Modern Random Access: an Age of Information Perspective on Irregular Repetition Slotted ALOHA
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Recent years have witnessed a steadily growing interest towards modern random access protocols for massive machine type communications in next generation wireless systems. Constructively embracing interference, these solutions have been shown to attain a spectral efficiency comparable to that of coordinated access schemes, while remaining true to the grant-free paradigm. On the other hand, their ability to maintain a fresh and up-to-date view at the receiver when devices transmit status updates, as typical in IoT applications, is still largely unexplored. In this paper we start to bridge such a gap, focusing on a scenario in which a large population of devices share a common wireless channel in an uncoordinated fashion, and studying the age of information (AoI) metric when medium access follows the irregular repetition slotted ALOHA (IRSA) protocol. By means of a Markovian analysis, we track the AoI evolution for a source at the receiver, prove that the process is ergodic, and derive a compact closed form expression for its stationary distribution. Leaning on this, we compute exact formulations for the average AoI and for the peak-age violation probability. The study reveals non-trivial design trade-offs for IRSA, highlighting the key role played by the protocol operating frame size. Moreover, a comparison with the performance of a simpler slotted ALOHA strategy highlights a remarkable potential for modern random access in terms of information freshness.
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