More and more large-scale M2M devices attempt to access the eNB (evolved Node B) in a short time. For the limited PA(Preamble) resources, the conflicts may easily occur when most M2M devices implement PA selection, thus resulting the failure of RA(Random Access). And when the conflicts become even more frequent, it is easy to cause network congestion. To solve the problem of low access probability for large-scale M2M devices, the clustered access method is proposed. Firstly the number of clusters for maximizing the access probability is determined, then M2M devices are clustered into different clusters with the k-means algorithm, then the proper cluster head in each cluster is selected according to the communication distance. The M2M devices that request access in an access slot are spatially divided into different packets, thus enabling multiple access processes to beaccomplished simultaneously at a low collision probability in an access slot. Simulation results indicate that compared with nonhierarchical access method, the hierarchical access method can improve the transmission success rate and resource utilization rate of PA, thus raising the access probability of large-scale M2M devices.%大规模M2M(Machine to Machine,M2M)设备短时间内接入基站的现象越来越普遍.由于可用于接入的前导码(Preamble,PA)资源有限,所以大部分M2M设备在PA选择时容易发生冲突,导致随机接入失败.当冲突发生得越来越频繁时,容易造成网络拥塞.为了解决大规模M2M设备接入成功率较低的问题,提出分层接入的方法.首先确定使接入成功率最大的分组数量,使用k均值聚类方法将小区内M2M设备分为不同组,每个分组中根据最佳通信距离选择合适簇头,然后组员通过簇头接入基站.将一个接入时隙内请求接入的M2M设备在空间上分到不同分组中,可以保证在一个接入时隙中以较低的冲突概率同时完成多个接入过程.仿真结果表明,相较于非层次接入方法,使用分层接入的方法可以提高PA的传输成功率和资源利用率,从而提高大规模M2M设备的接入成功率.
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