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.

The increasing demand for ubiquitous wireless connectivity has led to the widespread of Wi-Fi networks. However, due to the limited coverage of Wi-Fi networks, mobile stations (STAs) need to frequently search for neighboring Wi-Fi access points (APs). Inevitable inefficiency occurs during the Wi-Fi scanning since the STA typically does not have any prior knowledge of the neighboring APs, thus leading to unnecessary waste of time and energy. In this paper, we propose BlueScan, a scheme to boost Wi-Fi scanning using collocated Bluetooth radio. BlueScan enhances Wi-Fi scanning with low power Bluetooth radio by identifying the operating channels and target beacon transmission times (TBTTs) of neighboring APs, thus changing the Wi-Fi scanning from a blind search to an intelligent search. We implement a prototype of BlueScan using Ubertooth platform, and evaluate its performance through real experiments. Our results demonstrate that BlueScan reduces the scanning delay up to 77% compared to legacy Wi-Fi scanning.

Dense Wi-Fi and Bluetooth (BT) environments become increasingly common so that the coexistence issue between Wi-Fi and BT is imperative to solve. In this paper, we propose BlueCoDE, a coordination scheme for multiple neighboring BT piconets, to make them collision-free and less harmful to Wi-Fi. BlueCoDE reuses BT's existing PHY and MAC design, thus making it practically feasible. We implement a prototype of BlueCoDE on Ubertooth One platform and corroborate the performance gain via analysis, NS-3 simulations, and prototype-based experiments. Our experimental results show that with merely 10 legacy BT piconets, neighboring Wi-Fi network becomes useless achieving under 1 Mb/s throughput, while BlueCoDE enables the Wi-Fi throughput always remain above 12 Mb/s. We expect BlueCoDE to be a breakthrough solution for coexistence in dense Wi-Fi and BT environments.

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.

In this paper, we develop a contention window (CW) control scheme for practical IEEE 802.11 wireless local area networks (WLANs) that have node heterogeneity in terms of the traffic load, transmission rate, and packet size. We introduce activity probability, i.e., the probability that a node contends for medium access opportunities at a given time. We then newly develop a performance analysis model that enables analytic estimation on the contention status including the collision probability, collision time, back-off time, and throughput with comprehensive consideration of node heterogeneity. Based on the newly developed model, we derive the theoretically ideal contention status, and develop a CW control scheme that achieves the ideal contention status in an average sense. We perform extensive NS-3 simulations and real testbed experiments for evaluation of both the proposed performance analysis model and CW control scheme. The results show that the proposed model provides accurate prediction on the contention status, and the proposed CW control scheme achieves considerable throughput improvement compared to the existing schemes which do not comprehensively consider node heterogeneity.

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.

IEEE 802.11n WLAN supports frame aggregation called aggregate MAC protocol data unit (A-MPDU) as a key MAC technology to achieve high throughput. While it has been generally accepted that aggregating more subframes results in higher throughput by reducing protocol overheads, our measurements reveal various situations where the use of long A-MPDU frames frequently leads to poor performance in time-varying environments. Especially, since mobility intensifies the time-varying nature of the wireless channel, the current method of channel estimation conducted only at the beginning of a frame reception is insufficient to ensure robust delivery of long A-MPDU frames. Based on extensive experiments, we develop MoFA, a standard-compliant mobility-aware A-MPDU length adaptation scheme with ease of implementation. Our prototype implementation in commercial 802.11n devices shows that MoFA achieves the throughput 1.8x higher than a fixed duration setting (i.e., 10 ms, the maximum frame duration according to IEEE 802.11n standard). To our best knowledge, this is the first effort to optimize the A-MPDU length for commercial 802.11n.

Today, voice over IP (VoIP) service is emerging as a popular and important application in wireless local area networks (WLANs). While rate adaptation (or link adaptation) has been identified as a key factor determining the performance of WLANs, we have observed that most (if not all) rate adaptation algorithms have been developed to improve the throughput of data traffic, not the quality of service (QoS) of VoIP traffic. Accordingly, in this paper, we investigate the characteristics of VoIP traffic and the limitations of state-of-the-art rate adaptation algorithms, and then enhance the QoS of voice over WLAN (VoWLAN) by ameliorating the existing rate adaptation algorithms. Specifically, we design fast decrease to control the transmission rate of retransmissions, and retry scheduling to avoid the deep fading of the wireless channel as well as hidden terminal interference. We comparatively evaluate the QoS of the revised rate adaptation algorithms via ns-3 simulations and MadWiFi implementations in various communication environments, and demonstrate that the proposed schemes improve the R-score performance by up to 80 percent depending on the network scenarios.

Voice over IP (VoIP) service is emerging as a popular application in wireless local area networks (WLANs). However, the existing rate adaptation (or link adaptation) schemes for WLANs have been developed to improve the throughput of data traffic, not the quality of service (QoS) of VoIP traffic. In this paper, we investigate the characteristics of VoIP traffic and the limitations of state-of-the-art rate adaptation schemes, and then enhance the QoS of voice over WLANs (VoWLANs) by ameliorating the existing rate adaptation schemes. Specifically, we design fast decrease to control the transmission rate of retransmissions, and retry scheduling to avoid the deep fading of the wireless channel as well as hidden terminal interference. We comparatively evaluate the QoS of the revised rate adaptation schemes via ns-3 simulations and MadWiFi implementations in various communication environments, and demonstrate that the proposed scheme improves the R-score performance by up to 80% depending on the network scenarios.