David Michelson

Associate Professor

Relevant Degree Programs

 

Postdoctoral Fellows

  • Zahra Vali (Telecommunication Systems, Telemetry (Remote Sensing, Radar))

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Mar 2019)
Physical-layer security for visible-light communication systems (2017)

Visible-light communication (VLC) is an enabling technology that exploits the lighting infrastructure to provide ubiquitous indoor broadband coverage via high-speed short-range wireless communication links. On the other hand, physical-layer security has the potential to supplement conventional encryption methods with an additional secrecy measure that is provably unbreakable regardless of the computational power of the eavesdropper.The lack of wave-guiding transmission media in VLC channels makes the communication link inherently susceptible to eavesdropping by unauthorized users existing in areas illuminated by the data transmitters. In this thesis, we study transmission techniques that enhance the secrecy of VLC links within the framework of physical-layer security.Due to linearity limitations of typical light-emitting diodes (LEDs), the VLC channel is more accurately modelled with amplitude constraints on the channel input, rather than the conventional average power constraint. Such amplitude constraints render the prevalent Gaussian input distribution infeasible for VLC channels, making it difficult to obtain closed-form secrecy capacity expressions. Thus, we begin with deriving lower bounds on the secrecy capacity of the Gaussian wiretap channel subject to amplitude constraints.We then consider the design of optimal beamformers for secrecy rate maximization in the multiple-input single-output (MISO) wiretap channel under amplitude constraints. We show that the design problem is nonconvex and difficult to solve, however it can be recast as a solvable quasiconvex line search problem. We also consider the design of robust beamformers for worst-case secrecy rate maximization when channel uncertainty is taken into account.Finally, we study the design of linear precoders for the two-user MISO broadcast channel with confidential messages. We consider not only amplitude constraints, but also total and per-antenna average power constraints. We formulate the design problem as a nonconvex weighted secrecy sum rate maximization problem, and provide an efficient search algorithm to obtain a solution for such a nonconvex problem. We extend our approach to handle uncertainty in channel information.The design techniques developed throughout the thesis provide valuable tools for tackling real-world problems in which channel uncertainty is almost always inevitable and amplitude constraints are often necessary to accurately model hardware limitations.

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Characterization and simulation of non-line-of-sight fixed wireless links for smart grid applications (2016)

Electric power utilities are deploying wireless networks operating in mesh and point-to-multipoint configurations over near- and non-line-of-sight (NLOS) fixed links in tremendous numbers to support advanced metering and distribution automation applications. However, effective techniques for characterizing and simulating NLOS channels are required to support efficient implementation, deployment and operation of such networks.In Part I, we contribute to techniques for small-scale channel characterization by: 1) proving the equivalence of the Ricean fading distributions observed in the delay, spatial and frequency domains and their relationship to fading observed in the temporal domain, 2) demonstrating the advantages of estimating the Ricean K-factor from channel frequency response data, and 3) revealing the conditions under which the channel impulse response can be estimated from scalar frequency response data using the Hilbert transform. In addition, we propose a method for estimating the noise floor in measurement-based estimates of the channel impulse response that allows more accurate estimation of delay spread.In Part II, we contribute to the practical use of Loosely Synchronous (LS) pseudorandom codes to characterize dynamic MIMO channels by: 1) revealing the manner in which the autocorrelation and cross correlation properties of LS codes degrade when channel SNR is low and under reduced bit resolution using both simulation and measurement approach and 2) showing how fibre delay lines can be used to permit a single-port channel receiver to effectively measure the response of multiple receiving antennas simultaneously. This allows configuration of channel measurement equipment that captures the channel response in real time faster than the similar single-port channel sounders developed with switches.In Part III, we contribute to simulation of fixed wireless networks in suburban macrocell environments by demonstrating how shadow fading varies as a function of terminal height and building height distribution and the manner in which it affects the system coverage.The results contribute to the simulation and modeling framework developed by the National Institute of Standards and Technology (NIST) by more effective characterization and simulation of fixed wireless channels and better coverage and deployment cost estimation for Smart Grid applications.

