Ultra Wideband Impulse Radio for Wireless Sensing and Identification

Ultra Wideband Impulse Radio for Wireless Sensing and Identification
Ultra Wideband Impulse Radio
for
Wireless Sensing and Identification
Majid Baghaei Nejad
Doctoral Thesis
in
Electronic and Computer Systems
KTH – Royal Institute of Technology
Stockholm, Sweden, December 2008
Baghaei Nejad, Majid
Ultra Wideband Impulse Radio for Wireless Sensing and Identification
Doctoral Thesis in Electronic and Computer Systems
TRITA-ICT/ECS AVH 08:09
ISSN 1653-6363
ISRN KTH/ICT/ECS AVH-08/09--SE
ISBN 978-91-7415-153-4
Royal Institute of Technology
School of Information and Communication Technology
Department of Electronics, Computer and Software System
Forum 120
SE-164 40 Kista
Sweden
Copyright © Majid Baghaei Nejad, December 2008
ii
In The Name of God
I would like to dedicate this thesis to my family
iv
Abstract
Ubiquitous computing and Internet-of-Things (IoT) implies an untapped opportunity in
the realm of information and communication technology, in which a large number of
micro-devices with communication and/or computing capabilities, provides connectivity
for anything, by anyone at anytime and anywhere. Especially, these devices can be
equipped with sensors and actuators that interact with our living environment. Barcode,
smart contactless card, Radio Frequency Identification (RFID) systems, wireless sensor
network (WSN), and smart mobile phones are some examples which can be utilized in
ubiquitous computing.
RFIDs and WSN have been recognized as the two promising enablers for realization of
ubiquitous computing. They have some great features such as low-cost and small- size
implementation, non-line of sight operation, sensing possibilities, data storing ability,
and positioning. However, there are several challenges which need to be addressed, such
as limited life time for battery powered device, maintenance cost, longer operation
range, higher data rate, and operation in dense multipath and multiuser environment.
Ultra-Wideband Impulse Radio (UWB-IR) with its huge advantages has been
recognized as a great solution for future WSN and RFID. UWB-IR technique has the
possibility of achieving Gb/s data rate, hundreds of meter operation range, pJ energy per
bit, centimeter accuracy of positioning, and low cost implementation. In this work
utilization of UWB-IR in WSN and RFID is investigated.
A wireless sensor network based on UWB-IR is proposed focusing on low-cost and
low-power implementation. Our contribution is to imply two different architectures in
base station and sensor nodes to satisfy power, complexity and cost constraints. For
sensor nodes, an autonomous UWB-IR detection is proposed, which detects the UWB
signal autonomously and no restrict synchronization is required. It reduces the circuit
complexity significantly. The performance in term of bit-error-rate is compared with two
other common detection techniques. It is shown that the new detection is more
robustness to timing jitter and clock skew, which consequently reduces the clock and
synchronization requirements considerably.
A novel wireless sensing and identification system, based on remote-powered tag with
asymmetric wireless link, is proposed. Our innovative contribution is to deploy two
different UWB and UHF communication techniques in uplink and downlink
respectively. In the proposed system, tags capture the required power supply from
different environmental sources (e.g. electromagnetic wave transmitted by a reader) and
transmit data through an ultra-low power impulse UWB link. A new communication
protocol is devised based on slotted-aloha anti-collision algorithm. By introducing
several improvements including of pipelined communication, adaptive frame size, and
skipping idle slots, the system throughput of more than 2000 tags/s is achieved. To
prove the system concept a single chip integrated tag is implemented in UMC 0.18µm
CMOS process. The measurement results show the minimum sensitivity of -18.5 dB
(14.1 µW) and adaptive data rate up to 10 Mb/s. It corresponds to 13.9 meters operation
v
range, considering 4W EIRP, a matched antenna to the tag with 0dB gain, and free space
path loss. This is a great improvement in operation range and data rate, compared with
conventional passive RFID, which data rate is limited to a few hundreds of Kb/s.
System integration in a Liquid-Crystal-polymer (LCP) substrate is investigated. The
integration of a tunable UWB-IR transmitter and a power scavenging unit are studied.
Our contribution includes embedding and modeling the RF components and antenna in
substrate and co-optimizing the chip and package with on-chip versus off-chip passives
trade-offs. Simulation results verify the potential of system-on-package solution for
UWB integration.
The effect of antenna miniaturization in a UWB system is studied. Our focus is to scale
down a UWB antenna and optimize the performance through the chip-antenna codesign. A tunable impulse-UWB transmitter is designed in two cases - a conventional
50Ω design and a co-design methodology. The simulation results show that the standard
50Ω design technique can not reach the best condition in all cases, when a real antenna
is placed into the system. The performance can be improved significantly when doing
co-design. The antennas and UWB transmitter performances are evaluated in a given
UWB systems. It is shown that the operation distance at a target performance is reduced
with antenna scaling factor and it can be compensated by antenna-transceiver co-design.
The result proves the importance of antenna-transceiver co-design, which needs to be
addressed in the earliest phases of the design flow.
Keywords: Ubiquitous computing, Impulse Radio, Ultra wideband, RFID, Wireless
sensor, WSN, Antenna, System-on-Package.
vi
Acknowledgments
The highest praise is God’s for supporting me during this and all other steps of my life.
This thesis which is a result of four years study and work at ECS-KTH is achieved not
only by my own but also with the help and support of my family, colleagues, and friends
whom I would like to acknowledge them here.
First of all, I would like to thank my supervisor, Prof. Li-Rong Zheng, for all his
support during these years. He is such a respectful, helpful, knowledgeable, and
encouraging person whom I am very happy to be his student. I also would like to
appreciate Prof. Hannu Tenhunen, my second advisor, for welcoming me to KTH.
I acknowledge valuable discussion with all of my friends and colleagues at KTH. Dr.
Xinzhong Duo, Dr. Meigen Shen who helped me in the beginning to get familiar with
tools and lab. I appreciate my dear friends and colleagues, David S. Mendoza, and Zhuo
Zou, for their excellent work and collaboration in this work. All RaMSiS members,
especially, Jad Atallah, Martin Gustafsson, and Saúl Rodríguez Dueñas, for their
valuable help and discussion. I would like to thank all iPack members, especially
Roshan Weerasekera, Dr. Fredrik Jonsson, Dr. Julius Hållstedt, and Dr. Qiang Chen.
I would like to thank my dear friend and colleague, Soheil Radiom, from ESATKatholieke Universiteit Leuven for his close and excellent collaboration in the antenna
miniaturization work. Also, I would like to appreciate Prof. Georges Gielen from the
same department for his excellent guidance and comments during our corporation.
I would like to thank all former and current administrative staff at ECS, Lena Beronius,
Agneta Herling, Gunnar Johansson, Hans Larsson, and Alina Munteanu, for their
excellent administrative work. Thanks IT service group to keep servers and computers
alive.
My especial thank to the Ministry of Science, Research and Technology of Iran
(MSRTI), and Tarbiat Moallem University of Sabzevar, Iran, for awarding me a
scholarship and an opportunity to pursue my education towards PhD. I also appreciate
the financial support provided by iPack center for this research work.
Many special thanks go to my dear friend Bahman Korojy and his family for their
friendship. I also thank all of my Iranian friends and their families for their help and
many wonderful moments we shared together in Sweden.
Also, especial appreciation goes to my dear friend Jian Liu who helped me a lot during
my stay in Sweden.
I wish to express my deep appreciation to my father and mother and also my father-inlaw and mother-in-law for your infinite patience and love. Especially, I deeply
appreciate my father for his irreplaceable support during all steps of my life. Thank you
very much for being such a supportive father.
I also appreciate my brother and my sisters and also my brother-in-law and sisters-inlaw for their help and supports.
vii
And finally, but most importantly, I would like to give my deepest gratitude to Mitra,
my dear wife, for your infinite help and supports and also my beautiful daughters, Negar
and Neda. All of my achievements have an invisible part of your contribution.
For all those who helped me and I may have forgotten to mention, please forgive me
and I thank you in this sentence.
Majid Baghaei Nejad, 2008
viii
Contents
1
2
3
4
5
Abstract................................................................................................................................ v
Acknowledgments .............................................................................................................vii
List of Publications.............................................................................................................xi
Papers included in this thesis..........................................................................................xi
Related publications not included..................................................................................xii
Summary of the included papers ......................................................................................xiii
Introduction ..................................................................................................................... 1
1.1
Project Motivation ................................................................................................... 1
1.2
Introduction to Ultra Wideband Radio .................................................................... 3
1.3
Introduction to Radio Frequency Identification ...................................................... 6
1.4
Author’s Contribution.............................................................................................. 8
1.5
Outline of the Thesis................................................................................................ 9
Impulse Radio in Wireless Sensor Network.................................................................. 11
2.1
Introduction ........................................................................................................... 12
2.2
System Specifications............................................................................................ 13
2.3
Performance Estimation ........................................................................................ 15
2.4
Conclusions ........................................................................................................... 18
Remote-Powered UWB-RFID....................................................................................... 19
3.1
Introduction ........................................................................................................... 20
3.2
System Description................................................................................................ 20
3.3
Asymmetric UWB-RFID Architecture.................................................................. 22
3.4
Proposed Communication Protocol ....................................................................... 24
3.5
Implementation...................................................................................................... 26
3.5.1
Impulse UWB Transmitter ............................................................................ 26
3.5.2
Power Management Unit ............................................................................... 27
3.5.3
RF Demodulator ............................................................................................ 31
3.5.4
Clock Generator............................................................................................. 31
3.5.5
Logic Control................................................................................................. 32
3.6
Measurement Results and Discussion ................................................................... 33
3.7
Conclusion ............................................................................................................. 37
System Integration......................................................................................................... 39
4.1
Introduction ........................................................................................................... 40
4.2
LCP-based SoP Technology.................................................................................. 40
4.3
Case study 1: Power Scavenging Unit................................................................... 41
4.4
Case study 2: Impulse UWB Transmitter.............................................................. 42
4.4.1
UWB Antenna ............................................................................................... 42
4.4.2
SoC Integration.............................................................................................. 42
4.4.3
SoP Integration .............................................................................................. 43
4.4.4
Results and Discussion .................................................................................. 44
4.5
Conclusion ............................................................................................................. 46
System Miniaturization.................................................................................................. 47
5.1
Introduction ........................................................................................................... 48
5.2
PTMA UWB Antenna ........................................................................................... 48
5.3
Impulse UWB Transmitter-Antenna Co-Design ................................................... 50
ix
5.3.1
Results and Discussion.................................................................................. 52
5.4
Antenna Effects on UWB System Performance ................................................... 55
5.4.1
Correlation Detection .................................................................................... 56
5.4.2
Energy Detection........................................................................................... 58
5.4.3
Results and Discussion.................................................................................. 59
5.5
Conclusions ........................................................................................................... 62
6
Conclusion and Future Work ........................................................................................ 63
6.1
Conclusion............................................................................................................. 63
6.2
Recommendation for Future Work ....................................................................... 64
References ......................................................................................................................... 67
x
List of Publications
xi
List of Publications
Papers included in this thesis
1. Majid Baghaei Nejad and L.-R. Zheng, "An Innovative Receiver Architecture For
Autonomous Detection Of Ultra-Wideband Signals," in Proceedings of 2006 IEEE
International Symposium on Circuits and Systems, ISCAS 2006, pp. 2589-2591
2. Majid Baghaei Nejad, M. Shen, T. Koivisto, T. Peltonen, E. Tjukanoff, H.
Tenhunen, and L.-R. Zheng, "UWB Radio Module Design for Wireless Sensor
Networks" Analog Integrated Circuits and Signal Processing, vol. 50, pp. 47-57,
2007.
3. Majid Baghaei Nejad, Z. Zou, H. Tenhunen, and L.-R. Zheng, "A Novel Passive Tag
with Asymmetric Wireless Link for RFID and WSN Applications," in 2007IEEE
International Symposium on Circuits and Systems, ISCAS 2007, pp. 1593-1596.
4. Z. Zou, Majid Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "An Efficient Passive
RFID System for Ubiquitous Identification and Sensing Using Impulse UWB Radio",
Elektrotechnik and Informationstechnik Journal, Special issue by Springer Wien, vol.
124, pp. 397-403, Dec. 2007.
5. Majid Baghaei Nejad, Z. Zou, D. S. Mendoza, H. Tenhunen, and L.-R. Zheng,
"Enabling Ubiquitous Wireless Sensing by a Novel RFID-Based UWB Module," in
First International EURASIP Workshop on RFID Technology, Vienna, Ausria, 2007.
6. Majid Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "Chip-Package and Antenna
Co-Design of a Tunable UWB Transmitter in System-on-Package with On-Chip
versus Off-Chip Passives," in Electronics System integration Technology
Conference, 2006. 1st, 2006, pp. 291-298.
7. Majid Baghaei Nejad, Soheil Radiom, Guy A. E. Vandenbosch, L.-R. Zheng,
Georges Gielen, “Impulse UWB Antenna Size Reduction Due to TransmitterAntenna Co-Design”, in 2008 IEEE International Conference on Ultra Wideband,
ICUWB 2008, Hanover, Germany, 2008, vol. 2, pp. 37-40
8. Majid Baghaei Nejad, Soheil Radiom, Guy A. E. Vandenbosch, L.-R. Zheng,
Georges Gielen, “Miniaturization of UWB Antennas and its Influence on UWBTransceiver Performance”, Submitted to IEEE Transaction on Microwave Theory
and Techniques.
9. Majid Baghaei Nejad, D. S. Mendoza, Z. Zou, Soheil Radiom, Georges Gielen, L.-R.
Zheng and H. Tenhunen, , “A Remote-Powered RFID Tag with 10Mb/s UWB
Uplink and -18.5dBm-Sensitivity UHF Downlink in 0.18µm CMOS”, Accepted to
IEEE International Solid-State Circuits Conference, 2009
xii
List of Publications
Related publications not included
10. M. Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "Power Management and Clock
Generator for a Novel Passive UWB Tag," in System-on-Chip, 2007 International
Symposium on, Tampere, Finland, 2007, pp. 82-85.
