# Co-Channel Interference Suppression for LTE Passive Radar Based on Spatial Feature Cognition
###### Abstract  
Passive radars based on long-term evolution (LTE) signals suffer from sever interferences. The interferences are not only from the base station used as the illuminator of opportunity (BS-IoO), but also from the other co-channel base stations (CCBS) working at the same frequency with the BS-IoO. Because the reference signals of the co-channel interferences are difficult to obtain, cancellation performance degrades seriously when traditional interference suppression methods are applied in LTE-based passive radar. This paper proposes a cascaded cancellation method based on the spatial spectrum cognition of interference. It consists of several cancellation loops. In each loop, the spatial spectrum of strong interferences is first recognized by using the cyclostationary characteristic of LTE signal and the compressed sensing technique. A clean reference signal of each interference is then reconstructed according to the spatial spectrum previously obtained. With the reference signal, the interferences are cancelled. At the end of each loop, the energy of the interference residual is estimated. If the interference residual is still strong, then the cancellation loop continues; otherwise it terminates. The proposed method can get good cancellation performance with a small-sized antenna array. Theoretical and simulation results demonstrate the effectiveness of the proposed method. passive radar; long term evolution (LTE) signals; co-channel interference suppression; spatial feature cognition 202222110.3390/s2201017  
10 October 2021 20 December 2  
cancellation should first be performed to suppress the strong interference and increase the target signal to interference ratio. Many time-domain and spatial filtering algorithms have been developed to suppress these interference signals, such as the least mean square (LMS) [2; 12], extensive cancellation algorithm (ECA) [13], and subcarrier domain algorithm [14; 15]. These methods usually first collect the direct signal by using a narrow-beam antenna directed to the transmitter and then estimate the amplitude and delay of each interference signal. Finally, the interference signals are subtracted from the surveillance signals with its estimated parameters. In traditional passive radars, such as the passive radars based on FM and analog TV signals, the transmission adopts the frequency division multiple access (FDMA) technique and transmitters working on the same frequency are far away from each other. Therefore, the interference signals mainly include the DMI from the transmitter that is used as the IoO (T-IoO) [16]. The co-channel interference is usually ignored. However, the interference signals in LTE passive radar not only include the DMI from the base station used as the IoO (BS-IoO), but also the DMI from the co-channel base stations (CCBS) that work at the same frequency channel as the BS-IoO [10; 17]. This is because the base stations in 4G-LTE mobile communication systems are densely distributed and some of them share the same frequency band owing to the frequency re-use scheme in 4G-LTE. We name the DMI from the CCBS as the co-channel interference hereafter. In passive radar, the target echo is quite weak and the co-channel signals can be far stronger than the target echo, which causes severe interference on the target detection [18; 19]. The co-channel interference also arises in DTV passive radar, where the interference has the same waveform as the DMI from the T-IoO [20]. The traditional cancellation methods have been customized to suppress both the DMI and the co-channel interference in DTV passive radar. However, the co-channel interference in LTE passive radar usually has different waveforms with the DMI of the BS-IoO, therefore the traditional cancellation methods cannot be used to suppress this interference, and a new method has to be developed. In order to suppress the co-channel interference, the reference signal of each interference source should be obtained.