Browsing by Author "Knightly, Edward W"
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Item Client Beamforming for Rate Scalability of MU-MIMO Networks(2015-04-24) Yu, Hang; Zhong, Lin; Knightly, Edward W; Sabharwal, Ashutosh; Johnson, David BThe multi-user MIMO (MU-MIMO) technology allows an AP with multiple antennas to simultaneously serve multiple clients to improve the network capacity. To achieve this, the AP leverages zero-forcing beamforming (ZFBF) to eliminate the intra-cell interference between served clients. However, current MU-MIMO networks suffer from two fundamental problems that limit the network capacity. First, for a single MU-MIMO cell, as the number of clients approaches the number of antennas on the AP, the cell capacity often flattens and may even drop. Second, for multiple MU-MIMO cells, the multiple APs cannot simultaneously serve their clients due to inter-cell interference, so that the concurrent streams are constrained to a single cell with limited network capacity. Our unique perspective to tackle these two problems is that modern mobile clients can be equipped with multiple antennas for beamforming. We have proposed two solutions that leverage the client antennas. For the capacity scalability problem in a single MU-MIMO cell, we use multiple client antennas to improve the orthogonality between the channel vectors of the clients. The orthogonality between clients’ channels determines the SNR reduction from the zero-forcing beamforming by the AP, and is therefore critical for the capacity of a MU-MIMO cell to become more scalable to the number of clients. We have devised a 802.11ac-based protocol called MACCO, in which each client locally optimizes its beamforming weights based on the channel knowledge obtained from overhearing other clients’ channel reports. For the inter-cell interference problem in multiple MU-MIMO cells, we leverage multiple client antennas to assist the interfering APs to coordinately cancel the inter-cell interference between them. To achieve such coordinated interference cancellation in a practical way, We have proposed a two-step optimization including antenna usage optimization and beamforming weight optimization. We have devised another 802.11ac-based protocol called CoaCa, which integrates this two-step optimization into 802.11ac with small modifications and negligible overhead, allowing each AP and client to locally identify the optimal beamforming weights. We have implemented both MACCO and CoaCa on the WARP SDR platform leveraging the WARPLab framework, and experimentally evaluated their performance under real-world indoor wireless channels. The results have demonstrated the effectiveness of MACCO and CoaCa toward solving the capacity scalability and inter-cell interference problems of MU-MIMO networks. First, on average MACCO can increase the capacity of a single MU-MIMO cell with eight AP antennas and eight clients by 35%, compared to existing solutions that use client antennas differently. Second, for a MU-MIMO network with two cells, by cancelling the inter-cell interference CoaCa can convert the majority of the number of streams increase (50%-67%) into network capacity improvement (41%-52%).Item Controlling Race Conditions in OpenFlow to Accelerate Application Verification and Packet Forwarding(2014-10-24) Sun, Xiaoye Steven; Ng, T. S. Eugene; Knightly, Edward W; Zhong, LinOpenFlow is a Software Defined Networking (SDN) protocol that is being deployed in critical network systems. SDN application verification takes an important role in guaranteeing the correctness of the application. Through our investigation, we discover that application verification can be very inefficient under the OpenFlow protocol since there are many race conditions between the data packets and control plane messages. Furthermore, these race conditions also increase the control plane workload and packet forwarding delay. We propose Attendre, an OpenFlow extension, to mitigate the ill effects of the race conditions in OpenFlow networks. We have implemented Attendre in NICE (a model checking verifier), Open vSwitch (a software virtual switch) and NOX (an OpenFlow control platform). Experiments show that Attendre can reduce verification time by several orders of magnitude, and can significantly reduce TCP connection setup time.Item High-resolution Millimeter-wave Impulse-based MIMO 3D Imaging Radar in Silicon(2015-04-22) Chen, Peiyu; Babakhani, Aydin; Aazhang, Behnaam; Knightly, Edward W; Kono, JunichiroThe research on millimeter-wave (mm-wave) silicon-based integrated 3D imaging radar has gained tremendous attention in academia over the past decade. Compared with conventional 2D imaging, 3D imaging captures both 1D depth information and 2D intensity maps. Impulse-based 3D imaging radar can