Antennas and Propagation
for Body-Centric
Wireless Communications
Second Edition
For a listing of recent titles in the
Artech House Antenna and Propagation Series,
turn to the back of this book.
Antennas and Propagation
for Body-Centric
Wireless Communications
Second Edition
Peter S. Hall
Yang Hao
Editors
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A catalog record for this book is available from the U.S. Library of Congress.
British Library Cataloguing in Publication Data
A catalog record for this book is available from the British Library.
ISBN-13: 978-1-60807-376-4
Cover design by Vicki Kane
© 2012 Artech House
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10 9 8 7 6 5 4 3 2 1
Contents
Foreword
Preface
xi
xiii
CHAPTER 1 Introduction to Body-Centric Wireless Communications
1
1.1 What are Body-Centric Communications Systems?
1.1.1 Off- to On-Body Communications
1.1.2 On-Body Communications
1.1.3 Medical Implants and Sensor Networks
1.2 Overview of Systems 1.2.1 Narrowband Systems
1.2.2 Wideband Systems
1.3 Overview of Applications 1.4 New Trends and Progress Since the First Edition
1.4.1 Propagation Characterization and Control
1.4.2 Measurement Methods
1.4.3 Antenna De-embedding
1.4.4 Materials
1.4.5 Modeling of Body Dynamics
1.4.6 Standardization
1.5 Layout of the Book
References
1
5
6
6
8
8
10
11
11
11
12
12
13
13
14
14
15
CHAPTER 2 Electromagnetic Properties and Modeling of the Human Body
17
2.1 Electromagnetic Characteristics of Human Tissues
2.2 Physical Body Phantoms
2.2.1 Liquid Phantoms
2.2.2 Semisolid (Gel) Phantoms
2.2.3 Solid (Dry) Phantoms
2.2.4 Examples of Physical Phantoms
2.3 Numerical Phantoms
2.3.1 Theoretical Phantoms 2.3.2 Voxel Phantoms
17
18
21
22
22
23
27
27
28
v
vi
Contents
2.4 Numerical Modeling Techniques for Antennas and Propogation
2.4.1 Introduction of Numerical Techniques for Body-Centric Wireless
Communications
2.4.2 On-Body Radio Channel Modeling
2.5 Modeling of Dynamic Body Effects
2.5.1 Methodology
2.5.2 Measurements and Model Validation
References
29
29
36
50
50
52
56
CHAPTER 3 Antenna Design and Channel Characterization for On-Body Communications at Microwave Frequencies
3.1 Introduction
3.2 Measurement Methods
3.2.1 Connection Between Antenna and Measuring Instruments
3.2.2 Antenna De-embedding
3.3 Body-Centric Channel Measurement and Modeling
3.3.1 Path Gain 3.3.2 Channel Statistics
3.3.3 Channel Polarization Effects
3.4 Antenna Design
3.4.1 Performance Comparison
3.4.2 Antenna-to-Surface Wave Coupling
3.4.3 Antenna Match and Efficiency
3.5 Multiple Antenna Systems
3.5.1 Antenna Diversity
3.5.2 MIMO
3.5.3 Interference Cancellation
3.6 Systems Modeling
3.7 Conclusions
References
63
63
64
65
67
71
71
76
84
87
87
93
101
103
103
104
105
105
106
107
CHAPTER 4 Wearable Devices Using the Human Body as a Transmission Channel
113
4.1 Introduction of Communications Using Circuits in Direct Contact
with the Human Body
4.2 Numerical Analysis of Communication Devices Using Low Frequencies
4.2.1 Whole Body Models
4.2.2 Arm Models Wearing the Transmitter
4.2.3 Effective Electrode Structure
4.3 Experiments Using Human Phantoms
