## 4 The Proposed STCA Protocol
### STCA Without SIC  
First, this protocol considers three types of channels, which consist of superframes, frames, timeslots, and minislots. A competition access channel within a superframe contains \(I\) frames, and each frame is composed of three parts. The first part is the broadcast access information, which includes \(N_{m}\) minislots and one idle minislot used for waiting for transmission. The second part is the competition access, consisting of \(N_{C}\) minislots. The third part is the feedback access result, which also includes \(N_{m}\) minislots and one idle minislot. The service transmission channel contains \(N_{T}\) timeslots. More specifically, each minislot in the competition access channel lasts for \(T_{C}\) milliseconds, and each timeslot in the resource allocation channel lasts for \(XT_{C}\) milliseconds. The duration of this timeslot can be freely set, with minimal impact on the system. Each timeslot in the service transmission channel lasts \(XT_{C}\) milliseconds. To improve utilization efficiency, the service transmission channel is further divided into multiple sub-channels, with each air access control node allocated \(Y\) sub-channels. During the uplink transmission in the competition access channel, each ground terminal randomly selects a minislot at the beginning of frame \(i\) and sends a competition packet in the selected minislot. In the considered air-ground network, it is assumed that ground terminals of different air access control nodes can use orthogonal competition access channels to contend for the service transmission channel slots. Each minislot can have three states: successful, collided, and idle. The general process of the system is as follows: First, when the system is started for the first time, the satellite allocates service resources to the air access control nodes. Upon receiving the service channel resource allocation information, the air access control node broadcasts the service transmission channel index, competition access channel index, and access probability \(P_{i}\) to the ground terminals under its control. The ground terminals contend for access to the air access control node based on these indices and the probability \(P_{i}\). Subsequently, the air access control node performs decoding and provides feedback on the access results and the access probability for the next frame to the ground terminals. The ground terminals that have successfully gained access send data to the satellite through the service transmission channel based on the feedback information. The ground terminals that have failed to gain access wait for the next frame of the competition access channel to contend for access again. After one superframe of operation, the air access control node sends the estimated traffic load to the satellite, and the satellite allocates the service channel resources based on the load size. The flowchart is shown in Figure 5.  
The access process occurs between the ground terminals and the air access control nodes. When a data packet arrives at a ground terminal, this terminal must wait for the broadcast from the air access control node. The broadcast from the air access control node includes the access probability \(P_{i}\), and this ground terminal has a probability of \(P_{i}\) to send an access request. If it does not send the request, it will wait for the next frame. If it sends the request, it will select a minislot from the minislot indices broadcast by the air access control node to send the access information. Subsequently, the ground terminal waits for feedback from the air access control node, which includes the access result and the minislot in the service transmission channel where the data packet will be sent. If the ground terminal has a new data packet arriving during the access process, it will request access while sending the data packet.