IEEE JOURNAL ON SELECTED TOPICS IN SIGNAL PROCESSING (ACCEPTED) 1 Signal Processing for Passive Radar Using OFDM Waveforms Christian R. Berger, Member, IEEE, Bruno Demissie, Member, IEEE, J¨ org Heckenbach, Peter Willett, Fellow, IEEE, and Shengli Zhou, Member, IEEE Abstract—Passive radar is a concept where illuminators of opportunity are used in a multi-static radar setup. New digital signals, like Digital Audio/Video Broadcast (DAB/DVB), are excellent candidates for this scheme, as they are widely available, can be easily decoded to acquire the noise-free signal, and employ orthogonal frequency division multiplex (OFDM). Multicarrier transmission schemes like OFDM use block channel equalization in the frequency domain, efficiently implemented as a fast Fourier transform (FFT), and these channel estimates can directly be used to identify targets based on Fourier analysis across subsequent blocks. In this paper, we derive the exact matched filter formu- lation for passive radar using OFDM waveforms. We then show that the current approach using Fourier analysis across block channel estimates is equivalent to the matched filter, based on a piecewise constant assumption on the Doppler induced phase rotation in the time domain. We next present high-resolution algorithms based on the same assumption: first we implement MUSIC as a two-dimensional spectral estimator using spatial smoothing; second we use the new concept of compressed sensing to identify targets. We compare the new algorithms and the current approach using numerical simulation and experimental data recorded from a DAB network in Germany. Index Terms—Multi-static radar, radar processing, compressed sensing, sparse estimation, MUSIC, subspace algorithms. I. I NTRODUCTION A. Passive Radar: Motivation & Challenges In passive radar, illuminators of opportunity are used to detect and locate airborne targets. This is essentially the same as a bi-static radar setup, as sender and receiver are not co- located, and time difference of arrival (TDoA) measurements Manuscript submitted February 2, 2009, revised May 10, 2009, accepted October 1, 2009. This work was supported by ONR grant N00014-07-1- 0429. Parts of this work were presented at the International Conference on Information Fusion in Cologne, Germany, Jul. 2008, and at the Asilomar Conference on Signals, Systems, and Computers in Monterey, CA, Oct. 2008. Copyright c 2008 IEEE. Personal use of this material is permitted. How- ever, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org. C. Berger was with the Department of Electrical and Computer Engineering, University of Connecticut. He is now with the Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 USA (e-mail: crberger@ece.cmu.edu). B. Demissie is with the Fraunhofer Institute for Communication, Informa- tion Processing and Ergonomics FKIE, Neuenahrer Str. 20, 53343 Wachtberg, Germany (email: bruno.demissie@fkie.fraunhofer.de). J. Heckenbach is with the Fraunhofer Institute for High Frequency Physics and Radar Techniques FHR, Neuenahrer Str. 20, 53343 Wachtberg, Germany (email: joerg.heckenbach@fhr.fraunhofer.de). P. Willett, and S. Zhou are with the Department of Electrical and Computer Engineering, University of Connecticut, 371 Fairfield Way U-2157, Storrs, Connecticut 06269, USA (email: {willett, shengli}@engr.uconn.edu). Digital Object Indentifier 00.0000/JSTSP.2009.000000 -10 -5 0 5 10 -10 -8 -6 -4 -2 0 2 4 6 8 10 x-axis [km] y-axis [km] receiver illuminators target bistatic range Fig. 1. In passive radar, illuminators of opportunity are used in a multi-static setup; weak target signatures can be extracted from the dominating directly received radio/television signal based on their Doppler frequency, rendering bi-static range and range-rate information. localize targets on ellipses around the sender-receiver axis, c.f. Fig. 1 and [1]. It is the differences, though, that make passive radar attractive; i) as the illuminators are not part of the radar system, its presence is virtually undetectable; ii) illuminators of opportunity are often radio and TV stations, broadcasting in the VHF/UHF frequency bands otherwise not available to radar applications. The first point, in conjunction with the bi-static setup, makes it impossible for targets to know if they have been detected, while the operation in the radio/TV VHF/UHF frequency bands needs no frequency allocation, gives frequency diversity, and can help to detect low-flying targets beyond the horizon [2], [3]. Challenges connected to implementing a passive radar sys- tem are mostly due to using broadcast signals, which are not under control, for illumination. Therefore the transmitted signals are not known a priori, which means that a regular matched filter based receiver cannot be implemented easily. Second, although broadcast antennas are sectorized at times, since broadcast signals have to cover a broad area the transmit antennas are approximately isotropic and there is no significant transmitter gain. This can lead to constraints on the achievable range of a passive radar, if the transmit signal does not belong 0000–0000/00$00.00 c  2009 IEEE