4.3.1 Model for Assessments
4.3.2 Electric Field Distributions In and Around the Arm
4.3.3 Received Signal Voltage of the Receiver
4.4 Investigation of the Dominant Signal Transmission Path
113
120
120
122
123
125
125
126
128
131
Contents
vii
4.4.1 Calculation Model
4.4.2 Electric Field Distributions and Received Signal Voltages
4.5 Conclusions
References
131
134
135
136
CHAPTER 5 Ultrawideband Technology for Body-Centric Wireless Communications
139
5.1 Overview
5.2 UWB Antennas for Body-Centric Wireless Communication
5.2.1 Design and Analysis
5.2.2 Measurements
5.2.3 Concluding Remarks
5.3 Channel Simulation and Measurement Methodology
5.3.1 Simulation of the Radio Propagation in Body-Centric
Communication Scenarios
5.3.2 Measurement of the Radio Propagation in Body-Centric
Communication Scenarios
5.3.3 Concluding Remarks
5.4 Channel Characterization and Modeling
5.4.1 General Aspects
5.4.2 Personal Area Network Scenarios
5.4.3 Body Area Network Scenarios
5.4.4 UWB Multiband-OFDM Based System Modeling and Performance
Evaluation for Body-Centric Wireless Communications 5.6 Concluding Remarks
References
139
140
141
158
160
161
161
162
172
173
173
175
180
195
202
204
CHAPTER 6 Wearable Antennas: Advances in the Design, Characterization, and Application
6.1 Introduction
6.2 Review of the Literature
6.2.1 Antenna Types
6.2.2 Body Placement, Bending, and Crumpling
6.2.3 Fabric Material Properties and Antenna Manufacture Methods
6.3 Wearable Antennas: Critical Design Issues 6.4 Textile Materials
6.5 Effects of Substrate Materials: An Example of Fabric GPS Antenna
6.5.1 Effects of Ground Plane Size Attached to the Fabric Substrate
on GPS Antenna Performance
6.6 Effect on Various Conductive Materials of Patch Antennas:
An Example of WLAN Antenna on Fleece Fabric
6.7 Dual Frequency Wearable Antenna Design: An Example of a
U-Slot Patch
6.8 Wearable Electromagnetic Bang Gap Antenna (WEBGA):
An Example of WLAN Antenna
209
209
211
211
216
216
218
219
222
224
228
233
237
viii
Contents
6.8.1 Remarks on Antenna Bending
6.9 Wearable Antennas Near the Human Body: An Example of a
WLAN Antenna
6.9.1 Models and Methods
6.9.2 Results
6.10 Wearable Antenna Environmental Performance Issues
6.10.1 The Effect of Ice, Water, and Snow on Wearable Antenna
Performance
6.10.2 Example of Environmental Test During an Iridium Phone Call
6.10.3 Destructive Antenna Tests 6.11 Conclusions
Acknowledgments
References
239
244
246
247
250
252
255
256
261
262
263
CHAPTER 7 Body-Sensor Networks for Space and Military Applications
271
7.1 Introduction
7.2 Biosensor System and Basics of Biomedical RF Telemetry
7.2.1 Implantable Pressure Sensor
7.2.2 Integrated Inductor/Antenna
7.2.3 External Pick-Up Antenna
7.3 Antenna Design for Body Sensors
7.3.1 Implantable Antennas
7.3.2 Antennas for External Handheld Devices 7.4 Space, Military, and Civilian Applications
7.4.1 Sensors for Space Environment
7.4.2 Battlefield Sensors 7.4.3 Sensors in Hospitals and Smart Homes References
271
272
273
273
275
275
276
285
289
289
290
290
290
CHAPTER 8 Antennas and Propagation for Telemedicine and Telecare: On-Body Systems