11. Z. Zou, M. Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "Baseband Design for
Passive Semi-UWB Wireless Sensor and Identification Systems," in IEEE
International SoC Conference SoCC 07, 2007, pp. 313-316
12. Soheil Radiom, M. Baghaei Nejad, Guy A. E. Vandenbosch, Li-Rong Zheng,
Georges Gielen, “Antenna Miniaturization Influence on the Performance of Impulse
Radio UWB system”, in European Microwave Conference 2008, Amsterdam, 28-30
October 2008
13. S. M. David, M. Baghaei-Nejad, H. Tenhunen, and L.-R. Zheng, "Low Power
Tunable CMOS I-UWB Transmitter Design," in IEEE 2007 Norchip, 19-20
November 2007, Aalborg, Denmark, 2007, pp. 1-4
14. Y. Niu, M. Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "Design of a Digital
Baseband Processor for UWB Transceiver on RFID Tag," in 21st International
Conference on Advanced Information Networking and Applications Workshops,
2007, AINAW '07., 2007, pp. 358-361.
15. Z. Zou, M. Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "An Efficient Passive
RFID System for Ubiquitous Identification and Sensing Using Impulse UWB
Radio," in First International EURASIP Workshop on RFID Technology, Vienna,
Austria, 2007, pages: 4 pp
16. M. Baghaei Nejad, C. Chen, H. Tenhunen, and L.-R. Zheng, "An Innovative SemiUWB Passive Transponder for Wireless Sensor and RFID Applications," in First
International Conference on Industrial and Information Systems, 2006, pp. 310-315.
17. Chen, M. Baghaei Nejad, and L.-R. Zheng, "Design and Implementation of a High
Efficient Power Converter for Self-Powered UHF RFID Applications," in First
International Conference on Industrial and Information Systems, 2006, pp. 393-395.
18. L.-R. Zheng, M. Baghaei Nejad, S. Rodriguez, Z. Lu, C. Cairong, and H. Tenhunen,
"System-on-Flexible-Substrates: Electronics for Future Smart-Intelligent World," in
High Density Microsystem Design and Packaging and Component Failure Analysis,
2006. HDP'06. Conference on, 2006, pp. 29-36.
19. L.-R. Zheng, D. Xinzhong, M. Baghaei Nejad, R. Saul, M. Ismail, and T. Hannu,
"On-Chip versus Off-Chip Passives in Radio and Mixed-Signal System-on-Package
Design," in Electronics System integration Technology Conference, 2006. 1st, 2006,
pp. 221-232.
20. (invited paper) L.-R. Zheng, M. Baghaei Nejad, Z. Zou, David S. Mendoza, Zhi
Zhang, and Hannu Tenhunen, “ Future RFID and Wireless Sensors for Ubiquitous
Intelligence, IEEE Norchip Conference, 2008
Summary of the included papers
xiii
Summary of the included papers
Paper 1. M. Baghaei Nejad and L.-R. Zheng, "An Innovative Receiver Architecture For
Autonomous Detection Of Ultra-Wideband Signals," in Proceedings of 2006
IEEE International Symposium on Circuits and Systems, ISCAS 2006, pp.
2589-2591
In this paper, a non-coherent receiver architecture is proposed for autonomous
detection of ultra wideband signals. The new receiver will self-generate a
synchronous template and hence, neither local template nor transmitterreference synchronizer is required. We validate its performance via simulations
compared with coherent receivers and conventional transmitted-reference
receivers. The new architecture is found much more robust to timing noise and
skew, which greatly facilitates the synchronize problem in UWB receiver.
Contribution to the paper: The author is responsible for all related work to this
publication including of proposing the autonomous architecture, performance
analysis, and writing the manuscript.
Paper 2. M. Baghaei Nejad, M. Shen, T. Koivisto, T. Peltonen, E. Tjukanoff, H.
Tenhunen, and L.-R. Zheng, "UWB Radio Module Design for Wireless Sensor
Networks" Analog Integrated Circuits and Signal Processing, vol. 50, pp. 47-57,
2007.
An impulse-based ultra wideband (UWB) radio system for wireless sensor
network (WSN) applications is presented. Different architectures are studied for
base station and sensor nodes. The base station uses coherent UWB architecture
which offers high performance and good sensitivity. However, to meet
complexity, power and cost constraints, the sensor node uses a non-coherent
architecture that can autonomously detect the UWB signals without any restrict
synchronization requirement.
Contribution to the paper: The author is responsible for all related work to this
publication except circuit implementation. Contribution includes propose the
system architectures, performance and results analysis, and writing the
manuscript.
Paper 3. M. Baghaei Nejad, Z. Zou, H. Tenhunen, and L.-R. Zheng, "A Novel Passive
Tag with Asymmetric Wireless Link for RFID and WSN Applications," in
2007IEEE International Symposium on Circuits and Systems, ISCAS 2007, pp.
1593-1596.
In this paper, we present a radio-powered module with asymmetric wireless
links for RFID and wireless sensor applications. Our contribution includes using
two different UWB and UHF communication link in uplink and downlink
respectively. An embedded power scavenging in Liquid-Crystal Polymer (LCP)
substrate captures the required power supply from the incoming RF signal.
xiv
Summary of the included papers
However, in uplink, an UWB-IR transmitter is utilized. The module is designed
in a system-on-package solution and consists of a power scavenging unit, a RF
demodulator, an UWB-IR transmitter, a digital controller, and an embedded
UWB antenna.
Contribution to the paper: Propose the solution for UWB-RFID, designing and
modeling the embedded RF and passive component in substrate, circuit design
and simulation, writing the manuscript.
Paper 4. Z. Zou, M. Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "An Efficient
Passive RFID System for Ubiquitous Identification and Sensing Using Impulse
UWB Radio", Elektrotechnik and Informationstechnik Journal, Special issue by
Springer Wien, vol. 124, pp. 397-403, Dec. 2007.
This paper describes an efficient passive RFID system using impulse ultrawideband radio (UWB-IR) in uplink. By utilizing a specialized communication
protocol and a novel ALOHA-based anti-collision algorithm, such systems
enable a high network throughput (2000 tag/sec) under the low power and low
cost constraint.
Contribution to the paper: propose the UWB-RFID concept, define the system
specification and building blocks, define communication protocol, circuit design
and simulation analysis, and writing one section of the manuscript.
Paper 5. M. Baghaei Nejad, Z. Zou, D. S. Mendoza, H. Tenhunen, and L.-R. Zheng,
"Enabling Ubiquitous Wireless Sensing by a Novel RFID-Based UWB
Module," in First International EURASIP Workshop on RFID Technology,
Vienna, Austria, 2007.
In this paper, an integrated CMOS module for UWB-RFID is presented. An
on-chip power scavenging drives the power supply from the incoming RF signal
and an on-chip UWB transmitter is used to respond to the reader. The
communication protocol is proposed. The module consists of a power
management unit, an RF demodulator, a clock management unit, an UWB-IR
transmitter, and a digital baseband is designed in 0.18µm CMOS process.
Contribution to the paper: The author is responsible for all related work except
digital implementation.
Paper 6. M. Baghaei Nejad, H. Tenhunen, and L.-R. Zheng, "Chip-Package and
Antenna Co-Design of a Tunable UWB Transmitter in System-on-Package with
On-Chip versus Off-Chip Passives," in Electronics System integration
Technology Conference, 2006. 1st, 2006, pp. 291-298.
In this paper we investigate the system integration in a Liquid-Crystal Polymer
(LCP) based System on Package (SoP). Chip-package-antenna co-design is
performed in the presence of unwanted packaging parasitic effects. Our
contribution includes embedding and modeling of the RF passives and antenna
and co-optimizing the chip and package with on-chip versus off-chip passives
trade-offs. A tunable UWB-IR transmitter, a power converter are studied. The
Summary of the included papers
xv
results show the potential of SoP for UWB packaging. It also verifies the ability
of the tunable transmitter to compensate the parasitic effects of packaging and
antenna.
Contribution to the paper: the author is responsible for all related work
including of designing and modeling of RF passive components in substrate,
chip-package-antenna co-design, on-chip and off-chip analysis and trade-off,
writing the manuscript.
Paper 7. M. Baghaei Nejad, Soheil Radiom, Guy A. E. Vandenbosch, L.-R. Zheng,
Georges Gielen, “Impulse UWB Antenna Size Reduction Due to TransmitterAntenna Co-Design”, in 2008 IEEE International Conference on Ultra
Wideband, ICUWB 2008, Hanover, Germany, 2008, vol.2, pp. 37-40
In this paper, the benefit of a co-design between a Printed Tapered Monopole
Antenna (PTMA) and an UWB-IR transmitter is investigated. A comparison is
given between a 50Ω design and a co-designed version. The simulation results
show that with the co-design method the tunable UWB transmitter can reach the
bandwidth regulation for a much smaller antenna.
Contribution to the paper: Chip-antenna co-design, define simulation setup and
result analysis, writing 3 sections of the manuscript.
Paper 8. M. Baghaei Nejad, Soheil Radiom, Guy A. E. Vandenbosch, L.-R. Zheng,
Georges Gielen, “Miniaturization of UWB Antennas and its Influence on UWBTransceiver Performance”, Submitted to IEEE Transaction on Microwave
Theory and Techniques.
In this paper the effect of antenna miniaturization in an impulse UWB
system/transceiver is presented. A PTMA antenna is designed in different
scaling sizes. In order to evaluate the performance and functionality of these
antennas, the effect of each antenna is studied in a given impulse UWB system.
It includes an impulse UWB transmitter and two kinds of UWB receivers, one
based on Correlation Detection and one on Energy Detection. A tunable lowpower Impulse UWB transmitter is designed and the benefit of co-designing it
with the PTMA antenna is investigated for the 3.1-10.6 GHz band. A
comparison is given between a 50Ω design and a co-designed version. Our
antenna/transceiver co-design methodology shows improvement in both
transmitter efficiency and whole system performance. The simulation results
show that the PTMA antenna and its miniaturized geometries are suitable for
UWB applications.
Contribution to the paper: Transmitter-antenna co-design, transceiver-antenna
co-analysis, simulation and results analysis, writing three sections of the
manuscript.
xvi
Summary of the included papers
Paper 9. M. Baghaei Nejad, D. S. Mendoza, Z. Zou, Soheil Radiom, Georges Gielen,
L.-R. Zheng and H. Tenhunen, , “A Remote-Powered RFID Tag with 10Mb/s
UWB Uplink and -18.5dBm-Sensitivity UHF Downlink in 0.18µm CMOS”,
accepted to IEEE International Solid-State Circuits Conference, 2009
A remote-powered UWB RFID tag in 0.18μm CMOS is presented. The
innovation is to employ asymmetric communication links, i.e. UWB uplink and
UHF downlink in order to achieve extremely low power, high data rate and
accurate positioning. Measurement shows the tag can operate up to 10Mb/s with
minimum input power of 14.1μW, corresponding to 13.9 meters of operation
range.
Contribution to the paper: propose the idea, define system specification and
building blocks, define simulation setup, RF/Analog circuit design, circuit
implementation, PCB design, measurement, writing the manuscript.
CHAPTER 1
1 Introduction
1.1 Project Motivation
A new term of ubiquitous computing and communication is booming up which will
transform our future corporate, community and personal life [1]. Ubiquitous or
pervasive computing is an environment where people interact with various companion,
embedded or invisible computers [2]. Early form of ubiquitous information and
communication was happened in the use of mobile phones and nowadays it has become
a vital part of everyday life for many millions of people even more than internet. In the
last decade there has been a huge increase in the use of computing devices. Nowadays,
wirelessly connected organizers and smart phones have become popular and digital
computing is an integral part of many everyday appliances. Recently, many research and
developments are ongoing on to bring this phenomenon more into everyday life by
embedding smart devices into more objects which can interact to each other and people
through a wireless link [3-8]. It will provide connectivity for anything from anywhere,
any place and for anyone. These connections create a network between items which lead
to Internet of Things (IoT). Several kinds of information can be exchanged through the
network such as environment status, and location which make a huge field of novel
applications and market. To realize this vision, several technical innovations in different
number of fields are essential. In order to have an embedded module in almost
everything, a simple and low cost system is essential. On the other hand, the hardware
should be power-efficient and reliable to be able to operate without any maintenance for
a long time. Embedding sensor technology into the items allows the system to detect
changes in the physical status of things, which allows the system to change or modify
some parameters of the system. And finally, system miniaturization allows smaller
things and hence more things have the connectivity. A combination of all of these
developments will create the IoT which connect the world’s objects intelligently.
1
2
1.1 Project Motivation
Wireless Sensor Network (WSN) and Radio frequency Identification (RFID)
technologies are two promising solutions for realization of the IoT vision [9]. Wirelessly
connected sensor nodes in a WSN offer a powerful combination of distributed sensing
and computing, which provide huge applications. Environmental monitoring, warfare,
surveillance and agriculture are some examples. However, there are several challenges
such as life time, flexibility, maintenance, and data collection, which need to be
addressed. Figure 1-1 shows a scratch of deploying WSN on an volcano monitoring
[10]. Distributed sensor nodes monitor the environment and send event report to a base
station directly or through neighboring nodes. Due to multi-hop capability WSN system
can cover a wide area.
RFID technology is another possible enabler for IoT. It has been used mostly in supply
chain management and logistic for several years [11, 12]. However, recently RFID
based ubiquitous identification, localization and sensing systems are widely interested
[9, 13-17]. An RFID system identifies items using radio waves. A typical RFID system
includes two parts: a transponder or a tag which attached to the object to be identified,
and an interrogator or a reader which identifies the tags. Passive, semi-passive, active
tags have their own advantages and disadvantages and they are used in different
applications. Figure 1-2 illustrates some commercial RFID readers and tags [18].
Utilizing RFIDs in ubiquitous wireless sensing encountered to several challenges which
need to be solved, such as life time for active tags, operation distance, communication
data rate, and throughput in dense multipath and multiuser environment.
Figure 1-1: Wireless sensor network example
1.2 Introduction to Ultra Wideband Radio
3
Figure 1-2: Commercial RFID readers and Tags
Ultra-Wideband Impulse Radio (UWB-IR) with its advantages has been recognized as
a great solution for future WSN and RFID [19-24]. Information in impulse UWB system
is typically transmitted using a collection of short pulses with low duty cycle, resulting
in lower power consumption. UWB-IR technique has the possibility of achieving Gb/s
data rate, hundreds of operating range, low energy consumption per bit, centimeter
positioning, and low cost implementation [25, 26]. It has been shown that UWB-IR can
be a promising solution for next generation of RFID and WSN. In this thesis utilization
of impulse UWB for WSN and RFID is investigated.