also obtains more constitutional information of objects, like spectroscopy, so as to potentially have material identification functionality with 3D imaging simultaneously. The main objectives in the roadmapping of silicon integrated 3D imaging radar are higher image resolution, a larger image range and shorter acquisition time. With the dramatically improved performance of silicon transistors, mm-wave circuits using CMOS and BiCMOS technologies can generate picosecond-level impulses but with small RF power. Shorter impulses provide higher image resolution, but small RF power limits image range. Spatially coherent impulse combining from multiple silicon circuits is the solution to this problem. Compared with narrow-band phased-arrays that perform only 2D spatial filtering and have range-ambiguity problems, impulse-radiating arrays are capable of performing 3D spatial filtering that enhances the imaging sensitivity of a certain point in 3D space without sacrificing image resolution. Therefore, impulse-based MIMO imaging radar can achieve both high resolution and a large image range simultaneously. In this present work, a 60ps impulse radiator with an on-chip antenna is implemented in the IBM 130nm SiGe BiCMOS process technology. The impulse radiator is the core element of the synthetic arrays that are used to perform 3D imaging in this thesis. A pulsed-VCO-based architecture is designed based on an asymmetric cross-coupled pulsed VCO to convert a digital input signal to radiated impulses. The deliberate asymmetry in the pulsed VCO is introduced to minimize the timing jitter of the radiated impulses in order to achieve spatially coherent impulse combining with high efficiency. The radiated impulses have a record RMS jitter of 178fs with 64 averaging when the input trigger signal has a RMS jitter of 150fs. Two widely spaced impulse radiators are used to perform spatially coherent impulse combining with an efficiency of 98.7%. As the first step in demonstrating impulse-based MIMO 3D imaging radar, in this work, custom synthetic array imaging systems were built based on the proposed silicon-based integrated impulse radiator. 3D imaging of metallic and dielectric objects (rocks immersed in oil) have been performed successfully. A depth accuracy of 27um, a depth resolution of 9mm and a lateral resolution of 8mm at 10cm distance in the air have been achieved. To the author’s knowledge, this work demonstrates the first high-resolution 3D images that are generated by using synthetic array imaging systems based on a fully-integrated impulse radiator in silicon. Future work includes implementing fully integrated impulse transceivers and fully integrated impulse-based MIMO 3D imaging radar with independent time-delay controls.Item Large-Scale Software Defined Radio Systems: Design, Implementation, and Evaluation(2019-04-19) Guerra, Ryan Guerra Elliot; Knightly, Edward WSince 2012, Television White Space (TVWS) systems have been permitted unlicensed operation on unused television channels between 50 to 800 MHz utilizing radio spectrum sharing techniques. Often considered “beachfront property” radio spectrum for their advantageous propagation characteristics, 6 MHz TVWS channels are nevertheless narrow relative to other unlicensed radio frequency bands and often fragmented, resulting in low network throughput and limiting their usefulness for modern, high-bandwidth applications. However, new many-antenna radio technologies have been shown to improve spectral efficiency beyond 100 bits/s/Hz, mitigating the need for wide channel bandwidths by leveraging spatial multiplexing. This presents an opportunity to deploy new unlicensed wireless networks that are large-scale in both range and speed as well as the number of coherent radios utilized for Multi-User Beamforming (MUBF). In this thesis, we design and implement the first scalable, agile, Software-Defined Radio (SDR) platform designed to support multi-user beamforming on TVWS. This design addresses key physical layer implementation barriers such as: fast automatic gain control, ultra-wideband power transfer networks, and distributed clocking architecture for scalable MUBF. Through an extensive series of indoor and outdoor measurements using our new platform, we show that in comparison to other unlicensed frequency bands, measured TVWS channels have temporal characteristics that are more beneficial for MUBF while maintaining nearly the same amount of spatial diversity. We leverage this new experimental insight to design an opportunistic MUBF protocol for 802.11af-like networks that can avoid all overhead associated with MUBF channel estimation by exploiting the high stability of fixed TVWS channels. We emulate this protocol based on empirical measurements and show that our opportunistic channel