# DAB Signal Preprocessing for Passive Coherent Location
## 4 Discussion  
The PR and FIC symbols in the DAB frame constitute a deterministic part of the signal that results in periodic peaks in the autocorrelation function of the received signal. Another source of the ambiguity peaks is the presence of guard intervals, filled with the cyclic prefix, which is common in all systems with OFDM modulation. These unwanted peaks in the autocorrelation function can be removed by clearing certain parts of the received signal or filling them with noise. However, they correspond to relatively long distances in the range domain (150 km for the symbol duration, and 14,400 km for the frame duration \(T_{\text{F}}\)) [17] that lie far beyond the operating range of the considered passive radar [12] and thus have no visible influence on the plots with detection results. The main problem of the considered system was the presence of periodical artifacts in the bistatic velocity/range plane with a long integration time (5 s) applied to extend resolution in the bistatic velocity. Such artifacts can cause false detections, obscure echoes from objects moving at specific velocities, and disturb tracking. In some situations, these artifacts can even be more intensive than the weak signals from the targets, as shown in Figures 1 and 21, with different scales and target echo marked by a rectangle. Figure 21: Bistatic velocity/range plot with artifacts and weak target (close-up). Figure 20: PCL detection results after MSC symbol duplication in place of Null symbol: (**a**) bistatic velocity/range plane; (**b**) its cross-section at \(R=450\) m.  
The experimental results described in this paper show that clearing PR and FIC symbols in the DAB frame (see Figure 6), despite correcting the autocorrelation function, does not remove the mentioned periodical artifacts, but quite the opposite, it makes them even more intensive. The second method (replacing the Null, PR, and FIC symbols with a noise) not only does not remove the artifacts but also raises the noise floor in the bistatic velocity/range plane, which makes the detection of objects difficult, or even impossible, as demonstrated in Figures 16 and 17. Finally, the periodical artifacts in the bistatic velocity/range plane could be removed by copying part of the previous DAB frame (i.e., the selected MSC symbols) in place of the Null, PR, and FIC symbols in the current frame (see Figure 9), synchronously in both the reference and the surveillance signals. Although an additional peak in the autocorrelation function is introduced in this way (see Figure 10), its position can be shifted beyond the operating range of the PCL radar by an appropriate selection of the copied MSC symbols (8). The example plots with moving object's detection results after such a modification in the received signal are shown in Figures 18 and 19, and in animations S2, S4 provided in the Supplementary Materials. The animations S1 and S3 are available as a reference and show the initial detection results without modifications in the received signals. To evaluate the performance of this method, the Signal to Noise Ratio (SNR) was estimated for the object detected in the processed signal that was shown in Figure 19. The noise level was averaged over the bistatic velocity/range plane (excluding the clutter zone at \(v\approx 0\)), and the signal level was a peak value of the target area bounded with the red rectangles in Figure 19. The comparison of SNR values obtained this way with the raw signals (i.e., without any modifications) and with the method described in Section 3.3 is depicted in Figure 22, showing an average gain of 3.6 dB.