# Co-Channel Interference Suppression for LTE Passive Radar Based on Spatial Feature Cognition
## 4 Simulation Analysis  
In this section, we test the proposed method through simulations. The simulation scene is set as shown in Figure 8. It is known that in urban and suburban areas, the coverage of one LTE base station is approximately 0.3 to 1 km, so we set the distance between the BS-IoO and the radar receiver to 1 km. Six CCBSs are randomly distributed around the BS-IoO, and they are 5 km away from the BS-IoO. We simulate one target, which is 1.1 km away from the BS-IoO, and the DOA of the target is \(0^{\circ}\). It can be seen from the simulation scene that the surveillance signal received by the radar receiver consists of the DMI from the BS-IoO and those from the CCBSs and the target echo. In this simulation, we set 15 multipath signals for the BS-IoO and 10 multipath signals originating from each CCBS. The DOA of the BS-IoO is \(-45^{\circ}\) and those of the CCBSs are \(-30^{\circ},20^{\circ},70^{\circ},-66^{\circ},-20^{\circ}\), and \(10^{\circ}\), respectively. The energy of the target echo is set to 70 dB smaller than the co-channel interference. Such a weak target corresponds to an unmanned aerial vehicle (UAV) of which the typical radar cross section (RCS) is only 0.01 m\({}^{2}\). In the following, we set the number of the antenna array elements at 16. The spacing between two adjacent elements is half the wavelength. Because the location of the BS-IoO is known prior, we first form a beam in the direction of the BS-IoO to receive the direct signal and purify it by using the demodulation and reconstruction technique. We then use the direct signal extracted as the reference signal and cross correlate it with the surveillance signal received by one of the antenna array elements. The result is shown in Figure 9. It can be seen from Figure 9 that there are many peaks in the zero-Doppler slice. This is because of the DMI from the BS-IoO. Target echo is masked below the sidelobes of these peaks. Figure 8: Simulation scenario. Figure 9: The cross-correlation results of the surveillance signal. In the following, we use the traditional spatial-time interference suppression method and time-spatial method to suppress the DMI and co-channel interference. The range Doppler cross-correlation result between the direct signal from the BS-IoO and the surveillance signal after interference suppression is shown in Figure 10. It can be seen that the DMI is suppressed, as there is no apparent peak in the zero-Doppler slice. However, the target still cannot be detected. This is because the co-channel interference is not suppressed adequately by the traditional method. In the following, we use the proposed cascaded cancellation method shown in Figure 3 to suppress the interference signal. According to the processing procedure shown in Figure 3, the spatial spectrum of the surveillance signal is first constructed. The result is shown in Figure 11. It can be seen from Figure 11a that there is an apparent peak at the DOA of \(45^{\circ}\). (It should be noted that the signal amplitude in Figure 11a is that after energy normalization.) This corresponds to the direct signal of the BS-IoO, which has the biggest SNR. The results show that the proposed method can accurately estimate the DOA of the strongest interference signal, although the number of the interference signals is far greater than that of the array elements. According to the RIP criteria used in the sparse reconstruction, the number of interference signals of which the DOAs can be accurately estimated is 4, given a 16-element antenna array and a \(16\times 180\) dimensional measurement matrix. In this simulation, we extract two peaks from Figure 11 with the largest amplitudes to estimate the DOAs of the two strongest interference signals. The other peaks may correspond to noise and may not indicate the DOA of a specific interference signal. The estimated DOAs of the two strongest interference signals are \(-45\) and \(1\) degrees, respectively. We then form two beams to obtain the two interference signals.