卫星通信能够为M2M应用提供独特的通信优势，是实现天基物联网的关键所在，而其广覆盖、大时延、载荷受限等特性，给海量M2M用户的接入效率与性能提升带来严峻挑战。本项目针对卫星大规模M2M通信场景中存在的大量前导冲突、网络过载拥塞以及数据传输低效问题，从三个方面展开高效、大容量M2M随机接入技术的研究：1.竞争资源扩容。研究M2M随机接入冲突抑制方法，通过设计大容量M2M接入前导集及其动态划分方案，最大限度抑制前导冲突碰撞。2.接入机制优化。研究基于分布式队列的过载控制机制，并提出实时冲突解决与队列分组优化合并策略，进一步降低接入时延。3.资源高效复用。利用稀疏编码多址接入技术构建数传协同的随机接入过程，研究时延与容量最佳均衡的资源分配方法，在保证用户接入成功率的前提下提高资源利用率。本项目的研究将为实现天基物联网中高效、大容量的M2M用户接入提供关键技术支撑。

Satellite communication can provide unique advantages for machine-to-machine (M2M) applications, and consequently be the key to realize satellite-based Internet of Things (IoT). However, due to its characteristics of wide coverage, large delay and limited load, the improvement of access efficiency and performance will face severe challenges for a huge number of M2M users. To deal with the massive access preamble conflicts, network overload congestion, and low efficiency of data transmission in large-scale satellite-M2M communication scenarios, this project researches on the key technologies for efficient and high-capacity random access from the following three aspects: 1. Competition resource expansion. In this project, the method of M2M random access preamble conflict mitigation is studied, and the high-capacity M2M access preamble set and its dynamic partitioning schemes are designed to restrain the preamble conflict collision to the maximum extent. 2. Access mechanism optimization. An overload control mechanism based on distributed queuing (DQ) is proposed to further reduce access delay, by designing the schemes of real-time conflict resolution and optimization merging of queue grouping. 3. Efficient resources reuse. Sparse coding multiple access (SCMA) technology is used to construct the random access process compatible with data transmission, and the resource allocation method with the optimal balance of delay and capacity is presented to improve the resource utilization rate while ensuring the reliable success rate of M2M user access. Aforementioned research issues in this project will provide key technical support for the realization of efficient and high-capacity M2M user access in the satellite-based IoT.