PROGRAMA DE DOCTORADO EN TECNOLOGÍAS DE LA INFORMACIÓN Y LAS COMUNICACIONES
TESIS DOCTORAL
DISEÑO Y APLICACIONES DE SISTEMAS DE ANTENAS INTELIGENTES PARA REDES INALÁMBRICAS EN EL CONTEXTO DE LA INTERNET DE LAS COSAS (IoT)
Presentada por Alejandro Rafael Gil Martínez para optar al grado de Doctor
por la Universidad Politécnica de Cartage na
Dirigida por:
Dr. José Luis Gómez Tornero Codirigida por:
Dr. David Cañete Rebenaque
Cartagena, 2022
DOCTORAL PROGRAMME IN INFORMATION AND COMMUNICATION TECHNOLOGIES PhD THESIS
DESIGN AND APPLICATIONS OF SMART ANTENNA SYSTEMS FOR WIRELESS NETWORKS IN THE CONTEXT OF THE INTERNET OF THINGS (IoT)
Presented by Alejandro Rafael Gil Martínez to the Technical University of Cartagena in fulfilment of
the thesis requirement for the award of PhD
Supervisor:
Dr. José Luis Gómez Tornero Co-supervisors:
Dr. David Cañete Rebenaque
This PhD thesis is presented according to the Compendium of Publications mode, regulated under art. 20 of the Regulation of Official Doctoral Studies of the Technical University of Cartagena of March 24, 2021.
Publications
Article 1.-: A. Gil-Martinez, M. Poveda-Garcia, J. A. Lopez-Pastor, J. C. Sanchez- Aarnoutse and J. L. Gomez-Tornero, Wi-Fi Direction Finding with Frequency-Scanned Antenna and Channel Hopping Scheme IEEE sensors Journal, , vol. 22, no. 6, pp. 5210- 5222, 2022. DOI: 10.1109/JSEN.2021.3122232.
Article 2.-: A. Gil-Martinez, M. Poveda-Garcia, D. Cañete-Rebenaque, and J. L. Gomez- Tornero, Frequency-Scanned Monopulse Antenna for RSSI-based Direction Finding of UHF RFID tags IEEE Antennas and Wireless Propagation Letters,, vol. 21, no. 1, pp.
158-162, 2022. DOI: 10.1109/LAWP.2021.3122232.
Article 3.-: A. Gil-Martinez, M. Poveda-Garcia, J. Garcia-Fernandez, M. Campo-Valera, D. Cañete-Rebenaque, and J. L. Gomez-Tornero, Direction Finding of RFID tags in UHF Band Using a Passive Beam-Scanning Leaky-Wave Antenna IEEE Journal of Radio Frequency Identi cation, doi: 10.1109/JRFID.2021.3122233.
Article 4.-: J. L. Gomez-Tornero, A. Gil-Martinez, M. Poveda-Garcia and D. Cañete-
Rebenaque, ARIEL: Passive Beam-Scanning Antenna TeRminal for Iridiscent and
E cient LEO Satellite Connectivity in IEEE Antennas and Wireless Propagation Letters,
doi: 10.1109/LAWP.2022.3193040.
A mis padres, cuyo apoyo me ha dado inmensa seguridad a la hora de tomar
decisiones
Agradecimientos
En primer lugar, me gustaría agradecer enormemente a mi director José Luis Gómez Tornero y a mi codirector David Cañete Rebenaque. Gracias José Luis por todo el conociento que me has transmitido, pero sobre todo por mirar siempre por lo que era mejor para mí. Has sido mucho más que un simple tutor. Has aparecido en mi vida como profesor, psicólogo, confidente y amigo. Más allá de todo el conocimento profesional que me has aportado, me quedo con toda la sabiduría personal que has sido capaz de transmitirme. Eres una persona entrañable y te guardo un enorme cariño.
Musísimas gracias también David por ayudarme en todo lo que podías, guiándome en mis primeros pasos en la divulgación científica y enseñándome la parte más práctica de la investigación, que seguro me servirá en todo mi futuro profesional. Nunca cambiaría a mi pareja de directores, que han hecho de mi doctorado una experiencia inmejorable.
No podría haber hecho nada de esto sin mi familia. A mis padres y hermano que siempre han entendido y apoyado las diferentes decisiones que he tomado en la vida aunque para ellos tuvieran un resultado negativo. Han sido y serán el pilar que me sustenta. Gran parte de este trabajo es vuestra. También el resto de familia, que siempre han mostrado mucho interés en mi investigación para entender en qué consistía mi trabajo. Y por último, agradecer a la persona más dulce y servicial que he conocido, la cual llegó en un momento de total incertidumbre para brindarme su apoyo incondicional, gracias, Delia.
El otro gran pilar de personas que han favorecido a la realización de esta Tesis son integrantes del grupo "Formación tortuga". Son innumerables las memorias y recuerdos que me llevo de vosotros y por tanto, no podíais faltar una vez más en uno de
iii
mis agradecimientos, finalizando así esta trilogía. Joaquín, Huéscar y Alfredo, siempre os estaré enormemente agradecido por tantas horas de explicación sobre las diferentes asignaturas en las que me surgían dudas. Jesús, contigo descubrí el maravilloso mundo de la investigación, juntos comenzamos dando nuestros primeros pasos en un mundo en el que, seguro seguiremos por muchos años más. Y Sergio, gracias por haber terminado durante tantos fines de semana aquellas prácticas infumables en las que desde el día cero se sabía que no se esperaba en absoluto mi participación.
Por otro lado, no me olvido de mis compañeros de laboratorio, María, José, Miguel y Celia que hacían los días de trabajo más llevaderos, creando un ambiente muy agradable. Gracias también al resto de compañeros que he podido conocer durante mis estancias y que me han ayudado con mi investigación y evolución personal y profesional.
Finalmente, quiero agradecer a todos mis amigos. Los de toda la vida, por todas las
experiencias y momentos que hemos vivido durante estos años y por todo lo que nos
queda por vivir.
Resumen
Las antenas de onda de fuga (LWA) consisten en una estructura de guía de onda que permite la fuga de parte de la potencia a lo largo de la estructura. Por esta razón, la radiación de la antena se produce por la fuga de energía. Para producir una radiación coherente, es necesario controlar esta tasa de radiación a lo largo de la estructura radiante. Así, ajustando con precisión la tasa de radiación, se controla la forma del diagrama de radiación. Las LWAs han sido ampliamente estudiadas por la comunidad científica debido a sus ventajas, tales como, red de alimentación simple, alta directividad y escaneo en frecuencia pasivo. Sin embargo, presentan ciertas desventajas entre las cuales, la más importante a destacar es el efecto de beam-squinting. Éste se produce por la propiedad dispersiva inherente a este tipo de antenas. Además, presentan dificultades a la hora de generar radiación coherente en las direcciones broadside y endfire, aumentando la complejidad del diseńo para la radiación en dichas direcciones.