sounding protocol outperforms alternative 802.11af-based strategies for 4×4 or 8×4 MUBF when packets are short and modulation rates are high. For high-order MUBF like 32 × 16, opportunistic channel sounding outperforms alternatives by avoiding significant overhead that scales with the degree of spatial multiplexing. Through a comprehensive end-to-end system design addressing hardware, digital, and protocol challenges with final system validation, we develop a holistic new approach for leveraging TVWS to enable very large-scale, unlicensed wireless systems with gigabit network throughput.Item MAC Layer DATA/ACK Handshake in the Hybrid VLC-RF System(2016-04-20) Zhang, Junyi; Knightly, Edward WThe hybrid VLC-RF system utilizes a uni-directional VLC link for downlink and the legacy Wi-Fi link for uplink transmission. However, simply integrating the two links without any modi cation in the MAC layer would cause problems for the MAC layer DATA/ACK handshake. The acknowledgements for VLC packets have to be transmitted via ra- dio. Because of the Wi-Fi channel's contention based random access, the transmission will be delayed and degrades the performance of the legacy Wi-Fi, especially when the Wi-Fi channel is heavily loaded. In my the- sis, I design and analyze the Spoofed NAV Triggered Multi-client ACKs (STMA) scheme to transmit the delay-sensitive VLC ACK timely while keep the degradation of legacy Wi-Fi under-control. I implement the key components of STMA and evaluate its performance with a combination of over-the-air experiments and trace-driven simulations. The result shows that in a dense WLAN scenario, STMA reduces the response delay and legacy Wi-Fi degradation signi cantly in comparison to 802.11 contention- based approach.Item MU-MIMO WLANs in Diverse Bands and Environments(2015-04-24) Anand, Narendra; Knightly, Edward W; Sabharwal, Ashutosh; Zhong, Lin; Johnson, David BMulti-user MIMO (MU-MIMO) is a precoding technique that allows for an Access Point (AP) to transmit data to multiple receivers in parallel resulting in an overall capacity increase. However, achieving these gains requires significant overhead in first choosing the subset of users to serve and measuring the Channel State Information (CSI) between that user group and the AP. Management of this overhead and other Media Access Control (MAC) decisions are key to the performance of MU-MIMO transmissions. Previous solutions attempt all-purpose approaches which attempt to work well regardless of deployment environment or frequency band. However, through thorough analysis of MU-MIMO transmissions with respect to band and environment, we show that an all-purpose solution will not allow for MU-MIMO transmissions to reach their full potential. In fact, leveraging the differences in MU-MIMO transmission characteristics for different bands and environments to develop separate protocols will allow for increased overall system performance. To address transmission scenarios with high channel variability, we present Pre-sounding User and Mode selection Algorithm (PUMA), a transmission mode and user selection protocol that leverages the characteristics of the highly variable 2.4/5.8 GHz transmission scenarios for efficient MU-MIMO overhead amortization. While PUMA results in significant MU-MIMO capacity gains for 2.4/5.8 GHz indoor environments, it will not allow UHF-band indoor and outdoor MU-MIMO transmissions to reach their full potential. To that end, we design and implement Feedback Removal with Opportunistic Zero-overhead channel EstimatioN (FROZEN), a protocol that harnesses the channel stability of the UHF band to eliminate sounding overhead by relying on CSI measurements from previously received uplink packets.Item PERFORM: A Platform for Experimental Research in WLAN with Focus on Real Network Traffic and Multi-User Channel Access(2020-04-21) Da Silva Goncalves, Vinicius; Knightly, Edward WMulti-user (MU) - MIMO and OFDMA - is the next revolution in WiFi. Many experimental platforms today can support the exploration of physical layer aspects of MU but cannot run real-time experiments integrated into end-to-end network systems. However, recent work has shown that real network traffic regimes can be equally important to the MU performance of a WLAN. In this thesis we present PERFORM, a novel experimental platform for implementation and evaluation of WLAN MAC policies. This platform is the first end-to-end integrated system that carries real-time network traffic and, at the same time, allows for the flexible prototyping and evaluation of advanced WLAN MAC policies, including MU-MIMO, OFDMA, triggered uplink access, buffer status report, and data-driven policies. We present the platform validation experiments and also show the application of PERFORM in two uplink MU-MIMO