1.2 Introduction to Ultra Wideband Radio
In the first look, Ultra Wideband (UWB) radio is considered as a new technology
which enables something that was not possible before. However, there are no new
physical properties and in fact UWB is a new engineering technology. It can be said that
the earliest form of UWB was in 1893, when Heinrich Hertz used a spark discharge to
produce electromagnetic waves however, the dominant form of wireless communication
has been sinusoidal for several years [27]. Recently, UWB transmission has been
interested in both academic and industry. UWB offers many advantages such as high
data rate, low-cost implementation, and low transmit power, ranging, multipath
immunity, and low interference. Based on regulation approved in 2002 by Federal
Communication Commission (FCC) a signal is assumed to be UWB if the -10dB
bandwidth exceed 500 MHz. FCC reserved the unlicensed frequency band between 3.1
and 10.6 GHZ for indoor UWB wireless communication system [28]. Low transmitted
power regulated by FCC allows UWB system coexist with others licensed and
unlicensed narrowband systems. Therefore, the limited resources of spectrum can be
used more efficiently. On the other hand, with its ultra wide bandwidth, UWB system
has a capacity much higher than the current narrowband systems for short range
applications.
4
1.2 Introduction to Ultra Wideband Radio
Figure 1-3: FCC spectral mask for indoor applications
Two possible techniques for implementing UWB communication are Impulse Radio
(IR) and multicarrier UWB. UWB-IR is based on transmitting ultra short (in the order of
nanosecond) pulses. Usually in order to increase the processing gain more than one
pulse represents a symbol. Multi-carrier or multi-band UWB (MB-UWB) systems use
orthogonal frequency division multiplexing (OFDM) techniques to transmit the
information on each of the sub-bands. OFDM has several good properties, including
high spectral efficiency, robustness to RF and multi-path interferences. However, it has
several drawbacks. Up and down conversion is required and it is very sensitive to
frequency, clock, and phase inaccuracy. On the other hand, nonlinear amplification
destroys the orthogonality of OFDM. With these drawbacks MB-UWB is not suitable
for low-power and low cost application.
UWB-IR offers several nice advantages. It allows unlicensed usage of several
gigahertz of spectrum. It also offers great flexibility of spectrum usage. Adaptive
transceiver design can be used for optimizing system performance as a function of the
data rate, operation range, available power, demanded quality of service, and user
preference. Gb/s data-rate transmission over very short range is possible. Because of
ultra short pulses used in UWB, it is very robust against multipath, and more multipath
components can be resolved at the receiver, resulting in higher performance. Due to the
ultra-short duration pulses sub-centimeter ranging is possible. In UWB-IR no up and
down conversion is required therefore it reduces the implementation cost, and lowpower transmitter implementation is possible. Because of the short pulses and low
power transmission, it is very hard to eavesdrop the UWB signal.
1.2 Introduction to Ultra Wideband Radio
5
In spite of all the advantages, there are several issues which need to be considered. Due
to the short pulses, accurate synchronization and channel estimation is very difficult.
Several interferences such as multiple access interference and narrowband interference
should be detected and cancelled. Designing wideband RF components is a big
challenge. And for digital implementation high sampling rate ADC is very hard to
achieve.
Information in impulse UWB techniques is send by modulating short pulses. There are
several modulation options which depend on application, design specifications and
constraints, operation rage, transmission and reception power consumption, quality-ofservice, regularity, hardware complexity, data rate, and capacity. Some of known
modulation options in UWB-IR are Binary Phase Shift Keying (BPSK), Pulse
Amplitude Modulation (PAM), On-Off Keying (OOK), and Pulse Position Modulation
(PPM). The most popular modulation in UWB-IR is BPSK because of its better BER
and smooth power spectrum. However, accurate synchronization and channel estimation
is required for accurate pulse detection. Compared with BPSK, OOK and PPM are based
on presence or absence of signal and no channel estimation is required resulting in low
cost and easy implementation. Higher order modulations increase the data rate at the
cost of poor BER in a noisy channel. Therefore, for low power and low-data-rate
application lower order modulation is desired.
Generally, receivers for UWB-IR can be categorized in two kinds of Energy detectors
(ED), and Correlation Detectors (CD). Energy detectors are based on presence or
absence of signal and no channel estimation is required in low cost and easy
implementation at the cost of poor BER [26]. However, in correlation-based receiver a
template signal, generated locally according to the information acquired by bit and code
synchronization and possibly channel estimation, is multiplied by the received signal
and the result passed trough an integrator and decision block. To capture more multipath
components Rake receivers which include a bank of correlators and each finger is
synchronized to a multipath component can be used. A rake receiver with many fingers
can capture more multipath components and hence has higher performance. However,
channel estimation is a big challenge. Therefore, Rake receivers are too complicate and
consume much power, and hence they are not suitable for low-power and low-cost
implementation.
Different industrial standards have been developed. In 2002 the IEEE 802.15 formed a
Task group (TG4a) whose goal was to deliver the standard specification for Low Rate
Wireless Personal Area Networks (LR-WPANs). Finally On 22nd of March 2007,
P802.15.4a was approved as a new amendment to IEEE Std 802.15.4-2006 by the IEEESA Standards Board. It consisted of two optional Phys consisting of a UWB Impulse
Radio operating in unlicensed UWB spectrum and a Chirp Spread Spectrum operating in
unlicensed 2.4GHz spectrum [29]. Low data rate- IEEE 802.15.4a- provides emerging
applications of UWB such as wireless sensor network. It offers data rate from 50Kbps to
1 Mbps with ranges of 100 m with positioning capabilities. These features allow many
applications such as environment monitoring, tracking, localization, home control,
search and rescue, and security applications.
6
1.3 Introduction to Radio Frequency Identification
1.3 Introduction to Radio Frequency
Identification
RFID is a form of automatic identification (AutoID) technology that uses radio waves
to communicate between tags and a reader to identify items. It has been used for decades
and has become a hot topic since some big retail company like Wal-Mart announced to
introduce this technology in their supply chain. It can close the gap between physical
flow of goods and information flow in IT systems.
A typical RFID system is shown in Figure 1-4. It is composed by two main
components: A Reader and Tags (or transponders). In a passive tag system the reader
emits signals including command, energy and clock. All corresponding tags in the reader
field will detect the signal and use the energy from it to wake up and supply operating
power to its internal circuits. Once the tag has decoded a valid signal, replies its ID or
other information to the reader.
RFID transponders can be passive, semi-passive, or active. Passive tags do not have
any internal power supply and have no radio transmitter. Passive tags derive the required
power from a reader using either inductive coupling or electromagnetic capture. They
communicate to reader by utilizing load modulation or electromagnetic backscatter.
Semi-passive tags, also known as battery-assisted passive tags, have a local battery to
power the tag circuitry but they still have no radio transmitter and use backscatter
communication to send response. Active RFID transponders have a power source and a
conventional radio transmitter. They offer longer operation range, higher data-rate, and
larger memories than passive transponders however, they are expensive and usually with
big size.
Data
Power
RFID Reader
RFID Tag
Clock
Coupling
element
Application
Figure 1-4: RFID system block diagram
1.3 Introduction to Radio Frequency Identification
7
Passive tags are much cheaper and have virtually unlimited life time. Therefore, they
cover the majority of RFID transponders in used. They operate in different frequency
bands. The most commonly used frequency bands are the 125 KHz, 13.56 MHz, 860960 MHz, and 2.4 GHz.
Low frequencies (LF) and high frequency (HF) systems (125 KHz and 13.56 MHz)
offer an operating range up to 1 m. The coupling can be either capacitive or inductive.
For the capacitive coupling the resistance of coupling element is not important and can
be made from high resistive material such conductive ink. However they operate in very
short range of a few centimeters. Inductive coupling transponders operate in longer
distance up to 1 meter. Most of the RFID systems are of this type these days. Ultra High
frequency (UHF) transponders, however, operate in far-field region and radiative
coupling. They offer longer operation distance up to several meters [11].
Choosing LF, HF or UHF bands for RFID operation depends on requirements and
application. LF/HF tags use coil antenna with many turns while UHF tags use simple
dipole like antennas that are easily fabricated, but its size depends on the wavelength.
The operation range for LF/HF tags is comparable to antenna size, however UHF tags
provide operation range limited by the transmit power and when regulation allows it can
be increased. LF/HF tags operate in near-filed zones which are generally small and easy
to implement, however far-field operation in UHF tags are larger, but it is more complex
to implement. In UHF system nearby readers can interfere with each other and a
collision mechanism is required. LF/HF radiation penetrates into water while UHF
penetration is negligible in comparison to typical operation range. LF radiation can
penetrate thin layer of metal while HF and UHF radiation is shielded by metal layer
which limit their applications. Unlike LF tags which are limited to low data rate, HF and
UHF tags can supply tens or hundreds of kbps [12].
LF RFID is popular for animal, human, and objects ID, and access control. HF systems
are widely used for non-contact smart cards, access control, RFID-equipped passports
and travel documents, asset tracking, and supply management. UHF tags are widely
used in automobile tolling and rail-car tracking where a range of several meters is
desired. Recently, they are widely used in supply chain management, asset tracking, and
transport baggage tracking.
As in many other technologies, a lot of RFID standards exist. In the case of UHF
RFID, there is no accepted global standard. Transponder format, communication
protocols, frequency of operation and the code or ID can be parts of the standard [11].
Two major available standards are the EPC Global initiative [30] and the ISO 18000
standard [31]. Table 1 summarizes the common used RFID air interface standards.
8
1.4 Author’s Contribution
Table 1-1 Common used RFID air interface
Frequency
125/134 KHz 5-7 MHz 13.56 MHz 303/433 860-960
2.45 GHz
MHz
MHZ
Passive
ISO 11784/5
ISO
MIFARE
ISO 18000-6 ISO 18000-4
ISO 18000-2 10536 ISO 14443
EPC class 0,1 µ-chip
ISO
Ucode
18000-3
Semipassive
Maxim ISO 18000-4
Active
ANSI
ISO 18000-4
371.2
ANSI 371.1
ISO
18000-7
Tag Type
1.4 Author’s Contribution
The use of ultra-wideband impulse techniques for low-complex and low-cost wireless
sensing and identification system has been studied. The main goal of the thesis is to
propose a wireless sensing and identification system based on remote-powered tags
utilizing UWB-IR.
The study has been carried out in four parts. In the first part impulse UWB techniques
have been studied. Different impulse UWB receivers have been investigated and a novel
autonomous impulse detection has been proposed. Papers [32, 33] investigate the
utilizing of impulse radio in wireless sensor network. The author is responsible for al
related work for the paper [32], and propose two different architectures for base station
and sensor nodes, writing, and corresponding author in paper [33].
In part two a new wireless identification system, with remote-powered tag, utilizing
UWB-IR has been proposed. The innovative contribution is employing asymmetric
wireless links (UWB and UHF) in uplink and downlink. The system architecture has
been proposed, and the architecture and building blocks of the tag have been defined. RF
and analog circuits for the tag has been designed. A new communication protocol has
been defined. Simulation and measurements have been done and the results prove the
proposed concept. Three master theses have been supervised in the area of power
scavenging, impulse UWB transmitter, and a logic control and communication protocol.
Papers [34, 35] investigate the implementation of power scavenging and power
management unit. A tunable low-power UWB-IR transmitter has been proposed in
papers [36, 37]. Papers [38, 39] explore the implementation of the proposed
communication protocol. To proof of the concept, a single-chip tag including RF/Analog
circuitry and logic control has been implemented in CMOS 0.18µm technology, and the
measurement results show the feasibility of the proposed concept. The complete
proposed system has been published in papers [7, 40-42].
In part three system integration and packaging of the proposed system including an
UWB antenna in Liquid-Crystal Polymer (LCP) substrate has been studied. The
contribution is designing and modeling of the RF components and UWB antenna in the
1.5 Outline of the Thesis
9
package and co-optimizing the chip and package design with on-chip versus off-chip
passives trade-offs [43].
Finally, the effect of UWB antenna miniaturization in an impulse UWB
system/transceiver has been investigated. The author is involved in evaluation of the
antennas performance and their functionality in a given impulse UWB system including
of an impulse UWB transmitter and two kinds of UWB receivers based on correlation
detection, and energy detection [44-46].
1.5 Outline of the Thesis
The thesis is organized as follows: Chapter 2 presents the utilization of UWB-IR in
wireless sensor network. A new autonomous UWB receiver targeting low complexity
and low power consumption is presented. Simulation results are compared with other
UWB receivers in order to verify the proposed detection. Chapter 3 presents the
proposed UWB-RFID with remote-powered tag. The proposed communication protocol
is presented and performance analysis is discussed. Circuit implementation and
measurement results are given, which verify the system concept. A case study for
system integration is given in Chapter 4. Two concepts of system-on-chip and systemon-package are discussed. System miniaturization is discussed in chapter 5. It provides
the influence of UWB antenna shrinking on UWB system performance. Finally chapter
6 concludes the thesis.
CHAPTER 2
2 Impulse Radio in Wireless Sensor
Network
In this chapter, a wireless sensor network (WSN) based on ultra wideband impulse
radio is proposed. Different architectures are suggested for base station and sensor nodes
focusing on low-power and low cost implementation. The base station uses coherent
architecture due to the high performance and good sensitivity requirements. However, to
satisfy complexity, power and cost constraints, the sensor node uses a novel noncoherent architecture that can autonomously detect UWB signals with no restrict
synchronization requirements. The performance of the autonomous detection in term of
BER is compared with other detection techniques. The analysis results show that the
new architecture is more robust against timing jitter and time mismatch, and hence the
problem of skew in synchronization could be considerably reduced.
11
12
2.1 Introduction
2.1 Introduction
A wireless sensor network (WSN) is an infrastructure consisting of several sensing,
computing, and communication elements wirelessly connected, that provides the ability
to observe and react to events and phenomena in a specific environment. It has been
used in many applications such as environmental monitoring, health applications, home
automation, inventory control, vehicle tracking and detection, and so on. There are four
basic components in a WSN: sensor nodes; interconnecting network; central points of
information collecting; and computing resources to handle information [47]. Sensor
nodes or wireless nodes which are also called motes are connected via series of multihop or single-hop short distance and low-power wireless link.
