# Outage Performance of SWIPT-D2D-Based Hybrid Satellite-Terrestrial Networks
## 1 Introduction  
With the rapid development of communication networks, the application scope of the Internet of Things (IoT) has become increasingly extensive and diversified, and the traffic of mobile data services has also witnessed explosive growth. In general, the deployment of fifth-generation (5G) communication networks has further promoted the development of the IoT [1]. However, considering the limitations of infrastructure costs and geographical environment factors, terrestrial networks are mainly deployed in developed regions and densely populated urban areas, and are unable to cover the vast airspace, sea area, etc. This has significantly restricted the deployment and application scope of IoT devices. Therefore, the aspiration of this work is to formulate the design of sixth-generation (6G) communication networks, which enable ubiquitous communication [2, 3, 4]. Considering that satellite communication systems can provide global users with high-speed broadband access without geographical constraints, the architecture of hybrid satellite-terrestrial networks (HSTNs), formed by integrating terrestrial networks and satellite communication systems, can meet the requirements of the 6G communication networks to achieve ubiquitous coverage [5, 6, 7]. The HSTN framework is proposed in reference [8], which examines the role of the satellite communication system within HSTNs and outlines its future development directions. In the context of HSTNs, most research studies in the open literature have focused on such networks adopting amplify-and-forward (AF) and decode-and-forward (DF) relayingprotocols. In reference [9], the AF relaying HSTNs with a masked destination node have been analyzed over shadowed Rician fading and Nakagami-_m_ fading channels. The performance of the multiuser AF relaying HSTNs with opportunistic scheduling has been investigated in reference [10]. Reference [11] analyzes the outage performance of the non-orthogonal multiple access (NOMA)-based DF relaying HSTNs. Reference [12] studies the physical-layer security of the HSTNs composed of multiple DF relays and users in the presence of a terrestrial eavesdropper. Therefore, the terrestrial relaying network can effectively assist HSTNs in addressing the shadowing effect caused by obstacles, weather factors, angular differences, and other factors. However, considering the situation that the IoT devices affected by the shadowing effect are outside the coverage area of the terrestrial relay, the cooperation of the terrestrial relay will no longer be available in the downlink transmission of the HSTNs. In order to achieve the effective transmission of information between the satellite and the shadowed IoT devices, cooperative communication can be realized by forming a device-to-device (D2D) communication architecture with a neighboring user terminal (UT). However, considering that most UTs are energy-constrained in practical scenarios, it is unsustainable for the UTs serving as D2D transmitters to continuously provide cooperation for the shadowed IoT devices over an extended period. To address the issue of energy limitation, we consider introducing simultaneous wireless information and power transfer (SWIPT), which is proposed in reference [13], to extend the lifetime of the D2D communication. Without affecting the information decoding (ID), the energy-constrained UT harvests energy from a portion of the received radio frequency (RF) signal to deliver information to the D2D receiver. Both power splitting (PS) and time switching (TS) receiver architectures are proposed in reference [14]. By adopting PS-SWIPT, the UT splits the received signal into two portions, with one for energy harvesting (EH) and the other for ID [15,16]. By adopting TS-SWIPT, the UT switches in time between EH and ID [17, 18, 19]. Reference [20] investigates the performance of a two-way DF relay network over Nakagami-_m_ fading channels adopting PS and TS schemes.