# On the Communication Performance of Airborne Distributed Coherent Radar
## 1 Introduction  
The detection ability and anti-jamming ability of conventional monostatic radars are relatively weak. The complex electromagnetic environment of modern society necessitates greater requirements for the detection ability of radar [1]. Under the existing technical conditions, detection capability can be improved by increasing antenna aperture and signal bandwidth, but this will create problems such as large radar volume, limited mobility, high manufacturing cost, and difficult later maintenance [2]. ADCR can overcome the above shortcomings and significantly improve radar detection ability and anti-jamming ability [3,4]. It is usually composed of multiple radars with different functions and information fusion centers [5]. Compared with ground-based radar, ADCR generally offers the following advantages:  
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2. Radars with different frequency bands, power, polarization, and antenna patterns can be combined systematically to detect targets from multiple angles and in various ways. Meanwhile, coherent detection can also be realized. These advantages can improve the detection probability of the target [6]. 3. When a small number of radars are subject to inevitable electronic attack or failure, other radars can quickly form an effective detection network, which greatly improves the anti-jamming ability of the radar network [7]. 4. Compared with ground-based radar, ADCR is usually smaller, more flexible, and easier to deploy [8]. However, unlike distributed ground-based radar, the prerequisite for an ADCR radar to perform its mission is to maintain a stable communication connection [9]. This is necessary for the network to perform tasks independently or to be controlled by the ground station in real time. Meanwhile, compared with distributed ground-based radar, the ADCR communication system must be connected wirelessly, which is more unreliable thanwired communication. The factors that obviously affect the performance of airborne radar communication system mainly include the following three aspects:  
1. UAV position fluctuation: Ground-based radar is usually fixed, while ADCR is usually affected by forces such as wind, with its position fluctuating [10]. Taking the communication between UAVs as an example, communication usually depends on the directional antenna, and the mainlobe of the transmitting antenna should point to the connecting direction between the transmitter and the receiver by default. However, the fluctuation of the transmitter or the receiver will cause the receiver to deviate from the mainlobe of the transmitter, which will further cause the antenna gain of the communication link to decline and affect the communication performance. 2. Limited load of UAV: Unlike high-power ground-based radar, the load of ADCR is usually very limited [11; 12]. This mainly includes the energy carried by UAVs and their limited size. Limited energy leads to limited transmitting power and motion capability of UAVs, which will lead to limited communication coverage and motion range. Limited size often leads to performance limitations related to size and area. For example, fewer antenna elements will lead to wider mainlobe width. The limited size may also affect the performance of some precision components, such as the stability of the transmitter's power amplifier [12; 13; 14]. 3. Nature of wireless channel propagation: Since ADCR cannot rely on wired communication in the same way as ground-based radar, shadow fading, multi-path fading, and propagation loss will affect communication performance [15]. In view of the significant difference between ADCR and ground-based radar in the analysis of communication performance, it is important to build a comprehensive and accurate channel model for ADCR. At present, the performance analysis work of UAV communication system is emerging in endlessly.