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Recently, 802.11ah network has been widely used to support up to 8,192 nodes to serve various Internet of Things (IoT) communication applications. Because of the large number of nodes, it introduces a group-based MAC protocol, which divides nodes into groups to alleviate the hidden node problem. However, hundreds or even thousands of nodes are assigned into a group to contend the shared wireless channel. As a result, the problem is still serious and it causes a serious decline in system efficiency. Therefore, hidden nodes must be considered when grouping nodes in 802.11ah network. In this paper, the problem of minimizing the number of hidden nodes in group-based 802.11ah networks is expressed as 0/1 integer linear programming (0/1 ILP). In addition, the two additional problems extended from the above problem are also expressed as two 0/1 ILPs. Then, three approximation algorithms are proposed for these three NP-hard problems. The approximation ratio of the obtained feasible solutions to those of the three problems is competitively bounded.

Approximation Algorithms of Minimizing Hidden Pairs in 802.11ah Networks