Resumen de Robust and reliable millimeter wave wireless networks
Millimeter wave (mmWave) technology is one of the main pillars of the next generation Wireless Local Area Network (WLAN) and 5G mobile networks. The main reason lies in the quantum leap of capacity it provides with respect to wireless networks operating in the sub-6-GHz band. Nevertheless, efficient and reliable communication in the mmWave band demands novel techniques to tackle all the barriers associated with wireless propagation in this band. For example, material penetration and diffraction in the mmWave band are much weaker than the ones experienced in the sub-6-GHz band. As a result, wireless propagation in the mmWave band has a quasi-optical behavior in which the Line-of-sight component contributes to the majority of the received signal power. For these reasons, mmWave devices rely on either horn antennas or electronically steerable phased antenna arrays to establish directional beams and focus the energy towards a specific direction in space. Although directional communication compensates for the high power attenuation, it creates a new problem that is uncharted for wireless networks operating in the sub-6-GHz band. It makes mmWave wireless links susceptible to blockage, human mobility, and device rotation. Solving these issues requires developing algorithms to quickly find alternative paths for communication upon link interruption.
The IEEE 802.11ad protocol is the first WLAN standard that supports wireless networking in the unlicensed 60 GHz band. IEEE 802.11ad tackles the aforementioned problems by introducing new mechanisms at the medium access control (MAC) and physical (PHY) layers such as beamforming training and beam tracking, hybrid channel access scheme, relay operation mode, multi-band operation, etc. The performance of 60 GHz networks is the result of the interaction of all layers of the protocol stack with the wireless medium. To understand this interaction, it is fundamental to consider 60 GHz networks as a whole. Nevertheless, real-world experimentation with mmWave communication is not always feasible due to the significant amount of resources required and its associated costs. For these reasons, we develop in this thesis a high fidelity system-level model to simulate the IEEE 802.11ad standard. This allows us to study large-scale wireless networks operating in the 60 GHz band, taking into account all of the essential features supported by the standard. We investigate the networking aspects of various mmWave wireless network deployments and provide solutions to boost their performance. Additionally, since most of the mmWave devices are anticipated to adopt the carrier sense multiple access with collision avoidance as the primary channel access scheme, we propose two frame-aggregation policies that significantly improve network throughput and reduce end-to-end delay.
To complement our insights from the simulations, we analyze in-depth the performance of Commercial off-the-shelf devices that implement the full 802.11ad protocol stack. Fortunately, a growing number of 60 GHz devices supporting the IEEE 802.11ad standard have become recently available. However, the standard does not specify implementation-dependent characteristics that have a significant impact on device performance. For example, this includes the periodicity of beamforming training, which directly affects ongoing communication and performance under mobility. Also, the placement of the 60 GHz antenna in the device plays a fundamental role regarding self-shadowing, which in turn affects how the device shall be deployed to maximize coverage and improve spatial reuse. Understanding such implementation-dependent issues is crucial to correctly draw the aforementioned system-level insights. In this thesis, we characterize and compare commercial 60 GHz devices which are widely used for research purposes. Particularly, we look at different networking aspects, including spatial sharing, beam patterns synthesis, frame aggregation, and the interactions between carrier sense multiple access with collision avoidance (CSMA/CA) and Transmission Control Protocol (TCP) protocols within dense network settings. In summary, we find significant discrepancies in terms of behavior and performance between these devices and what theory suggests. Additionally, achieving a gigabit of throughput and maintaining low-latency require adopting cross-layer solutions operating between the MAC layer and the transport layer.
The firmware running on these devices provide some experimental capabilities based on the operation of the IEEE 802.11ad protocol. These capabilities include reporting the Channel State Information (CSI) per antenna element for the connected antenna array. This CSI information contains valuable information about the spatial environment. Additionally, this information is sensitive to any minor changes in the environment. This inspires us to build a collaborative scheme consisting of spatially distributed APs that sense the environment and pinpoint the location of an obstacle without involving the end users.
In this thesis, we first delve into the operation of the IEEE 802.11ad protocol and analyze its performance both in simulations and practice. Second, we propose a set of solutions and recommendations to boost its efficiency. Finally, we extend its scope beyond typical wireless communication and utilize it to build a passive localization system.