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Mitigating the effect of propagation impairments on higher layer protocols and algorithms in wireless sensor networks (2016)

Wireless sensor networks (WSNs) range from body area networks (BANs) that involve a relatively small number of nodes, short paths and frequent update rates to precision agriculture wireless sensor networks (PAWSNs) that involve a relatively large number of nodes, long paths and infrequent update rates. They are distinguished from wireless access networks by: 1) their mesh architecture and reliance on higher layer protocols and algorithms to perform routing, scheduling, localization, and node placement, 2) their need to operate for long periods of time with only limited access to battery or scavenged power. Energy conservation has long been an important goal for developers of WSNs and the potential for reducing energy consumption in such networks by reducing the strength and/or frequency of transmission has long been recognized. Although the impact of propagation impairments on the physical and media access control layers of WSNs has long been considered, few previous studies have sought to assess their impact on higher layer protocols and algorithms and devise schemes for mitigating or accounting for such impacts. Here, we present four case studies that demonstrate how higher layer protocols and algorithms can be devised to achieve greater energy efficiency by accounting for the nature of the propagation impairments experienced. In the first two case studies, we focus on BANs and: 1) propose a routing protocol that uses linear programming techniques to ensure that all nodes expend energy at a similar rate and thereby maximize network lifetime and 2) propose a scheduling algorithm that accounts for the periodic shadowing observed over many BAN links and thereby reduce the transmit power required to transfer information and thereby maximize network lifetime. In the second two case studies, we focus on PAWSNs and 3) propose an efficient localization algorithm based on the Bayesian model for information aggregation and 4) demonstrate that path loss directionality observed in sites such as high density apple orchards greatly affects WSN connectivity and, therefore, energy consumption and must be considered when designing node placement in agricultural fields.

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Configuration of multiple input multiple output antenna arrays for wireless communications in underground mines (2013)

In recent years, the underground mining community has begun to embrace standards-based short-range wireless communications technology as a key part of their strategy for enhancing the safety and productivity of their operations. Here, we show how the significant differences between wireless propagation in conventional surface environments and underground mines affect the design of modern wireless communications systems based upon multiple-input multiple-output (MIMO) antenna array technology. In order to achieve this goal, we have employed a variety of approaches to characterize wireless propagation (and MIMO-based wireless system performance) in underground environments representative of those found in modern hard rock mines, including: 1) field measurements collected using a custom-designed channel sounder in both a building service tunnel at the University of British Columbia and an underground lead-zinc mine at Myra Falls, BC, 2) simulations based upon ray-tracing in representative environments and 3) theoretical models based upon waveguide mode expansion in representative environments. We have used the results obtained: 1) to determine the reduction in the angular spread of multipath signals that arrive at the receiver in an underground mine compared to that observed in conventional surface environments and the manner in which it decreases with increasing transmitter-receiver separation and 2) to show that the antenna elements in MIMO antenna arrays used in underground environments must therefore be separated by several wavelengths (rather than the customary half-wavelength used in surface environments) in order to achieve acceptable performance. Further, the separation between the antennas must increase as the transmitter-receiver separation increases, higher order modes attenuate and, as a consequence, angular spread decreases. Other outcomes of this work include: 1) demonstration that the power azimuth spectrum (PAS) in underground mine environments can be modeled by a Gaussian distribution and 2) development of a novel technique based upon particle swarm optimization (PSO) for assessing and optimizing the performance of distributed-MIMO antenna systems in underground mine environments.

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Master's Student Supervision (2010-2017)
Characterization of persistent multipath components in indoor and outdoor environment at 30 GHz (2018)

Millimeter-wave (mm-wave) frequency bands are under active consideration for use as short-rangemobile broadband links in fifth generation (5G) cellular access networks. Although channel characteristicssuch as path loss, delay spread and fading distributions have been extensively studied formm-wave channels, the study of the time-varying nature of the channel is still in its early stages.In this work, we studied the lifetime of multipath components of the mm-wave channel, usually referredto as persistence. An important time-varying characteristic of the mm-wave channel, persistencemay affect the capacity, and beam training and beam tracking process of mm-wave systems.We developed a 30-GHz vector-network-analyzer-based channel sounder suitable for characterizingmultipath persistence and verified its performance through a three-stage verification procedure;time and frequency domain verifications, two-ray verification, and measurements conducted usingthe National Institute of Standards and Technology (NIST) mm-wave channel sounder verificationartifact.The primary goal of this work was to characterize multipath persistence based on measurementsconducted at 30 GHz in indoor and outdoor urban microcell environments. Through analysis of ourmeasurement data, we confirmed that the log-logistic distribution provides an accurate descriptionof persistence and showed how the physical attributes of the channel influence the parameters ofthe distribution. We also verified that a weak correlation exists between average received powerand length of the persistent path. We further showed that the rate of angular change of a multipathcomponent throughout its lifetime follows a Laplace distribution and that the angular rate dependson the distance of reflectors from the transmitter-receiver path. We used these results to proposea simulation model that can be used to make simple ray tracing simulations more realistic and toassess the effect of persistence and variations in the angular rate on the capacity, and beam trainingand tracking process of mm-wave systems.

