Random access design for clustered wireless machine to machine networks

In this paper, we consider the infrastructure mode wireless machine-to-machine (M2M) network, such as the machine-type communication (MTC) in the LTE-Advanced, where there could be tens of thousands of machines within a macro cell and intending to access the same macro base station. Since most of the M2M communications are of low duty cycle and the machine traffic is usually bursty, the bottleneck for M2M communications is usually at the random access stage. In the LTE-Advanced, the access class barring (ACB) and extended access barring (EAB) have been proposed for the random access of MTC. For the one-shot random access attempts which correspond to the extremely low-duty-cycle scenarios where the network has enough time before the next traffic burst to resolve random access collisions, ACB and EAB have been shown to be sufficient if the machines can tolerate long access delay. However, when the machine traffic is recurrent with higher duty cycle, the ACB and EAB will fail to resolve collisions before the next wave of traffic comes in. To accommodate the M2M traffic and reduce the random access delay under limited random access resource, a clustered network structure is considered for which the machines in a macro cell are divided into clusters, and the machines belonging to a cluster communicate to the cluster head which then aggregates the traffic and relays to the macro base station. This clustered M2M network spatially reuses the random access resource. Thus it can increase the number of machines supported by the network and/or reserve more random access resource for the conventional (human) devices such that their qualities of service will not be affected too much by the M2M communications. Our analysis shows that full reuse of the random access resource among the clusters is feasible. In addition, simulations are conducted to study the random access performance of the clustered network, and to determine the number of clusters that should be formed and the number random access preambles that should be allocated to the machines when the total number of machines is given.