Las LWA han sido relativamente poco utilizadas en aplicaciones prácticas hasta la fecha, a pesar de sus ventajas. Las pocas aplicaciones en las que se han utilizado son los radares de onda continua modulada en frecuencia y los sistemas de enfoque controlado en frecuencia de campo cercano. Esta tesis propone el uso de las LWAs en aplicaciones prácticas aprovechando las ventajas mencionadas anteriormente y teniendo en cuenta los inconvenientes de este tipo de antenas para que su uso no sea limitado. Recientemente, las LWAs han sido propuestas para aplicaciones de localización de bajo coste, ya que permiten el diseńo de estructuras planas con haces directivos. Además, debido al aumento exponencial del uso de la tecnología, es necesario encontrar nuevas tecnologías para una transmisión de datos mayor, más rápida y más eficiente, manteniendo bajos costes de fabricación. Por lo tanto,
v
las LWAs pueden ser una solución crucial al mezclar bajos costes de fabricación, alta integrabilidad en diferentes sistemas debido a su tecnología impresa planar y alta directividad al mismo tiempo que se aprovecha su característica dispersiva que proporciona un escaneo pasivo en frecuencia. En este contexto, la principal aportación de esta Tesis consiste en el estudio, análisis, diseńo e integración de LWAs en aplicaciones reales y prácticas.
Esta Tesis presenta las siguientes tres contribuciones principales, definidas en los tres bloques principales de este documento:
• Estudio y análisis de LWAs para su uso en sistemas de estimación de dirección de llegada basados en técnicas de amplitud de monopulso. Comparar las características y prestaciones de las LWAs junto con las antenas comerciales más utilizadas. Para ello, diseńar y fabricar las HWM-LWAs con el fin de comparar sus prestaciones con las antenas de panel adquiridas comercialmente. Dado que cada aplicación requiere el diseńo de una HWM-LWA nueva y diferente, estudiar y proponer una técnica eficiente de análisis y diseńo de antenas para obtener fácilmente diagramas de radiación monopulso escaneados en frecuencia.
• Una vez analizado que las HWM-LWA son una solución factible para su uso en aplicaciones reales de localización debido a sus diversas ventajas. Integrar las HWM-LWAs diseńadas en sistemas digitales para estimación del ángulo de llegada en interiores. Por lo tanto, diseńar, desarrollar, configurar e integrar las LWAs en diferentes sistemas basados en las bandas de frecuencia Wi-Fi ISM de 2,4 GHz y 5 GHz. Finalmente, comparar los resultados de estimación obtenidos con otras soluciones propuestas para corroborar que los LWAs pueden ser utilizados en aplicaciones reales.
• Asimismo, debido a su bajo coste de fabricación y a su principal propiedad de escaneo en frecuencia. Ampliar el uso de las LWAs para la localización angular en redes de sensores inalámbricas (WSN) utilizando la banda de frecuencias UHF de 900 MHz. Utilizando así etiquetas RFID pasivas. También estudiar su aplicabilidad en WSNs utilizando etiquetas LoRa activas.
Este documento se presenta como una Tesis por compendio, por lo que se presentarán
y explicarán brevemente los 4 artículos de revistas que se han publicado durante
el programa de doctorado. Además, también se presentarán algunos artículos de
conferencias y otros trabajos en revisión para exponer algunas de las investigaciones
que no han sido publicadas en revistas hasta la fecha de depósito de tesis. El
documento está organizado como se indica a continuación: En la Introducción, se
presenta una contextualización del estado del arte y una explicación rigurosa sobre
las LWAs y las aplicaciones anteriormente mencionadas. Las dos partes siguientes se
dedican a presentar y explicar brevemente los trabajos publicados que contribuyen a esta Tesis. En la parte II, se presentan los cuatro artículos que conforman el compendio. Esto es, el análisis de las LWAs para la estimación de la dirección del ángulo de llegada y la integración de las LWAs en sistemas de localización digital usando el protocolo Wi-Fi en el Capítulo 1, la banda de frecuencias ISM UHF 900 MHz se utiliza junto con los HWM-LWAs en el Capítulo 2, luego se implementa en un sistema en tiempo real para la estimación de la dirección de llegada de múltiples tags pasivos en el Capítulo 3 y la integración de LoRa en el Capítulo 4. Finalmente, en la Parte III, se discuten las conclusiones generales y las futuras líneas de investigación.
vii
Abstract
Leaky-Wave Antennas (LWA) consist on a waveguide structure which allows the leakage of part of the power along the structure. For this reason, the radiation of the antenna is produced by the leakage of power. In order to produce coherent radiation, it is necessary to control this leakage rate along the radiating structure. Thus, precisely adjusting the leakage rate, the shape of the radiation pattern is controlled. LWAs have been widely studied by the scientific community due to their advantages, such as, simple feeding network, high directivity and passive frequency-scanning performance.
However, they present certain disadvantages among which, the most important to highlight is the beam-squinting effect. TThis is due to the inherent dispersion property of this type of antenna. In addition, LWAs present difficulties when generating coherent radiation in broadside and endfire directions, increasing the complexity of the design for radiation in these directions.
LWAs have been relatively unused in practical applications to date, despite of their benefits. The few applications in which they have been used are frequency modulated continuous wave radars and near-field frequency controlled focusing systems.This thesis proposes the use of LWAs in practical applications by exploiting the advantages mentioned above while taking into account the drawbacks of this type of antennas so that their use is not limited.
Recently, LWAs have been proposed for low-cost localization applications, as they
allow the design of planar structures with directive beams. In addition, due to
the exponential increase in the use of technology, it is necessary to find new
technologies for higher, faster and more efficient data transmission while maintaining
low manufacturing costs. Therefore, LWAs can be a crucial solution mixing low
manufacturing costs, high integrability in different systems due to their planar printed technology and high directivity while taking advantage of their dispersive characteristic that provides passive frequency scanning.
In this context, the main contribution of this Thesis consist of the study, analysis, design and integration of LWAs in real and practical applications.
This Thesis presents the following three main contributions, defined in the three main blocks of this document:
• Study and analysis of LWAs for its use in direction of arrival estimation systems based on monopulse amplitude techniques. Compare the characteristics and performance of LWAs along with widely used commercial antennas. For this purpose, design and manufacture the HWM-LWAs in order to compare their performance with commercially acquired panel antennas. Since each application requires the design of a new and different HWM-LWA, a main objective of this block is to study and propose an efficient antenna analysis and design technique to facilitate obtaining frequency-scanned monopulse patterns.
• Once analyzed that LWAs are a feasible solution for its use in real localization applications due to their several advantages, integrate the designed half-width microstrip (HWM-LWAs) in digital indoor angle-of-arrival estimation systems.
Therefore, design, develop, configure and integrate LWAs in different systems based on the Wi-Fi ISM 2.4 GHz and 5 GHz frequency bands. Finally, compare the obtained estimation results with other proposed solutions to corroborate that LWAs can be used in real applications.