Implementations of WSNs have to address a set of technical challenges. One of the
current research challenges is to develop low-power and low-cost wireless nodes. Lowpower consumption is a key factor in ensuring long operation time for battery powered
nodes. Today’s commercially available radio transceivers consume typically several tens
of milliWatts. To maintain the required power consumption, the nodes must sleep most
of the time. This can be realized by using low duty cycle operation such as a 1% or 0.1%
duty cycle. In addition to low duty cycle operation, utilizing a power efficient
transceiver can also increase the power efficiency.
Impulse UWB communication has the potential of achieving high data rate, long
operating range, low transmit power consumption, and low cost implementation [25]. It
also offers the ability of accurate ranging and localization. It has been recognized to be
cost-effective to be integrated in the WSN instead of traditional radio transceivers.
Although, low-power implementation of impulse UWB transmitter is possible, but
UWB receivers usually consume much more power, due to the detection of ultra
wideband pulses. High performance UWB receivers, such as rake/correlation receivers
are too complex and expensive to be implemented in low-cost devices such as wireless
sensor nodes. Non-coherent receivers such as energy detection can be a solution.
However, they require higher signal-to-noise-ratio (SNR) which can be achieved in
short-range communication, especially in low data-rate applications. Therefore, in order
to have a power efficient WSN with long life time sensor nodes, single-hop or
multipoint-to-point (start-based) system connectivity is used. It can be realized in shortrange applications such as home, factory, or human body.
An example of a wireless sensor network using UWB links is shown in Figure 2-1,
where the sensor nodes gather data autonomously, and the network passes this data to
one or more base stations through UWB links, and then forwards to sensor network
servers.
2.2 System Specifications
13
Internet
Network
Server
Base
Station
Base
Station
UWB links
n1
n4
n3
n2
n6
n5
Sensor Nodes
Figure 2-1: Sensor network architecture with UWB link
2.2 System Specifications
Two architectures for base station and sensor node are shown in Figure 2-2 and
Figure 2-3 respectively. The idea is to bring the complexity and power consumption
from the sensor node to the base station, since there is not such a severe restriction on
power and cost in the base station. Therefore, in the base station a coherent receiver is
used, which offers higher performance. However, the power consumption and the cost
are more critical in the sensor node. As a result, a non-coherent architecture is utilized in
the sensor node. Autonomous non-coherent receiver architecture is proposed for sensor
node as shown in Figure 2-3. The template waveform employed in detection process is a
squared replica of the received signal. In this way, the complexity of the synchronization
and channel estimation is eliminated.
Antenna
Power
Management
Timing Circuit
Pulse Generator
LNA
Switch
Amplifier
τ
∫
T
0
A
D
C
Baseband
Process
Template Signal
Figure 2-2: Impulse-based UWB architecture used in the base station
14
2.2 System Specifications
Antenna
Power
Management
Pulse Generator
BPF
Td
Amplifier
∫
A
D
C
Baseband
Baseband
Process
Process
Switch
Autonomous detection
Figure 2-3: Impulse-based UWB architecture used in the sensor node
The proposed receiver architecture in the sensor nodes has a number of attractive
properties:
• Same as other non-coherent receivers, the synchronization is only in symbol
level, and neither frame nor pulse level synchronization is needed. Therefore the
complexity of the receiver is reduced.
• Multi-path gathering is achieved. Since all components in the received signal go
trough the same system, the produced template pulse is synchronized (but
unfortunately noisy) to the received signal without any channel estimation or the
need for a rake receiver with many branches.
• Since the template pulse is the same as the received signal, changing in channel
due to the movement does not affect the receiver performance. This can be a
significant advantage for systems operating in a mobile environment.
• Because the template pulse is produced from the received signal, this architecture
does not suffer from timing jitter. On the other hand, this architecture is very
robust to timing mismatches, and the problem of skew in synchronization could
be considerably reduced.
• Compared with TR-UWB, in this architecture the transmitter does not send any
reference pulses. Therefore the entire signal power carries data which improves
the performance.
Since the power consumption is critical in the sensor node, the wideband LNA can be
also eliminated from the sensor module for short range applications (as shown in Figure
2-3). Therefore, the power consumption is significantly reduced and the lifetime of the
modules is consequently increased. In addition, a power management block has been
utilized to wake-up the circuit and also controls the various operation modes such as
working mode and sleep mode, to reduce the power consumption as much as possible.
To have a long lifetime operation, transmission power of the sensor node is often set
to as low as possible. It reduces the signal strength on the base station. However, as
mentioned before, the coherent receiver utilized in the base stations can detect the weak
incoming signal.
2.3 Performance Estimation
15
2.3 Performance Estimation
In order to verify the performance of the proposed architecture in the sensor node, the
receiver performance is evaluated in term of bit error rate (BER) and compared with
conventional coherent detection and transmitted-reference UWB (TR-UWB) scheme.
Monte-Carlo analysis in MATLAB is used to evaluate the performances. The analysis
parameters are as follows:
• Pulse shape is a second derivative of the Gaussian pulse with 250ps duration as
shown in Figure 2-4.
• Channel model is Additive White Gaussian Noise (AWGN) and no interference
is considered.
• No multi-path effect is considered, though we expect better multi-path gathering
effects for the proposed autonomous architecture.
• Receiver bandwidth and noise bandwidth is set to 5 GHz.
• Noise figure is set to 8dB.
• Pulse rate is 100 MHz.
• In coherent and autonomous receivers, BPSK modulation and in TR-UWB twopoints-signal is utilized
Figure 2-5 shows the BER performance versus the SNR for three structures. It is
clearly seen that the coherent receiver has the best sensitivity. That is because coherent
detection is able to moderate the noise and maximize the SNR if a matched template is
available, while in autonomous and TR-UWB detection the template is generated from
the incoming signal and is very noisy.
Figure 2-4: Monocycle pulse
16
2.3 Performance Estimation
The autonomous receiver and the TR-UWB show similar performance, even though
more improvements are expected if timing mismatch and phase noise are considered. In
reality, time mismatch should be considered. In coherent receiver, that is a time skew in
synchronization. But in TR-UWB and autonomous receiver architectures, time
mismatch is the difference of Td from the expected value. In autonomous receiver, the
delay value equals to the delay of the mixer. Therefore, the accuracy of this delay is only
related to the receiver and it is much easier to control during designing and manufacture.
Figure 2-6 compares the performance of the receivers in the presence of time mismatch.
As can be seen, any time mismatch degrades the performance. Although, the TR-UWB
seems most tolerant to time mismatch, but as mentioned before the time mismatch in
autonomous receiver can be easily eliminated during manufacture (since it is not related
to the transmitter). Coherent receiver and TR-UWB system usually suffer from timing
jitter of the oscillator in transmitter and receiver [48]. But the autonomous architecture is
not affected by that. Figure 2-7 shows the performance in the presence of timing jitter
for three architectures. In coherent receiver, jitter in both transmitter and receiver are
considered. Unlike coherent and TR-UWB receivers, the performance of our
autonomous receiver is independent of timing jitter. It reduces the clock and
synchronization requirements in UWB transceiver, and consequently decreases the
power consumption and implementation cost.
0
10
-1
BER
10
-2
10
-3
10
-4
10
-30
Auto UWB 10Mbps
Auto UWB 100Mbps
Coherent 10Mbps
Coherent 100 Mbps
TR-UWB 10Mbps
TR-UWB 100 Mbps
-25
-20
-15
-10
-5
SNR (dB)
0
5
10
15
Figure 2-5: Performance of three detection techniques for different data rates
2.3 Performance Estimation
10
17
0
Coherent
TRUWB
Auto. UWB
BER
10
10
10
10
-1
-2
-3
-4
0
5
10
15
20
25
Time missmatch %Tp
Figure 2-6: Time mismatch analysis (100 Mbps)
10
0
Coherent
TR UWB
Auto. UWB
BER
10
10
10
10
-1
-2
-3
-4
0
5
10
15
20
25
30
35
40
45
50
Jitter (rms)Ps
Figure 2-7: Performance of three types of UWB receiver in the presence of jitter (100 Mbps)
18
2.4 Conclusions
2.4 Conclusions
In this chapter, impulse-based UWB radio system for wireless sensor networks has
been investigated. Because of different performance and cost constraints, different
impulse UWB architectures are utilized in base stations and sensor nodes respectively.
The base stations utilize coherent architecture due to the requirement of high sensitivity
and high performance detection. Whereas, in sensor nodes, because of its power and cost
constraints, we have proposed a new autonomous non-coherent architecture. This new
receiver can detect UWB signals autonomously with no restrict synchronization in pulse
or frame level. It is found to be extremely immune to timing jitter. Comparison between
this new architecture and conventional coherent and TR-UWB system has been
presented. Simulation results show that the BER performance of the novel autonomous
receiver architecture is close to TR-UWB system when timing jitter is not considered.
However, the new architecture is more robust against timing jitter and time mismatch,
and hence the problem of skew in synchronization could be easily reduced.
CHAPTER 3
3 Remote-Powered UWB-RFID
In this chapter, a novel wireless sensing and identification system is proposed, in which
the nodes, such as conventional UHF-RFID systems, capture required power supply
from the received RF signal transmitted by a reader. However, to overcome the
limitation of conventional passive RFID systems, instead of backscattering, in uplink an
Ultra-Wideband Impulse-Radio (UWB-IR) link is employed. Because of the asymmetric
links, a new communication protocol is proposed based on slotted-ALOHA anticollision algorithm. Simulation results show the throughput more than 2000 tags/s,
which is a great improvement compared with normal RFID system (less than 1000
tags/s). An integrated tag is fabricated in UMC 0.18µm CMOS process to verify the
proposed concept. It consists of a power management unit, an RF demodulator, a clock
generator unit, a low-power UWB-IR transmitter, and a logic control. Measurement
results prove the system concept and show the potential of UWB impulse radios for lowcost, low-power, and battery-less implementation, especially in low data rate
applications.
19
20
3.1 Introduction
3.1 Introduction
Radio Frequency Identification (RFID) has been one of the most rapidly growing
segments in automatic identification and data collection industry. An RFID system
identifies the unique tags’ ID or detailed information saved in them. They are widely
used in asset monitoring, access control, supply chain, and many other applications.
Wireless sensing and positioning are new added functions highly demanded in future
RFID technology [9]. Current solutions for RFID are mostly based on backscattering or
load modulation with data rate limited to a few hundreds of kb/s [11]. It causes large
latency when there are many tags in the reading filed. On the other hand, position
accuracy for backscattering system is not better than 70 cm [16] .
A solution is inclusion of a radio transmitter in the tag which is usually done in active
tags using existing narrowband radio. However, carrier generation is power consuming
which requires battery operation, which increases the implementation and maintenance
cost compared with passive tag.
Advanced design allows passive operation at long distances in order of 10 meters [49].
However, the longer operation range, the more tags have to be read, implying larger
capacity for the system, which is difficult to achieve with backscattering system.
Impulse Ultra wideband (UWB-IR) technique using short pulses for data transmission
has been recognized as a powerful candidate for future RFID systems. It has the
potential of achieving MB/s throughput, operating range of hundreds of meters,
centimeters positioning, low power consumption, and low cost implementation. Previous
works have revealed that UWB transmitter is extremely area and power efficient,
whereas the receiver is still area and power hungry which requires internal power source
which increases the implementation cost, size and maintenance cost [50].
In this work, we present a 10Mb/s impulse UWB RFID tag in 0.18μm CMOS. The tag
is remotely powered by UHF wave with minimum input RF power as low as 14.1μW.
Our innovative contribution is to employ two different UWB-IR and UHF
communication links in uplink and downlink respectively. This is because the amount of
data or instruction from a reader to a tag is very few and a low data rate communication
link as conventional UHF-RFID at 900 MHz can be used as downlink. UHF also
provides remote power to the tag. The uplink requires higher data rate and precise
positioning capability, therefore an UWB-IR transmitter is employed [7, 40, 41].
3.2 System Description
Figure 3-1 shows the proposed system concept. In general the required power supply
can be provided by any kind of energy sources such as thermal energy, vibration,
movement, solar energy and so on. Available power may be too low to provide
continuous power for circuitry, thus a tag captures energy for a long time, stores it in a
storage capacitor and uses this energy in operation period. As a result, the amount of
available energy is limited and the operation should be done in a time and power
3.2 System Description
21
efficient manner. Impulse UWB is a promising solution to achieve high data rate along
with low-transmit power consumption [7, 40].
The system can operate in two scenarios. In Burst mode, there is no communication
link from the readers to the tags and the tags transmit their ID and other information
when they capture and store enough energy. To realize multiple access each tag sends
data after a random delay generated locally. The burst mode results in simple
implementation and operation however, in a dense multi user environment there will be
a huge collision, which degrades the overall system throughput.
In order to reduce the tags collision, Acknowledgement mode can be used. In
acknowledgement mode, such as conventional RFID communication protocol, tags send
their data after receiving a request from a reader and go to halt mode after receiving an
acknowledgment to reduce tags collision, resulting in higher throughput. In this mode
the tags can capture the required power supply from the received RF signal transmitted
by the reader, which carries data to the tags. In this thesis the operation in
acknowledgement mode is considered and the electromagnetic wave is used as the
power source.
An example of a generic network using asymmetric wireless links is shown in Figure
3-2. The network consists of hundreds, or thousands of tags and several readers, which
covers the whole area of interest. The area is divided into several overlapped clusters. A
reader in each cluster provides remote power for the tags and sends command, data, and
synchronization clock using a UHF electromagnetic wave. The tags respond their
information via an UWB-IR link and readers forward data to a network server through a
standard link e.g. LAN or WLAN link. Adjacent reader should handle an anti-collision
protocol to resolve reader collision.
Figure 3-1: Block diagram of the system concept
22
3.3 Asymmetric UWB-RFID Architecture
Internet
Server
Tag2
Semi-UWB
Reader2
Tag N
e te
10 m
Reader1
Tag1
Tag 3
rs
Reader2
Reader2
Tag5
Tag6
Tag4
Figure 3-2: Generic Network with Semi-UWB Tags
3.3
Asymmetric UWB-RFID Architecture
RFID applications hold some notable characteristic which are different than usual
communication systems:
• Huge number of tags might appear in the reading field simultaneously.
Therefore, an efficient multiple-access algorithm is essential for system
efficiency.
