# Intelligent Beam-Hopping-Based Grant-Free Random Access in Secure IoT-Oriented Satellite Networks
## 2 System Model and Procedure of BH-GFRA  
### System Model  
In this paper, we consider the forward link transmission of the multi-beam satellite system consisting of multiple NGSO satellites, as depicted in Figure 1. There are \(N\) satellites serving the ground area, which contains a total of \(M\) cells. The sets of satellites and cells are denoted by \(\mathcal{N}=\{n\mid n=1,2,\ldots,N\}\) and \(\mathcal{M}=\{m\mid m=1,2,\ldots,M\}\). In particular, the set of cells covered by the \(n\)-th satellite is represented by \(\mathcal{M}_{n}\), and we assume that each satellite can cover \(J\) cells (i.e., \(|\mathcal{M}_{n}|=J,\forall n\in\mathcal{N}\)). The cell index set under the coverage of one satellite is denoted as \(\mathcal{J}=\{j\mid j=1,2,\ldots,J\}\). Due to the multiple coverage characteristics of the NGSO satellite communication system, one cell may be covered by multiple satellites, such that \(\mathcal{M}=\bigcup_{n\in\mathcal{N}}\mathcal{M}_{n}\) and \(\mathcal{M}_{i}\cap\mathcal{M}_{j}\
eq\emptyset,\forall i,j\in\mathcal{N}\). In this system, each satellite can simultaneously generate beams to serve distinct IoT scenarios using a time division multiple-access approach, with each beam covering a single scenario. This manuscript concentrates on time-slot-driven scheduling for terrestrial IoT scenarios, wherein the beams of satellites must be scheduled for each time slot, and the time-frequency resource grid is allotted through a beam-hopping technique. The collection of served cells during each time slot is referred to as the BH pattern. We focus on the GFRA random access protocol for IoT-based satellite access network systems, where a large number of IoT devices send data packets to the satellite intermittently, but can only communicate when the beam slot is lit by the beam. In our system, the sequence of lit slots is structured as a hopping beam. Various time slots are allocated for distinct satellite services; therefore, it is necessary to create a frequency-time resource grid and designate the index of each slot together with the corresponding sent information, referred to as the BH-GFRA block. Initially, we posit that the GFRA blocks has \(G=R\cdot L\) resources in the header and \(R\cdot R\) resources in the data segment. Each resource block is tasked with occupying a time slot, the length of which is dictated by the subcarrier interval designated for transmission. This interval is represented by the symbol \(T_{slot}\). The load factor, represented by \(L\), clearly shows the traffic situation of device in \(\mathcal{J}\) cell, with each corresponding to the \(i_{th}\) GFRA group, under the assumption that Internet of Things devices can attempt to execute GFRA within \(\mathcal{J}\) cell. Each device can transmit data at the \(i_{th}\) random access occasion (RO) during the \(I\cdot T_{slot}\) time interval. Subsequently, we will introduce the proposed BH-GFRA approach within a singular GFRA block, ensuring the prompt elucidation of our explanation. If at least one data packet is chosen for transmission, all active devices within the \(i_{th}\) GFRA group are operational. In the absence of this condition, the gadget remains inactive. Consequently, the quantity of active devices within each GFRA block varies in line with the variable \(v\). This study's findings indicate that device activation probability varies due to the sporadic need for data transfer from devices situated in distinct places. The activation probability \(v\) influences the quantity of active devices within a particular GFRA group. This is approximately characterized as \(v\) multiplied by \(R\) and \(L\). In each GFRA group, let \(\textit{preamble}=\{\textit{preamble}_{1},\textit{preamble}_{2},\dots,\textit{ preamble}_{\tau}\}\) denote a designated preamble set, where \(\tau\) signifies the number of uniformly-length preamble sequences available.