Loaded Reciprocal Filter for OFDM-based Passive Radar Signal Processing

The Range-Doppler maps of Passive Bistatic Radar (PBR) are often evaluated by means of the suboptimal batches algorithm. For OFDM waveforms of opportunity, a typical choice is to fragment the signal in batches length equal to the OFDM symbol length. This is shown to provide Signal-to-Noise Ratio (SNR) losses for both Matched Filter (MF) and Reciprocal Filter (RF), especially for long target ranges. We investigate the case of “non-OFDM fragmentation”, with special attention to the case of batch length much longer than a single OFDM symbol that can potentially reduce the losses at long ranges. While this is effective for the MF, we show that the RF filter in this case still provides high Peak-to-Sidelobes Ratio (PSLR) values but at the expense of significant SNR losses. Therefore, we propose the Loaded Reciprocal Filter (LRF) as a modified version of the RF for the non-OFDM fragmentation case, which is shown to provide a trade-off between SNR losses and PSLR. Finally, a simulated case study is presented for a DVB-T-based PBR, which clearly shows the effectiveness of the proposed LRF scheme when using the non-OFDM fragmentation.

I. Introduction

While the concept of passive radar, also known as passive bistatic radar (PBR), dates back to the early days of radar in 1935, in the last decades it has received a renewed interest from the research community. In fact, the availability of largely increased computational capabilities at low cost compared to the past, has made possible the real-time implementation of very long coherent integration times (CIT) and effective adaptive signal processing techniques able to provide sensibly improved performance. This allowed to capitalize on the well-known advantages of PBR over conventional radar, among which the exploitation of already existing illuminators of opportunity, covert operation, low environmental impact, and reduced costs. Among the available and commonly considered illuminators of opportunity, the digital transmitters of DVB-T television, DAB radio, LTE mobile communications and Wi-Fi represent an attractive choice for their wide availability [1]–[4]. In particular, most of the modern digital signals, including all the above-mentioned ones, are based on the OFDM scheme. Therefore, the peculiarities of OFDM-based PBR are of a special interest and their specific issues are considered in this paper.