# BeiDou Short-Message Satellite Resource Allocation Algorithm Based on Deep Reinforcement Learning
## 2 System Model  
We consider the following scenario. In a snapshot [40] of an SMSCS, there are \(n\) short-message satellites (SAT) that cover the ground area; these are recorded as a set \(\mathbf{SAT}=\{SAT_{1},SAT_{2},\cdots,SAT_{n}\}\), where \(n\) is the total number of short-message satellites in the \(\mathbf{SAT}\). At the same time, there are \(m\) short-message terminals (ST) in the coverage area; these are recorded as a set \(\mathbf{ST}=\{ST_{1},ST_{2},\cdots,ST_{m}\}\), where \(m\) is the total number of short-message terminals in the \(\mathbf{ST}\). The communication link of the system adopts the Gaussian white noise channel, and the task requests of the short-message terminal have a Poisson distribution. Our system model is shown in Figure 1 and consists of the BDS-3 short-message satellite constellation, ground station, and short-message terminal. The short-message satellite constellation adopted is the Walker \(24/3/1\) constellation, which is used to collect short-message transmission requests and return related information such as the working status to the ground station, and then wait for control commands from the ground station. The ground station can collect information on the working status, resource information, and transmission control instructions from the short-message satellite. The short-message terminal initiates task requests and receives short messages from other terminals. The system workflow is divided into three stages. (1) The short-message terminal and the short-message satellite establish uplink and downlink communication links. (2) The short-message satellite establishes a communication link with the ground station, and the inbound information from the short-message terminal is sent to the ground station. (3) The short-message satellite sends the short message to the target terminal through itself or via the intersatellite link depending on the service instructions provided by the ground station. Short-message satellites can establish intersatellite links to achieve intersatellite information exchange. The red arrow in Figure 1 shows the end-to-end communication flow of a complete short-message terminal. First, \(ST_{2}\) sends a short-message task request. Secondly, \(SAT_{2}\) collects the request and informs the ground station, and the ground station  
Figure 1: Short-message satellite communication system model. sends the request to \(SAT_{2}\) so that it can respond. Finally, \(SAT_{2}\) transmits the short message from \(ST_{2}\) to \(ST_{3}\) through the intersatellite link established with \(SAT_{3}\). In the system model, the task requests from each short-message terminal can only be answered by a unique short-message satellite. Multiple short-message terminals can be distributed within the coverage area of each short-message satellite in the short-message satellite constellation. There is a one-to-many mapping relationship between a short-message satellite and the short-message terminal. As the short-message satellite has the characteristics of a large ground coverage area, the vast majority of short-message terminals are covered by multiple short-message satellites. Therefore, there is competition among short-message satellites to respond to short-message terminal tasks. For example, the task request of \(ST_{1}\) shown in Figure 1 can be responded to by \(SAT_{1}\) or \(SAT_{4}\). In different snapshots of the above model, due to the characteristics of short-message satellite coverage and the uneven distribution of short-message communication traffic in different regions, the resource utilization of the SMSCS and the energy efficiency of the short-message terminal are low. Consequently, our optimization objectives include the following:  
1. To reduce the transmission energy consumption of short-message terminals;
2. To improve the resource utilization of short-message satellites;
3. To produce adequate short-message terminal service quality.