Very high throughput (VHT) WLAN, known as IEEE 802.11ac, has attracted extensive attentions for achieving transmission rate over 1G bps. Due to the introduction of MU-MIMO and maximum 160MHz bandwidth transmission, the conventional carrier sensing mechanism emerges some drawbacks especially in the overlapping BSS scenario. In order to address the issue of redundant or useless network allocation vector (NAV) setting, this paper proposes a two-level NAV mechanism which only needs small modification to current standard draft. Theoretical analysis is also performed on the proposed mechanism and the formulas of the achieved gain is derived. Numerical results confirm the validation of our theoretical analysis and show that the proposed scheme obtains obvious throughput gain over the conventional mechanism.
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