# Co-Channel Interference Suppression for LTE Passive Radar Based on Spatial Feature Cognition
## 3 Algorithm Description  
Traditional passive radar interference suppression methods include the time-domain and spatial-domain cancellation methods. To achieve the best effect of interference suppression, joint space-time cascaded processing is usually adopted [21; 22; 23; 24]. According to the order of the time domain processing and spatial domain processing, the joint space-time cascaded processing can be divided into spatial-time processing [21; 22] and time-spatial processing [23; 24]. The processing flow charts of the two methods are shown in Figure 2.  
The spatial-time processing first performs spatial filtering to suppress the interference signals and then conducts the time-domain adaptive filtering to further suppress the DMI of the BS-IoO. In contrast, the time-spatial processing first conducts time-domain adaptive filtering to cancel the DMI of the BS-IoO and then performs spatial filtering to further suppress the interference residual. The traditional methods above only consider the suppression of DMI from the BS-IoO but never consider the suppression of co-channel interferences. That is, the time-domain cancellation in their processing flow charts only uses the reference signal of the BS-IoO to conduct the cancellation. It cannot suppress the co-channel interference signals, as the co-channel interference signals have different waveforms from the reference signal of the BS-IoO. The spatial filtering not designated to cancel the co-channel interference may suppress the co-channel interference to some extent. However, the CCBSs in LTE communication systems are densely distributed and the number of interference signal sources are usually very large. In order to get adequate suppression for traditional spatial filtering, the number of antenna array elements required must be larger than that of the interference signal sources, which will lead to over-large antenna size. Figure 2: Flow charts of the (**a**) spatial-time and (**b**) time-spatial processing. In the following, we propose a cascaded cancellation method based on the spatial feature cognition of the interference signals, which can get adequate cancellation performance with a small sized antenna array. The processing flow chart of the proposed method is shown in Figure 3.  
It can be seen from the figure that the proposed algorithm consists of several cancellation loops. Each loop mainly includes the following processing steps:  
(1) DOA and energy estimation of the strong interference:  
In practice, the DOA of the BS-IoO is usually known a priori. However, it is not easy to know the positions of all the CCBSs. Therefore, we propose to estimate the DOAs of the interference signals from the CCBSs by processing the received surveillance signal. We utilize the cyclostationary characteristics of LTE signals and the compressed sensing technique to get a good DOA estimation performance with only a small sized antenna array. In this step, we also estimate the energy of the interference signals. (2) Beamforming in the direction of strong interference:  
With the DOA of the strong interference signals, we use the beamforming technique based on the worst-case performance optimization [25] to generate a beam directing to each strong interference. This beamforming technique can generate a beam directing to the desired interference direction; it can also suppress the interference signals from the other directions, so as to maximize the signal-to-noise ratio. At this step, we can get the direct signal of each interference base station. (3) Purification of the direct signal based on demodulation and reconstruction:  
The direct signal of each interference base station obtained by using the beamforming technique above can inevitably be contaminated by the other interference residual. Here, we demodulate the direct signal to get the signal bits and then reconstruct the direct signal according to the LTE protocol [26] to get the clean direct signal. At this step, the clean direct  
Figure 3: The proposed co-channel interference cascaded cancellation algorithm.