Analysis of asymmetric hidden node problem in IEEE 802.11ax heterogeneous WLANs

In this paper, we investigate issues of fairness and spatial reuse in heterogeneous wireless local area networks (WLANs) based on IEEE 802.11ax. According to discussions in the evolving IEEE 802.11ax standardization, the value of carrier sensing threshold (CSTH) needs to be increased, in order to improve spectral efficiency of WLANs by means of spatial reuse. We reveal a serious problem of unfairness, referred to as asymmetric hidden node problem, arising when legacy devices that have smaller CSTH coexist and compete for channel access with new devices that have larger CSTH; legacy devices hardly access the wireless channel and the channel is mostly occupied by new devices. The reasons of this problem are analyzed from two viewpoints of asymmetry of carrier sensing and unintended operation of binary exponential backoff. We also show that this problem is not effectively resolved by the ready-to-send/clear-to-send (RTS/CTS) mechanism, which is a well-known solution to the hidden node problem. Via simulation study, we validate this problem and evaluate the trade-off between spatial reuse and fairness with respect to the value of CSTH.