Analysis of PDCCH Performance for M2M Traffic in LTE

As Long-Term Evolution (LTE) is starting to be widely deployed, the volume of machine-to-machine (M2M) traffic is increasing very rapidly. From the M2M traffic point of view, one of the issues to be addressed is the overload of the random access channel. The limitation in the physical downlink control channel (PDCCH) resources may severely constrain the number of devices that an LTE Evolved Node B (eNB) can serve. We develop a Markov model that describes the evolution of the Message 4 queue in the eNB formed by several users performing the random access procedure simultaneously, and then, we study its stability and performance. Our model explicitly takes into account the four initial steps in the random access procedure. By utilizing the model, we are able to determine the stability limit of the system, which defines the maximum throughput and the probability of failure of the random access procedure due to different causes. We observe that the sharing of the PDCCH resources between Messages 2 and 4 with different priorities makes the performance of the whole random access procedure deteriorate very rapidly near the stability limit. However, we can extend the maximum throughput and improve the overall performance by increasing the PDCCH resource size. Furthermore, we estimate the upper limit of the number of devices that can be served by an LTE eNB and determine the minimum PDCCH resource size needed to satisfy a given traffic demand.