• Unlike other RF communication systems, the traffic loads in RFID are highly
asymmetric between the uplink from a tag to a reader and downlink from a
reader to a tag. Data (e.g command, synchronization) from a reader to tag is
very few, but the traffic transmitted from a great number of tags is heavy.
Therefore, higher data-rate link in uplink is demanded.
• Due to the demand of low cost implementation, tags have very limited
resources such memory, computational ability, and power supply, but a reader
can be a powerful device and more expensive.
Based on this consideration, we propose an asymmetric UWB-RFID system
architecture shown in Figure 3-3. UWB and UHF techniques are used in uplink and
downlink respectively. Due to the nature of the impulse UWB radio, it provides a high
speed and high secure uplink under a low power and low complexity implementation.
Since the wideband UWB receivers usually consume too much power, the traditional RF
receiver such as UHF-RFID is applied as the downlink. On the other hand, unlike other
communication systems, RFID and WSN applications are dominated by uplink
communication, and the low downlink traffic becomes insignificant for the system
efficiency. As a result, the low data-rate narrowband radio is adequate.
3.3 Asymmetric UWB-RFID Architecture
23
Figure 3-3: Block of the Reader and Tag with Semi-UWB link
The reader broadcast command to the tags by UHF (870-960 MHz) signal using
Amplitude-Shift Keying (ASK) modulation with modulation depth from 30% to 100%.
Binary data is encoded as pulse width modulation of the low amplitude pulse. Low
intervals of 1.5µs and 4.5µs represent bit 0 and bit 1 respectively. Figure 3-4 depicts the
modulation and data encoding parameters for reader to tag communication.
A tag replies information by transmitting UWB signal. The UWB pulse rate and data–
rate are adapted by the reader, based on the available power and the desired operation
distance. In long range operation, when the available power to the tag is low, lower
pulse rate and data rate is chosen, resulting lower power consumption. On the other
hand, in short range applications, higher pulse rate with higher data rate can be
transmitted since the available power is high enough.
Generally, BPSK modulation achieves the best BER performance [51]. However the
circuit complexity is the highest. Furthermore, BPSK detection is very sensitive to clock
jitter and skew. It demands an accurate synchronized clock on-chip, which increases the
complexity and power consumption in the tag. Either OOK or PPM modulation can be
used in the tags to modulate the UWB pulses. Because of the simplicity, in this work,
On-Off Keying (OOK) modulation is utilized which is insensitive to clock jitter. It has
less performance than BPSK, but it reduces the complexity and power consumption
significantly, which is a great advantage for the tag.
UHF envelope
Low Interval
Bit 0
Bit 1
Mod
Figure 3-4: Reader-to-tag signaling
24
3.4 Proposed Communication Protocol
3.4 Proposed Communication Protocol
A new communication protocol is proposed for the system based on slotted-ALOHA
anti-collision algorithm. Five functions are defined in the proposed protocol, which are
listed in Table 3-1.
Table 3-1:Functions in communication protocol
Function
Wakeup
Request
Write
Modify
Kill
Description
Identify all tags in the reading field.
Identify the tags that have not been
identified in the reading field.
Program tag’s memory unconditionally.
Program a specific tag with access
control.
Delete a specific tag.
A frame which represents an operation initiated by readers is composed by four phases:
powering up, start of frame (SOF), commands, and response. A frame time is divided
into discrete time intervals, called slots. A tag randomly selects a slot number in the
frame and responds to the reader. A procedure called acknowledgment is required to
resolve collisions or failed transmissions. Collided tags retransmit in next frames. Three
improvements are employed to enhance the system performance.
Due to the great asymmetry between the downlink and the uplink, the
acknowledgement from the reader to tags becomes a bottleneck that decreases the
network throughput. This problem is solved by using a pipelined method that poses the
data packet and its corresponding acknowledgement in two adjacent slots. As can be
seen in Figure 3-5, a tag sends data in the K slot and receives the ACK in the K + 1 slot
[8, 39].
Figure 3-5: Frame format and the pipelined communication
3.4 Proposed Communication Protocol
25
Because the global clock is scalable controlled by the reader, it provides a possibility to
skip idle slots. By detecting the incoming signals at the beginning of each slot, the
reader can determine if there is any transmission in this time slot. If it is an idle slot, the
reader skips this slot by adjusting the clock frequency and transits into the next cycle
(slot) immediately. Figure 3-6 compares the system efficiency with idle skipping
approach and normal approach. As can be seen higher efficiency can be achieved.
System Efficiency at the frame size=256
0.7
None Idle Slot Skipping
Idle Slot Skipping
0.6
System Efficiency
0.5
0.4
0.3
0.2
0.1
0
0
100
200
300
400
500
600
Number of Tags
700
800
900
1000
Figure 3-6: Simulation result of the system efficiency with idle slot skipping
Since the system efficiency depends on the frame size when the number of tags in the
reading filed is varied, the Frame sizes can be regulated by the reader to improve the
performance in a dynamic circumstance. With certain algorithms, the reader can
estimate the number of tags based on states of previous frames, and set the frame size in
a Wakeup or Request command. Figure 3-7 shows the effect of adaptive frame size
compared with fixed frame size. As can be seen, with adaptive frame size more than
2000 tags/s can be processed.
The Processing Delay Simulation
2500
Frame Size=256
Frame Size=512
Frame Size=1024
Adaptive Size
Processing Delay (ms)
2000
1500
1000
500
0
0
200
400
600
800
Number of Tags
1000
1200
1400
Figure 3-7: Simulation result of system efficiency with adaptive frame size and idle slot
skipping
26
3.5 Implementation
3.5 Implementation
In order to achieve the desirable properties of the tag as well as to verify the proposed
system concept, the tag circuitry is implemented in UMC 0.18µm CMOS technology.
Figure 3-8 shows the detailed block diagram of the tag. It consists of a power
management unit, an RF demodulator, a clock generator unit, an impulse UWB
transmitter, and logic control.
3.5.1 Impulse UWB Transmitter
Digital pulse generator, up conversion or (Finite Impulse Response) FIR transmitter is
not feasible, since they required on-chip high frequency clock [52, 53]. On the other
hand, these circuits consume high power which is also unaffordable in the proposed
battery-less tag. Therefore, we use continuous-time filter architecture. Figure 3-9 shows
the schematic of the impulse UWB transmitter. Since one pulse is generated at each
falling edge of the incoming clock, the pulse rate can be adjusted by the logic control
easily based to the available power and desired operation distance.
Both duration and amplitude of the output pulse are tunable by two control inputs.
These controls enable the module to compensate the process variations, packaging
effects, and frequency response of the antenna. On the other hand, it offers the ability
Antenna
Power Managment Unit
Matching
Power
converter
Voltage
sensor and
switches
RF Demodulator
Envelop
Detector
Clock Generator
Injection Divider
PWM Det
Vdd
Low drop
Regulator
Power on
Reset
Logic Control
Data Rx
Global Clock
Gen.
Clock
HF Clock
Tx Enable
UWB
Ant
UWB Tx
Data Tx
Figure 3-8: Block diagram of the UWB-RFID tag
3.5 Implementation
27
Figure 3-9: Schematic of the impulse UWB Transmitter
to trade-off between data rate, output power, power consumption, and operation range.
For example, in short range applications, when the incoming RF signal provides higher
power to the tag high data rate transmission is chosen. On the contrary, to transmit data
in longer distance, low data rate transmission results in low-power consumption. In both
cases, amplitude and duration controls enable the module to adjust the transmitted signal
meeting bandwidth and power regulations [37, 43].
3.5.2 Power Management Unit
The power management unit provides the power supply for the whole circuitry from
the incoming electromagnetic wave. A power scavenging unit (PSU) rectifies the
incoming 900MHz RF signal to DC in a storage capacitor. The instantaneous power
consumption of the circuits is too high to be operated constantly by remote-power.
Therefore, a low-power voltage-sensor (Vsen) activates the operation only when the
voltage in the storage capacitor reaches a certain value (e.g 2.5 V). The required energy
for the operation is provided by the stored energy in this storage capacitor. The size of
the storage capacitor depends on the operation time and the required current, which can
be in the order of hundreds of nano-farad and therefore off-chip capacitor is used. While
the chip is working, voltage over the storage capacitor is degraded; therefore, a lowdrop-out (LDO) voltage regulator is utilized to provide regulated voltage of 1.8V for the
circuitry. When the capacitor voltage becomes less than a certain threshold (e.g. 1.8V)
the operation is stopped, and the storage capacitor is charged again. A Power-on-Reset
(PoR) circuitry creates a reset signal for the logic control to eliminate the transient
response of the voltage sensor and LDO [35].
The operation distance of a tag depends on the efficiency of power scavenging unit. A
diode-connected MOS multiplier is chosen here. The schematic of the power scavenging
unit is shown in Figure 3-10. It includes a chain of 7 stages NMOS voltage multiplier
and a voltage limiter which keeps the output voltage less than the breakdown voltage.
The design goal is to achieve the required output voltage and current with minimum
input power resulting in longer operation range. Figure 3-11 shows the equivalent circuit
of the antenna and the load represented by the input impedance of the tag. In general Rant
28
3.5 Implementation
Figure 3-10: Schematic of power scavenging unit
and Xant can include the matching network effects as well. Assuming a free space path
loss, operation distance can be written as (3-2).
λ2
Pa = EIRP.Ga .
(4πd ) 2
d=
(3-1)
EIRP.Ga λ
.
Pa
4π
(3-2)
Where EIRP is the Equivalent Isotropic Radiated Power of the transmitter, Pa is the
available power at the receiving antenna, Ga is the receiving antenna gain, λ is the wave
length, and d is the distance. Increasing of the operation distance is expected. Normally
Ga can not be increased since the antenna should be ideally isotropic to operate in all
direction. EIRP is also dictated by the regulation. What remains is reducing the Pa.
Considering a matched antenna to the IC’s input impedance, Pa can be written as
Equation 3-3.
Xant
Rant
Vi
Vant
Cic
Ric
Figure 3-11: Equivalent circuit of the antenna and tag
3.5 Implementation
29
2
V
Pa = i
2.Ric
(3-3)
Where Vi is the voltage across the IC’s input and Ric is the equivalent parallel input
resistance. To decrease Pa, we can reduce Vi and increase Ric. Vi depends on the required
output voltage and the number of stages of multiplier. Ric depends on the load current
and the leakage of the circuit. Using the voltage sensor in our proposed tag, which
switch off the ship during scavenging, reduces the load current considerably and
consequently increases the Ric. For a fixed output voltage and load current, increasing
the number of stages reduces the required Vi. However, more number of stages increases
the leakage which decreases the Ric. On the other hand, the conversion factor and the
leakage current of the multiplier depend on the size of the transistors. Larger transistors
increase the conversion factor reducing the required Vi. But it also increases the leakage
current which decreases the Ric. Therefore, the optimum values should be found for the
number of stages and the transistor size. It has been shown that for the low load current
the capacitors size does not have much effect on the performance if it is larger a certain
value [54]. So, it has not been considered as the optimization parameters.
The voltage sensor is the only operating part of the chip in powering phase, therefore
its static current is very critical. Figure 3-12 shows the schematic of voltage sensor
including a reference voltage generator, a Schmitt trigger comparator and a power
switch. Resistor based and bandgap references need very big resistors in order to reduce
the power consumption, which occupy large area [55]. Therefore, in this work a CMOS
based reference voltage is used [56].
Figure 3-12: Schematic of voltage sensor
30
3.5 Implementation
Because the voltage across the storage capacitor is degraded during operation mode, a
voltage regulator is utilized to provide fixed 1.8V voltage for the chip. Unlike traditional
RFID, shunt regulator is not suitable for this module because it is power consuming.
Thus, a low-drop-out (LDO) voltage regulator is designed. The schematic of LDO is
shown in Figure 3-13. The same voltage reference generator as the voltage sensor is
utilized.
Figure 3-13: Low drop output voltage regulator
To avoid the transient response of the LDO and the voltage sensor a Power-on-Reset
(PoR) circuitry is used to create a reset signal for the control logic in the beginning of
the operation. Figure 3-14 shows the schematic of the PoR. A capacitor charged by a
current source generates a pulse which is delayed compared with the Vsen switch.
Vcc
Vbias
Td
From Vsen
Figure 3-14: Power-on-Reset circuit
3.5 Implementation
31
3.5.3 RF Demodulator
The envelope of the UHF signal contains data and clock for the tag. An envelop
detector similar to power scavenging unit but with only 2-stages extracts the envelope.
Figure 3-15 shows the block diagram of the RF demodulator including envelope detector
and data-clock recovery circuitry. It supports the 900 MHz ISM band with data rates up
to 160 kbps. The extracted clock is used for logic control and no local oscillator is
needed which reduces the power consumption significantly.
RFin
I Bias
Envelope
Det.
+
Vref 1
Vref 2
Feadback
Network
+
-
Data
out
Clock
Figure 3-15: Schematic of RF demodulator
3.5.4 Clock Generator
UWB transmitter requires a high frequency clock of 100 MHz for data transmission.
LC oscillators occupy large area and consume much power. On the other hand, ring
oscillator show large variation across the process, temperature and voltage as well as
huge phase noise [57]. Utilizing PLLs which are common in communication systems are
not applicable because of their high complexity and power consumption. Therefore, a
low power harmonic injection locked divider (HILD) is utilized. Injection-locked
dividers realized a high operation frequency along with low power consumption. In
particular harmonic injection locked divider realizes a division order of more than two
which is effective for power consumption reduction [58]. A low-power harmonic
injection locked (HIL) divide-by-9 is used to down convert the 900 MHz carrier
frequency to 100 MHz clock for UWB transmission which is shown in Figure 3-16. it
composed by cascading two divide-by-3 HILD circuits. To reduce the power
consumption, the circuit is designed at 1 volt power supply.
32
3.5 Implementation
Figure 3-16: HIL Divider schematic
3.5.5 Logic Control
A logic control is designed to execute the specified communication protocol. Figure
3-17 illustrates the block diagram of the logic control. A 128-bit memory is organized in
three segments: 64 bits ID, 16 bits CRC and 48 bits header (32 bit preamble and 16 bit
reserved word). The pseudo number generator (PNG) and the slot counter are used to
implement the transmission protocol and the anti-collision algorithm.
