## 1 Introduction
Based on such assumptions, \(UAV_{L}\) can overhear information successfully from the suspicious UAVs only when the received signal-to-noise ratio (SNR) (and accordingly the achievable data rate) at \(UAV_{L}\) is no smaller than that at \(UAV_{SR}\), since in this case \(UAV_{L}\) can decode the data that can be decoded at \(UAV_{SR}\)[10]. Let \(R_{L}\) and \(R_{S}\) denote the achievable data rate of the legitimate eavesdropping link form \(UAV_{ST}\) to \(UAV_{L}\) and the communication rate of the suspicious link form \(UAV_{ST}\) to \(UAV_{SR}\), respectively. Then, \(UAV_{L}\) can decode transmitted signal correctly (with arbitrarily small error) if, and only if, \(R_{L}\) is no smaller than \(R_{S}\). We define the eavesdropping rate \(R_{E}\) as the suspicious data rate that \(UAV_{L}\) can successfully decode, which is given as \(R_{E}=R_{S}\) if \(R_{L}\geq R_{S}\), and \(R_{E}=0\) if \(R_{L}<R_{S}\). \(UAV_{ST}\) and \(UAV_{SR}\) are assumed to fly following a collision-free formation, where they keep a prescribed relative distance and angle. There are four cases for \(UAV_{L}\) to successfully eavesdrop suspicious communication link. Case 1, as shown in Figure 2a, \(UAV_{L}\) eavesdrops suspicious \(UAV_{ST}\) by sending jamming signals to \(UAV_{SR}\). In this case, \(UAV_{ST}\) increases transmission power in order to sustain \(R_{S}\) at its original level, thus increasing \(R_{L}\) inevitably in the eavesdropping link. When \(R_{L}\) is no smaller than \(R_{S}\), \(UAV_{L}\) is able to decode the whole information that can be decoded at \(UAV_{SR}\) to fulfill eavesdropping missions. Case 2, as shown in Figure 2b, \(UAV_{L}\) eavesdrops suspicious \(UAV_{ST}\) by sending jamming signals to \(UAV_{SR}\). Take the time-division-duplex (TDD) multi-antenna transmission scheme as an example, where \(UAV_{ST}\) designs its transmit beamforming vectors based on the reverse-link channel estimation from \(UAV_{SR}\). In that case, \(UAV_{ST}\) can spoof the reverse-link transmit signals received by \(UAV_{ST}\), such that \(UAV_{ST}\) estimates a fake channel, and changes its beamforming direction towards \(UAV_{L}\) and away from \(UAV_{SR}\)[11]. This approach increases \(R_{L}\) and decreases \(R_{S}\), and accordingly improves \(R_{E}\). Case 3, as shown in Figure 2c, \(UAV_{L}\) eavesdrops suspicious \(UAV_{SR}\) by sending jamming signals to \(UAV_{SR}\). In that case, \(UAV_{SR}\) increases transmission power in order to sustain \(R_{S}\) at its original level, thus increasing \(R_{L}\) inevitably in the eavesdropping link. When \(R_{L}\) is no smaller than \(R_{S}\), \(UAV_{L}\) is able to decode the whole information that can be decoded at \(UAV_{ST}\) to fulfill eavesdropping missions. Case 4, as shown in Figure 2d, \(UAV_{L}\) eavesdrops suspicious \(UAV_{SR}\) by sending jamming signals to \(UAV_{ST}\). Take the time-division-duplex (TDD) multi-antenna transmission scheme as an example, where \(UAV_{SR}\) designs it's transmit beamforming vectors based on the reverse-link channel estimation from \(UAV_{ST}\). In that case, \(UAV_{SR}\) can spoof the reverse-link transmit signals received by \(UAV_{SR}\), such that \(UAV_{SR}\) estimates a suspicious channel, and changes its beamforming direction towards \(UAV_{L}\) and away from \(UAV_{ST}\)[12]. This approach increases \(R_{L}\) and decreases \(R_{S}\), and accordingly improves \(R_{E}\). We have previously discussed the first approach to eavesdrop suspicious communication link by jamming \(UAV_{SR}\), as shown in Figure 2a[13], so this paper mainly focuses on the other three eavesdropping and jamming cases, as shown in Figure 2b-d. In practice, UAV's trajectory period depends on the battery charge. Low power consumption can make sure the UAV fly in a relative long period. In this paper, we aim to (1) minimize the power consumption at \(UAV_{L}\), and to (2) maximize the eavesdropping rate at \(UAV_{L}\).