# Photonic Systems and Devices for Linear Cell Radar
## 5 Possible Interference in Linear Cell Radars [32]  
Spectrum congestion in millimeter-wave bands are not yet severe; however, frequency bands are allocated to various radio services. In addition to remote sensing and radio astronomy, high-speed wireless data transmission technologies using 90 GHz bands have been actively developed for connecting high-speed trains and remote access units (RAUs). This implies that radio interference at 90 GHz would be an issue in the near future. In the linear cell radar system, comprehensive RAU control would be useful to suppress interference between the RAUs. The beam direction of each RAU could be controlled by a central control unit to avoid such interference. All the RAUs could be  
Figure 11: Field trial at Narita International Airport [6]. Figure 12: Linear cell radar image [6]. connected to the central control unit through optical fiber links. However, strong interference would occur, by strong radio-wave reflection from a smooth surface on an aircraft. Figure 13 shows an interference model in a liner cell radar system to calculate the ratio of a desired signal from an FOD and an undesired signal from another RAU located at the opposite side of the runway. The beam cross-section was assumed to be Gaussian with a 1-degree FWHM (full width half maximum). The radar cross-section of the FOD was assumed to be \(-20\) dBsm. Figure 14 shows the ratio between the desired and undesired signals (D/U ratio). The possibility of interference would be roughly estimated by this calculated D/U ratio. If the beam angle difference was smaller than 1 degree, the D/U ratio would be smaller than 0 dB. Thus, the angle difference between the RAUs should be larger than 1 degree to suppress the interference described in Figure 13.  
Here, we discuss the possibility of interference due to stray beams generated by reflection on aircraft body surfaces. The angles and locations of such surfaces should be st  
Figure 14: Desired and undesired signal (D/U) ratio [32]. Figure 13: Interference model [32]. they depend on the angles and locations of the aircrafts. For simplicity, their distributions are assumed to be uniform. An interfering beam from a surface illuminates a RAU with a particular azimuth angle illumination. The probability that the azimuth angle difference is smaller than 1 degree would be 1/360. The probability that aircrafts reflect the beams would be 0.05, when the time interval of landings and take-offs is 90 s. The average aircraft length and speed are respectively assumed to be 60 m and 50 km/h. If the antenna rotation period is 4 seconds, the false alarm rate per day would be approximately 3.2. The false rate would be 0.08, by taking into account the factors of shadowing and misalignment of elevation angle, which are assumed to be 1/4 and 1/10, respectively. A hybrid sensing system consisting of high-resolution cameras and radars can be used to reduce the false alarm rate. The direction of cameras would be controlled by information obtained by radars. In Reference [33], an interference mitigation technique was reported, where false images were suppressed by using various frequency sweep speeds for neighboring RAUs. Conceptualization and writing T.K. ; device analysis T.U. ; system analysis A.K. and P.T.D. ; project administration, N.Y. The research work reviewed in this paper was partially supported by the Japanese Government funding for \"R&D to Expand Radio Frequency Resources\" from the Ministry of Internal Affairs and Communications, and by JSPS KAKENHI Grant Number JP18H01454. The authors would like to thank [PERSON], [PERSON], and [PERSON] of Hitachi Kokusai Electric Inc., Japan, [PERSON] of Electronic Navigation Research Institute, Japan, and [PERSON] and [PERSON] of the National Institute of Information and Communications Technology, Japan, for their collaboration with the experiments and their fruitful discussions. The linear cell radar field trial at NRT was supported by the Narita International Airport Cooperation (NAA), Japan. The authors declare no conflict of interest.