293
8.1 Telemedicine and Telecare Applications
8.1.1 Physiological Signals for Patient Monitoring
8.1.2 Technologies for Ward-Based Systems
8.1.3 Technologies for Home-Based and Full Mobility Systems
8.1.4 Emerging Technologies and Novel Applications
8.1.5 Wireless Telemedicine Link Design
8.2 Antennas and Human Body Interaction in Personal Telemedicine
8.2.1 Antenna–Body Effects (< 1 GHz)
8.2.2 Antenna–Body Effects (> 1 GHz)
8.2.3 Emerging Antennas
8.3 System Design Issues
8.3.1 Channel Effects
8.3.2 Radio Frequency Interference and Inter-BAN Interference
293
295
296
297
297
298
300
303
307
310
314
314
318
Contents
8.4 Conclusion
References
ix
319
320
CHAPTER 9 Medical Implant Communication Systems
325
9.1 Introduction
9.1.1 Inductive Coupling
9.1.2 MICS Standard
9.1.3 The 2.4-GHz ISM Band
9.2 Antennas in Lossy Dispersive Medium
9.2.1 Matter
9.2.2 Material Data and Measurements
9.2.3 Phantoms
9.2.4 Skin Depth
9.2.5 Wave Propagation: One-Dimensional FDTD Simulations
9.2.6 Influence of Patient
9.2.7 Phantom Influence on Antenna
9.3 Low-Profile Antennas for Implantable Medical Devices
9.3.1 What Is the Antenna?
9.3.2 Antenna Efficiency Calculations in Matter
9.3.3 Electric vs. Magnetic Antennas
9.3.4 Implantable Antennas Designs
9.3.5 Dependence on Insulation Thickness
9.3.6 SAR
9.4 Conclusion
References 325
326
327
328
328
329
330
331
334
334
337
338
339
341
341
343
347
353
353
355
355
CHAPTER 10 Conclusions
359
About the Authors
365
Index 375
CHAPTER 1
Introduction to Body-Centric Wireless
Communications
Peter S. Hall and Yang Hao
1.1 What are Body-Centric Communications Systems?
The ever-growing miniaturization of electronic devices, combined with recent developments in wearable computer technology, is leading to the creation of a wide
range of devices that can be carried by their user in a pocket or otherwise attached
to their body [1–3]. This can be seen as a continuation of a trend spearheaded by
the mobile phone, which has become smaller and more convenient for personalized
operation over the last few decades. Alongside this trend, there have been a number
of body-centric communication systems for specialized occupations, such as paramedics and fire fighters, as well as continuing interest for military personnel. The
development of the mobile phone can be characterized from the user perspective by
three phases. The first equipment was large and heavy, and was used only by those
people whose job required it. Then, the business community saw it as a way of
improving business operations. Finally, the mobile phone became popular with the
general population, who used it for social and entertainment purposes, and, more
recently, as a fashion accessory. It is quite possible that body area networks (BANs)
will follow the same path. Figure 1.1 shows a wearable computer, an early form
of BAN, developed at the University of Birmingham [4], as a test bed for a wide
range of studies into hardware and software architectures, and user applications. It
is obviously bulky and inconvenient to use and has wired interconnections. However, further miniaturization has taken place; as a result, it was clearly desirable to
remove the wired interconnections. It is also likely that power will be provided
at each body unit and that data transfer will use high capacity wireless communications [5]. Other current uses of such systems include warehouse operators
and garage mechanics. Figure 1.2 shows wearable computers as a fashion accessory.
Body-worn equipment is also used for health monitoring [6]. Figure 1.3 shows
a smart wireless electrocardiography (ECG) patch [7]. It is a hybrid system, combining electronic assembly on a flexible substrate with textile integration. The wireless ECG patch measures a bipoloar ECG signal between two electrodes separated
1
2
Introduction to Body-Centric Wireless Communications
Figure 1.1 University of Birmingham wearable computer (a) headset with video display, microphone, and earpiece and (b) miniaturized PC.
by a distance varying for 3 to 4 cm. The ultralow-power consumption of the ASIC
allows it to maintain the overall power consumption of the system relatively low
(around 2 mA). The patch transmits to a local base station and is an example of
an off-body channel, as explained in Section 1.1.1. When both ends of the wireless link are on the body, the term on-body channel is used, as explained in Section
1.1 What are Body-Centric Communications Systems?
Figure 1.2 Wearable computer as a fashion accessory.
Figure 1.3 Smart wireless ECG patch. (Image courtesy of IMEC.)
3
4
Introduction to Body-Centric Wireless Communications
1.1.2. Figure 1.4 shows an example. The Medtronic MiniMed Paradigm REALTime Revel System [8] has a glucose sensor integrated with a wireless communications link at one position on the stomach of the patient and a glucose pump system
at another for real-time diabetes control. The wireless transmitter sends information from the glucose sensor to the glucose monitor for readings every 5 minutes,
24 hours a day. Users specify the amount of insulin they want the pump to deliver
based on the readings and their meals.
Medical implants for monitoring, diagnosis, and activity have been studied
for some time. The opportunities created by nanotechnology and microtechnology
now open up the possibility of much more widespread use and application. In addition to sensors and drug delivery mechanisms, communication is a vital part of
this implementation process, for monitoring of internal body conditions and for
signalling actions to be taken by the implant. These implants might also be used
in conjunction with a body area network, as shown in Figure 1.5 [9]. Optimum
design of such BANs and implants means that a full understanding of antennas and
propagation into and through the body is needed.