Introduction: Wireless communication in the mmWave band is a crucial enabler for the next generation wireless technologies such as 5G Mobile Networks. Thanks to the massive amount of spectrum available, mmWave can be used for a wide range of scenarios where, traditionally, wired solutions have been utilized, such as ultra-high definition video streaming, virtual-reality headsets, or wireless front-hauling and back-hauling in mobile networks. With the mature advancements and developments in electronics components and Radio Frequency (RF) circuits operating at these frequencies, manufacturing relatively low-cost mmWave electronics circuits became possible. This resulted in many commercial standards operating in the 60GHz mmWave band such as the IEEE 802.15.3c, and the IEEE 802.11ad. However, mmWave communication is extremely challenging compared to traditional microwave communication. The main reason lies in the mmWave signal characteristics which include high attenuation, susceptibility to blockage and human mobility, and the necessity of a LOS path for stable communication. The high signal attenuation in the mmWave band mandates devices to utilize phased antenna array to establish a directional communication link. However, the management of highly directional mmWave links poses some challenges that are alien to the more traditional sub-6-GHz bands. Namely, beam training or user association methods, triggered upon link degradation due to blockages, may incur substantial channel time wastage, particularly in highly-dynamic scenarios with moving receivers, transmitters, and reflectors. As a result, efficient wireless communication in this band is challenging and requires adequate network planning and efficient design of wireless networking protocols.
In this thesis, we propose various techniques to tackle the underlying challenges associated with wireless networking in the mmWave band. First, we create a high-fidelity open-source model for simulating the IEEE 802.11ad protocol in network-simulator ns-3. Using this model, we study the performance of the MAC layer of the 802.11ad protocol, and we identify several deficiencies that penalize its performance for various deployment setups. For each identified problem, we provide novel solutions that enhance wireless networking performance and improve protocol operations and efficiency. Next, we study and analyze the behavior of practical 802.11ad COTS devices. Particularly, we look at different networking aspects including TCP performance over carrier sense multiple access with collision avoidance for dense mmWave network, spatial reuse when utilizing practical phased antenna arrays, beam patterns synthesis for different frequency channels, etc. For each networking aspect, we study its impact on the overall network behavior and performance. Then, we propose standard-based solutions to tackle each issue independently. Finally, we go beyond just a typical wireless communication in the mmWave band. We build a mmWave sensing system based on COTS 60 GHz devices. The system is capable of sensing human presence and determines its location passively and transparently to the communicating devices.
Motivations and Contributions: This thesis is dedicated to investigate and improve the performance of the IEEE 802.11ad protocol in simulations and practice. Additionally, it proposes a novel mechanism to utilize the IEEE 802.11ad protocol beyond communication to understand the surrounding environment and provide reliable communication links. To achieve these objectives, the main motivations and contributions in this thesis are as follows:
Networking Simulation Tool: Experimental evaluation of networking in the mmWave band is extremely costly, and available hardware has minimal capabilities. Despite the availability of commercial devices utilizing the IEEE 802.11ad protocol, these devices provide only limited access to the operations of the lower layers of the protocol stack, which hinders in-depth analysis and development of innovative solutions. In such cases, resorting to network simulation is a very useful alternative which abstracts implementation details while providing a good grade of realism. However, there are no publicly available simulation tools supporting IEEE 802.11ad in the mmWave band. For these reasons, in Chapter 3, we present the implementation of a high-fidelity model for simulating the IEEE 802.11ad protocol using network simulator ns-3. The model allows network researchers to understand the interactions and behavior of mmWave devices. Additionally, it paves the way for the development of innovative wireless networking solutions. Our work is the first to provide an open source model for the IEEE 802.11ad amendment in ns-3.
At the time of writing, the WiFi Alliance is finalizing the next generation multi-gigabit standard to support wireless networking at 60 GHz, the so-called IEEE 802.11ay. IEEE 802.11ay is envisioned to support extremely high data-rates of up to 300 Gbps, achieved through new complex physical layer techniques including multiple-input and multiple-output (MIMO) communication, channel bonding and aggregation, and high order modulation schemes. Simulating the IEEE 802.11ay standard in a network-level simulator requires accurate abstraction models to incorporate the effects of those techniques. However, ns-3 still lacks support for multi-user MIMO (MU-MIMO) communication. Additionally, it requires generating environment dependent Signal-to-Noise Ratio (SNR)-to-bit error rate (BER) look-up tables to accurately simulate single-user MIMO (SU-MIMO) communication. At the end of Chapter 3, we propose a hybrid implementation that includes minimum signal processing blocks to accurately simulate IEEE 802.11ay SU/MU-MIMO communication in ns-3 with high accuracy and reduced computational complexity.
In Chapter 4, we demonstrate the capabilities of the IEEE 802.11ad model in ns-3. First, we study the performance of each newly introduced technique in 802.11ad independently and validate its behavior. Then, we study the performance of the IEEE 802.11ad protocol for various deployment settings using realistic phased antenna arrays and quasi-deterministic channel models. More particularly, we look at the impact of LOS blockage and the use of Non-line-of-sight (NLOS) paths on link performance. Besides, we show the benefits of deploying multiple access points per room to guarantee gigabit throughput per user. Finally, we evaluate the performance of the IEEE 802.11ad protocol in a typical high-density scenario.