• Extending the use of antennas for angular localization in sensor networks using the 900 MHz UHF frequency band: the main properties of low manufacturing cost and passive frequency beam scanning can be used in other applications. Thus, the localization estimation of passive RFID tags is studied, as well as their application in Wireless Sensor Networks (WSNs) using active tags with LORA technology.
This document is presented as a Thesis by compilation, so the 4 journal articles that have been published during the Ph.D program will be presented and briefly explained.
Besides, some conference articles and other work under review will be also presented to expose some of the research that has not been published in journals.
The document is organized as outlined hereafter: In Part I, a state-of-the-art contextualization, a rigorous explanation about LWAs and the previous applications mentioned above is presented. The next two parts are dedicated to present and briefly explain the published works included in this Thesis and their main contributions.
In Part II the explanation of the four papers which compose the compendium
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are presented. This is, LWAs analysis for direction of arrival estimation and the
integration of LWAs in digital Wi-Fi localization systems in chapter 1, the UHF 900
MHz ISM frequency band is used in conjunction with HWM-LWAs in chapter 2,
then, it is implemented in a real time system for direction of arrival estimation of
multi RFID tags in chapter 3 and LoRa integration in chapter 4. Finally, in Part III,
the overall conclusions and the future research lines are discussed.
Contents
Agradecimientos iii
Resumen vii
Abstract ix
Contents x i
Introduction and Objectives 1
I Introduction and Objetives of the Thesis 1
1 Leaky-Wave Antennas . . . . 4
2 Design of Leaky-Wave Antennas for Localization . . . 10
Objective 1 . . . 12
3 IoT Wi-Fi 2.4 GHz and 5 GHz Applications . . . 14
Objective 2 . . . 14
4 IoT Applications in 900 MHz, RFID and LoRa . . . 15
Objective 3 . . . 16
xi
Contents
II Compendium 19
1 Wi-Fi Direction Finding with Frequency-Scanned Antenna and
Channel Hopping Scheme 21
1.1 Summary . . . 35
2 Frequency-Scanned Monopulse Antenna for RSSI-based Direction
Finding of UHF RFID tags 39
2.1 Summary . . . 45
3 Direction Finding of RFID tags in UHF Band Using a Passive
Beam-Scanning Leaky-Wave Antenna 47
3.1 Summary . . . 60
4 ARIEL: Passive Beam-Scanning Antenna TeRminal for Iridiscent and Efficient LEO Satellite Connectivity 63
4.1 Summary . . . 69
III Conclusions and Future Lines 71
1 Conclusions 73
2 Future Lines 77
IV Appendices 79
1 Impact factor of articles published in journals 81 2 Other works and collaborations during the Thesis 85
V References 88
Part I
Introduction and Objetives of
the Thesis
Introduction and Objectives
Introduction and Objectives
1 Leaky-Wave Antennas
Antennas have become one of the main elements of communications during the last decades. An antenna is an interface between a guided medium (cable) and a radiated medium, thus the antenna is responsible for converting an electrical signal into an electromagnetic wave. This allows information to be transmitted from one point, the transmitter, to another point, the receiver, without the use of cables, known as wireless communications.
As mentioned above, antennas are one of the most important elements in modern communication systems. This is because the different electronic communication devices are connected to a network to transmit data. In order to be able to transmit information over the network wirelessly, it is necessary for the different devices to integrate radiating systems. In addition, there are different types of networks and protocols for data transmission, which requires the integration of different antennas in each case. There are numerous types of antennas, such as:
Yagi Uda, Horn antennas, patches, Dipoles, Parabolic or nanophotonic antennas, among others. Therefore, they are used in different wireless communications, such as: mobile telephone networks, satellite communications, RADAR, astrophysics, THz communications, etc. In addition, antennas can be classified according to their manufacturing, whether planar technology, waveguide, hybrid, etc. Finally, these antennas must be integrated in different types of devices, like: cell phones, buildings, satellites, computers, radio telescopes, etc. Hence, each application will have different requirements or specifications in terms of bandwidth, power handling, volume and size, gain, etc. Thus, a different design and fabrication needs to be developed for each application.
Figure 1 shows different types of antennas. (Figure 1(a)), illustrates a parabolic antenna. This type of antennas are widely used due to their high directivities. Their main uses are for satellite communications, long distance terrestrial communications although they are also used for radio astronomy applications in space research.
Figure 1(b) shows the antennas integrated in a cell phone. Which usually are printed antennas, due to their low cost and low profile to be easily integrated in small, thin and lightweight devices. On the other hand, Figure 1(c) presents a horn antenna used in radars due to its very high directivity. Figure 1(d) displays a dipole antenna commonly used as a reference antenna since the polarization is clearly defined. Also, panel antennas are frequenctly used as reference, shown in Figure 1(e) and log-periodic antennas, Figure 1(f), are widely used in ultra-wideband (UWB) applications due to their wide bandwidth.
4
1 Leaky-Wave Antennas
Figure 1: Different antenna types. (a) Parabolic. (b) Yagi Uda. (c) Patch. (d) Horn
Another type of antennas that have barely seen the light of day in real applications
are the well-known leaky-wave antennas (LWA) which are a type of traveling-wave
antenna (TWA), which are usually fabricated in planar technology. These antennas
can produce directive beams in different directions in space while maintaining a simple
feeding network [18]. This type of antenna allows the radiation of energy to
the space by using a mechanism in which the electromagnetic wave propagates
along the antenna acting as a partially open transmission line. In the beginning,
this type of antennas were based on rectangular waveguides with apertures that
allowed the leakage of the energy to the space [19], [20]. These types of LWAs were
large and heavy, based on waveguide technology, so they were hardly used in
commercial applications. However, LWAs can now be manufactured in printed
technology [21]-[22]. This makes LWAs highly integrable in different systems due
to their low manufacturing cost and low profile. Also, their feeding has been
greatly simplified compared to other antennas, such as phased-array antennas that
require complex feeding networks to produce directional beams in different
directions in space or even avoiding the use of
Introduction and Objectives
Figure 2: Schematic of a single port leaky-wave antenna. (a) Radiation schematic. (b) Dispersion curve.
electronically tunable components. Since LWAs are dispersive antennas, they have the inherent property of being antennas that generate frequency-scanned directive beams.
That is, depending on the frequency at which the antenna port is fed, directional beams pointing in different directions in space will be produced.
This can be seen in Figure 2(a), which shows the schematic of a LWA of length L
Afed by a single port with a determined frequency. While the opposite port is connected to a matching load to absorb the remaining power that has not been radiated. It can be observed that a steered-beam is generated with a certain scanning angle θ
RADand bandwidth ∆θ. Therefore, for different frequencies, different radiation angles will be obtained. This can be seen in the Figure 2(b), where, for a lower feed frequency results in a lower pointing angle and vice versa for higher frequencies.
