two different transmitters of opportunity enhance the case for deciding whether or not targets are present. Air-truth data provided by a mode S/ADS-B IFF receiver is used for comparison with the 2-D bistatic results.

Passive Bistatic Radar (PBR) using FM radio illuminators of opportunity

The presence of clutter in passive radar returns imposes a minimum floor level on the ambiguity surface, masking weak returns and reducing dynamic range. When the illuminating signal is a form of OFDM, orthogonality between subcarriers can be exploited to produce a zero floor within certain delay bins. In this paper we compare and contrast several such ambiguity processing techniques for general OFDM signals. The methods are applied to simulated and real DVB-T passive radar returns having a wide range of delays. It is shown that an OFDM-specific method can outperform a standard pulse-Doppler processing method under certain conditions. An example is provided using real German DVB-T data from Fraunhofer FHR.

Evaluation of the ambiguity function for passive radar with OFDM transmissions

A MFH-PR concept has been presented. The rationale for pursuing such a hybrid unit has been explained in terms of enhancing detection range and coverage over conventional PRs. The important distinctions between MFNs and SFNs have been analysed. Section VI has described the advantages of signal reconstruction and has shown that, in certain situations, adequate interference suppression can be achieved without use of additional adaptive filters. An important consideration of MFH-PR is the potential range coverage extension. This has been demonstrated in Section VII using an advanced radar planning tool. Frequency and spatial diversity have been discussed in the context of improving tracking performance. The military utility of a MFH-PBR has also been discussed in Section XIII.

Daniel W. O'Hagan

Papers

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

Passive radar from history to future

Passive Bistatic Radar (PBR) using FM radio illuminators of opportunity

Evaluation of the ambiguity function for passive radar with OFDM transmissions

A multi-frequency hybrid passive radar concept for medium range air surveillance