Target wake time (TWT) has been introduced to provide the low power consumption mode for stations (STAs) with low traffic and periodic data transmissions in IEEE 802.11ax networks. The TWT operation starts with negotiating wake and doze periods between an access point (AP) and STAs. Thanks to the nature of wake and doze period negotiation, TWT operation enables time scheduled access, like time-division multiple access (TDMA) without modification overhead. In this paper, we demonstrate TWT scheduling issues that include scheduling requirements and results notifications from the AP to STAs, using standard-compliant methods. Then we establish TWT scheduling strategies by exemplifying two well-known schedulers, max-rate and proportional fairness schedulers. Through ns-3 simulation, we evaluate the performance of TWT scheduling and highlight the benefits of TWT for a large number of STAs from the fairness and power consumption perspective.

LTE-Unlicensed (LTE-U) supports LTE downlink operation in 5GHz unlicensed bands where Wi-Fi has been a traditional user for a long time. To achieve fair coexistence with Wi-Fi, LTE-U employs carrier sense adaptive transmission (CSAT), but it does not guarantee perfectly fair coexistence. Therefore, many studies have dealt with unfair coexistence problems of Wi-Fi and LTE-U. However, in this paper, our experiment results show that a Wi-Fi station not only suffers from unfair coexistence but also wastes energy and air time when it coexists with LTE-U. To cope with this problem, we propose AWARE, a station-driven adaptive Wi-Fi power save operation coexisting with LTE-U, which detects LTE-U and adjusts Wi-Fi power state adaptively. We implement AWARE on a commercial 802.11n device, and our evaluation shows that a Wi-Fi station achieves almost the same throughput while reducing its power consumption by up to 33% and enhancing the throughput of neighbor Wi-Fi stations by up to 43%.

State-of-the-art IEEE 802.11ac supports wide bandwidth operation, which enables aggregating multiple 20 MHz channels up to 160 MHz bandwidth, as a key feature to achieve high throughput. In this paper, our experiment results reveal various situations where bandwidth adaptation without changing the receiver's baseband bandwidth, called operating channel width (OCW), leads to poor reception performance due surprisingly to time-domain interference not overlapping with the incoming desired signal in frequency domain. To cope with this problem, we develop RECONN, a standard-compliant and receiver-driven OCWadaptation scheme with ease of implementation. Our prototype implementation in commercial 802.11ac devices shows that RECONN achieves up to 1.85× higher throughput by completely eliminating time-domain interference. To our best knowledge, this is the first work to discover the time-domain interference problem, and to develop OCW adaptation scheme in 802.11ac system.

IEEE 802.11n/ac wireless local area network (WLAN) supports frame aggregation, called aggregate medium access control (MAC) protocol data unit (A-MPDU), to enhance MAC efficiency by reducing protocol overhead. However, the current channel estimation process conducted only once during the preamble reception is known to be insufficient to ensure robust delivery of long A-MPDU frames in mobile environments. To cope with this problem, we first build a model which represents the impact of mobility with a noise vector in the I-Q plane, and then analyze how the mobility affects the A-MPDU reception performance. Based on our analysis, we develop STRALE, a standard-compliant and mobility-aware PHY rate and A-MPDU length adaptation scheme with ease of implementation. Through extensive simulations with 802.11ac using ns-3 and prototype implementation with commercial 802.11n devices, we demonstrate that STRALE achieves up to 2.9x higher throughput, compared to a fixed duration setting according to IEEE 802.11 standard.

Wi-Fi Direct, now part of Android smartphones, makes it possible for Wi-Fi devices to communicate directly without passing through an access point. Before starting actual data exchange, two devices should apparently find each other through a device discovery process, called find phase. After identifying the limitation of the legacy find phase, incurring long discovery delay, we propose a simple and efficient scheme, called Listen Channel Randomization (LCR), in order to expedite device discovery. Both the legacy find phase and LCR are evaluated with our elaborate Markov model and ns-3 simulations for comprehensive comparisons. Moreover, we have implemented a prototype of LCR in Android smartphone. Our experimental results reveal that LCR can significantly reduce the device discovery delay, yielding up to 72% delay reduction compared to the legacy find phase.

IEEE 802.11 has evolved from 802.11a/b/g/n to 802.11ac in order to meet ever-increasing high throughput demand. The newest standard supports various channel widths up to 160 MHz by bonding multiple 20 MHz channels. In order to efficiently utilize multiple channel widths, 802.11ac defines two operations, namely, dynamic channel access (DCA) and dynamic bandwidth operation (DBO). In this paper, we reveal that the use of DCA improves channel utilization significantly, and DBO not only partly overcomes secondary channel hidden interference problems but also achieves better channel utilization. However, when a transmitter attempts to send an enhanced RTS frame before data transmission as part of DBO, the station has no way to know how much bandwidth will be used, thus leading to malfunction of virtual carrier sensing at neighboring stations. On the other hand, the use of enhanced RTS/CTS without hidden traffic wastes airtime significantly. To address these problems, we first define how to calculate an appropriate value of duration field for enhanced RTS/CTS, and then develop an algorithm which adaptively enables/disables DBO considering the (secondary) hidden interference. Through ns-3 simulations, we demonstrate that the proposed scheme achieves up to 2x higher throughput compared to the baseline of 802.11ac.