Other possibility, which is used in the contributions throughout this Thesis is shown in Figure 3(a). The schematic of a two port LWA fed with three different frequenies is displayed. For each of the frequencies a steering beam with a different pointing angle is produced. For example, for a lower frequency f
1the red filled steering beam is obtained when the antenna is fed by the port 1 while when the antenna is fed through port 2 using the same frequency f
1, the unfilled red directive beam is obtained. The same is resulting when feeding the antenna with another frequencies, provided that the frequency with which the antenna is fed is within the frequency band in which the LWA is designed. The dispersion curve is shown in Figure 3(b), where it can be seen that it has been extended for a case of a LWA bidirectionally fed with two ports.
Thus producing frequency-scanned mirrored steering beams.
There are also different types of planar LWAs, from substrate-integrated waveguides (SIW) [23] to simple half-width microstrip-LWAs (HWM-LWA).
SIW-LWAs are
6
1 Leaky-Wave Antennas
Figure 3: (a) Example of a planar leaky-wave antenna with an angular scanning range from 10 to 50 degrees. (b) Dispersion curve of a bi-directionally fed LWA.
waveguide-based antennas, manufactured in planar technology, which allow to control the leakage rate along the aperture length by modifying the spacing between the radiating edge posts. On the other hand, HWM-LWAs also manufactured in printed technology, are a simpler solution because they do not have a posts-wall at the radiating edge, thus eliminating design variables. However, they do not allow control of the leakage rate throughout the structure. The work of this Thesis is focused on the study, analysis, design and integration of HWM-LWAs in different technologies and communication protocols. Figure 4 shows examples of antennas fabricated throughout this Thesis based on HWM technology printed on different substrates. Figure 4(a) presents a HWM-LWA printed on a FR4 substrate board, which is low dispersive, cheap and used for low cost applications in bluetooth-low-energy (BLE) technology.
While Figure 4(b) displays a HWM-LWA printed on a more expensive and more dispersive AD1000 substrate board designed for applications in the 2.45 GHz ISM Wi-Fi frequency band.
The complex propagation constant k = β−jα [24]. determines the controlled radiation in a LWA. Which is composed by a real part β, measured in rad/m which determines the phase variations and an imaginary part, α, measured in nep/m, directly related with the leakage rate along the antenna. In addition, as discussed above, LWAs are highly dispersive, so the complex propagation constant depends on the frequency at which the antenna is fed, k = k(f). The pointing angle, θ
RAD, of the input frequency is determined by the phase constant, as:
sinθ
RAD= β
k
0(1)
Introduction and Objectives
Figure 4: HWM-LWA topologies. (a) HWM-LWA printed in a FR4 substrate. (b) HWM-LWA printed in an AD1000 substrate.
where k
0is the free space wavenumber k
0= 2π/λ
0. Due to their inherent dispersive ability, LWAs can produce frequency-scanned steering beams in several space directions depending on the input frequency. Beta allows the control of the scanning angle of these beams. LWAs become an interesting option for application in RADAR systems [25] or near-field focusing applications [26].
The half-power-beamwidth (HPBW, ∆θ) is determined by the aperture length and the scanning angle θ
RAD, according to the following equation:
∆θ ≈ 1
LA
λ0
cosθ
RAD(2)
where L
Ais the length of the aperture and λ
0is the free space wavelength. It should be noted that for the above equation to be effective, the antenna aperture must be properly illuminated. HWM-LWAs have exponentially decreasing illumination. Being mostly illuminated the initial part of the aperture over which the antenna is fed and decaying exponentially as the energy is radiated along the aperture. Therefore, this energy reaching the end of the aperture without being radiated will determine the radiation efficiency of the antenna. This amount of power that remains in the structure when it is not radiated can be observed in Figure 5. A uniform bidirectionally fed leaky-wave antenna of length 6.75λ
0is presented in the figure. Figure 5(a) shows the uniform α and β of the complex propagation constant. Having a uniform distribution in the LWA structure results in an exponentially decreasing illumination. This can be better understood as follows: if the leakage rate, α, is constant along the structure and the structure does not present any modulation the energy will generally be radiated at the beginning of the aperture while when the energy advances through the structure,
8
1 Leaky-Wave Antennas
it will be less and therefore less will be radiated. The previous explanation is displayed in Figure 5(b), where it is observed that when the LWA is fed by port 1 the energy is generally radiated at the beginning of the structure, while at the end of the structure, barely arrives energy and therefore a smaller amount of power will be radiated. The radiation efficiency of a LWA can be expressed as follows:
η
RAD= P
RADP
IN= 1 − e
−2αLA(3)
Figure 5: Dispersive response of a uniform bidirectionally fed leaky-wave antenna of length 6.75λ
0. (a) Complex propagation constant. (b) Illumination along the length of the antenna.
So alpha must be properly adjusted in order to produce high radiation efficiencies.
Typically a design compromise in such antennas is that they are designed for η
RAD=
90%. Finally, it should be noted that the length of the antenna will change the
Introduction and Objectives
beamwidth at the desired frequency, due to that, long antennas will produce narrow beams while short antennas will create broader beams.
For each application, to be able to control the scanning velocity of a LWA is of key importance in order to fulfil the restrictions in terms of frequency band and angular scanning range. Different techniques will be applied in the designs presented in this Thesis to either increase or reduce the scanning velocity, fitting the dispersion to the restrictions imposed in the different applications, which mainly are the frequency bandwidth and the scanning range.
The three main objectives of this Thesis are detailed below. The first objective deals with the study, analysis and design of leaky-wave antennas for localization applications. A second objective in which these leaky-wave antennas are integrated in localization systems using the Wi-Fi protocol for both 2.4 and 5 GHz. Finally, the aim is to integrate LWAs in RFID and LoRa systems in the 900 MHz ISM band to reduce costs in location applications.
2 Design of Leaky-Wave Antennas for Localization
The location of devices in wireless networks is performed by the global navigation satellite system (GNSS) when they are in outdoor scenarios. However, the localization of these devices cannot be performed correctly when they are in indoor scenarios.
In addition, numerous technologies are emerging for the so-called internet of things (IoT) that are generally developed in indoor scenarios. Therefore, indoor real-time localization systems (RTLSs) are one of the key technologies for the new generation of communications, comprehended within the IoT framework. This is why the need arises to integrate these indoor location systems together with commercial wireless networks. In this context, there are several solutions for indoor RTLS in commercial wireless networks using Wi-Fi wireless local area network (WLANs) [27], [28] and [29], wireless personal area network (WPAN) networks based on BLE beacons [30], [31] and [32] also in WSN wireless sensor networks using Zigbee sensors [33]
and in RFID networks using passive tags [34].