Body-centric communication takes its place firmly within the sphere of personal area networks (PANs) and (BANs). The content of a BAN or PAN contains a
range of communications needs and requirements. These can be classified as:
Figure 1.4 Medtronic MiniMed Paradigm REAL-Time Revel System. (Courtesy Medtronic.)
1.1 What are Body-Centric Communications Systems?
5
Figure 1.5 Wearable medical support network.
••
Off-body: because the channel is off of the body and in the surrounding
space, only one antenna in the communications link is on the body. This is
referred to as the off-body domain.
••
On-body: most of the channel is on the surface of the body and both antennas will be on the body. This is called the on-body domain.
••
In-body: a significant part of the channel is inside the body and implanted
transceivers are used. This is called the in-body domain.
The italics show our shorthand nomenclature that we use in overviewing the
book in Section 1.5. This nomenclature implies a partitioning of the PAN and BAN
space into three areas. The first is where most of the channel is off the body and in
the surrounding space, and where only one antenna in the communications link is
on the body, which we call the off-body domain. The second is where most of the
channel is on the surface of the body, and both antennas will be on the body, called
the on-body domain. The last is where a significant part of the channel is inside
the body and implanted transceivers are used, and we call this the in-body domian.
While this is not a perfect subdivision, it does serve to highlight some of the different challenges for antennas and propagation in the body-centric system.
1.1.1 Off- to On-Body Communications
Communications from localized base stations, or broadcast stations, to transceivers located on the body have been studied extensively [10–12]. In the mobile phone
area, such investigations include studies of the propagation characteristics of urban
and rural environments. and the performance of body worn antennas in the face of
variations of body proximity and orientation. There are many publications on this
topic, and we have included little of it in this book, except that which relates to the
6
Introduction to Body-Centric Wireless Communications
use of ultrawideband techniques in Chapter 5. However, we have noticed that the
topic of fabric-based antennas (covered in Chapter 6) is now gaining prominence,
as fabric-based antennas can significantly increase the performance of body-worn
equipment in communications to local base stations. Such antennas need to orient the radiation pattern away from the body while simultaneously providing all
around coverage. In addition, it is important to screen the antenna from the body in
order to prevent the losses in the tissue from degrading the antenna efficiency. This
can be achieved by using larger ground planes than are possible in miniaturized
equipment. The challenges for fabric-based antennas are to maintain good performance in the face of changes in the body posture and to be unobtrusive.
1.1.2 On-Body Communications
There are now many examples of wearable computers in general use today, some
of which still incorporate wired interconnections. This is undesirable, due to reliability issues surrounding constantly flexing cables and connectors, the weight of
such cables, and the general inconvenience to the user. A number of other connection methods have been proposed for this purpose, including smart textiles and
communication by the currents in the user’s body. Each of these methods has its
own advantages and drawbacks. Among the drawbacks of the smart clothes, for
example, is the need for a special garment to be worn, which may conflict with the
user’s personal preferences. Similarly, body current communication is limited because it has a relatively low capacity. For real-time video transfer around the body,
very high data rates are required.
Wireless radio connectivity is an obvious option for connecting body-worn
devices. Several standards for wireless connections between small, closely spaced
devices have been developed, including Bluetooth, BodyLAN, and Zigbee. These
types of connection can provide high levels of flexibility and comfort to the user,
and therefore have received a lot of attention. There are three primary criteria for
wireless modules for on-body communications. Firstly, they must support the high
data rates expected in the future. Secondly, they must be small and lightweight.
Both of these suggest the use of high frequencies. Thirdly, they must consume
minimum power, which implies highly efficient links. In terms of antennas and
propagation, efficient design requires a good understanding of the properties of the
propagation channel involved and the development of optimized antennas.
1.1.3 Medical Implants and Sensor Networks
In recent years, several exciting developments, such as submicron electronics, nanotechnology and microelectromechanical systems (MEMS), have emerged, which
will have a profound impact on medicine. These technologies will allow the construction of intelligent microscopic implantable sensors, mobile robots, and drug
release devices, which will perform in vivo diagnostic and therapeutic intervention.
