PHY/MAC Layer Strategies for High-Efficiency Dense WLANs
손영욱 · Seoul National University Open Repository (Seoul National University) · 2020
Among lots of performance issues on IEEE 802.11 wireless local area networks (WLANs), also referred to as Wi-Fi, the top priority concern in recent years has been to achieve better user experience and higher efficiency in densely deployed, userpopulated areas.In this context, IEEE 802.11ax standard has newly adopted groundbreaking PHY/MAC protocols as core features, including uplink multi-user transmission (UL MU) and spatial reuse (SR) operation.These new technologies enable multiple stations within a common cell or across neighboring cells to transmit simultaneously at a time, for elevating the spectral efficiency and overall network capacity.However, since the real-world performance is highly dependent on specific built-in algorithms and implementation which stipulate actual device behaviors, the upcoming standard still holds plenty of practical issues regarding its real deployment.In this dissertation, we consider the following three challenges to be addressed for desired operation of IEEE 802.11WLANs: (i) Symbol timing synchronization for UL MU, (ii) understanding and exploitation of a CS anomaly named preamble-passing for promoting valid transmissions in high-density WLANs, and (iii) enabling viable simultaneous transmissions via SR protocol with intelligent operating strategies.First, we spotlight the symbol timing synchronization problem for UL MU in IEEE 802.11axWLANs specifically.While UL MU enables multiple stations to transmit simultaneously to a common access point (AP), tight timing synchronization is required among transmitted signals for the receiver AP to correctly decode them, which is not fully addressed in the standard specification.Based on the observations and analysis, we present a novel receiver-side symbol timing synchronization mechanism with enhanced PHY functionality to capture a desirable symbol timing, which accommodates asynchronously arriving signals and mitigates ISI and ICI.Second, we shed light on preamble-passing anomaly in actual carrier sensing (CS) i behaviors, which makes neighboring devices blind to each other and transmit simultaneously.Through experimental study, we reveal both sides of preamble-passing, which heavily affects the overall network performance.Based on the observations, we design REFRAIN, a standard-compliant PHY/MAC framework, to cope with and further exploit the anomaly for better spatial reuse.Our prototype using NI USRP and commercial devices shows the effectiveness of our approach, while extensive simulation results demonstrate that REFRAIN achieves up to 1.57× higher average throughput by promoting valid transmissions, without modifying 802.11CS specification at all.Finally, we investigate detailed operation of the SR protocol in IEEE 802.11ax, with relation to preamble-passing anomaly.Identifying both detrimental effects and potential for spatial reuse, we develop AdOPT, a standard-compliant operating framework for the SR protocol.AdOPT enables only viable simultaneous transmissions opportunistically, by adjusting data rate at each transmission attempt.Our extensive simulation results verify that AdOPT brings significant performance gain over baseline 802.11 and other comparison schemes, up to 1.82× especially for the worst-case stations suffering from starvation and poor link conditions.In summary, we propose a symbol timing synchronization algorithm for UL MU, and two frameworks for better spatial reuse, REFRAIN and AdOPT, each working with conventional CS mechanism and 802.11axSR protocol.Through this research, we present practical operating strategies to achieve desirable real-world performance in high-density WLANs.The feasibility and performance of our approaches are validated via various methodologies including system-level and link-level simulation, and prototype using commercial Wi-Fi devices and NI USRP software-defined radio.