experiments to showcase its capabilities and the new research that it enables.Item Performance Evaluation of Angularly Mismatched Leaky Wave Antenna-based Terahertz Links(2022-01-27) Ahmad, Mohammad Furqan; Knightly, Edward WLeaky wave antennas (LWAs) have been experimentally demonstrated in recent years as a promising candidate for THz-scale networking. They operate on the principle of angular dispersion, where higher frequencies radiate with maximum power towards smaller angles. These devices offer frequency support upto 1 THz, where this coupling has enabled dynamic beam steering through frequency tuning and path discovery. Obtaining these benefits at both the transmitter and receiver requires deploying LWAs at both nodes. However, in such a link, since angular dispersion affects both transmission and reception, a given frequency is maximally coupled out of the transmitter and into the receiver only if the emission and reception angles are equal. In this work, we perform the first performance evaluation of angularly misaligned THz LWA-LWA links, utilizing a mix of numerical studies and over the air experiments, with the following key contributions. We first introduce a numerical model yielding data rate in a LWA link, capturing how the fall in data rate arising from coupling loss due to misalignment is not only angle dependent, but is non-linearly dependent on both frequency and bandwidth. Using this model, we perform the first numerical study to quantify the rate and coverage penalty of angularly mismatched LWA links, demonstrating the tradeoff that the link is more robust to misalignment at large emission angles, but has access to lower data rates, and that perfect alignment does not maximize data rate. We next evaluate the performance of using both multiple angularly separated LWA slots per sector, and opportunistically utilizing non line of sight (NLoS) paths as a means of obtaining multiple options in emission and reception angles, to overcome the rate loss due to misalignment. We show that the higher the emission angle, the fewer the number of slots required to overcome mismatch to a given extent, and that the average rate achievable with N+1 additional NLoS paths is comparable to that attainable with N additional slots. Finally, we perform the first experimental study of the performance of angularly mismatched THz LWA links, where we show that as compared to the numerical model, not only is data rate maximized with even greater misalignment, but that the experimental link is far more robust to angular mismatch.Item Performance Evaluation of MU-MIMO WLANs Under the Impact of Traffic Dynamics(2016-04-14) Nayak, Peshal; Knightly, Edward WDownlink multiuser MIMO (DL MU-MIMO) has shown a great promise to enhance the performance of a wireless network by serving multiple users simultaneously. While researchers have focused on tackling the challenges involved in deploying these networks in the real world, little is known about their performance under the impact of traffic dynamics. In this paper, we evaluate the performance of DL MU-MIMO under the impact of traffic dynamics. Specifically we study how the system behaves under closed loop (FTP/TCP flows) and open loop traffic (UDP flows). We extend the network simulator ns 3 to include support functionalities for 802.11ac compliant DL MU-MIMO simulations. By performing extensive simulations, we find that 802.11ac MU-MIMO achieves extremely small queue lengths ($\approx$ 2 packets) under TCP flows. Moreover, the contention based channel access reduces the user set size for downlink transmission thereby affecting the multiplexing gain as well. Consequently, the downlink performance is extremely poor. Unfortunately we find that even under an ideal network and channel conditions, the system performs worse than a SISO system. We show that while the downlink performance can be improved by enabling frame aggregation for uplink transmissions, the multiplexing gain remains constrained due to the contention based channel access mechanism. This shows that 802.11ac MU-MIMO has severely limited throughput under closed loop traffic. Furthermore, we find that with the AP's co-ordination the system performance shifts closer to the maximum achievable throughput which is not possible under a contention based access mechanism. Next we show that with the help of a multi-user uplink, 802.11ac compliant DL MU-MIMO achieves the maximum achievable throughput under closed loop traffic. Finally, we explore how the system performs under open loop traffic burstiness. We find that the aggregate downlink throughput shows a exponential decrease with an increasing amount of traffic burstiness. While the system does achieve a high throughput at lower levels of traffic burstiness, at higher levels there is no gain