# On the Communication Performance of Airborne Distributed Coherent Radar
## 2 Materials and Methods
### System Model
#### 2.1.1 The Overall Topology of the ADCR Communication System  
Figure 1 shows the topology of the ADCR communication system. As shown in the figure, the communication system consists of several radar UAVs, a communication UAV, and a ground base station. Each radar UAV sends electromagnetic waves to detect the target and receives a target echo. The target echo is represented by a brown arrow in Figure 1. At this time, each radar UAV needs to transmit the echo information to the ground base station. However, since the terrain on the ground is too complex and the radar UAV is far from the ground base station, at least one communication UAV is required to act as a relay. To simplify the analysis, we only set up one communication UAV. In general, the signal received by the communication UAV from each radar UAV may be the echo signal, or it may be the result of the preliminary signal processing by the radar UAV (such as the target range, velocity, etc.). In this paper, the radar UAV transmits the coherent target echo signal to the communication UAV. The communication UAV adopts the decode-and-forward (DF) mode for the information transmitted by each radar UAV, and forwards the information to the ground base station. The communication link between the radar UAV and the communication UAV is named communication link 1, and the communication link between the communication UAV and the ground base station is named communication link 2. It should be emphasized that we assume that each radar UAV performs a coherent detection task for the target, and each radar UAV is of the ability to perform coherent processing on the echo signal. At this time, the echoes obtained by each radar can be directly summed at the communication UAV. That is to achieve coherent accumulation of echoes and provide a great gain for the final target detection at the base station. Since each  
Figure 1: Schematic diagram of ADCR communication system. radar UAV transmits coherent radar echoes to the communication UAV, if the clocks of all UAVs are synchronized, these communication signals can also be directly summed at the communication UAV. For the convenience of subsequent representation, we define the following variables: the number of radar UAVs is \(N_{radar}\), the corresponding transmitting power of \(i\)-th radar UAV is \(P_{r}^{i}\) (where \(i=1,2,\cdots,N_{radar}\)), and the power gain of transmitting antenna is \(G_{r}^{i}\). The transmitting power of the communication UAV is \(P_{c}\), and the power gain of the receiving antenna is \(G_{r}\). For the communication link between the \(i\)-th radar UAV and the communication UAV, its corresponding fading is \(F_{1}^{i}\) and the distance is \(L_{1}^{i}\). For the communication link between the communication UAV and the base station, its corresponding fading is \(F_{2}\) and the distance is \(L_{2}\). The noise variance introduced by the receiver of the communication UAV is \(N_{c}\), and the noise variance introduced by the receiver of the base station is \(N_{b}\). In order to simplify the problem, we assume that the propagation coefficients of all paths are \(\alpha\). We assume that for each radar UAVs, \(G_{r}\) is an independent random variable. Meanwhile, since the radar UAV is usually far away from the communication UAV, we believe that there is \(L_{1}\approx L_{1}^{i}\), that is, the distance between each radar UAV and the communication UAV is roughly the same. Since the receiving antenna of the base station is usually large in size and the effective receiving range of angle arrival is often wide, the impact of the position error of the communication UAV on the communication performance can be ignored. Based on the above definition, it is necessary to determine the receiving signal model at the communication UAV and base station under the DF mechanism.