Such devices offer great potential to improve the quality of life for many patients;
there are now a number of conditions where implants are used to improve patient
lifestyle, such as pacemakers [13] and cochlea implants [14]. There are also significant future aspirations, such the projects within the EC Integrated Project, Healthy
Aims, [15] which include pressure sensing in the brain cavity, glaucoma sensor, and
1.1 What are Body-Centric Communications Systems?
7
retinal implants. Other future aims are the lab-on-a-chip for internal diagnosis or
automatic drug delivery [16] and interfacing to nerve endings for communications
to the brain [17]. As an example of the significance of such developments, current
contact with consultants in the local hospital of the editors of this book indicates
that there are patients with severe spinal injuries who are fully aware but unable to
communicate because they have no muscular control. Implanted sensors connected
to the spinal cord, or even the brain, could give them back the ability to communicate. This would have an immeasurable impact upon the quality of life for these
patients.
There are, however, many problems that must be overcome if this implant
technology is to be widely exploited and used commercially. There is now much
work to develop new sensor technology, much of which is being enabled by nanosystem and microsystem research [15, 16]. There is less work being done on other
important aspects of the implant system, namely communications. This will have a
crucial effect on the practicability of future implant systems, where the potentially
very small size of the sensor needs to be matched by miniaturization in the communication technology. Electromagnetics is one of the enabling technologies that
has to be applied within the framework of communications networks and user
needs. There is also the concept of a hierarchy of implants, as shown in Figure 1.6,
applied to the head, where the higher levels, some of which may be outside or on
the body, have high intelligence. Because of this, high communication rates are
needed, while those at the lower levels become smaller and have less communication requirements. The subdermal sensors could read body data, as well as being
relays to the single, or group, neuron (or other body data) sensors which are ideally
at the submicron scale. At this size, sensor noise, be it electrical or mechanical, will
limit performance. The larger implants may be active—that is to say, with internal
power sources—and the smaller ones may be passive; in either case, communications are important. Sensors are now becoming available for all levels. The feasibility of providing communications will be limited both by the fundamentals of electromagnetics and the antenna and transceiver technology needed to implement it.
Hairnet sensor
array + external
power device
Subdermal
sensors
Figure 1.6 Communications to medical implants.
Neuron group
sensors
8
Introduction to Body-Centric Wireless Communications
1.2 Overview of Systems
1.2.1 Narrowband Systems
The definition of narrowband or wideband systems for body-centric wireless communications is not straightforward. Here, we differentiate between the two by their
operational bandwidth only. Narrowband systems have been dealt with in most of
chapters in this book (Chapters 3–5, 7–9), and represent current market trends in
body-centric wireless communications. Such systems use a wide selection of frequency bands, as shown in Table 1.1.
1. UHF/VHF bands: pacemakers and implantable RFID (Chapter 9) use the
inductive link with carrier frequencies between 9 to 315 KHz, with a data
rate of up to 512 kb/s. The range of communication is, in practice, constrained to “touch” range, and hence limits its usefulness. The problem
can be slightly alleviated by using a higher frequency (~10 MHz) and further separation between the transmitter and receiver. Chapter 4 proposes a
waveguide transmission channel based on surface waves propagating along
the human body. The communication system uses the near field region of
the electromagnetic wave generated by the devices, which is eventually coupled to the human body via electrodes. An average of 20 dB attenuation
for each electric and magnetic field component is reported at a distance
of 10 cm away from the signal electrode. This value is comparable to the
ones presented in Chapter 3 in the case of on-body link when the channel
is static.
