Analysis of a Persistent Multi-Channel Slotted ALOHA Protocol Without Acknowledgment

In this paper, we focus on a persistent medium access mechanism for a wireless multichannel scenario without a return channel. It is a version of the legacy slotted ALOHA protocol in which each active user chooses a sub-channel (SC) to transmit its messages and, in each slot, it either keeps transmitting in the same SC (with persistence probability p) or chooses another SC among those left idle in the previous slot. This decision, however, is taken without any explicit information about the success or failure of the previous transmission, but only using high-level busy/idle information provided by, e.g., carrier-sensing mechanisms. This scenario is of interest when direct feedback is impractical, e.g., with broadcast traffic or unconfirmed data like status report update messages in massive Internet of Things (IoT). Despite the simplicity of the protocol, its performance analysis may become cumbersome because of the combinatorial nature of the multi-access problem. We overcome this difficulty by applying the Equilibrium Point Analysis (EPA) to estimate some key performance indicators, such as throughput and Age of Information (AoI). The resulting model, though approximate, turns out to be accurate and scalable, as proved by comparing the mathematical results with the outcome of computer simulations when changing the key system parameters (channel load, persistence probability).