PNG
Din
Memory
Control Logic
(FSM)
Slot Counter
CLK_Tx_En
128 Bits
Tx Buffer
Dout to Tx
Figure 3-17: Block diagram of the logic control
3.6 Measurement Results and Discussion
33
The proposed tag is implemented in UMC 0.18µm CMOS process. The chip
micrograph is shown in Figure 3-18. Since all the sub blocks are placed independently
and there are several test blocks, I/O, and filtering capacitors on chip, the chip size is 4.5
mm2, but the active area is less than 1 mm2 .The chip is packaged in a Quad Flat No
leads 48 (QFN 48) to minimize the parasitic inductance effects on UWB transmitter
output.
Figure 3-18: Die micrograph
3.6 Measurement Results and Discussion
A printed circuit board (PCB) is designed to add the external storage capacitor and
other interconnection between the sub blocks. A photograph of the test board is shown
in Figure 3-19.
Figure 3-19: Photo of the test PCB for the measurements
34
3.6 Measurement Results and Discussion
To measure the required input power for the power scavenging unit, the Agilent Vector
Network Analyzer (VNA 8753ES 6GHz) is used and the calibration is done to de-embed
the PCB trace. The measurement setup is shown in Figure 3-20. The input sensitivity of
-18.5 dBm (14.1 μW) has been measured. It corresponds to 13.9 meters operation range
considering 4W EIRP and a matched antenna with 0dB gain, which is a great
improvement compared with existing passive RFID [59]. This improvement is due to the
new proposed operation method which reduces the power consumption during the
harvesting time and improves input sensitivity by using a voltage sensor. Figure 3-21
shows the PSU and the PoR outputs with a storage capacitor of 211nF (four capacitors
in parallel 100+68+33+10 nF) at 10 MHz pulse rate. The charging time is 31 ms and the
operation time is 1.9 ms and 0.184 ms for 10 MHz and 100 MHz pulse rate respectively.
The output impulse of the UWB transmitter has an amplitude of 220 mVPP and a
duration of 620 ps as illustrated in Figure 3-22. The output pulse shape is sampled by the
83484A dual channel 50 GHz digital oscilloscope from Agilent. At 10 MHz pulse rate it
consumes 51µ[email protected] (91.8μW) and the power spectral density satisfies the FCC
indoor regulation. This corresponds to 9.2 pJ/pulse which is much less than recently
reported work [53]. The FSQ26 spectrum analyzer from Rohde-Schwarz is utilized to
measure the output power spectral density which is shown in Figure 3-23.
Figure 3-20: Measurement setup
3.6 Measurement Results and Discussion
35
Figure 3-21: measurement results: (a) output voltage measured at rectifier and power-on-reset and (b)
output voltage at LDO regulator and PoR at 10MHz UWB clock
0.15
0.10
Volt
0.05
0.00
-0.05
-0.10
-0.15
0
0.5
1
1.5
Time (ns)
2
2.5
3
Figure 3-22: Measured output pulse shape of UWB transmitter
36
3.6 Measurement Results and Discussion
-45
dBm/MHz
-55
FCC
PSD
-65
-75
-85
0
1
2
3
4
5
6
7
Frequency (GHz)
8
9
10
Figure 3-23: Measured output power spectral density @10MHz pulse rate compared with
FCC indoor mask
ASK modulated RF signal such as explained in section 3-3 is used to measure the data
and clock recovery performance. Measurement results for receiving bit 0 and 1 are
shown in Figure 3-24. As can be seen data can be sampled in the falling edge of clock.
Figure 3-24: Measurement result for ASK demodulator
3.7 Conclusion
37
Measurement result of the harmonic injection locked divider shows the locking
frequency range of 82-92 MHz, which has a frequency shift from the expected 100
MHz. The total power consumption is 30µA and the minimum input power for locking
is measured to be -19dBm. Figure 3-25 shows the output phase noise before and after
locking measured by the Rohde-Schwarz FSQ26 spectrum analyzer. Due to the OOK
modulation used in UWB transmitter, UWB transmission is not sensitive to the jitter,
although the measurements show the jitter less than 7ps and phase noise of -87dBc/Hz at
100Hz offset.
Figure 3-25: Measured Phase noise of HIL Divider before and after locking
3.7 Conclusion
A novel system with asymmetric wireless links has been presented for wireless sensing
and identification. The innovative contribution is to employ two different
communication links (UWB and UHF) respectively in uplink and downlink of the tag. It
allows long-range remote-power operation along with high data-rate and precise
positioning capability. Table 3-2 summarizes and compares the measurement results
with two related works. The input sensitivity is measured to be -18.5 dBm (14.1μW)
corresponding to 13.9 meters operation range considering 4W EIRP and a matched
antenna with 0dB gain. The UWB transmitter consumes 918 µW instantaneous-power at
100 MHz pulse rate which corresponds to 9.2 pJ/pulse. Adaptive data rate up to 10 Mb/s
has been achieved for uplink. The new proposed communication protocol allows more
than 2000 tags/s to be proceeded, which is a great improvement compared to existing
passive RFID systems.
38
3.7 Conclusion
Table 3-2: Performance summary of this work in comparison with passive UHF tag
[59] and high data-rate HF tag [60]
This work
[60]
[59]
Technology
0.18µm
0.18µm
0.13 µm
Die area
(Active area)
Downlink
4.5 mm2
(1 mm2)
900MHz ISM
band
40-160 kb/s
2.7 mm2
(0.75 mm2)
13.56 MHz
0.55 mm2
106 kb/s
900MHz
ISM band
40-160 kb/s
Pulse rate
UWB
3.1-10.6Ghz
10 ~100MHz
HF, Load
modulation
-
UHF,
backscatter
-
Data rate
Up to 10Mbps
3.4 Mb/s
40-640 kb/s
-
-
220 mVpp
-
-
620 ps
-
-
91.8 µW,
9.2 pJ/pulse
918 µW,
9.2 pJ/pulse
-
-
-
-
-
-
Data rate
Uplink
UWB transmitter
Pulse amplitude
Pulse width
Power
consumption
10Mhz
100Mhz
Power scavenging
Vout
2.75 V
NA
1.45 V
Iout
1.5 µA
NA
NA
-18.5 dBm
(14.1µW)
13.9 meters
(@4W EIRP)
NA
-14 dBm
(39.8µW)
7 meters
Input Sensitivity
Typical Distance
Typical Throughput
2000 tags/s
10 cm
NA
EPC C1G2
(880 tags/s)
CHAPTER 4
4 System Integration
In this chapter, integration and packaging of the proposed system including an UWB
antenna in Liquid-Crystal Polymer (LCP) package is investigated. Chip-antenna codesign is performed in the presence of unwanted packaging parasitic effects. Our
contribution includes modeling of the RF components and antenna in package and cooptimizing the chip-package with on-chip versus off-chip passives trade-offs. A tunable
low power UWB-IR transmitter, a power scavenging unit, and an UWB antenna are
studied. Simulation results show the feasibility of system-on-package integration for
UWB implementation.
39
40
4.1 Introduction
4.1 Introduction
A typical wireless node in WSN or RFID system may includes several components
such as digital processor, memories and ASICs, analog front end, RF and microwave
components, discrete components, micro-electromechanical components and even user
interface. Integration all of these components in a single system-on-chip (SoC) is not
necessary the best solution in all cases. Instead, system-on-package (SoP) could be a
better option. Different components can be realized in different technologies with less
constraint and cost than SoC.
SoP offers embedded passive components on substrate. Therefore, the expensive and
low quality on-chip passive components can be moved off the chip, which can improve
the performance and decrease the total cost [61, 62]. However, due to the parasitic that
may limit the performance, designing with embedded passives can be a complex task.
An optimum solution must be found by doing chip-package co-design with precise
trade-offs for on-chip versus off-chip components [63-65].
In this chapter, SoP integration of the power scavenging unit and the UWB-IR
transmitter are studied as case studies. Embedding and modeling the RF components
and co-optimizing the chip with on-chip versus off-chip passives trade-offs are
investigated.
4.2 LCP-based SoP Technology
Liquid Crystal Polymers (LCPs) is a thermoplastic material. It has been considered as a
low-cost substrate material for high-performance packaging. It has relatively stable
dielectric constant of 3.1 over the frequency range up to 110 GHZ. It has a very low
tangent-loss of 0.002 to 0.0045 in this frequency range, which makes it very suitable for
high frequency application [66]. Previous research in our lab has investigated its
potential for electronic packaging [67]. This technology allows implementation of small
size system at very low cost, along with a small antenna and high quality embedded
passive components such as inductor, resistor, and capacitor [68]. The concept of SoP
implementation is shown in Figure 4-1. It can include an antenna, RF chip, sensors, a
baseband IC, and embedded passives.
MEMS Sensor
L
RF IC
R
Baseband IC
C
Antenna
Figure 4-1: SoP implementation with embedded passive components and antenna
4.3 Case study 1: Power Scavenging Unit
41
4.3 Case study 1: Power Scavenging Unit
To realize the feasibility of circuit implementation in LCP substrate, as a case study a
power scavenging unit (PSU) is studied. It is a voltage multiplier using surface mounted
Schottky diodes (The Metelics MSS30-242). Figure 4-2 shows the schematic and layout
of the PSU. As can be seen because of using low forward voltage drop Schottky diode,
the required output can be achieved by less number of stages (2 stages) compared with
CMOS on-chip implementation (7 stages). All passive components including the
coupling capacitors and the matching network are integrated in substrate. Since, the
storage capacitor is too big to be implemented in substrate a surface mounted capacitor
is utilized. The circuit is matched to a 50Ω antenna by an embedded matching network.
ADS momentum is used to extract the model for passives in package.
Vout
D4
D3
C2
L2
L1
C1
D2
D1
C4
SMD
C3
Figure 4-2: Power scavenging unit in LCP substrate
Figure 4-3 shows the simulation results of the PSU. It can provide 2.5V and 1.5µA
output current with minimum -17.1dBm (19.5 µW) input power. It corresponds to 12
meter operation range with 4W EIRP emission, which is close to the on-chip integration.
Considering free space propagation loss and assuming 0dB receiving antenna gain. At
this distance maximum efficiency of 34 % is achieved at 1.5V/5µA.
Figure 4-3: Simulation results of power scavenging unit
42
4.4 Case study 2: Impulse UWB Transmitter
4.4 Case study 2: Impulse UWB Transmitter
In this study, integration of the impulse UWB transmitter in LCP substrate is
investigated. The study is done in two cases of SoC and SoP integration. In SoC solution
all passive components except antenna are considered to be implemented on chip.
However, in SoP integration, passive components are moved off the chip. The
performances of the transmitters in two solutions are compared in the presence of an
UWB antenna.
4.4.1 UWB Antenna
In this study the knight’s helm shape antenna is chosen. It is a double-slotted small size
antenna, which shows stable characterizations over the UWB frequency band [69].
Figure 4-4.a shows the geometry of the antenna in LCP substrate. Layout of the antenna
is simulated in ADS momentum and Figure 4-4.b shows the S11 return loss and the gain
of the antenna. These parameters are used to co-design the chip and antenna.
Figure 4-4: Geometry, S11 and gain of the antenna on LCP substrate
4.4.2
SoC Integration
Figure 4-5 shows the circuit model for SoC solution. Except antenna all the other
passive components are integrated on chip. Flip-chip packaging using solder bump is
used due to the lower parasitic inductor (less than 0.08nH) compared with bound wire,
which is modeled by Rbump, Lbump and Cbump [70]. The design goal is to have an output
pulse with less substantial late-time ringing and high bandwidth, while it complies with
the FCC spectral regulation. The required operation condition can be achieved through
the amplitude and duration tunability, which is one of the advantages of the above
transmitter design. Figure 4-6 shows the output pulse shape and its power spectral
density in different pulse repetition rates.
4.4 Case study 2: Impulse UWB Transmitter
43
Vcc
On Chip
Components
ESD
Protection
L1
Duration
Control
Solder
Bump Model
Rp
Cp
Amplitude
Control
Lbump
Rbump
C1
L2
Cbump
Rp
Cp
Driver
Solder Bump
Model
Antenna
Model from
Momentum
Solder Bump
Model
Figure 4-5: Schematic of System-on-Chip implementation of UWB transmitter
(a)
(b)
Figure 4-6: Output voltage (a) and radiated power spectral density (b) for SoC
implementation
4.4.3 SoP Integration
In this case, all passive components are moved off the chip, in order to validate SoP
solution for UWB packaging, and to confirm the ability of the module to compensate the
parasitic effects of the packaging. Figure 4-7 shows the circuit model used for SoP chippackage and antenna co-design. ADS momentum simulation has been used to extract the
models for the passive components and the antenna in substrate. Figure 4-8.a shows the
output pulse shape and corresponding power spectral density in different pulse repetition
rates. As can be seen, the tunable transmitter can adjust the output pulse shape to satisfy
the FCC regulation in different pulse rates.
44
4.4 Case study 2: Impulse UWB Transmitter
On Chip
Components
On-chipVcc
ESD
Protection
Duration
Control
Solder
Bump Model
Rp
Vcc
Cp
Amplitude
Control
Lbump
Rbump
Rp
Driver
Cp
Solder Bump
Model
Solder Bump
Model
3.4 mm
Cbump
1 mm
Off-Chip
Inductor
Antenna
Model from
Momentum
Off-Chip
Cap.
Figure 4-7: Schematic of System-on-Package implementation of UWB transmitter
(a)
(b)
Figure 4-8: Output voltage (a) and radiated power spectral density (b) for SoP
implementation
4.4.4 Results and Discussion
Usually FoM (Figure of Merit) is used to describe the overall performance of RF
circuits. In order to describe the performance of our modules, a figure of merit is
proposed. To establish a performance figure of merit, several key parameters must be
taken into account. For low power applications such as RFID and wireless sensors, low
standby and operating power consumption are desired. In impulse UWB system, short
duration pulses represent data. Therefore, the output amplitude and settling time of the
output pulse are taken into account. Although, UWB radio offers extremely large
bandwidth, usually transmitters can not cover the whole available bandwidth. Therefore,
the power spectral efficiency (PSE), which shows how much of the available spectrum is
used by the output signal, is considered here. For road mapping purpose, it is preferable
to have a performance measure independent of frequency. To meet the FCC regulation,
4.4 Case study 2: Impulse UWB Transmitter
45
when the chip rate is varied, the output amplitude should be scaled by square root of the
pulse rate frequency. On the other hand, power consumption is linearly increased by the
pulse rate frequency. Assuming all of these parameters a FoM is defined as follow:
FoM UWB
PSE ⋅ Vo ⋅ f 3
=
TSET ⋅ P ⋅ PSTB
(4-1)
Where
PSE
Power spectral efficiency
VO
Output amplitude in volt
TSET
Settling time in ns
P
Power consumption in nW
PSTB
Standby power in nW
f
Pulse repetition rate in MHz
The performance merits and FoMs of SoC and SoP integrations are summarized in
table 4-1. As we expected, the SoP module has lower FoM because of the larger
parasitics introduced by the embedded passives. Although, higher FoM is expected if
implementation cost and chip area are also considered. On the other hand, the results
confirm the potential of SoP for UWB packaging and confirm the ability of the proposed
tunable UWB transmitter to compensate the parasitic effects of the packaging and the
antenna.
