# DAB Signal Preprocessing for Passive Coherent Location
## 1 Introduction  
Broadcast radio and television transmitters, e.g., Frequency Modulated (FM), Digital Audio Broadcast (DAB), and Digital Video Broadcasting-Terrestrial (DVB-T) are commonly used as illumination sources for Passive Coherent Location (PCL) [1, 2, 3] due to their excellent availability, high level of emitted power, and significant area coverage. Moreover, the broad (and time-invariant) frequency spectrum of digital broadcast signals (DAB, DVB-T) has a positive impact on the performance of PCL detection [4, 5] in terms of range, Doppler resolution [6], and predictable characteristics, when compared to analog FM transmission [7, 8]. The performance calculations and initial trials of DAB-illuminated PCL are presented in [4, 9, 10]. The results of experiments with DAB signals acquired during the NATO campaign APART-GAS 2019 (Active Passive Radar Trials Ground-based, Airborne, Seaborne) in Poland [11] showed encouraging results in detecting airplanes of different sizes flying in the range up to 20 km [12] from the moving PCL platform [13]. However, due to the time-domain properties of the DAB signal, there were periodical artifacts sometimes visible in bistatic velocity/range plots that obscure the images with detection results. Examples of such images are shown in [12] and in Figure 1 as a reference. These disturbances appeared with an integration time extended to 5 s. They were repeated on the bistatic velocity axis with a period \(v_{\mathrm{a}}\approx 14.3\) m/s that is strictly related to the signal's wavelength (\(\lambda\) = 1.37 m for the received DAB center frequency: 218.64 MHz) and the DAB transmission frame duration [14, 15] (\(T_{\mathrm{F}}\) = 96 ms)  
\[v_{\mathrm{a}}=\frac{\lambda}{T_{\mathrm{F}}}. \tag{1}\]Due to this connection with DAB signal timing, it is necessary to investigate the DAB signal structure to find its influence on the quality of PCL detection images. Several signal processing methods are proposed in the literature to reduce the ambiguity peaks in the Orthogonal Frequency Division Multiplex (OFDM) signal (mainly DVB-T) for passive radar applications. However, it must be noted that the DAB signal, despite the same modulation scheme (OFDM), has a different structure [16] due to its synchronous design. In [17], methods for reducing the ambiguity peaks in the cross-ambiguity function were investigated for the PCL system with DVB-T illumination signals. Three methods were proposed to replace the elements responsible for the ambiguity peaks with ideal 64-Quadrature Amplitude Modulation (QAM) symbols, zero power components, and finally, random segments. The results show a significant reduction of the ambiguity peaks but with some undesired reduction in detection capabilities. Since DVB-T signal is assumed, whose structure differs from DAB signal [16], some effects resulting from the DAB signal structure (i.e., repeatable symbols at the beginning of each frame and interrupted RF transmission) were not considered. In this paper, we focus mainly on these DAB-specific issues. As demonstrated in [18], the side peaks of the ambiguity function of OFDM signals are caused by the periodicity of pilots. The solution was proposed to eliminate the side peaks in the received signals to improve the performance of cognitive radio-based PCL. A theoretical analysis was provided for general OFDM signals (e.g., WiFi, WiMax, DVB-T) and confirmed by a series of simulations, but no experimental results with the real signals were presented. In [7], a multistage processing algorithm was developed for disturbance cancellation and target detection based on projections of the received signal in a subspace orthogonal to both the disturbance and previously targets. The described work focused on the use of FM radio signals, and experimental verification was performed only with FM-illuminated PCL. An efficient architecture of a coherent integration processor for DVB-T-based passive bistatic radar was proposed in [19].