Indoor positioning techniques can be divided into four methods [35], [36], which are fingerprinting (FP), lateralization, direction of arrival (DoA) and time-of- arrival (ToA):
1. Fingerprinting (Figure 6(a)): It consists on the correlation of the received data with a set of previous measurements which are used as a reference. This is used to build a reference radio map that is then used to compare the instantaneous received strength signal indicator (RSSI). Thus, obtaining information about the
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2 Design of Leaky-Wave Antennas for Localization
received power variations as a function of position. That is, to be able to estimate the location of a device by using the fingerprinting t echnique. First, the system is calibrated by receiving the RSSI values in the set of test points where it is desired to locate a device, thus creating a radio map. Then, in order to estimate the location, the instantaneous RSSI data of the device to be located are obtained and correlated with the RSSI data stored in the radio map. This estimation is usually performed with the use of a trained machine, known as machine learning (ML) such as a neural network (NN) or an artificial intelligence (AI). This FP technique has been used in different protocols in recent years, with BLE beacon networks [37], [38] in Wi- Fi WLAN networks [39], [37] and WSN Zigbee [40].
2. Lateration (Figure 6(b)): Lateralization consists of estimating the distance from each of the APs receiving power to the device. Once the distances from each of the access points (APs) to the device are obtained, then the location estimation is obtained as the intersection of the radius that can be traced from the APs, [41], [42].
3. Direction of arrival (DoA) (Figure 6(c)): It measures the angle or direction in which the device is located with respect to the receiving AP. Complex antenna arrays are required to scan all possible directions [43]. There are two techniques to measure the angle of arrival. One consists of receiving phase information from the different receiving antennas to estimate the location of the device. This is done using algorithms such as MUSIC [44] or ESPRIT [45].
On the other hand, DoA estimation can also be performed using amplitude information [46]. These techniques are based on RSSI reception, simplifying the system and reducing its cost. This DoA estimation can be done in multiple ways.
One approach would be to use mechanically scanned antennas [47], [48]. Another possibility is to make use of electronically scanned antennas, this type of antennas are known as switched beam antenna (SBA) and are explained in depth in [49], [50]. This category also includes electronically-steereable parasitic array radiator (ESPAR) antennas [51], [52] and electronically-scanned amplitude-monopulse antennas [53], [54].
4. Time of arrival (ToA) (Figure 6(d)): ToA consists of measuring the time it takes for a signal to reach the AP receivers. With this time it is possible to know the distance to the device to be located. As in lateralization, at least three APs are required to triangulate the received signal. Due to the enormous decompensation between the speed of light and the short distances. Estimation errors can be large since a small error in the time-of-arrival estimation can mean a huge distance error.
Therefore, complex receivers and complex phase synchronization [55] between the
different A Ps t hat p erform t he e s timation. R ecently, i t h as b een d emonstrated to
be possible to reduce the estimation error to one meter by using round-trip-time
(RTT) in conjunction with the 802.11mc Wi-Fi standard [56], [57] and [58].
Introduction and Objectives
Figure 6: Different localization techniques. (a) Fingerprinting. (b) Lateration. c) DoA. d) ToA.
Finally, frequency-scanned LWAs have been studied in recent years for DoA applications. These are a compact, passive and very low-cost solution compared to the other solutions mentioned above. These antennas can produce beam scanning by simply modifying the RF frequency fed to the input antenna. This passive scanning of space has been of great interest for study and integration in indoor location applications [59], [60].
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2 Design of Leaky-Wave Antennas for Localization
Objective 1
The first objective of this Thesis is to study the advantages and disadvantages of LWAs for its use in localization applications. Secondly, to perform an optimization of the synthesis technique for a fast and efficient analysis and design of frequency-scanned LWAs. The main contributions of this Section are detailed below:
1. Theoretically design of a bi-directional compact HWM-LWA with frequency-scanned directive beams in the central frequency of the ISM Wi-Fi 2.45 GHz frequency band. This design must be optimized to synthesize two mirrored steered-beams generating an amplitude monopulse pattern. Therefore, the HWM-LWA can be compared theoretically with two commercial apertures with unscanned beams pointing in the broadside direction. These apertures should be in a tilted configuration in order to perform the monopulse amplitude pattern.
Also compare both designs in near-field and far-field regions. [8]
2. Design, manufacture and measure of a bi-directionally fed HWM-LWA to shape an amplitude monopulse pattern using the Wi-Fi 2.45 GHz ISM band. Acquire, assembly, adjust and measure two commercial panel antennas in tilted configuration to perform the same monopulse pattern as the HWM-LWA. Integrate both antennas into a digital Wi-Fi system, making use of an access point (AP) with multiple-input multiple-output (MiMo) features. Use the system to characterize, measure and estimate the location of commercial Wi-Fi devices, cell phones, etc. Make a study of location estimation with both systems in both near-field and far-field situations.
In order to compare both systems and obtain the advantages and disadvantages of both in applications of sensor networks or indoor localization, among others. The design of the antenna and the results are shown in [12].
3. Theoretically analyze a novel synthesis technique for HWM-LWAs with frequency-scanned monopulse amplitude patterns. This synthesis should take into account the next input variables of LWAs such as, desired operating frequencies, scanning angle of the design frequencies and beamwidths and bi-directionally fed antennas. The result of the synthesis is to obtain the parameters of the substrates on which these input variables can be designed in addition to the design parameters of the HWM-LWA, width, W and length, L
A. [6]
4. Fabricate two different designs of HWM-LWAs analyzed by the novel
synthesis technique, presented for the first time in [6]. The first design should be for
a low-cost application, where fast frequency scanning is not required, so a narrow
field-of-view (FoV) will be obtained. This will result in a design on a low-cost
substrate with low permittivity. The second design should be for a wide FoV
application where fast frequency scanning is required. Resulting in a design on a
high permittivity,
Introduction and Objectives
higher cost substrate. Subsequently, measure both designs to verify the correct performance of the novel synthesis technique for the design of frequency-scanned HWM antennas for localization applications. [13]
3 IoT Wi-Fi 2.4 GHz and 5 GHz Applications
Another objective of this Thesis is to extend the study of the use of frequency-scanned HWM-LWAs for use in IoT applications using the Wi-Fi ISM 2.45 GHz and Wi-Fi 5 GHz protocol. The possible IoT applications of the 802.11 Wi-Fi WLAN standard are multiple. From location, security and context aware services [27]. There are also other paradigms such as the use of efficient communications. Based on enhanced utilization of energy transmission during communication. For this, one solution would be to focus all the energy towards the device that requires communication instead of radiating energy to all directions in space, known as isotropic radiation. Thus taking advantage of a huge amount of the transmitted energy. Therefore, in the IoT paradigm, it is desirable for a single AP to be able to service devices while being able to estimate the location of these devices and thus focus energy towards them. Thus generating an efficient communication. Many of the APs that perform DoA estimation make use of MiMo and channel state information (CSI) [61], [62].
However, CSI is not directly accessible in the vast majority of commercial APs. So it becomes a complex solution to which to add external hardware and software.
But RSSI is directly available in most commercial APs. Therefore, the objectives of this chapter will focus on DoA using RSSI to perform location estimation.