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Ultra low profile VHF antennas for M2M communications via satellite (2016)

Government transport regulations and practical considerations limit the height of terminal antennas used in satellite-based mobile asset tracking applications to less than 2.5 cm. For Orbcomm systems that operate at 138 MHz (downlink) and 150 MHz (uplink), this implies an antenna that is just over one-hundredth of a wavelength tall. Achieving good efficiency and operating bandwidth with such an ultra low profile antenna is fundamentally difficult. Here we consider the possibility of using a multi-arm normal mode cylindrical helix antenna to achieve a significant fraction of the performance of a full size Orbcomm reference antenna in a more compact form. In order to simplify impedance matching, we introduced an internal magnetic coupling loop that can be increased or decreased in radius in order to achieve a good match. In order to identify the optimum design, we assessed the radiation efficiency and bandwidth of the antenna as a function of the key design parameters (helix height, radius, number of arms, number of turns, feed loop radius, presence or absence of crossbars that connect the arms at the top of the helix) using simulations and validated the results by measuring the performance of selected hardware prototypes. Further, we developed an equivalent circuit model that allows one to extract key design information much more quickly than would be possible by simulation or measurement. Our design curves show that bandwidth can only be increased with height and is independent of radius at ULP height. We found that efficiency increased significantly with helix radius but the presence of crossbars yielded only a marginal increase in efficiency. A pair of ULP antennas is required for uplink and downlink. To connect two such antennas to a standard Orbcomm modem with a single antenna port, we considered three duplexer designs: a carrier operated relay, a diplexer filter, and a novel complementary feed. The latter is both simpler and more effective than the first two. Over a 72-hour period, our best ULP antenna design (four arms, no cross bar, helix radius ~0.04 wavelengths) was able to exchange 45% of the messages that a standard Orbcomm reference antenna could.

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Toward cognitive vector network analyzers for conducting wireless stimulus-response measurements in open-area environments (2015)

Vector network analyzers (VNAs) are often used to measure antenna performance, channel response, and shielding effectiveness in open-area environments. In such applications, external interference from other users may sporadically occupy portions of the frequency band of interest and thus compromise the integrity of the measurements. The simple strategies for avoiding such interference that are commonly employed may be ineffective because: 1) clear channels within the band may not be available, 2) it may be difficult to find suitable antennas for use in adjacent clear bands, 3) the other users in the band may be uncooperative, 4) the interference encountered in the band may be intolerable even during off-peak hours or 5) it may not be possible or convenient to move to a different measurement location. Here, we show that the reliability and accuracy of VNA-based wireless measurements performed under such circumstances can be significantly improved by applying cognitive radio concepts where uncooperative wireless systems are cast as primary users and the VNA is cast as the secondary user. For the case of long-burst interference, i.e., scenarios dominated by voice and video transmissions that are much longer than the VNA measurement dwell time, we propose and demonstrate a scheme that uses carrier sensing to: 1) avoid collisions between VNA and primary user transmissions and 2) identify and reject corrupted measurements. For the case of short-burst interference, i.e., scenarios dominated by data packet transmissions that are much shorter than the VNA measurement dwell time, we show that identification and rejection of corrupted measurements is more difficult but can be accomplished by modifying the interference-aware VNA to apply robust estimation to the results. The main limitation of the second scheme is the time required to collect the additional measurement data required. In both cases, re-purposing existing hardware within the VNA and making relatively minor enhancements to the firmware would both simplify implementation and significantly decrease the data collection time. Both schemes represent an important step toward realizing a fully cognitive VNA that is capable of sensing its environment and configuring itself to conduct interference-free wireless measurements as quickly and effectively as possible.

