# Co-Channel Interference Suppression for LTE Passive Radar Based on Spatial Feature Cognition
## 2 Signal Model  
Consider the LTE passive radar shown in Figure 1. A linear antenna array with M elements is exploited. The spacing interval between the adjacent elements is one half of the wavelength. For this antenna array, the range of the unambiguous angle measurements is from \(-90^{\circ}\) to \(90^{\circ}\). The received signal of each antenna array element can be represented as:  
\[s_{\text{m}}[n]=\text{t}_{\text{tar}_{\text{m}}}[n]+z_{m}[n]+I_{m}[n]\text{ }n=1,\ldots,Nm=1,\ldots M \tag{1}\]  
where  
\[\text{t}_{\text{tar}_{\text{m}}}[n]=d_{1}[n-\tau_{ar}]e^{j\pi m\sin(\theta_{ tar})} \tag{2}\]  
\[I_{m}[n]=\sum_{s=1}^{N_{s}}\sum_{i=1}^{N_{s}^{s}}a_{s_{i}}d_{s}[n-\tau_{s_{i}}, i]e^{j\pi m\sin(\theta_{s_{i}})}\text{ }n=1,\ldots,Nm=1,\ldots M \tag{3}\]  
where \(\text{t}_{\text{tar}_{\text{m}}}[n]\) is the target echo; \(I_{m}[n]\) is the co-channel interference signal from the CCBSs; \(z_{\text{m}}[n]\) is thermal noise; \(N\) is the total number of the signal samples; \(d_{1}[n]\) is the reference signal of the BS-IoO; \(\tau_{ar}\) is the delay of a specific target; \(\theta_{tar}\) is the target's direction of arrival (DOA); \(d_{s}[n]\) is the direct signal from the \(s\)-th CCBS, where we assume the base station 1 is the BS-IoO and 2-Ns are the CCBSs; \(\tau_{s_{i}}\), \(i\), \(s_{i,\,jm}\) and \(\theta_{s_{i}}\) are the delay, complex amplitude, and DOA of the \(i\)-th multipath signal from the s-th CCBS; \(N_{l}^{s}\) is the number of the multipath signals from the \(s\)-th CCBS; and \(N_{s}\) is the number of the CCBSs within the field of surveillance.  
As can be seen from Equation (2), interference signals in LTE passive radar not only include the DMI from the BS-IoO, but also the co-channel interference signals from the CCBSs. This is different from traditional passive radars such as the FM and analog TV passive radars, where only the DMI from the BS-IoO is considered. Usually, the waveforms of the co-channel interference signals from different CCBSs are different, and they are different from that of the direct signal of the BS-IoO. Therefore, it is not possible to suppress the co-channel interference signals with the direct signal of the BS-IoO. The direct signal of each interference should be collected. A straightforward way to collect the direct signal of an interference base station is by forming a narrow beam to the direction of the transmitter. In this circumstance, the direction of arrival of the interference base station should be  
Figure 1: Schematic diagram of radar target detection structure for LTE passive radar.  
known. In the following, we will introduce a spatial spectrum cognition method to estimate the spatial distribution of the interference signal sources. We then use the beamforming technique to form a narrow beam to the direction of the interference base station and get the direct signal. A clean version of the direct signal is further reconstructed with the signal bits demodulated from the received signal. With the direct signal, each interference signal is cancelled and the remaining interference signal to noise ratio is estimated through a cyclic autocorrelation method to assess the inference level. If the INR is smaller than a predefined threshold, then the cancellation terminates. Otherwise the cancellation procedure repeats.