Table 1.1 A Summary of Available Technologies in Body-Centric Wireless Communications
Chapter
Standard
Frequencies (MHz) Data Rate
Max. Power Range (m)
Number
UHF/VHF
~10
Very low
Very low
<= 0.5
4
Medical Implant
Communications
Service (MICS)
402-405
Low
Low
<= 2
8, 9
Wireless Medical
Telemetry Services
(WMTS)
420–429
440–449
(Japan)
Low
Low
~10
8, 9
608–614
1395–1400
1427–1429.5
(USA)
BodyLAN
900
32 kb/s
0 dBm
2–10
8
Bluetooth
2400–2480
1 Mb/s
0 dBm
0.1–10
3, 7, 8
ZigBee
2400
250 kb/s
Low
1–100
3
915
40 kb/s
868
20 kb/s
WLAN
2400, 5200
10-50 Mb/s
0 dBm
30–50
8
UWB
3100–10,600
1 Gb/s
-41 dBm/
MHz
10
5
1.2 Overview of Systems 9
2. Implant and telemetry bands: The Medical Implant Communication System (MICS) band is 402–405 MHz [18], and the Wireless Medical Telemetry Services (WMTS) [19] operates on various bands between 420 and
1430 MHz. Chapters 7 and 9 present wireless communications for medical
implants and swallowable sensors, which have distinct nature of channel
behavior. First of all, electric and magnetic fields experience high attenuation inside the human body due to increasing conductivity of human tissue
with frequencies of interest. Secondly, channel properties cannot be experimentally characterized easily and can only be determined by numerical
modeling. Furthermore, while most current studies examine in-to-out body
channels, in-body channel characteristics are useful when the link is locally
established among various implants in the same patient. Figure 1.7 shows
comparison of electric field at MICS and ISM (868 MHz and 2.45 GHz)
bands in a cut plane along human stomach, clearly demonstrating the lossy
behavior of human tissues at different frequency bands. Since communication links are usually needed between the implants and body worn sensors, Chapters 7 and 9 also address important channel characteristics from
in-body to on-body, and even off-body. Chapters 8 and 9 discuss various
antenna types for implantable devices and Chapter 8 specifically demonstrates practical antenna miniaturization techniques for body sensors.
3. ISM, Bluetooth, and WLAN: on-body propagation measurement, models and simulations are presented and discussed extensively in Chapters
3, 6, and 7. It is noted that the antenna properties will be modified due
to the presence of the human body. In addition, communication nodes on
the body are often in the near radiation field region; therefore, on-body
Figure 1.7 E-field inside the body in horizontal axis of a human model with full stomach.
10
Introduction to Body-Centric Wireless Communications
radio behavior is strongly influenced by antenna types. Chapter 6 discusses
various wearable textile antennas for space exploitation and military applications. Examples for mobile phones, GPS, and WLAN applications are
presented together with antenna performance enhancement technique using EBGs. One of distinct features for the on-body radio channel is its variability in path loss and delay profile due to antenna placement and posture
changes. The comparison between the standing and sitting body activities
has shown that the location of the transmitter should be chosen wisely, as
its orientation can be altered by posture change. This may adversely affect the signal strength for receivers on the back. In most cases, statistical
analysis of the measured on-body radio channels regarding path loss and
deviation can be fitted to known distributions. The dependency of system
performance on the applied data rate has been demonstrated and it was
learned that an acceptable BER can be achieved for power levels as low as
–40 dBm for low data rates mainly used in health monitoring applications.
1.2.2 Wideband Systems
The wideband body-centric wireless communication system referred to in Chapter
5 is associated with UWB regulations issued by the Federal Communications Commission (FCC) in April 2002. Under Part 15 of the Commissions rules, the FCC permits ultrawideband intentional emissions, subject to certain frequencies and power
limitations that will mitigate interference risk to those sharing the same spectrum
[20]. The wideband body-centric wireless communication system can directly deploy UWB signals between 3.1 and 10.6 GHz at power levels up to –41 dBm/MHz,
with higher degree of attenuation required for the out-of-band region. The bandwidth of such systems can also be defined as more than 25% of a center frequency
or more than 1.5 GHz. Clearly, this bandwidth is much greater than the bandwidth
used by any current technology for communication. The large bandwidth of UWB
signals provides robustness to jamming and has low probability of detection properties. UWB devices usually require low transmit power due to control over duty
cycle, thus allowing longer battery life of handheld equipment. In addition to its
high data rate and short-range features (which makes UWB a suitable candidate for
the wireless body-centric network), the low power level increases the compatibility
of this technology with current exiting wireless systems.
For narrowband body-centric wireless systems, comparison of the results taken
in the anechoic chamber to those in the laboratory shows a noticeable increase
in signal variability outside the anechoic chamber due to multipath fading. Outside the chamber, the propagation channel is not stable, even when the body is
still; this instability is caused by other people moving about the room. Preliminary
frequency-domain measurements indicate that UWB on-body, like its narrowband
counterpart, suffers significant radio link variation due to different body postures.
However, it is noted that the human body attenuates the UWB signal more seriously, so at non-LOS situation, received signal level becomes very low. The results
also indicate that the dominant propagation channel for UWB on-body network is
still the free space path, although it depends on the antenna types used.
