## 4 The Proposed STCA Protocol
### STCA with SIC  
The following section introduces the process of a system with SIC capability, and Figure 6 shows the system flowchart. First, when the system is started for the first time, the satellite evenly allocates the number of sub-channels to each air access control node. Upon receiving the 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 Serial Interference Cancellation (SIC) decoding and provides feedback on the access results and the access probability for the next frame to the ground terminals. The successfully accessed ground terminals send data to the satellite through the service transmission channel based on the feedback information. The ground terminals that fail to access will 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 number of sub-channels proportionally based on the load. The access process occurs between the ground terminals and the air access control nodes. When a data packet arrives at a ground terminal, the 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 randomly generate \(l\) copies and select \(l\) minislots from the minislot indices broadcast by the air access control node to send the access information. Subsequently, the 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 a new data packet arrives at the ground terminal during the access process, it will request access while sending the data packet. Additionally, due to the difference in the duration between minislots and timeslots, when a traffic data packet in the service transmission channel is completed, several ground terminals will have already completed their access in the competition access channel. A transmission queue will be generated in the service transmission channel. When the normalized throughput of the access process is sufficient, the channel utilization will always be 100%. The access probability \(P_{i}\) can be adjusted according to the access load to ensure maximum normalized throughput. The air access control node is unaware of the actual access load. Therefore, it needs to estimate how many pieces of SUE have data packets to send in the current frame. In  
Figure 6: System flowchart of STCA with SIC. discrete-time channels, the access load is proportional to the number of idle minislots. The probability of a minislot being idle is  
\[P_{iulte}=M_{i}P_{i}\left[\sum_{l=1}^{N_{C}}\left(r_{l}\frac{(N_{C}-1)}{(N_{l})} \right)\right]^{M_{i}P_{i}}. \tag{13}\]  
Thus, the estimated number of idle minislots in a frame of the CAS is  
\[n_{iulte}=N_{C}P_{iulte}=f(M_{i}). \tag{14}\]  
For the ACN, the only unknown variable in the above equation is \(M_{i}\); thus, it can be expressed as a function \(f(M_{i})\).