# On the Communication Performance of Airborne Distributed Coherent Radar
## 1 Introduction
We divide these works into two main categories: one is to focus on the impact of antennas and beams on communication performance; the second is to focus on the impact of cascading and fading on communication performance. Antenna and beam: the impact of beam misalignment caused by fluctuating UAV position on communication performance is studied. [PERSON] et al. studied the challenges faced by UAVs when applying millimeter wave or terahertz directional transmission [16]. They pointed out that the misalignment between the transmitting and receiving antennae of the UAVs could easily lead to communication outage in many outdoor measurement experiments. [PERSON] et al. [17] focused on formation of an adaptive beam to overcome the unstable beam pointing between UAVs. They combined the feedback information of beam deviation measured by different sensors, the expected data rate and greedy geometry algorithm to propose a beam design method. [PERSON] et al. [18] analyzed the impact of beam misalignment on communication performance in non-orthogonal multiple-access downlink systems. In addition, [PERSON] et al. also conducted a lot of in-depth research on the position fluctuation of UAV. For example, in [19] they established the position fluctuation of UAV as Gaussian random variable, and modeled the channel fading with Nakagami distribution. The analytic expression of OP was derived under three different topology models. [PERSON] et al. further enriched and improved previous work in [20], and expanded the antenna model from the original uniform linear array to any planar array. On the basis of performance analysis, they also studied the optimal antenna pattern and the optimal geometric position of the relay UAV. Cascading and fading: This part mainly focuses on the cascading between different communication links, and considers the impact of different shadow fading or multi-path fading on communication performance. At present, the dual-hop link is the most studied field. For example, [PERSON] et al. conducted a lot of research on the dual-hop system in an urban environment. In [21], they proposed a communication system in which an UAV acted as a relay. [PERSON] also conducted a lot of in-depth research on the performance of the dual-hop communication system combining UAV and intelligent reflecting surface (IRS). For example, in [22], the IRS fixed on the building was used to improve the severe fading between the ground transmitting source and UAV. The OP, bit error rate and ergodic channel capacity at the destination terminal were derived. In addition, [PERSON] et al. [23] studied the performance of multi-hop communication links where UAVs acted as relays. Considering the propagation loss, shadowing, and multi-path fading of the communication system, the OP and other performances of the communication system composed of ground-to-air (G2A), air-to-air (A2A), and air-to-ground (A2G) were derived. The rationality of the proposed OP was verified by numerical simulation under different terrain conditions (such as hills, mountains, and oceans). From the above research, the following two deficiencies can be summarized:  
1. At present, there is still little research into communication performance analysis for ADCR. Since the target detection by ADCR is usually coherent, and the data acquired by radar UAVs needs to be sent (possibly through relay) to the ground information fusion center for processing. In this case, if the clocks between UAVs can keep strict synchronization, the communication signals transmitted by each radar UAV can be directly summed at the communication UAV. Therefore, the topology of ADCR is usually complex. Thus, the above existing research and conclusions are not directly applied. This will be explained carefully in the modeling of Section 2. 2. At present, few studies comprehensively consider factors such as UAV transmitting power error [24], UAV position error [19; 20; 25], multipath fading [26; 27]. Most research only considers one or several of them.