S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Electrical and Computer Engineering (전기·정보공학부) Theses (Ph.D. / Sc.D._전기·정보공학부)
Robust and Interference-Resilient MAC/PHY Layer Strategies for WLANs
- 공과대학 전기·컴퓨터공학부
- Issue Date
- 서울대학교 대학원
- IEEE 802.11 wireless local area network (WLAN); Wi-Fi; aggregate MPDU (A-MPDU); mobility; channel estimation; baseband; operating channel width; automatic gain control (AGC); prototype design
- 학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2018. 2. 최성현.
- Thanks to the explosive growth of mobile devices such as smartphones and tablet PCs, IEEE 802.11 wireless local area network (WLAN), often referred to as WiFi, has
become one of the most successful wireless access technologies, supporting ever increasing demand for high data rates at relatively low cost. Encouraged by this remarkable success, the state-of-the-art IEEE 802.11 WLAN provides a physical layer (PHY) data rate of Gb/s to a single user in the 5 GHz unlicensed band, by enabling multi-input and multi-output (MIMO) technology, which utilizes multiple antennas at both transmitter and receiver, and channel bonding which aggregates multiple 20 MHz channels up to 160 MHz bandwidth. Furthermore, as a key feature to enhance medium access control (MAC) efficiency, IEEE 802.11 standard defines frame aggregation called aggregate MAC protocol data unit (A-MPDU), which amortizes PHY protocol overhead over multiple frames by packing several MPDUs into a single frame.
In this dissertation, we propose the following three strategies to enhance throughput performance in practice: (1) Mobility-aware PHY rate and A-MPDU length control,
(2) Receiver-driven operating channel width adaptation, and (3) Receive architecture for eliminating time-domain interference not overlapping with the desired signal
Firstly, a significant growth of mobile data traffic volume, primarily generated by portable devices, has led to a change of WLAN communication environments
wireless channel condition in WLAN system is no longer quasi-stationary over the duration of a single frame reception. Especially, frame aggregation, i.e., A-MPDU, which lengthens frame duration significantly, causes the channel state information (CSI) obtained at the preamble can be no longer valid for successfully decoding the latter part of A-MPDUs, when the channel condition substantially changes during the A-MPDU reception. To cope with this problem, we analyze the wireless channel dynamics considering mobility through extensive measurements, and we then build a model which represents the impact of mobility with a noise vector in the I-Q plane, to investigate 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 2.9 higher throughput, compared to a fixed duration setting according to IEEE 802.11 standard. STRALE simply requires to update device driver only at one end of the wireless link (i.e., transmitter), thus allowing it to be applicable to any kind of platforms.
Second, IEEE 802.11ac supports bandwidth of 20, 40, and 80 MHz as a mandatory feature, and optionally supports 160 MHz bandwidth. To transmit and receive packets using such wide bandwidth, the 802.11ac devices need to increase the size of fast Fourier transform (FFT), equivalently, the baseband bandwidth, referred to as
operating channel width (OCW). However, our experiment results reveal various situations where bandwidth adaptation without changing the receivers 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 OCW adaptation scheme with ease of implementation. Our prototype implementation in commercial 802.11ac devices shows that RECONN achieves up to 1.85x 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.
Finally, based on the observation that time-domain interference causes 1) packet detection and synchronization failure, 2) undesirable receive locking problem, and 3) automatic gain control (AGC) failure, we propose a receive architecture called REACTER to eliminate the impact of time-domain interference: REACTER digitally extracts the desired preamble signal not affected by time-domain interference, and provides
interference-resilient A-MPDU reception performance by real-time AGC level adaptation during A-MPDU reception. The proposed receive architecture extensively evaluated
via IT++ based link-level simulator, and the simulation results show that REACTER significantly improves the frame reception performance by completely eliminates
the impact of time-domain interference.
In summary, we identify the two existing problems through the extensive measurement and simulations, and we then propose compelling algorithms to improve the throughput performance. We demonstrate the feasibility of our approaches by implementing prototypes in off-the-shelf commercial 802.11n/ac devices, showing that our proposed algorithms fully comply with the 802.11 MAC and requires no PHY modification such that it can be applicable to the existing hardware platform by simply updating the device driver only at one end of the wireless link. Furthermore, we present a novel receive architecture which shows the ability to fundamentally enhance the performance of wide bandwidth operation with very low cost and complexity.