Table 4-1: Performance Merits
Solution
SoC
SoP
Freq(MHz)
50
250
50
250
Merits
Amplitude Cont. (v) 0.75 1.12
1
1.24
Duration Cont. (v)
0
0.4
0
0.6
I av (µA)
206 479 119 373
Pav (nW)
371 863 215 673
PSE
19.25 31.73 10.97 15.71
Settling time (ns)
0.81 0.68 0.82 0.48
Vopeak (mV)
160
54
115
45
Iav-STB (nA)
1.83
2.3
Pdc-STB (nW)
3.3
4.1
FoM
1.10 3.50 0.62 2.11
46
4.5 Conclusion
4.5 Conclusion
System integration in LCP substrate has been investigated. As two case studies, a
power scavenging unit and a tunable UWB transmitter have been considered. The power
scavenging unit has been implemented by the surface mounted Schottky diode and all
other passive components including coupling capacitors and matching network have
been implemented in substrate. Simulation results show that with less number of stages
(2 stages) the required performance can be achieved compared with on-chip 7 stages
standard CMOS integration. That is because of using the Schottky diode with low turn
on voltage and low junction capacitor. The UWB transmitter has been implemented in
two cases. First, in a System-on-Chip integration all components are considered to be
integrated on chip however, in System-on-Package case passive components are moved
off the chip. Chip-package co-design has been performed and by defining a figure-ofmerit two solutions have been compared. Simulation results show lower FoM for SoP
solution however higher FoM is expected if cost analysis is also performed. It also
shows that the tunable UWB transmitter is able to compensate the parasitic of the
packaging.
CHAPTER 5
5 System Miniaturization
In this chapter, the effect of antenna miniaturization in an impulse UWB
system/transceiver is investigated. A Modified small-size Printed Tapered Monopole
Antennas (PTMA) are designed in different sizes. In order to evaluate the antennas
performance and their functionality, the effect of each antenna is studied in a given
impulse UWB system. It includes an impulse UWB transmitter and two kinds of UWB
receivers based on correlation detection, and energy detection. The tunable low-power
Impulse UWB transmitter is designed and the benefit of its co-design with the PTMA is
investigated. A comparison is given between a 50Ω design and a co-designed approach.
Our co-design methodology shows improvement in both transmitter efficiency and
whole system performance. The simulation results show that the PTMA antenna and its
miniaturized geometries are suitable for UWB applications.
47
48
5.1 Introduction
5.1 Introduction
In many applications such as RFID the size of the system is dominated by the antenna
dimension. Therefore it is very desirable to miniaturize the antenna in order to scale
down the system size. In most small-size applications, the optimal solution would be to
have the antenna integrated on the printed circuit board (PCB). However, the small-size
antenna usually does not meet the traditional requirement of 50 Ω input impedance [71].
In addition, when the antenna is scaled down, its response is changed, which may cause
a destructive effect on the pulse shape and consequently degrade the system
performance. Therefore, the antenna characteristics should be considered in the earliest
design phase, when extracting the individual requirement for each sub-system in a
transceiver chain. However, the impact of the antenna until now has been given only
limited attention [72, 73].
In this chapter, the miniaturization of the modified small-size PTMA antenna [74] is
investigated for co-design with UWB transceiver in order to find the optimum trade off
between antenna size miniaturization and its tolerable effect on system performance. A
complete UWB transceiver including an impulse-UWB transmitter and two types of
UWB receiver architectures based on Correlation Detection (CD) and Energy Detection
(ED) are investigated. The tunable low-power Impulse UWB transmitter is co-designed
with the antenna and the benefit of its co-design with the PTMA is investigated. A
comparison is given between a 50Ω design and a co-designed approach. The whole
system performance is estimated in terms of BER and the results are compared in two
cases of co-design and normal 50Ω design methods.
5.2
PTMA UWB Antenna
In this section the antenna structure, a small-size, low-cost, PTMA antenna is
described. The antenna structure consists of a tapered radiating element fed by a
microstrip line is shown in Figure 5-1. The antenna is designed for the 3.1-10.6 GHz
band on a high-resistive silicon substrate material (2000 Ωcm) with a dielectric constant
εr of 11.9. The thickness of the copper metal layers is 5µm and its conductivity is 5.8e7
S/meter.
The original area of the PTMA antenna, 22×15.7 mm2, is still too large for integration
on PCB with the transceiver [75]. Thus, the antenna should be miniaturized in a manner
which has minimum degrading effects on the antenna performance. Regularly scaling
down the antenna in the x and y dimensions changes the impedance as it works below
the resonance. This can be seen in Figure 5-2. The antenna’s real impedance is lower
than 50 Ω, and both the antenna’s real and imaginary impedance are shifted to upper
frequencies when the antenna scaling factor (S) decreases from 1 to 0.4.
Figure 5-3 shows the maximum antenna directivity as a function of frequency for
different antenna sizes. It is clear that the return loss is not included in the directivity
calculation. Thus, using a smaller antenna, the directivity at each frequency doesn’t
5.2 PTMA UWB Antenna
49
change significantly due to the fact that the antenna works similar to the main full
PTMA.
To estimate the received pulse shape at receiver end, a transmission link composed of
two modified PTMA antennas placed in front of each other at distance of 10 cm is
simulated and extracted S-parameter is utilized to model the transmission link. Figure
5-4 shows the extracted S21 for different sizes of the PTMA antenna.
y5
y4
y
y3
z
x
y2
y1
Figure 5-1: Top and bottom layer view of the UWB Antenna
Figure 5-2: PTMA antenna impedance vs. size scaling
50
5.3 Impulse UWB Transmitter-Antenna Co-Design
m axim u m d irectivity (d B )
5
4
3
2
S=0.4
S=0.55
S=0.7
S=0.85
S=1
1
0
3
4
5
6
7
8
Frequency (GHz)
9
10
11
Figure 5-3: Antenna directivity vs. size scaling
S
21
(dB) Excluding the effect of S
11
-20
s=0.4
s=0.55
s=0.7
s=0.85
s=1
-25
-30
-35
-40
-45
-50
-55
3
4
5
6
7
Freq. (M H z)
8
9
10
Figure 5-4: S21 for different sizes of the PTMA antenna
5.3 Impulse UWB Transmitter-Antenna CoDesign
The analysis setup is shown in Figure 5-5. The impulse UWB transmitter is the same
as previous chapter. In order to cope with unwanted package parasitic effects and to
optimize the transmission efficiency, chip-antenna co-design has been performed for
5.3 Impulse UWB Transmitter-Antenna Co-Design
51
different-size antennas. The design goal is to have an output pulse with less substantial
late-time ringing and high bandwidth, while it complies with the FCC spectral
regulation.
The antennas with different sizes are modeled as a 1-port network by their extracted
scattering parameters (SP). These 1-port networks are used as the load of the transmitter
and the optimum design parameters are found for each antenna. The Effective Isotropic
Radiated Power (EIRP) of the antenna is estimated as follows:
EIRP =
GA
(1 − S )
2
11
×
Ra
2 Ra + jX a
2
× PSD(Va )
(5-1)
Where GA, Ra, Xa, S11, Va are the gain, the resistance, the reactance, the S-parameter,
and the applied voltage of the antenna respectively. In order to prove our co-design
methodology, the circuit is designed in two approaches. First, as in the conventional
50Ω approach, the circuit is designed with a pure 50Ω load and the performance of the
circuit is examined in the presence of the real designed antennas. Secondly, in the codesign approach the extracted model for the designed antennas are considered as the
load and for each antenna optimum design parameters are found. As can be seen later,
the required operation condition can be achieved through the amplitude and duration
tunability while all the components are kept constant, which is one of the advantages of
the above transmitter design.
Vcc
On Chip
Components
Solder
Bump Model
Rp
Duration
Control
Cp
+
Xa
L1
Amplitude
Control
Lbump
Va
Cbump
-
Rbump
C1
L2
Rp
Cp
Ra
Driver
Antenna
Model
Solder Bump
Model
Solder Bump
Model
(a)
(b)
Figure 5-5: Schematic of the UWB-IR transmitter with the antenna (a), Antenna model for
EIRP estimation (b)
52
5.3 Impulse UWB Transmitter-Antenna Co-Design
5.3.1 Results and Discussion
Figure 5-6 shows the output power spectral density (PSD) for the 50Ω design approach
at 100MHz pulse repetition rate. As can be seen, although the PSD fits the FCC
regulation with 50Ω load, in the presence of the real antennas there is a large
degradation in radiated power, because of the antenna return loss. Especially for the
small-size antenna the PSD is much less than FCC limit, which reduces the system
performance and operation distance.
In the co-design approach, as mentioned before, the antenna models are considered
as the load and the transmitter is optimized to reach the FCC limit and increase the
overall performance. Table 5-1 summarizes the resulting control voltages for the 100
MHz pulse repetition rate. The output pulse shapes and their radiated power spectral
densities are shown in Figure 5-7 and Figure 5-8 respectively. As can be seen, due to the
co-design and by tuning of voltage controls, the power spectral densities for all antennas
are adjusted to have the maximum EIRP while they meet the FCC limitation as well.
A Figure-of-Merit (FoM) as equation 5-2 is used to compare the circuit performances.
P
FoM = Rad ( watt ) =
Pdc (uW )
10.6 e 9
∫
3.1e 9
EIRP.df
(5-2)
Pdc
-40
-45
PSD (dBm/MHz)
-50
-55
-60
Ant S1
Ant S0.85
Ant S0.7
Ant S0.5
Ant S0.4
Ant Ideal
FCC
-65
-70
-75
-80
1
2
3
4
5
6
7
Frequency
8
9
10
11
12
9
x 10
Figure 5-6: Output PSD for the 50Ω-design case for different antenna sizes
5.3 Impulse UWB Transmitter-Antenna Co-Design
53
Tx output pulse shape @100MHz
Ant Ideal
Ant S1
0.1
0
-0.1
0.5
1
1.5
2
0.05
0
-0.05
-0.1
0.5
1
1.5
2
-9
-9
x 10
Ant S0.85
x 10
Ant S0.7
0.1
0.1
0
0
-0.1
0.5
1
1.5
2
-0.1
0.5
1
1.5
2
-9
-9
x 10
Ant S0.55
x 10
Ant S0.4
0.2
0.1
0
0
-0.1
-0.2
0.5
1
1.5
2
0.5
1
1.5
2
-9
-9
x 10
x 10
Figure 5-7: Co-designed output pulse shape for a 100MHz pulse rate for different antenna
sizes
-40
-45
PSD (dBm/MHz)
-50
-55
-60
-65
Ant S1
Ant S0.85
Ant S0.7
Ant S0.5
Ant S0.4
Ant Ideal
FCC
-70
-75
-80
1
2
3
4
5
6
7
Frequency
8
9
10
11
12
9
x 10
Figure 5-8: Co-designed PSD for 100MHz pulse rate for different antenna sizes
54
5.3 Impulse UWB Transmitter-Antenna Co-Design
Table 5-2 compares the normalized FoM for the 50Ω and the co-design cases. The
ideal antenna used as reference is a 50Ω antenna with 0dB gain. As can be seen, the
FoM decreases fast in the 50Ω design case, while with the co-design approach higher
FoM can be achieved. The antenna with scaling factor 1 has a FoM slightly higher than
the reference antenna, because of its higher gain. As can be seen, most of the output
power is around 6 GHz frequencies. Therefore the antenna with scale 0.7, which has
higher radiation resistance in that frequency (Figure 5-2) radiate the same power with
less current consumption as can be seen in Table 1. Therefore the antenna with scale 0.7
has higher FoM than scale 0.85 because of the reduction in current. In co-design case,
the FoM improvement is clear, especially for smaller antennas. So in the case of antenna
miniaturization the importance of co-design becomes more obvious.
Table 5-1: Co-Designed Simulation results summary
Parameters Amplitude
Control
(V)
Antenna
Duration
Control
(V)
Idc
(μA)
50 Ω
1.05
0.98
329
Scale 1
1.03
0.97
350
Scale 0.85
1.02
0.96
355
Scale 0.7
1
0.9
337
Scale 0.55
1
0.9
338
Scale 0.4
0.5
0.6
573
Table 5-2: Normalized FoMs
Design
Antenna
Ideal
Scale 1
Scale 0.85
Scale 0.7
Scale 0.55
Scale 0.4
FoM
50Ω
design
CoDesign
1
0.92
0.7
0.6
0.49
0.187
1
1.03
0.85
0.88
0.75
0.4
5.4 Antenna Effects on UWB System Performance
55
5.4 Antenna Effects on UWB System Performance
In order to study the effect of miniaturizing the antenna on the UWB system
performance, the designed antennas are entered in a given UWB transceiver and the
performances in terms of BER are compared. Figure 5-9 shows a simple block diagram
of the UWB transceiver. The described transmitter is considered as the UWB-Tx. The
antenna and channel loss are estimated by a CST Studio analysis at a reference distance
(d0=0.1 meter). An Additive White Gaussian Noise (AWGN) channel is considered in
this study. Two kinds of UWB receiver with 50Ω input impedance are considered as the
receiver as described later. No multipath fading and no interference are assumed.
UWB Tx
Antenna and
channel model
Vrx
Uwb Rx
Zin=50O
Figure 5-9: Block diagram of UWB transceiver
Figure 5-10 shows the received pulse shape at reference distance d0=10 cm. For the ideal
antenna a free space path loss is considered. By utilizing the free space path loss formula, the
received signal at reference distance can be scaled for different distances by equations (3) and
(4). [26]
Pr / d
⎛d ⎞
= Pr / d 0 .⎜ 0 ⎟
⎝d ⎠
Vrx / d (t ) = Vrx / d 0 (t ).
