# Intelligent Beam-Hopping-Based Grant-Free Random Access in Secure IoT-Oriented Satellite Networks
## 2 System Model and Procedure of BH-GFRA
### Procedure of GFRA  
Figure 2 illustrates the procedure of the proposed GFRA protocol. To execute more grant-free random access in an S-IoT network, the GFRA mechanism undergoes the following phases:  
Step 0: When the spotbeam of the satellite only lights the beam position, the satellite will send a signal to \(\mathcal{J}\) cell. Also, 3 GPP defines that after receiving the satellite downlink Synchronization Signal Block (SSB), the device starts to select the preamble and sends an access request to the appropriate random access occasion (RO). Here, we assume that the device must start receiving the SSB after transmitting data, and the probability of the device being active defines the process by which the device successfully completes synchronization.  
**Step 1**: Each of the \(L_{device}\) devices randomly selects a preamble from the assigned preamble P and transmits it within the first slot of the GFRA group.  
**Step 2**: The satellite receives the preamble and then allocates the time-frequency resources for data transmission to this device based on the received preamble index and the IoT device identifier of the device. The specific frequency-hopping pattern will be mentioned later. At the beginning of the access slot, the satellite uses the signal power and the first received channel state information (CSI) to identify the state of the preamble sequence. There are three possible preamble sequence occupancy states:  
Case 1: If no UE has selected the preamble, the preamble is referred to as an idle preamble.  
Case 2: If only one UE has selected the preamble, the preamble is referred to as a single-instance preamble. The satellite detects the single preamble state and assigns the frequency-hopping pattern for data transmission immediately after the current preamble transmission frame ends.  
Case 3: If a single preamble is selected by multiple devices, the contending user transmits data according to the resource-hopping pattern specified by the preamble.  
**Step 3**: When the beam again covers this beam position, the satellite transmits the preamble usage of the previous access period to all users in the beam position, and each UE can independently detect whether it has a conflict. Each conflicting UE then selects a random preamble sequence from the set of preambles and updates the frequency-hopping mode of the device that selected the previous preamble, and then no feedback will be provided by the satellite until the conclusion of the last access time session.  
Finally, the satellite will record the time from the start of access to the correct transmission of data during the first slot scan cycle of the recorder. If the device does not collide when initiating the first access and correctly selects the preamble of the single instance, the access delay is T1; if the device collides when selecting the preamble and successfully transmits data with the satellite during the second scan of the beam for this beam position, the access delay of this device is T1 + T2 + T3. The number of devices in each beam position is different, and in the following statistics, the average access delay of the users in a selected beam position is selected for the statistics.