  IEEE 802.11ax as the newest Wireless Local Area Networks (WLANS) standard
brings enormous improvements in network throughput, coverage and energy
efficiency in densely populated areas. Unlike previous IEEE 802.11 standards
where power saving mechanisms have a limited capability and flexibility,
802.11ax comes with a different mechanism called Target Wake Time (TWT) where
stations (STAs) wake up only after each TWT interval and different STAs can
wake up at different time instance depending on their application requirements.
As an example, for a periodic data arrival occurring in IoT applications, STA
can wake up by following the data period and go to sleep mode for a much longer
time, and STAs with high traffic volume can have shorter TWT interval to wake
up more frequency. Moreover, as multi-user transmission capability is added to
802.11ax, multiple STAs can have the same TWT interval and wake up at the same
time, and hence there is a great opportunity to have collision-free
transmission by scheduling multiple STAs on appreciate TWT intervals to reduce
energy consumption and also increase network throughput. In this paper, we
investigate the problem of STAs scheduling and TWT interval assignment together
to reduce overall energy consumption of the network. We propose an algorithm
that dynamically selects STAs to be served and assigns them the most suitable
TWT interval given their traffic and channel conditions. We analyze our
algorithm through Lyapunov optimization framework and show that our algorithm
is arbitrarily close to the optimal performance at the price of increased queue
sizes. Simulation results show that our algorithm consumes less power and
support higher traffic compared to a benchmark algorithm that operates randomly
for TWT assignment.
