# Covert Communication for Integrated Satellite-Terrestrial Relay Networks with Cooperative Jamming
###### Abstract
To further improve the system's covert performance, an optimization strategy for the system capacity was proposed in [24]. The authors of [25] used a full-duplex receiver to transmit a jamming signal to confuse the eavesdropper while receiving the information sent by the transmitter. In [26], the authors performed a similar investigation, differing from the previous study in that a delay constraint was also considered. The covert performance of a wireless relay communication system consisting of a transmitter, receiver, relay and eavesdropper was investigated in [27], where the relay could operate in full-duplex mode or convert to half-duplex mode. The authors of [28] studied whether a self-sustained relay could forward source information while secretly transmitting covert information of it. In that paper, its feasibility was demonstrated, and the performance limits as well as associated costs were obtained. Moreover, not only covert communication between terrestrial nodes but also UAV-based air-terrestrial covert communication has been widely investigated [29; 30; 31]. Inspired by the above works, we note that covert communication with positive rate is usually achieved by various uncertainties inherent in the system or the assistance of the jammer. In view of better controllability and performance, jammer-assisted approaches to achieve covert communication have been widely adopted and studied. Nevertheless, almost all the previous works of others have been based on independent terrestrial networks. Moreover, existing assisted jamming schemes mainly allow transmitters or receivers to be equipped with full duplex antennas as jamming nodes or deploy specific jammers to emit artificial noise to confuse eavesdroppers. Although all of the above mainstream approaches can better achieve covert communication, they inevitably have the following shortcomings:  
1. The artificial noise can degrade the quality of legitimate communication. 2. Legitimate communication resources are tied up or costly to achieve. 3. It is easy to arouse the suspicion of eavesdroppers. Based on this, the relay cooperative scheme is proposed. Given its own characteristics, it can partially or completely solve the above problems and has been considered as one of the key means to achieve effective covert communication. In addition, the introduction of a relay can also increase the diversity of channel uncertainty, which can bring gains to the covert performance of the system. Thus far, the performance of covert communication with relay-assisted cooperative jamming, particularly for the ISTRN, has not been reported. Therefore, this article aims to study the impact of cooperative jamming on covert communication for the ISTRN, where the relay is employed to aid information transmission and acts as a jammer to cheat the eavesdropper. Further, the probability of detection error (PDE) of the eavesdropper is deduced. In addition, we propose a cooperative scheme to realize efficient covert communication in the ISTRN. To quantify the system's covert performance, the closed-form expression of outage probability (OP) and effective covert rate are derived. Lastly, we obtain the optimal power allocation factor, leading to the maximum effective covert rate under covert constraints.