Stephen Searle

DVB-T Passive Radar Signal Processing

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.

A fast reconstruction algorithm for deterministic compressive sensing using second order reed-muller codes

This paper proposes a deterministic compressed sensing matrix that comes by design with a very fast reconstruction algorithm, in the sense that its complexity depends only on the number of measurements n and not on the signal dimension N. The matrix construction is based on the second order Reed- Muller codes and associated functions. This matrix does not have RIP uniformly with respect to all k-sparse vectors, but it acts as a near isometry on k-sparse vectors with very high probability.

Evaluation of adaptive filter algorithms for clutter cancellation in Passive Bistatic Radar

Passive Bistatic Radar exploits the illumination of a scene by a local communications transmitter in order to perform radar processing without dedicated transmitter hardware. The direct transmission and strong stationary clutter are often present in the surveillance signal, reducing dynamic range and masking returns from targets. In this study various adaptive filters are used to estimate the direct path and clutter components and cancel them from the signal. Performance metrics particular to radar processing are defined, and the investigated filters are evaluated by application to Passive Bistatic Radar with real DVB-T data.

Remodulation of DVB—T signals for use in Passive Bistatic Radar

Passive Bistatic Radar employs a local communications transmitter as an illuminator. A template for matched-filtering is obtained by steering a beam at the transmission source. However this signal is subject to sensor noise and may also contain delayed attenuated copies of the transmitted signal. Demodulation and remodulation of the signal provides a clean template, however this also removes transmitter effects, resulting in a less than ideal mismatched filter. Estimation of the carrier drift allows transmitter effects to be re-introduced to the template signal and results in ambiguity surfaces of improved quality.

Evaluation of the ambiguity function for passive radar with OFDM transmissions

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.

Enhanced CDMA communications using compressed-sensing reconstruction methods

We propose a simple method for downlink communications based on second order Reed-Muller sequences which generalize the Walsh sequences that are used in orthogonal CDMA. In our approach, coding occurs at the chip level (i.e. we use a spreading factor of 1) and different users are not orthogonalized. Our decoding algorithm is borrowed from work on fast reconstruction of signals for compressed-sensing. This algorithm allows for low-complexity multiuser detection.

A novel polyphase code for sidelobe suppression

A binary code sequence attained by modification of a Golay code is proposed. This code has sidelobes which are out of phase with the main lobe, a property which facilitates their removal. Unknown delay-dependent phase terms proclude the multiplexing of a Golay pair in frequency. However the modified Golay code is found to have an autocorrelation whose square is complementary with that of the codes original Golay partner. This enables complementary behavior to be achieved when the modified code is multiplexed at equal offsets both above and below the partner. Applications include sidelobe removal in radar returns.

Signal processing considerations for passive radar with a single receiver

Processing of passive radar signals is hindered by the presence of clutter and sensor noise in the reference signal, and by strong direct and zero-Doppler clutter returns in the surveillance signal. Previous research has addressed methods of removing or mitigating unwanted components in either signal. However what is considered as interference in one signal may be considered as information in the other. In this study we investigate under what conditions both surveillance and reference information may be extracted from a single signal, thus enabling passive radar processing with a single receiver.

The use of complementary sets in MIMO radar

In multstatic radar it is desirable to transmit waveforms which are completely mutually orthogonal in order to minimise cross-transmitter interference. Perfect orthogonality is impossible to achieve, and in reality adaptive processing is necessary to remove interference, with some mismatch loss in main lobe size. This study proposes a method of composing waveforms from mutually-orthogonal complementary sets. Each transmitter is assigned a set of complementary waveforms to be separated in frequency. Nonlinear processing of matched filter outputs can completely remove cross-transmitter interference and suppress autocorrelation sidelobes without cost. Cross terms introduced by the nonlinear operation are handled via frequency agility and Doppler processing. The principle is demonstrated by simulation.

Doppler processing of square-complementary coded radar signals

Sidelobes in radar filter output may be avoided by using a set of modified Golay-coded pulses separated in frequency, and performing a nonlinear filtering operation on the radar returns. However the nonlinear operation introduces cross-terms which hamper detection in clutter. This study considers the Doppler processing of a train of frequency-separated square-complementary pulses. It is shown that the cross-terms have a dependence on offset frequency, and that the manifestation of cross-terms in Doppler can be controlled by slowly varying the offset frequency with each pulse. Simulations demonstrate the ability of this method to expose weak returns masked by strong clutter.