Frame Aggregation in Wireless Networks: The impact of frame aggregation on WLAN performance increases dramatically with higher data rates. The key problem is that the transmission time of packets decreases while the medium access, preamble, and packet header overhead remain the same. Recent 802.11 standards address this issue using frame aggregation, i.e., grouping multiple data frames in a single transmission to reduce the overhead. This already provides substantial efficiency gains in networks operating in the 2.4 GHz and 5 GHz bands, and for the 60 GHz networks such as 802.11ad, gains are even more pronounced due to the order-of-magnitude higher data rates. In 802.11ad, frame aggregation becomes crucial to achieve the multi-Gbps data rates that are possible in theory, since medium access overhead can be 20 times larger than the time required to transmit a single packet. While frame aggregation is essential, it largely depends on the traffic patterns present in the wireless network, and a node may not always have enough packets in the transmit queue to achieve a sufficiently large aggregated frame size. A particularly harmful case occurs when new packets arrive just after a node started the transmission of a small group of aggregated packets. In Chapter 5, we investigate in which case 802.11ad enabled devices should wait to construct a larger aggregated packet before starting the channel access procedure. We present a simple waiting policy for the uplink case that either waits for a minimum number of packets or for a maximum amount of time, whichever comes first. For the downlink case, we utilize a maximum weight scheduling policy with a maximum waiting time. Our results show that both policies significantly improve medium utilization, thus increasing throughput and reducing end-to-end delay.
MAC and Transport Layer Aspects in Practical COTS 60 GHz Devices: mmWave communication promises high spatial reuse at multi-Gbps data rates in dense wireless networks. Existing work studies such networks using commercial hardware but is limited to individual links. In Chapter 6, we study the performance of dense mmWave deployments featuring up to eight stations. We use a testbed of commercial IEEE 802.11ad mmWave devices that provide access to lower layer parameters. This enables us to analyze the impact of these parameters and other deployment considerations on network performance. We analyze issues such as the impact of channel contention on the buffer size at the transport layer, the effect of frame aggregation, the directivity of practical phased antenna arrays at different frequency channels, and the efficiency of spatial sharing. Our results show that using large buffer sizes with TCP is harmful due to channel contention despite the multi-gigabit-per-second data rates. For COTS, frame aggregation is only beneficial up to a certain level due to higher error rates for large frames. Besides, utilizing beamforming codebook with static entries has an impact on the overall network performance based on the operating frequency. Finally, spatial reuse is limited by imperfect beam patterns with many side lobes and is only significant for interferer distances of more than 10 m. In contrast, selecting beam patterns that maximize spatial reuse allows doubling the throughput.
mmWave Sensing: mmWave communication systems, albeit having attained great momentum, pose challenges that are alien to the traditional sub-6 GHz bands due to the channel sparsity induced by highly-directional links. Namely, beam (re)alignment methods, triggered upon detection of link blockages, may incur substantial overhead. In line with related literature, we argue that to establish pervasive connectivity, mmWave systems shall integrate mechanisms to predict and react proactively to potential blockages. Towards this goal, in Chapter 7 we design and build BeamScanner, a collaborative mechanism across quasi-stationary mmWave APs to detect and track obstacles in indoor scenarios, such as an office room or a living room. The nexus of our approach is the ability of BeamScanner to detect weak reflections, even from human bodies, with readily available commodity hardware. Around this, we design a system controller that operates in two stages. First, BeamScanner collects CSI from custom-built narrow beams spurred across a set of distributed APs. Second, we use k-nearest neighbor (KNN) classifier to infer the location of objects in the environment based on fine-grained data from signal reflections. We evaluate our approach in a typical 6x11-meter indoor room with four commodity APs with a median estimation error of less than 3 m when detecting human obstacles.
Beam Tracking: mmWave devices must use highly directional antennas to achieve Gbps data rates over reasonable distances due to the high path loss. As a consequence, it is crucial to align the antenna beams between sender and receiver precisely. Even minor movement or rotation of a device can result in beam misalignment and thus a substantial performance degradation. Existing work as well as standards such as IEEE 802.11ad tackle this issue through antenna sector probing. This comes at the expense of a significant overhead, which may significantly reduce the performance of mmWave communication, particularly in mobile scenarios. In Appendix A, we present a mechanism that can track both movement and rotation of 60 GHz mobile devices with zero overhead. To this end, we transmit part of the preamble of each packet using a multi-lobe beam pattern. Our approach does not require any additional control messages and is backward compatible with 802.11ad. We implement our scheme on a 60 GHz testbed using phased antenna arrays to obtain SNR measurements. We use these measurements to perform simulations in ns-3 to validate our approach in a wide range of scenarios. Using our scheme, we achieve up to 2 times throughput gain.