1.3 Overview of Applications 11
1.3 Overview of Applications
Body-centric wireless communications are aimed at providing systems with constant availability, reconfigurability, and unobtrusiveness. High levels of processing
and complex network protocols are needed to provide the powerful computational
functionalities required for advanced applications. These requirements have led to
increasing research and development activities with the main interests being healthcare, patient monitoring, personal identification, navigation, personal multimedia
entertainment, and task-specific/fully compatible wireless wearable computers.
Body-centric wireless communications can be applied to many fields and a summary of their applications can be found in Table 1.2. Some of them include:
••
Medical applications: smart diagnosis, treatment, drug delivery system, patient monitoring, and aging care;
••
Wireless access/identification systems: wireless transactions and identification of individual peripheral devices;
••
Navigation support and location based services: tourism, security, and intelligent transportation system;
••
Personal multimedia entertainment: wireless DVD and wearable computing;
••
Military and space applications: smart suits, battlefield personnel care and
intelligence, and biosensors for astronaut monitoring.
1.4 New Trends and Progress Since the First Edition
The previous sections sought to present taxonomies for body-centric channels and
frequencies, which may help highlight the possibilities and challenges in each. This
book, as a whole, attempts to address all of these areas, but also includes the additional dimensions of the ever-widening area of applications, with the emphasis on
medical, personal, and special uses (such as space).
Since 2006, when the first edition of this book was published, there have been
some noticeable trends in the topic. While the content of different chapters in this
book show varying degrees of maturity and remaining issues to be addressed, we
have identified some topics which have changed our perspectives or directions, or
which represent significant difficulties that required further intense study. A brief
description of these is now given, followed by a section describing the contents of
each chapter.
1.4.1 Propagation Characterization and Control
There is now much data worldwide characterizing channels around the human
body, and this is discussed in various places throughout this book. Different frequencies, antennas, channels, and size and shaped bodies have been studied. In general, frequencies above 10 GHz remain an uncharted area; however, there is now
increasing interest in both understanding the various propagation modes that can
exist on the body and in trying to control them by using materials covering the body
12
Introduction to Body-Centric Wireless Communications
for wave guidance. The principles of radio wave propagation on lossy bodies have
been the subject of intense study for many decades; it is still controversial, and this
spirit is now being seen in the various names for energy propagation modes around
the body. As noted above, it is interesting to see that wave guidance, or trapping,
on the human body is being investigated and that concepts, such as the smart vest
[21], are emerging.
1.4.2 Measurement Methods
There is now an increased interest in better methods for measuring the channel
performance on the body. Cable-based methods have been widely adopted due to
their low cost, flexibility, and simplicity, and have allowed the basic properties of
channels to be established. However, there are improved methods, such as fiber
optic and data logging modules. Much smaller components are now available, as
shown in Chapter 3. While these are not problem-free, it is likely that there will be
better acceptance of statistical models obtained this way.
1.4.3 Antenna De-embedding
System designers would like to separate antenna performance from channel behavior, so that antenna design is simplified and the other constraints that RF designers
face (such as antenna size, weight, cost, bandwidth, and efficiency) can be dealt
with more easily. There has been some acknowledgment among researchers that
this is a desirable thing to do. However, some studies described in Chapter 3 show
that we are still some way off from freeing the antenna designer from needing to
test each new antenna in the channel which it is intended for. The challenges remain
and, so far, there are only solutions for a limited number of cases.
Table 1.2 A Summary of Potential Applications in Body-Centric Wireless Communications
Wave Propagation
Standard
Applications Scenarios
Mechanism
Chapter Number
UHF/VHF
Medical implants,
personal access,
Inductive and
near field coupling
4, 9
personal identification system
Medical implant
(licensed)
Medical implants,
Near/far field,
lossy media
7, 8, 9
telemedicine
BodyLAN
Telemedicine
Free space wave
7
Space/surface waves
3, 6, 8
Space/surface waves
5
Bluetooth/ ZigBee/ Personal multimedia
WLAN
entertainment, security,
smart clothing,
location-based services
UWB
Wireless DVD,
wearable and ubiquitous
computing
1.4 New Trends and Progress Since the First Edition
13
1.4.4 Materials
While many antennas used in body-centric communications are produced using the
pervasive manufacturing process of the printed circuit board, the use of alternative
materials continues to progress. There is significant work on fabric based antennas,
in terms of antenna design, but also in the materials themselves and in production
techniques. Figure 1.8 shows a typical fabric-based wearable antenna. Frequently
surprising developments have occurred, such as the “sticking plaster” antenna [22],
in which usability seems to drive new concepts.
