# Design of Wideband FMCW Radar Transceiver System Using Photonic Elements
## 1 Introduction  
As the integration of low-probability of intercept (LPI) technology into unmanned aerial vehicle (UAV) gradually increases with the development of relevant technologies [1; 2; 3], the demand for research to prevent the threats posed for these aircraft is growing [4], and to detect the target in advance is of paramount importance. However, the detection of LPI targets, which are small and have low radar cross-sections (RCS), is difficult when using conventional radar systems. A radar transceiver system using photonics technology has recently attracted attention as a fundamental solution to the LPI target detection issue by using the capability of wideband signal generation [5]. Microwave photonic radar technology has some advantages in low-frequency-dependent loss and wideband frequency characteristics, and this technology has been adopted for obtaining high-frequency signals for signal processing using photonics elements. In [6], a photonic-based phased array radar architecture is proposed to obtain the target image through digital beamforming. Moreover, various studies have been conducted including increase in detection range [7; 8], use of high-frequency bands for low-RCS target detection [9; 10; 11]. In addition, there have been substantial efforts to implement various systems using microwave photonics technology for high-resolution images [12; 13; 14; 15]. For example, in [14], a linearly chirped microwave generator using photonic elements was introduced for SAR systems with large-range resolution demand, such as Earth surveillance and monitoring. In [15], the use of photonic generation and transmission of linearly chirped microwave waveform was examined for modern radar systems.  
However, most of the preceding studies have remained at the level of showing the feasibility of microwave photonics by employing indoor experiments [9; 10; 11; 12; 13]. Although several researchers have conducted experiments to verify the performance of the technology in a real environment, most of their studies have used targets with large RCS such as airplanes [8]. Thus, these results are not suitable for LPI targets with low RCS, and such research has limitations in detecting and identifying a target in a real-field environment.  
In this paper, we propose a frequency modulated continuous wave (FMCW) radar transceiver using photonic elements with wideband property to obtain high-resolution images. The proposed transceiver system with photonic elements does not use various electrical elements such as mixers and filters which are used in conventional radar transceiver systems to obtain wideband characteristics with low-frequency-dependent loss [15]. To immediately apply the proposed system to a real environment, the performance of the transceiver system is observed and optimized using budget analysis and simulators. Inorder to verify the proposed system in terms of detection and inverse synthetic aperture radar (ISAR) images acquisition, the proposed system is fabricated and its performance is verified in real-field test conditions. The results demonstrate that the proposed radar transceiver with photonic elements is suitable for radar systems aiming to detect low-RCS targets and high-resolution images.