# DAB Signal Preprocessing for Passive Coherent Location
## 1 Introduction
The proposed architecture consists of a frequency-domain pulse compression module and a decimated keystone transform module based on the chirp \(z\)-transform to compensate for the range migration effect caused by high-speed targets. The effectiveness and validity of the proposed design have been verified through real-data experiments. A modular and reconfigurable passive radar architecture was also shown in [20] based on a software-defined approach. Thanks to efficient implementation in a multi-core machine, the system was able to perform real-time processing of one or two surveillance channels and one reference channel. The described signal processing pipeline consisted of beamforming, adaptive filtering, cross-ambiguity function calculation, adaptive detection, and plot extraction. In [21], a signal pre-processing method based on  
Figure 1: Bistatic velocity/range plot with artifacts (**a**) and its cross-section for \(R=450\) m (**b**). a generalized multi-channel adaptive filter was developed to suppress the sea clutter in a passive radar. The effectiveness of the proposed approach was verified using real-data results with the illumination from a digital television broadcast transmitter. A multi-stage procedure designed to remove clutter and strong echoes from moving targets was shown in [22]. The proposed solution was intended to improve the dynamic range in PCL and noise radars and thus facilitate the detection of weak targets. However, it was verified using only simulations. A space-time Constant Modulus Algorithm (CMA) for multipath removal on the reference signal in PCL was proposed in [23]. The adaptive equalization of the reference signal based on an array of antennas and multiple receiving channels was shown in simulations to recover the performance loss caused by multipath contribution. In [24], the reconstruction of the reference OFDM signal by demodulation and remodulation was presented to produce the perfect copy of the transmitted signal and thus to remove the influence of multipath propagation. Additionally, digital signal equalization was proposed to remove the spurious peaks in the ambiguity function caused by the presence of pilot sequences. The DVB-T signal's cross-ambiguity function was also analyzed, and several methods of its improvement for passive radar were proposed in [25; 26; 27; 28; 6]. A preprocessing technique designed to improve the range resolution of PCL and to reduce the unwanted side peaks in the ambiguity function by exploiting multiple signals (i.e., several broadcast channels) was proposed in [6; 30]. However, since the DVB-T signal was considered, the effects resulting from the specific structure of the DAB signal were not taken into account, and the obtained results cannot be directly translated to DAB-illuminated PCL. A target detection method by correlating a surveillance signal with a reconstructed reference signal in DAB-based PCL was introduced and validated in [8]. It was experimentally confirmed that reference signal reconstruction could be effectively employed for direct signal interference suppression and PCL detection algorithms; however, the integration time was only set to 500 ms, which limits the Doppler resolution. The preprocessing method for DAB signal in PCL application was described in [31]. However, the presented results were limited to only eliminating the periodic ambiguity peaks in the signal's autocorrelation function, with no investigation into the influence on the PCL detection results. In this paper, we continue the work presented in [31]. We begin with a short review of the essential properties of the DAB signal. Then we present four preprocessing methods that improve the correlation properties of the DAB signal. Finally, we investigate their influence on the quality of bistatic velocity/range plots with PCL detection results obtained from real signals acquired in the field. We show that two of the presented methods eliminate the periodical artifacts from the bistatic velocity/range plots reported in [12].