1.4.5 Modeling of Body Dynamics
There is a significant problem in the world of simulation, due to the fact that onbody channel performance is greatly affected by body motion. This issue is, of
course, present in other forms of communications, such as cellular phones and
indoor WiFi systems, and seems to be getting increased attention. Several software
providers offer whole body phantoms that can be moved into different postures.
There is also a physical phantom with moving legs to mimic walking. Body scanners
and motion capture equipment are now being used. Perhaps the most challenging
area is the one of analysis, and workers are now considering canonical solutions for
fields around moving objects. There is still much to do, and Chapter 2 gives some
details of recent work.
Figure 1.8 A fabric-based wearable antenna. (Courtesy of Jon Pinto, BAE Systems.)
14
Introduction to Body-Centric Wireless Communications
1.4.6 Standardization
The IEEE 802.15 Task Group 6, which was formed in November 2007, is actively
developing a standard for wireless body area networks [23]. The standard is targeted for relatively low data rate (100 kpbs to 1 Mbps) transmission to devices
attached around the body (human or other) or implanted. Low-power medical and
consumer applications will benefit from this development. Possible bands include
the Medical Implant Communications Service band (MICS) at 402‐405 MHz, the
Wireless Medical Telemetry Services (WMTS) band at various frequencies in different
parts of the world between 420 and 1430 MHz, the 2.45 and 5.8 GHz Industrial, Medical
and Scientific (ISM) bands, and the ultrawideband (UWB) from 3.1 to 10.6 GHz band.
The proposed standard will include reference to applications, bands, data rates, physical
layer details, and some of the MAC proposals.
1.5 Layout of the Book
The chapters and authors in this book have been selected to give overviews of
work in the three areas of body-centric communications, as noted in Section 1.1.
A guide to their content now follows, and can be read in conjunction with Figure
1.9, which shows the relationship of the chapters to each other and to the topic of
personal and body area networks. The figure shows that Chapter 2, in addition to
the Introduction and Summary, is relevant to all parts of body-centric systems. Only
Chapters 3, 4, and 9 deal with specific domains, while all the rest relate to overlap
areas between domains.
Chapter 2 discusses the electromagnetic properties of the body, including material properties. An overview of the various types of phantoms is then given. Chapter 2 also presents an introduction to numerical modeling techniques for antennas
and methods for modeling moving bodies.
Chapter 3 gives an overview of on-body channels at microwave frequency
bands and a brief review of applications of microwave on-body communications
from current handset-headset communications to future applications, such as support for medical sensor networks and emergency personnel. It gives information
about on-body channel measurements, channel characterization and modeling, antenna and system design, and link budget analysis.
Chapter 4 deals with on-body wireless communications at low frequency bands
and gives an overview of on-body channel performance at these frequencies. Chapter 4 presents equivalent circuit models, numerical analysis, experiments using human phantoms, transmission mechanism, and overviews various applications.
Chapter 5 gives an overview of UWB as an enabling technology for bodycentric communications. UWB antennas are discussed and analyzed. UWB channel
measurement and simulation methodology, as well as channel characterization and
modeling for PANs and BANs are presented.
Chapter 6 introduces wearable antennas for cellular and WLAN communications, and is aimed primarily at communications from on to off the body. Chapter
6 presents different antenna solutions for this communications methods and smart
antenna fabrics.
1.5 Layout of the Book
15
Figure 1.9 Interaction of the book chapters.
Chapter 7 deals with body-sensor networks, including the basics of biomedical
RF telemetry and biosensor systems. Antenna designs for body-sensors and different applications in military and space are also presented.
Chapter 8 gives an overview of antennas and propagation for telemedicine.
Various telemedicine applications are presented, and some principles of numerical
modeling of the human body for telemedicine together with system design principles are introduced.
Chapter 9 introduces antennas and propagation for wireless implants. Chapter
9 deals with RF biotelemetry and focuses especially on low-profile antennas for
implantable medical devices and at antennas that can operate successfully in a lossy
dispersive medium.
Chapter 10 gives an overview of this book, together with conclusions on the
state of the art of body-centric wireless communications. Finally, some future challenges are presented.
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