Age of Information in Ultra-Dense Computation-Intensive Internet of Things (IoT) Systems

In this paper, a dense Internet of Things (IoT) monitoring system for computational intensive applications is studied in which a large number of devices with computing capability pre-process the collected raw status information into update packets and contend for transmitting them to the corresponding receivers, using a carrier sense multiple access (CSMA) scheme. Depending on whether the pre-processing operation completes when each device senses a channel, two policies are considered: pre-process-then-Sense policy (PtS) and preprocessing-while-Sensing policy (PwS). Particularly, under policy PtS, each device must complete the pre-processing operation before sensing a channel; while under policy PwS, it performs the pre-processing operation and senses a channel concurrently. Here, for policy PwS, if the pre-processing operation is incomplete while a sensed channel is available to be used, then each device will still occupy the channel by sending dummy bits. For both policies, the closed-form expressions of the average age of information (AoI) are characterized. Then, a mean-field approximation framework with guaranteed accuracy is developed to study the asymptotic performance for the considered system in the large population regime. Simulation results validate the analytical results and show that the proposed mean-field approximation under policy PtS is accurate even for a small number of devices. It is also observed that policy PtS achieves a smaller average AoI than policy PwS, revealing that it is unnecessary for each device to occupy the channel before the pre-processing operation completes.