Objective 2
There are many contributions to the IoT Wi-Fi localization in this Thesis. Hereunder, each of the contributions that have been pursued in this area are presented:
1. Study of the feasibility of the use of LWA arrays to increase the directivity and data transmission when integrating the array in a MiMo AP. Analyze, design and simulate a 4-element array in HWM-LWA technology to synthesize highly directive frequency scanned beams in both H-plane and E-plane. This proposal is shown in the published article [5].
2. Propose a novel technique for Direction-of-Arrival (DoA) angle estimation in the Wi-Fi 802.11 protocol. Implement a real case of 1-D localization using the MUSIC alogorithm in the ISM 2.45 GHz Wi-Fi frequency band. Analyze, design and manufacture a frequency-scanned HWM-LWA using three equidistant channels in the ISM 2.45 GHz band and integrate it in an AP MiMo. Use a MUSIC algorithm
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4 IoT Applications in 900 MHz, RFID and LoRa
to estimate the angle-of-arrival of the system and compare the results with other previous published solutions. This proposal is presented in the published article [1].
3. Extend the previous 1-D location research by employing a frequency-scanned HWM-LWA designed in the 2.45 GHz ISM Wi-Fi band. Program, measure and make use of round trip time (RTT) to extend the localization to two dimensions. In this case, measure angle and distance to the reference antenna in order to estimate the location of Wi-Fi devices in 2-D. Integrate it into a digital system and compare the results with the rest of the 2-D localization literature. [14]
4. Theoretically perform an analysis and design of a two-dimensional (2-D) Fabry-Perot leaky-wave antenna with an enhancement in the frequency scanning response angle-of-arrival estimation applications in the ISM 5 GHz Wi-Fi frequency band. Prove the possibility of using a high impedance surface (HIS) to improve the frequency scanning of dispersive 2-D FP-LWAs in both planes theoretically.
This paper is published in [9].
5. Manufacture a two-dimensional frequency-scanned Fabry-Perot LWA formed with a HIS structure to enhance the scanning response in the 5 GHz ISM Wi-Fi frequency band. Measure 2-D radiation patterns in anechoic chamber. Perform a study of localization estimation with conical beams using monopulse amplitude techniques extended to the 2-D case. [15]
4 IoT Applications in 900 MHz, RFID and LoRa
As discussed above, one of the most important reasons for the use of smart antennas is that emerging technologies require an improvement in the efficient use of the energy being transmitted. That is why smart antennas have been widely used for sensor networks in different technologies and protocols, such as Zigbee, RFID or LoRa among others.
For the case of WSNs using Zigbee 2.45 GHz technology, smart antennas have been
proposed to increase the functionalities of the protocol [63], [64]. They have
been used to improve the quality of the communication link [65], [66], as well as
to bring improvements in communication security [67], [68]. Smart antennas have
also been used in conjunction with the Zigbee protocol for location estimation
applications [69], [70]. Most smart antennas used in WSN Zigbee are phased-arrays
[69], [71], electronically steerable parasitic array radiator (ESPAR) [66], [46] or
switched beam antennas (SBAs)[73], [74].
Introduction and Objectives
There are also WSNs that employ RFID and LoRa technologies in the 900 MHz ISM UHF frequency band. Most of the localization systems implementing these technologies are based on the use of phased-arrays [75], [76] or systems based on phase matching [77], [78]. On the other hand, mechanically steered antennas are also used to estimate the location of the different tags [79]. However, all these techniques used for localization applications require high costs either for power consumption or for electronic mechanisms such as phase-shifters that increase the cost of the systems and their implementation. Therefore, the objective of this part of the Thesis is to use frequency-scanned printed antennas to simplify the systems and reduce their costs. Another problem that generally arises in sensor networks is that the devices or sensors are located in positions close to the reference antenna, i.e., they are in near-field, so it will also be the objective of study in this chapter of the Thesis the performance of these systems based on HWM-LWAs in near-field scenarios.
Objective 3
The main objective of this chapter of the Thesis has been divided into several parts:
1. Theoretically analize and design a HWM-LWA in the ISM 900 MHz RFID frequency band for RSSI-based amplitude monopulse angle estimation techniques. The aim is to study the possibility of using frequency-scanned beam-steered LWAs to reduce the cost, profile and complexity and increase the efficiency of the current developed systems. The paper is presented in the published article in [7].
2. Design, manufacture and measure a compact HWM-LWA in the 900 MHz RFID band for an amplitude-monopulse localization scenario. Perform a study in an anechoic chamber of angle-of-arrival estimation using amplitude monopulse techniques. Subsequently, study the performance of the system in two operating modes, in narrow field mode, in which very close beams are used and therefore the field-of-view is reduced. And a second wide-field mode, in which the beams are equispaced and the field-of-view is increased. Already published in [2].
3. Extend the previous research to implement the above manufactured HWM-LWA, in a location system based on digital RFID 900 MHz technology. Integrate the antenna with a MiMo RFID commercial reader and program an RSSI-based amplitude monopulse algorithm to estimate the direction finding o f d ifferent pa ssive RFID UHF tags. Extend the research for a case of an outdoor and multipath scenarios and study the feasibility of the system when multiple tags must be located in real time. Published in [3].
4. Propose a frequency-scanned, passive, simple, low-cost and high-gain beam-steered LWA in LoRa 900 MHz UHF-ISM band for global data connectivity using
16
4 IoT Applications in 900 MHz, RFID and LoRa
Low-Earth-Orbit (LEO) satellites constellations. Present a frequency-scanned passive power-efficient LW A to av oid co mplex an d hi gh co st me thods su ch as electronically-scanned antennas, mechanical rotation or beam-switching antennas which requires elevated power-demanding methods. Published in [4].
5. Perform a theoretical study of a series-fed array of 1-D leaky-wave antennas and its radiation characteristics. Study the possibility of using shorter fabricated antennas and connect them by coaxial cable (taking into account the intermediate phase shift suffered by the wave) to attach them in series with other short a ntennas. This could solve many problems with the fabrication of long antennas on substrates, which generally have predefined dimensions and cannot have the desired size. [82].
6. Manufacture two short HWM-LWAs to conect them in a series fed configuration for UHF Wi-Fi frequency-scanning smart applications. Measure the series array in an anechoic chamber to verify the proper operation of the antenna array. Study near-field effects and their possible applications in near-field communications. [16]
7. Develop a laboratory session for post-graduate students of Electrical Engineering
to introduce them in the field o f r esearch. To d o s o, d esign a s imple HWM-LWA
in the UHF 900 MHz ISM frequency band for localization applications using a
commercial RFID reader and passive RFID tags. Manually manufacture the
low-cost hand-made HWM-LWA, measure it and integrate it in a real RFID
system for direction-of-arrival estimation using RFID tags. [17]
Part II
Compendium
Chapter 1
Wi-Fi Direction Finding with Frequency-Scanned Antenna and Channel Hopping Scheme
©2021 IEEE. Reprinted, with permission, from A. Gil-Martinez, M. Poveda-Garcia, J. A. Lopez-Pastor, J. C. Sanchez-Aarnoutse and J. L. Gomez-Tornero, “Wi-Fi Direction Finding with Frequency-Scanned Antenna and Channel Hopping Scheme”
IEEE Sensors , January 2022.