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Deployment of wireless sensor and actuator networks for precision agriculture (2013)

Wireless sensor and actuator networks (WSAN) are emerging as a key enabling technology for precision agriculture, a technique for maximizing crop yield and quality through targeted application of resources such as water, fertilizer and pest control agents by exploiting temporal and spatial variability in crop and soil conditions. In this thesis, we make three contributions to the field. First, we assess the state of the art in deployment and configuration of wireless sensor and actuator networks for precision agriculture, including the relevance and suitability of existing propagation models, lessons learned from previous demonstrations and field trials, and the potential for improving network performance through suitable deployment strategies, and physical, medium access control (MAC) and network layer design. We reveal an urgent need to assess airlink design of such networks to account for the unique nature of the wireless propagation environment and to consolidate proposed improvements to and best practices for WSAN design in the form of an industry standard. Second, we show that the conventional practice of employing wireless transceivers that operate at 800 MHz or above incurs significant penalties for achievable range and/or power consumption and propose that low-power short-range wireless devices intended for use as sensor nodes in precision agriculture be allowed to share the 433 MHz sub-band currently authorized for use by active radio frequency identification (RFID) devices at cargo terminals, port facilities and warehouses so that they may experience less path loss and achieve greater range and reliability while consuming less power. Finally, we analyze 2450 MHz channel impulse responses that we measured in a high-density apple orchard and consider the implications of both their form and scale for the design and deployment of WSANs and our understanding of the propagation environment. Of particular note is the vastly reduced delay spread compared to that observed in traditional residential, commercial and industrial environments

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Characterization of fixed wireless channels for use in distribution automation networks (2011)

Distribution Automation (DA) is the use of advanced communications network infrastructure in combination with intelligent power equipment and intelligent electronic devices (IEDs) in substations and on distribution feeders to monitor, protect and control the electrical power distribution network. Because wireless networks are generally less expensive, easier to deploy and more resilient than alternatives such as fibre-optic and power line carrier (PLC) networks, they have attracted considerable interest from DA network designers. However, designers must take careful account of the manner in which: 1) the useful range of the links between wireless devices impacts the formation of wireless networks, 2) the depth of fading varies across the range of frequencies available to power utilities for use in such networks, and 3) the degree of shadow fading varies as the height of wireless devices used in such networks rise from pedestrian to pole-top level. Here we show that: 1) the manner in which the distribution assets in BC Hydro's electrical power distribution network are geographically distributed affects the ease with which such assets can be formed into either conventional base-station-to-pole-top fixed wireless macrocell networks or pole-top-to-pole-top fixed wireless mesh networks, 2) the depth of fading experienced on fixed wireless macrocell channels is generally proportional to the carrier frequency and 3) the degree of shadow fading experienced on fixed wireless macrocell channels often increases as the terminal height is raised from pedestrian to pole-top level. These results will help power utilities design reliable and cost-effective wireless networks in support of DA.

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Characterization of radiowave propagation in indoor industrial environments (2010)

In recent years, the successful introduction of short-range wireless technology in both consumer and commercial markets has attracted considerable interest from designers of industrial plants and factories. Wireless technology can be used to achieve both flexibility and cost reduction when installed and utilized for industrial process control networks and factory automation systems. Effective application of wireless devices in industrial environments requires careful assessment of the potential uses for such devices, methods for characterizing the wireless channel, and accurate models for the impairments introducted by the wireless channel. Here, we show that: (1) Although conventional wireless technologies such as ZigBee and WiFi are currently used in many industrial applications, ultrawideband (UWB) wireless technologies offer unique capabilities that may lead to their playing key roles in future industrial applications. (2) Our computer-assisted technique for fitting the Saleh-Valenzuela model to measured UWB channel impulse responses (CIRs) offers a more rigorous and reproducable method for characterizing UWB channels than existing manual techniques can. (3) Although relatively little propagation data has been collected in indoor industrial environments to date, we combine these results to form a single rationalized UHF/UWB propagation model that is useful to designers and fills an important immediate need of designers while revealing gaps in our current understanding that need to be completed by future researchers.

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