| Low-Earth-Orbit(LEO)Internet of Things(IoT)is one of the main research directions of satellite communication technology at present,and it plays an important role in asset monitoring,smart city,logistics and environmental data perception on a global scale.With the rapid development of the diversity of IoT application scenarios,the scale of ground IoT terminal access is rapidly increasing.However,a single satellite access point can only serve a limited number of terminals,making it difficult to achieve concurrent access and massive information transmission for IoT terminals under high loads.In the current LEO satellite IoT communication system,due to limitations in terminal size and power consumption,most IoT terminals adopt random access to establish connections with base stations.In low-load scenarios,random access protocols can flexibly realize communication between terminals and base stations.However,as regional loads increase,using random access will cause different terminals to occupy the same channel resources leading to signal collisions that not only reduce system communication capacity but also increase communication delay.Additionally,due to long propagation delays between LEO satellites and terminals inexpensive IoT devices are unable to achieve strict network-wide time synchronization.Therefore,synchronous multiple-access techniques cannot be directly applied in LEO satellite-based IoT.Therefore,modifying traditional random access protocols for LEO Satellite-based IoT has become one of the hotspots in research.In response to the problem that traditional access protocols are not suitable for LEO satellite IoT,this paper combines Coordinated Multi-Point(Co MP)technology with random access protocol.By utilizing the characteristics of single-area multi-satellite coverage and diversity gain brought by multisatellite reception,it analyzes and designs two adaptive transformation schemes for random access protocols in LEO satellite scenarios to address issues such as packet collision caused by resource contention among terminals.The overall communication capacity and performance of the LEO satellite IoT system are improved by increasing the complexity of receiver processing through these adaptation schemes.The main research contents of this paper are as follows:(1)A cooperative beamforming ALOHA scheme based on linear constraint minimum variance criterion(CBA)is proposed to address the issues of packet collision and "deadlock" in LEO satellite IoT multi-satellite scenarios.In the co-visibility area of multiple satellites in LEO,packets only need to be sent once and can be received by multiple satellites,forming the concept of spatial domain replication.By jointly processing data packet replicas from multiple satellites,a collision resolution scheme combining continuous interference elimination and cooperative beamforming is designed to effectively solve packet collisions through iterative methods at signal gateways.In addition,robustness improvements are made to the solution for potential wave angle mismatch problems during signal beamforming under worst-case scenarios.Finally,throughput and packet loss rate performance of the proposed solution are derived and simulated.Simulation results show that compared with baseline problems,the proposed solution can effectively solve "deadlock" issues and improve random access performance.(2)Aiming at the avalanche effect caused by packet collision in high load of LEO satellite IoT network,an asynchronous collision-tolerant ACRDA scheme based on satellite-selection collaboration-beamforming(SC-ACRDA)is proposed.To solve the problem of satellite selection in multi-satellite cooperative beamforming,a multi-satellite cooperative selection algorithm based on maximizing signal-to-interference-plus-noise ratio(SINR)is proposed,which can be implemented as a non-convex optimization problem.In order to achieve the solvability of non-convex optimization problems,Charnes-Cooper transformation is used to transform the proposed problem into a convex optimization problem with unknown parameters.Moreover,since the value of unknown parameters determines both the number of selected satellites and SINR performance,an iterative binary search algorithm is designed to obtain optimal unknown parameters.Finally,combining this multi-satellite cooperative selection algorithm with ACRDA protocol effectively solves packet collision problems in gateway stations.Simulation results show that compared with baseline schemes,our proposed approach reduces overall system power consumption through satellite selection processing while also effectively addressing avalanche effects caused by random access protocols in satellite IoT networks and improving throughput and packet loss rate performance. |