2
d0
d
(5-3)
(5-4)
Where Vrx/d0 is the received pulse at reference distance of d0 and Vrx/d is the received
pulse shape at given distance of d.
Two kinds of receiver, one based on Correlation Detection (CD) and one based on
Energy Detection (ED) are considered as the receiver and for both cases the BER is
evaluated. The analysis parameters are as follows:
• The pulse shape is the received pulses from Figure 5-10.
• The noise figure of the receiver is 8 dB.
• The channel is an AWGN channel and no interferences are considered.
56
5.4 Antenna Effects on UWB System Performance
• Each bit is represented by Ns=10 pulses, resulting in a bit rate of 10 Mbps
corresponding to 100MHz Pulse Repetition Rate (PRR).
• The modulation is Binary Phase Shift Keying (BPSK) and On-Off Keying
(OOK) for the CD and ED receiver respectively.
• No transmission code is utilized in this study.
• The system is assumed to be synchronized perfectly.
-3
2
x 10
Received pulse shape @ D=10 cm
-3
Ant Ideal
x 10
Ant S1
2
0
0
-2
0
0.5
1
1.5
-2
0
0.5
1
-9
-3
x 10
-9
x 10
-3
Ant S0.85
x 10
x 10
Ant S0.7
1
1
0
-1
0
0
0.5
1
1.5
-1
0
0.5
1
-9
-3
x 10
0
-4
Ant S0.55
0
0.5
1
1.5
1.5
-9
x 10
1
-1
1.5
10
5
0
-5
x 10
0
x 10
Ant S0.4
0.5
1
-9
x 10
1.5
-9
x 10
Figure 5-10: Received pulse shape at distance d0=10 cm
5.4.1 Correlation Detection
Figure 5-11 shows a simple block diagram of correlation detection. The incoming
signal r(t) is multiplied by a template waveform w(t), and is integrated over the entire Ns
pulses. The same pulse shape as the transmitted pulse is used as the template waveform
[25].
5.4 Antenna Effects on UWB System Performance
57
Figure 5-11: Correlation Detection block diagram
The received signal can be written as
N s −1
r (t ) = ∑ ∑ bi pr (t − i.N s .T f + j.T f ) + n(t )
i
(5-5)
j =0
where pr(t) is the received pulse shape, bi=±1 is the data bit for BPSK modulation, Tf is
the pulse period, Ns is the number of pulses per bit, and n(t) is the additive white
Gaussian noise with double-sided spectral density of σn2=N0/2. The signal at the
detection block can be written as
Ti
Tf
Tf
0
0
0
rd / i (t ) = ∫ r (t ).w(t )dt =N s ∫ bi . pr (t ).w(t )dt + Ns∫ n(t ).w(t )dt
(5-6)
where Ti is the integration window, Tf is the pulse period and w(t) is the local template
pulse which is 2nd derivative Gaussian pulse shape in this study. The mean value and
variance of rd(t) can be written as
Tf
μi = E (rd / i (t )) = N s .bi .∫ pr (t ).w(t ).dt
0
bi = ±1
Tf
σ 2 = E (rd2 ) − μ 2 = σ n2 N s .∫ w2 (t ).dt
0
(5-7)
(5-8)
The probability density function of rd(t) can be expressed as
⎛ − ( x − μi ) 2 ⎞
1
⎟⎟ i=0,1 for bit 0 and 1
pdf ( x) i =
exp⎜⎜
2
2π σ
⎝ 2σ
⎠
(5-9)
The error probability can be estimated by combining the two conditional probability
functions of p(1/0) and p(0/1) as:
⎛μ ⎞
BER = Pe = 0.5 p (1 / 0) + 0.5 p (0 / 1) = Q⎜⎜ i ⎟⎟
⎝σ ⎠
where
(5-10)
58
5.4 Antenna Effects on UWB System Performance
⎛ x ⎞
Q( x) = 0.5.erfc⎜
⎟
⎝ 2⎠
(5-11)
5.4.2 Energy Detection
A nominal energy detection block diagram is shown in Figure 5-12. The incoming
signal after amplification and filtering is squared and integrated over the symbol period.
The decision is made based on whether there is a pulse or not [26].
Figure 5-12: Energy Detection block diagram
The received signal can be written as
N s −1
r (t ) = ∑ ∑ bi pr (t − i.N s .T f + j.T f ) + n(t ) = si (t ) + n(t )
i
(5-12)
j =0
where bi=0/1 is the data bits for OOK modulation.
The signal at the detection block can be written as
T
Tp
Tp .N s
0
0
0
rd / i (t ) = ∫ r 2 (t )dt =N s ∫ si2 (t ).dt + 2∫
si (t ).n(t ).dt + ∫
Tp .N s
0
n 2 (t ).dt
(5-13)
where Tp is the pulse duration. The first part is the signal part which is deterministic.
The second term is a zero mean Gaussian noise and the last part is a non-central chisquared random process with 2M=2.Bw.Ti.Ns+1 degrees of freedom where Ti is the
integration window and Bw is the signal bandwidth [76]. It has been shown that the chisquared probability distribution function (pdf) can be approximated as a Gaussian
process as the degrees of freedom is increased [77]. Then the means and variances of
the signal rd(t) for bits 0 and 1 can be expressed as
μ 0 = M .N 0
(5-14)
σ 02 = M .N 02
(5-15)
μ1 = M .N 0 + Eb
(5-16)
σ 12 = M .N 02 + 2.Eb .N 0
(5-17)
5.4 Antenna Effects on UWB System Performance
59
where μ0 and σ0 are the mean value and variance of signal for bit 0, μ1 and σ1 for bit 1
and Eb is the received energy per symbol as
Eb = ∫
N s .T p
0
pr2 (t ).dt
(5-18)
As can be seen from equation (5-17) and (5-18), the larger the integration window, the
higher the variance in the signal. The optimum value for the integration window can be
found as to increase the signal to noise ratio [78]. In this study the signal is integrated
over the pulse duration.
The decision threshold γ is located at the intersection of two Gaussian pdfs for bits 0
and 1, and can be evaluated from
pdf 0 (γ ) = pdf 1 (γ )
(5-19)
The BER can be evaluated by the two probabilities of false alarm and detection, which
can expressed as follows [76] .
⎞
⎛
Eb N 0
⎟
BER = Pe ≈ Q⎜
⎜ M + M + 2E N 0 ⎟
b
⎠
⎝
(5-20)
5.4.3 Results and Discussion
The performances of the designed antennas are evaluated in the two described systems.
In order to evaluate the performance versus antenna scaling factor, the transmitted pulse
shapes for each antenna are applied to the system and the system performances are
evaluated in terms of BER and compared. The achievable operation distance with BER
better than 10-5 is shown in Figure 5-13.
As can be seen, the operating distance is decreased by the scaling factor of the antenna.
This reduction is caused by the degradation in the antenna efficiency, gain and radiation
properties introduced by the smaller antennas. Figure 5-14 and Figure 5-15 show the
BER performance versus distance for the two kinds of receivers and compare the results
for the non-co-design and co-design methods. It shows the improvement in performance
by doing co-design between antenna and transmitter compared to a standard 50Ω design.
Figure 5-16 shows the amount of improvement in terms of operation distance at BER
less than 10-5 in comparison with the non-co-design case. As can be seen, in the codesign method distance improvement is huge (up to 100%) for the small size antenna, at
the expense of an average current consumption up to 74% compared with the non-codesigned current. It shows that the co-design methodology is highly important in smallsize antenna.
60
5.4 Antenna Effects on UWB System Performance
2
10
OOK
OOK-Codesign
BPSK
BPSK-Codesign
1
Distance (meter)
10
0
10
-1
10
1
0.85
0.7
Antenna Scale Factor
0.55
0.4
Figure 5-13: Operation distance at BER<10-5 @10Mbps
BPSK @100Mhz and 10 Mbps
0
10
Scale 1
Scale 0.85
-1
Scale 0.7
Scale 0.55
Scale 0.4
Co-des-S1
Co-des-S0.85
Co-des-S0.7
Co-des-S0.55
Co-des-S0.4
10
-2
BER
10
-3
10
-4
10
-5
10
-6
10
2
4
6
8
10
Distance (meter)
12
14
16
18
Figure 5-14: Comparison of BER of correlation receiver versus distance with and without codesign
5.4 Antenna Effects on UWB System Performance
61
OOK @100MHz and 10Mbps
0
10
Scale 1
Scale 0.85
Scale 0.7
Scale 0.55
Scale 0.4
Co-des-S1
Co-des-S0.85
Co-des-S0.7
Co-des-S0.55
Co-des-S0.4
-1
10
-2
BER
10
-3
10
-4
10
-5
10
-6
10
0.5
1
1.5
2
2.5
Distance (meter)
3
3.5
4
4.5
Figure 5-15: Comparison of BER of ED receiver versus distance with and without antenna
co-design
100
Distance-OOK
Distance-BPSK
Current Consumtion
90
80
Increment (%)
70
60
50
40
30
20
10
0
1
0.85
0.7
Antenna Scale factor
0.55
0.4
Figure 5-16: Co-design performance improvement and power consumption increment
62
5.5 Conclusions
5.5 Conclusions
The effect of antenna miniaturization in a UWB system has been investigated. A
PTMA UWB antenna has been scaled down, and the chip-antenna co-design has been
performed. The antennas and the UWB transmitter performances have been evaluated in
two types of UWB systems in an AWGN channel: Correlation Detection and Energy
Detection UWB have been studied. The simulation results of the UWB-IR transmitter
and antenna co-design show that the standard 50Ω design technique can not reach the
best condition in all cases when a real antenna is placed into the system. Performance
can be improved significantly when doing co-design. It has been shown that the
operating distance at a target performance is reduced with the antenna scaling. To
compensate this reduction, the transmitting power can be increased, but only up to the
limit of violating the mask regulations. The results show that antenna-chip co-design
needs to be considered in the earliest phases of the design flow in order to obtain the
maximum system performance, especially when the small size antenna is desired.
CHAPTER 6
6 Conclusion and Future Work
6.1 Conclusion
In this thesis, we have investigated the design and implementations of ultra-wideband
impulse radio for wireless sensing and identification systems. A wireless sensor network
based on UWB impulse radio has been proposed, focusing on low power and low cost
implementation. To meet the power and cost constraint in sensor nodes, a novel
autonomous UWB detection has been proposed. The performance of the autonomous
detection has been evaluated in term of BER and it shows the performance close to TRUWB, when timing jitter is not considered. It also shows that the proposed detection is
very robustness to timing jitter and time mismatch, which reduces the synchronization
and clock requirement significantly.
A novel wireless identification and sensing system based on UWB impulse radio has
been proposed. Our innovative contribution is to employ two different communication
links (UWB and UHF) respectively in uplink and downlink of the tag. This is because
the amount of data or instruction from a reader to a tag is very few and a normal
communication link as conventional UHF-RFID at 900 MHz can be used as downlink.
UHF signal also provides remote power to the tag. The uplink requires a link with higher
data rate and precise positioning capability, therefore an UWB-IR transmitter is
employed. A logic control core has been designed for the proposed communication
63
64
6.2 Recommendation for Future Work
protocol based on slotted-ALOHA anti-collision algorithm. Simulation results show the
throughput more than 2000 tags/s resulting in a great improvement compared with
normal RFID system which is at most 1000 tags/s. To verify the system concept, a
single chip implementation of the tag has been fabricated in UMC 0.18µm CMOS
process. Measurements results show the input sensitivity of -18.5 dBm (14.1 μW) and
adaptive data rate up to 10 Mb/s. It corresponds to 13.9 meters operation range
considering 4W EIRP, a matched antenna to the tag with 0dB gain, and free space path
loss, which is a great improvement in operation distance and data rate, compared with
existing passive RFID.
System integration in a LCP substrate has been investigated. As two case studies, a
power scavenging unit (PSU) and a UWB transmitter have been studied. Simulation
results of the PSU show that the required performance can be achieved with less number
of stages (2 stages) compared with standard CMOS on-chip integration (7 stages). That
is because of the usage of Schottky diode with low forward voltage drop. The UWB
transmitter has been designed in two cases of System-on-Chip (SoC), and System-onPackage (SoP). Chip-package-antenna co-design has been performed. Although SoP
shows lower FoM but, higher FoM is expected for SoP if cost analysis is performed. On
the other hand, the simulation results show the feasibility of SoP implementation for
UWB system.
Miniaturize system integration have been studied through out UWB antenna
miniaturization and the effects of antenna scaling have been investigated. A PTMA
UWB antenna has been scaled down and the chip-antenna co-design of a tunable
impulse-UWB transmitter has been done. The antennas and UWB system performance
have been also evaluated. The simulation results show that the standard 50Ω design
technique can not reach the best condition in all cases when a real antenna is placed into
the system. Performance can be improved significantly when doing co-design. The
results show that antenna-chip co-design needs to be considered in the earliest phases of
the design flow in order to obtain the maximum system performance, especially when
the small size antenna is desired.
6.2 Recommendation for Future Work
In following of this thesis and to make the complete proposed system work, further
research is required to be done. The following are suggestions:
• Reader implementation: Asymmetric reader with UWB and UHF uplink and
downlink is required. Different UWB receiver architectures need to be investigated
to find the optimum solution for the proposed system in different applications.
• Synchronization: Since the available power for transmission from a tag to a reader
is limited, a fast and efficient synchronization algorithm is essential.
• Localization: For localization application, accurate synchronization in pulse level is
demanded to increase the positioning accuracy. Since the transmitted pulse by the
6.2 Recommendation for Future Work
65
tag is synchronized to the incoming RF signal, the idea is to use the RF signal in the
reader to estimate the time-of-arrival (ToA) of the pulses.
• Security: The feasibility of time hopping can be investigated as a possible solution
for adding more security to the system.
• Standard: To make the tag compatible to other existing UWB devices, it would be
very interesting to study the feasibility of implementing the IEEE 802.15.4.a (LRWPAN) standard in proposed battery-less tag.
• Low power sensor: Implementation of low power sensors, which can operate with
limited available remote power, on chip or on package is an attractive topic.
• Antenna integration: Different technologies can be considered for antenna
integration such as on-chip antenna and printing technology focusing on low cost
and small form factor integration.
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