Signal strength assistant grouping for lower hidden node collision probability in 802.11ah

Recently, 802.11ah with restricted access window for collision attenuation has been proposed for Internet of Things (IoT) applications. Random grouping of nodes in different access window is utilized to reduce the hidden node collision probability. However, when the coverage area of the sink node is large, the occurrence probability of hidden node problem is still high and severely impacts the system energy efficiency. Hence, in this paper, we propose a Signal Strength Assistant (SSA) grouping scheme to further alleviate this problem. In the proposed SSA grouping, several Reference Nodes (RN) with known locations are installed in the application scenario evenly. RNs sequentially transmit beacon signals, while the normal nodes monitor the signal strength from different RNs and join the group identified by the RN with the strongest average signal strength. We formulate the analytical model of the hidden node collision probability in SSA and analyze the optimal number of RN nodes to satisfy specific hidden node collision probability requirement. Furthermore, the performance of SSA is evaluated through extensive simulation. It is shown that mathematical and simulation results of SSA match well and the proposed SSA grouping can significantly reduce the hidden node collision probability in 802.11ah.