# Efficient Multipath Clutter Cancellation for UAV Monitoring Using DAB Satellite-Based PBR
## 4 Performance Analysis
### Suppression Capability
Equation (33) reflects the attenuation degree of target echoes and clutters after processing, in which the approximate form under extreme cases should be taken into consideration for the parameter selection:  
1. For fast-moving targets, \(SF(f_{d})\approx|\chi_{0}(f_{d})|\), as \(f_{d}\) is located in the filter's stopband and \(H(f_{d})\approx 0\). Thus, the batch segmentation needs to be thorough enough to ensure the reference signal's Doppler tolerance range. Unfortunately, the flatness of \(|\chi_{0}(f_{d})|\) near the origin makes the corresponding SNR improvement reduce sharply with larger \(B\), while the total computation burden and temporary memory usage still maintain a fixed increase factor. In order to avoid unnecessary overhead, the targets' Doppler range and allowed SNR loss should be designed first, with which the up-limited batch size can be determined based on the ambiguity function of the reference signal. 2. For slow-moving targets/clutters, \(SF(f_{d})\approx|1-H(f_{d})|\), as \(|\chi_{0}(f_{d})|\approx 1\). Since the clutter components need to be covered by the filter's passband to avoid the masking effect mentioned above, target echoes around zero Doppler are suppressed to the same extent. A suitable notch width is crucial to preserving the detectability of low-speed targets. The filter weights can be designed based on the estimated suppression range for applications with stable clutter characteristics. While for time-varying scenarios, the passband can be adjusted in real-time according to the residual clutter power. Compared with ECA-B, the Doppler ambiguity caused by segmentation is well prevented for ECA-C/LF, as the unambiguous range is much broader than the tolerance range if the allowed SNR loss caused by \(|\chi_{0}(f_{d})|\) is limited. Moreover, ECA-C/LF can form a steeper notch edge, which is of great value for improving the detectability of low-speed targets. To better show these features, the suppression factor (SF) of a 3 MHz bandwidth DAB signal along the Doppler domain is compared between different ECA versions at a sampling frequency of 3.25 MHz. The ECA parameters are \(N=1.625\times 10^{6}\), \(K=1\), while the batch numbers of ECA-B, ECA-S, and ECA-C/LF are 10, 50, and 1000, respectively. The length of the sliding window used by ECA-S is \(N_{\mathrm{s}}=3.25\times 10^{5}\). A 999-order symmetric lowpass filter with a 3 dB bandwidth of 8.8 Hz designed via the LS method is utilized by ECA-C/LF. As shown in Figure 4, since the total sampling time is \(N/f_{s}=0.5\,\mathrm{s}\), the orthogonal frequencies are an integral multiple of 2 Hz, where ECA-CF and ECA-LF have the same performance. In contrast, for other frequencies within the filter's 3 dB bandwidth, the performance of ECA-CF deteriorates obviously with a maximum value of 47 dB at \(f_{d}=1\,\mathrm{Hz}\) and the area where SF is better than \(-20\) dB is reduced from \([-3.6\,\mathrm{Hz},3.6\,\mathrm{Hz}]\) to \([-2.6\,\mathrm{Hz},2.6\,\mathrm{Hz}]\). It can also be found that the difference between ECA-CF and ECA-LF is negligible outside the filter's passband, and the sharp edge is well preserved. However, the SF provided by ECA-CF for moving clutters with \(|f_{d}|<1.75\,\mathrm{Hz}\) is worse than ECA-B and ECA-S, which might influence the robustness and must be counted in practice to determine the applicability of ECA-CF. As mentioned above, the process lag of ECA-LF can be reduced with asymmetric weights, for which the performance can be verified by a filter obtained with the same LS setting except for a reduced group delay of 100. Notice that it is precisely the extra delay caused by ECA-S when the sliding step is 1. The result shows that the asymmetric filter decreases the notch width a little while effectively retaining the sharp edge and that the SF deterioration is much weaker than that in ECA-CF. If \(N_{\mathrm{s}}\) can be divided by the step size, ECA-S can be approximated with ECA-LF using constant filter weights, for which the bandwidth can only be adjusted by the filter's order.