IEEE SENSORS JOURNAL, VOL. XX, NO. XX, MONTH X, XXXX 1
XXXX-XXXX © XXXX IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
Abstract— A novel and simple technique for Direction of Arrival (DoA) estimation in 802.11 Wi-Fi networks is proposed. It is based on the use of a dual-port frequency-beam scanning antenna, which is connected to single MIMO access point. By simply performing a channel hopping scheme and acquiring Received Signal Strength Information (RSSI) from each channel, the DoA can be estimated in a wide Field of View of 180º with a mean angular error of 5º.
Due to its simplicity, a single commodity Wi-Fi AP can be used without the need of Channel State Information (CSI), or complex electronically reconfigurable or mechanically revolving antennas.
Index Terms— localization with 802.11, direction of arrival (DoA), channel hopping, frequency-scanning antenna.
I. Introduction
IRECTION FINDING in IEEE 802.11 Wi-Fi Wireless Local Area Networks (WLAN) finds multitude of applications for indoor localization, user location tracking, context-aware services, and security [1]-[2]. Besides, in the paradigm of a Smart Home and the Internet of Things (IoT), it is desired that a single Access Point (AP) performs the Wi-Fi sensing and direction finding of mobile devices [1]. In this context, several single-AP Wi-Fi direction finding systems have been proposed in the literature [3]-[18]. Most of them estimate the Direction of Arrival (DoA) with MIMO (Multiple Input Multiple Output) antennas and using Channel State Information (CSI). CSI provides complex data of the received signal, including phase information from which phased-array techniques [5], [7] can be applied to estimate the DoA.
However, CSI data is not directly accessible on commodity APs, and ad-hoc hardware and software modification for dedicated synchronization is needed to calibrate the system [6], [8], [9], [16], [19].
On the contrary, Received Signal Strength Information (RSSI) is directly accessible from commercial Wi-Fi APs [15], [20], and it does not rely on complex time and/or phase synchronization. For this reason, some single-AP WiFi RSSI- based DoA estimation systems have been proposed to avoid the need of CSI data. When using RSSI (amplitude) data without
Manuscript submitted Dec. XX, 2020. Manuscript received Dec. XX 2020. This work has been supported by Spanish National projects PID2019-103982RB-C42/AEI/10.13039/501100011033, Spanish MICINN DIN2018-009815, and Murcia regional projects INFO 2018.08.ID+I.0015.
phase information, a directive beam antenna able to scan the angular space is required to estimate the DoA. One common solution is to use Switched Beam Antennas (SBAs) [4], [10]- [13], which require an electronic control circuit to acquire RSSI data from several sectorial antennas, each one scanning at different adjacent angular zones. Also, amplitude-monopulse antennas have been proposed to implement RSSI-based DoA estimation for Wi-Fi [14], [17], [18]. However, since they show a limited Field of View (FoV), more than one AP [14] or a revolving antenna [3], is needed to eliminate ambiguities in wider angular zones. However, both electronically SBAs and mechanically revolved antennas are active-antenna solutions, which increase the cost and complexity of the DoA estimation system since they are not directly integrable with commercial MIMO Wi-Fi AP functionalities.
In this paper, we propose a new RSSI-based direction finding technique for Wi-Fi WLANs, which avoids the use of CSI data, or the need of complex active-scanning beam antennas. The proposed novel technique is based on the use of a simple passive frequency-scanning antenna (FSA), which can be directly connected to a single MIMO Wi-Fi AP without any external control hardware, in order to perform the beam scanning requested for the RSSI-based DoA estimation. As a consequence, the proposed technique can be applied to any commercial MIMO Wi-Fi AP with OpenWRT libraries.
A. G-M., M. P-G., J.C. S-A. and, J.L. G-T are with the Department of Communication and Information Technologies, Technical University of Cartagena, Cartagena 30202 Spain (e-mail: JoseL.Gomez@ upct.es).
J.A.L-P is with Neuromobile company, Murcia, Spain.
. Calibration.
Steering vectors acquisition Device
0°
MUSIC Application
Angle determination
Wi-Fi Direction Finding with Frequency-Scanned Antenna and Channel-Hopping Scheme
Alejandro Gil-Martínez, Miguel Poveda-García, Jose Antonio López-Pastor, Student Member IEEE, Juan Carlos Sánchez-Aarnoutse, and José Luis Gómez-Tornero, Senior Member IEEE
D
Chapter 1. Wi-Fi Direction Finding with Frequency-Scanned Antenna and Channel Hopping Scheme
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Fig. 1. Scheme of the frequency-scanned antenna connected to a single MiMo Wi-Fi Access Point performing channel hopping for DoA estimation.
TABLE I SYSTEM MODULES DESCRIPTION
Acr Name Function Feasibility
MD Mobile
Device
- Connect to the AP using the announced Wi-Fi channel.
Any IoT Wi-Fi mobile device
AP MIMO
Wi-Fi AP
- Switch among a set of N Wi-Fi channels.
- Read the RSSI at two antenna ports.
- Send RSSI data b(P#n) to CPU.
Any commercial MIMO AP
with two antenna ports and OpenWRT
installed FSA
Frequency- Scanned Antenna
- Create directive frequency- scanned beams.
Any FSA tuned to the Wi-Fi
band
CPU
Control &
Processing Unit
- Store calibrated MUSIC steering vectors a().
- Receive RSSI b(P#n) from AP.
- Estimate the DoA with MUSIC
Any microprocessor connected to or embedded in the Wi-Fi AP
The main novelty of the proposed DoA Wi-Fi technique, is that the beam scanning is based on a sequential channel- hopping scheme, which can be directly performed by commercial APs whose operating channel can be configured. In our case, we use OpenWRT libraries to perform the channel hopping and RSSI acquisition. As sketched in Fig.1, the FSA antenna can be connected to a single MIMO Wi-Fi AP to produce directive beam scanning over a wide FoV by simply hopping among the available Wi-Fi channels. This is demonstrated in Section II of this paper, which describes the system and the calibration process needed to apply the MUSIC technique [21]. In Section III, this technique is applied to estimate the DoA of Wi-Fi signals in the 2.4 GHz band. It is shown that, by using the 11 available Wi-Fi channels, the DoA can be estimated in a wide Field of View of 180º with a root mean square error of 14º (mean angular error of 5º). Finally, Section IV compares this performance with other single-AP Wi-Fi direction finding systems.