from simultaneous data transmission.Item Pseudo Lateration: Millimeter-Wave Localization Using a Single Infrastructure Anchor(2016-04-22) Chen, Joe; Knightly, Edward WWhile radio-based indoor localization schemes achieve decimeter-scale accuracy, they typically require precise reference measurements or multiple infrastructure nodes with redundant localization anchors. In this paper, we propose Pseudo LATeration (PLAT), an indoor localization protocol that requires only a single infrastructure anchor and does not require prior knowledge of the environment. PLAT leverages the directionality and propagation characteristics of millimeter-wave transmissions to relax the requirement of multiple infrastructure anchors and constructs virtual anchors for multilateration from reflected signal paths. By combining these virtual anchors with time-of-flight measurements for distance estimation, PLAT can localize user devices in indoor environments with only a single infrastructure node. Our evaluation reveals centimeter scale location accuracy for typical office environments. In testbed measurements and simulations, localization errors are below 10 cm for distances up to 1.5 m and beamwidths below 10 degrees. Although accuracy decreases with distance, we show that multiple reflection paths can mitigate this effect.Item Stealthy off-target couple control plane jamming(2022-08-05) Gupta, Shreya; Knightly, Edward W; Sabharwal, Ashutosh; Ng, EugeneMulti- and single-user beamforming is a key feature for realizing a high data rate in next-generation Wi-Fi such as IEEE 802.11ax. In this paper, we study for the first time, a jammer that strategically attacks layer two (L2) control frames associated with beamforming to realize a denial-of-service (DoS) attack on transport (L4) and application layer (L7) control planes. By coupling the attack across control plane layers, the attacker targets throughput or availability DoS while also maintaining stealth via low airtime jamming and “off-target jamming” in which the targeted higher layer message is never directly jammed. With end-to-end application layer experiments, we show that such a jammer can reduce TCP throughput to 1% with less than 0.1% of jamming air-time. Moreover, the attack can yield seconds-to-minute scale outages by targeting the L4 or L7 setup messages while leaving a minimal footprint.Item Triggered Uplink MU-MIMO and Access Adaptation for 802.11 WLANs(2022-11-30) Da Silva Goncalves, Vinicius; Knightly, Edward WUplink Multi-User (MU) MIMO transmissions allow clients to simultaneously transmit independent data streams to the Access Point (AP), effectively multiplying the capacity of the wireless channel for uplink access. The IEEE 802.11ax amendment defines the Triggered Uplink Access (TUA) mechanism as the only way to initiate an uplink MU-MIMO transmission in Wi-Fi, which enables an AP to start and time synchronize simultaneous uplink MU transmissions by broadcasting a trigger frame containing the resource allocation information. Unfortunately, the aforementioned procedure introduces challenges in the design and implementation of medium access control policies for 802.11 wireless networks, including contention and scheduling problems. In this thesis we present the following three contributions. First, we design, implement, and validate PERFORM, a novel experimental platform for implementation and evaluation of WLAN MAC policies. This platform is the first end-to-end integrated system that carries real-time network traffic and, at the same time, allows for the flexible prototyping and evaluation of advanced WLAN MAC policies, including MU-MIMO, TUA, and buffer status reports. Second, we experimentally study the role of real application traffic on the performance of TUA. While TUA gains for fully backlogged traffic are well established, we show that bursty closed-loop traffic radically transforms performance. Last, we introduce Client-side Access Manipulation (CAM) as a mechanism that enables clients to dynamically adapt their channel access priority without explicitly coordinating state with the AP to realize an efficient uplink multi-user WLAN. Our results start by presenting the PERFORM platform validation, which verifies that the platform can achieve the necessary performance targets. Then, using PERFORM, we find that TUA significantly reduces file transfer latency compared to legacy single-user uplink, but unfortunately the standardized method for low-overhead backlog reporting leaves substantial benefits unrealized. Last, we show that CAM achieves gains in throughput and up to 65% reduction in average latency. Moreover, with the use of the standard’s defined access adaptation mechanism on the same scenarios, the aggregate throughput decreases, and the average latency increases sharply.