II. SYSTEM OPERATION A–SYSTEM DESCRIPTION
Recently it has been proposed a radio direction finding technique using frequency-scanned antennas (FSA) together with multi-tone analog RF signals [22], [23]. This concept was lately applied for DoA estimation in Bluetooth Low Energy (BLE) digital wireless networks [24]. Here we demonstrate for the first time its application also for Wi-Fi systems. As shown in Fig.1, the system prototype is formed by four basic modules.
The main flowchart of the proposed system operation is illustrated in Fig.2, and a description for each module and its function is given next and summarized in Table I.
The first module is a mobile Wi-Fi device (MD) which must be connected to a Wi-Fi AP, and whose angular location w.r.t.
the AP is to be estimated. Any Wi-Fi compatible mobile device can be used; in our case we use a Raspberri Pi with a conventional Wi-Fi card. The second main module is a MIMO Wi-Fi AP. It is in charge of providing access to a Wi-Fi network using one of the 11 available Wi-Fi channels in the 2.4 GHz band. Any commercial MIMO Wi-Fi AP with at least two external antenna ports and with OpenWRT installed can be used for this application. In our case we use a Compex WPJ531HV- A Multi-Function QCA9531 Embedded Board [25] with a mini-PCIe Atheros AR9380 Wireless 3X3 MIMO card. This AP must be programmed to perform a sequential channel hopping scheme over a set of N predefined channels, and sample the RSSI data obtained from the two antenna ports and for any operating channel. The “hostapd cli chan_switch”
command (provided by bash language into the Open WRT/Linux operating system running in the AP), allows to sequentially change the Wi-Fi channel every 100 ms. The MD follows this dynamic channel hopping assignation scheme, since the MD is connected to the Service Set Identifier (SSID) provided by the AP and uses the channel announced by the AP.
Then, every millisecond the AP monitors the RSSI obtained at the two antenna ports using the the “iw dev” Linux bash command. In this way, a total of 200 RSSI samples (100 samples per each antenna port), are collected during the 100 ms that the AP is fixed to a given channel. This process is repeated for all the channels being swept as summarized in the operating flowchart in Fig.2.
The third module is the FSA, which is a passive antenna with two radio-frequency (RF) ports (port A and port B). These two ports are connected to the two RF interfaces of the MIMO Wi- Fi card using coaxial cables, as sketched in Fig.1. The FSA is similar to the ones used in [22] and [24], but tuned to operate in the Wi-Fi 2.4 GHz band. The main novelty of the proposed system is the use of the FSA. This makes that, as the MIMO Wi-Fi AP changes the operating Wi-Fi channel, the system eventually performs directive beam scanning using a simple passive antenna without extra hardware. This is illustrated in Fig.1, which represents six frequency-scanned beams corresponding to the sweep along Wi-Fi channels #1, #6 and
#11, and using port A or B. As will be explained later in detail, each port allows beam scanning along opposite angular quadrants (positive or negative directions). Besides, the change of the Wi-Fi channel makes the antenna to steer the beams from the perpendicular direction towards lateral angles as the channel is increased.
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Fig. 2. Flowchart of the system operation steps.
The fourth module is a Control and Processing Unit (CPU), which is in charge of collecting the RSSI data received from the MIMO Wi-Fi AP at both antenna ports P (A and B) as the channels #n are being swept. Once all the N Wi-Fi channels have been swept, the CPU collects this information in the form of a vector of RSSI samples b(P#n). Also, the CPU controls the turntable in the calibration process. As will be described in the next subsection and summarized in Fig.2, the calibration is needed to obtain the MUSIC steering vectors a(). These are the reference RSSI values for the two antenna ports and all Wi- Fi channels, and for all possible DoA angles . Using the RSSI data b(P#n) and the calibrated steering vectors a(), the CPU performs the DoA estimation using a modified version of the MUSIC algorithm [21].
For this demonstrator prototype we use a Personal Computer Laptop as the CPU. The Laptop is connected to the AP with an Ethernet cable to receive the RSSI data using UDP sockets, and we have implemented the MUSIC algorithm in Matlab.
Although we have used an external Laptop to acquire the RSSI data and perform the MUSIC DoA signal processing, all this could have been done by the same AP CPU using bash language over OpenWRT and avoiding additional hardware. Table I and Fig.2 summarize the steps and the role of each module in the calibration, acquisition, and DoA estimation. Also, the feasibility analysis of each module is included in Table I to highlight that the proposed technique can be used with Commercial-Off-The-Shelf (COTS) Wi-Fi hardware, thus maintaining the desirable standard IEEE network compatibility and integrability.
B–SYSTEM CALIBRATION
To apply the MUSIC DoA estimation algorithm [21], the system must be characterized and calibrated in an anechoic chamber, in order to construct the reference steering vectors a() as depicted in the flowchart in Fig.2. The FSA connected to the Wi-Fi AP is mechanically rotated using a turntable controlled by the CPU, as shown in Fig.3a.
Fig. 3. a) Calibration in anechoic chamber b) Measured digital beam patterns in Wi-Fi channels #1, #6 and #11.
In the calibration process, a reference MD transmits frames using the Wi-Fi channel-hopping scheme dictated by the AP.
The MIMO AP card collects the RSSI levels for different azimuthal angles as it is being rotated, and for all the possible 11 Wi-Fi channels. In this way, the digital radiation patterns are obtained for different Wi-Fi channels (from #1 to #11), and for the two RF ports (A and B). The normalized patterns are shown in Fig.3b, demonstrating the successful generation of directive beams, which are digitally steered by just sweeping the selected Wi-Fi channel. As can be seen in Fig.3b, Wi-Fi channel #1 (with central frequency 2.412 GHz) is received in port B with a directive sectorial beam directed at +20º. As the Wi-Fi channel is increased, the beam direction is steered to higher positive-scanning angles ( +30º for channel #6 with 2.437 GHz, and +40º for ch.#11 with 2.462 GHz). The rest of the beams associated to all 11 Wi-Fi channels are not plotted for the shake of clarity; however they show a smooth continuous scanning response of their peak gain direction from +20º to
+40º. Similarly, channel-scanning to negative angles is performed when acquiring Wi-Fi frames from port A of the MIMO Wi-Fi card, due to the mirror-symmetric electromagnetic response of the FSA [22].
As it was mentioned in the introduction, directive beam scanning or beam switching among different directions is necessary to perform DoA estimation using only amplitude information (RSSI), thus avoiding the need of complex CSI data for phase acquisition. As will be explained, the frequency- beam steering technique illustrated in Fig.3b can be directly performed by a commercial MIMO Wi-Fi AP without extra hardware. This is a key advantage when compared to electronically switched-beam antennas (SBA) [4], [10]-[13] or mechanically rotating antennas [3], which request for active external hardware. For instance, the SBA in [12], [13] is formed by 8 antennas, and it requires a complex RF switch network and an external hardware to electronically control the sectorial beam switching and acquire the associated RSSI data.
Chapter 1. Wi-Fi Direction Finding with Frequency-Scanned Antenna and Channel Hopping Scheme