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Ubiquitous Healthcare Using MAC Protocols in
Wireless Body Area Sensor Networks (WBASNs)
Final Year Project Report
Presented
by
Muhammad Sarim Hayat
CIIT/FA08-BET-131/ISB
Nadir Ali Khan
CIIT/FA08-BET-089/ISB
Umair Rafiq
CIIT/FA08-BET-132/ISB
In Partial Fulfillment
of the Requirement for the Degree of
Bachelor of Science in Telecommunication Engineering
DEPARTMENT OF ELECTRICAL ENGINEERING
COMSATS INSTITUTE OF INFORMATION TECHNOLOGY,
ISLAMABAD
June 2012
i
Ubiquitous Healthcare Using MAC Protocols in
Wireless Body Area Sensor Networks (WBASNs)
Final Year Project Report
Presented
by
Muhammad Sarim Hayat
CIIT/FA08-BET-131/ISB
Nadir Ali Khan
CIIT/FA08-BET-089/ISB
Umair Rafiq
CIIT/FA08-BET-132/ISB
In Partial Fulfillment
of the Requirement for the Degree of
Bachelor of Science in Telecommunication Engineering
DEPARTMENT OF ELECTRICAL ENGINEERING
COMSATS INSTITUTE OF INFORMATION TECHNOLOGY,
ISLAMABAD
June 2012
ii
Declaration
We, hereby declare that this project neither as a whole nor
as a part there of has been copied out from any source. It is
further declared that we have developed this project and
the accompanied report entirely on the basis of our
personal efforts made under the sincere guidance of our
supervisor. No portion of the work presented in this report
has been submitted in the support of any other degree or
qualification of this or any other University or Institute of
learning, if found we shall stand responsible.
Signature:______________
Muhammad Sarim Hayat
Signature:______________
Nadir Ali Khan
Signature:______________
Umair Rafiq
COMSATS INSTITUTE OF INFORMATION TECHNOLOGY,
ISLAMABAD
June 2012
iii
Ubiquitous Healthcare Using MAC Protocols in
Wireless Body Area Sensor Networks (WBASNs)
An Undergraduate Final Year Project Report submitted to the
Department of
ELECTRICAL ENGINEERING
As a Partial Fulfillment for the award of Degree
Bachelor of Science in Telecommunication Engineering
by
Name
Registration Number
Muhammad Sarim Hayat
CIIT/FA08-BET-131/ISB
Nadir Ali Khan
CIIT/FA08-BET-089/ISB
Umair Rafiq
CIIT/FA08-BCE-132/ISB
Supervised by
Dr. Nadeem Javaid
Assistant Professor
Department Of Electrical Engineering
CIIT Islamabad
COMSATS INSTITUTE OF INFORMATION TECHNOLOGY,
ISLAMABAD
June 2012
iv
Final Approval
This Project Titled
Ubiquitous Healthcare Using MAC Protocols in
Wireless Body Area Sensor Networks (WBASNs)
Submitted for the Degree of
Bachelor of Science in Telecommunication Engineering
By
Name Registration Number
Muahammad Sarim Hayat CIIT/FA08-BET-131/ISB
Nadir Ali Khan CIIT/FA08-BET-089/ISB
Umair Rafiq CIIT/FA08-BET-132/ISB
has been approved for
COMSATS INSTITUTE OF INFORMATION TECHNOLOGY,
ISLAMABAD
_____________________
Supervisor
Dr. Nadeem Javaid
______________________ ______________________
Internal Examiner-1 Internal Examiner-2
______________________
External Examiner
Name,
Designation
_____________________
Head
Department of Electrical Engineering
v
Dedication
We first thank ALLAH ALMIGHTY for giving us the courage and wisdom to complete
this project. We dedicate this project to our parents for their love and moral support.
Without their inclusive support and commitment, this project would not have been
possible. They have been our true motivation and inspiration for the satisfactory
completion of this project. We also dedicate this project to all our respected teachers who
guided us all the way through this project with their support and motivation.
vi
Acknowledgements
We are grateful to Allah Almighty, who helped us and gave us courage at every stage of
life. All esteems are for His Prophet Muhammad (SAW) whose teachings have served as
a beacon of light for the humanity in the hours of despair and darkness and provided us
with regular guidance in every phase of life.
We might never be able to find words to thank our parents for their exceptional love,
unlimited support and unconditional prayers that made us accomplish success in our
academic life. We may not be able to express our deepest and heartiest feelings towards
them.
Furthermore, we are really grateful to Dr. Nadeem Javaid, our project supervisor for his
kind of shore up an ultimate guidance during each and every point of this project. We are
also thankful to Dr. Safdar Hussain Bouk for guiding us through this project. We are also
obliged to all teachers for providing us with knowledge which enabled us to reach this
level today. We are also really thankful to our friends and class mates and all those who
in some way or the other became a source of encouragement.
Muhammad Sarim Hayat
Nadir Ali Khan
Umair Rafiq
vii
List of Publications
Nadir Ali Khan, Jaffar Kulachi, Umair Rafiq, Ayesha Bibi, Zahoor
Ali Khan, Nadeem Javaid, ``Ubiquitous Healthcare in Wireless Body
Area Networks'', 11th IEEE International Conference on Ubiquitous
Computing and Communications (IUCC-2012), Liverpool, UK, 25-27
June 2012.
Sarim Hayat, Abida Shareef, Anzar Mahmood, Zahoor Ali Khan,
Safdar Hussain Bouk, Nadeem Javaid, ``Energy Efficient MAC
Protocols in Wireless Body Area Sensor Networks'', 14th IEEE
International Conference on High Performance Computing and
Communications (HPCC-2012), 25-27 June, Liverpool, UK, 2012.
Submitted Papers
Analysis of Routing Protocols from Wireless Sensor Networks (WSN)
to Wireless Body Area Networks (WBAN), 9th IEEE International
Conference on Ubiquitous Intelligence and Computing (UIC),
FUKUOKA, Japan, 04-07 September 2012.
LEACH,TEEN, DEEC,SEP and HEED: A Performance Study in
Wireless Sensor Networks, 9th IEEE International Conference on
Ubiquitous Intelligence and Computing (UIC), FUKUOKA, Japan,
04-07 September 2012.
viii
Table of Contents
1. Practical perspective of WBASNs 1
1.1 Introduction…………………………………………………1
1.2 Wireless Body Area Sensor Networks………………………1
1.3 Applications of Wireless Body Area Networks………....2
1.3.1 Cardiovascular diseases…… …………………………2
1.3.2 Diabetes…………… …………………………………2
1.3.3 Cancer detection………………………………………3
1.3.4 Asthma………………………………………………..3
1.3.5 Artificial retina……………………………………….3
1.3.6 Battlefield…………………………………………….3
2. Energy Efficient MAC Protocols in WBASNs 4
2.1 Introduction……………………………………………..........4
2.2 Related Work…………………….…………………………...6
2.3 Energy Minimization Techniques in MAC Protocols for
WBANs………………………………….…….……………...6
2.3.1 Low Power Listening………………………………….7
2.3.2 Scheduled Contention…………………………………7
2.3.3 Time Division Multiple Access……………………….8
2.4 Energy Efficient MAC Protocols……………………………10
2.4.1 Okundu MAC Protocol… … ……………………….10
2.4.2 MED MAC Protocol……… … …………………….10
2.4.3 Low Duty Cycle MAC Protoc. ………………………11
2.4.4 B-MAC Protocol…………………………… ………11
2.4.5 Ta-MAC Protocol……………………….…………...12
2.4.6 S-MAC Protocol……………………… …………….13
2.4.7 T-Mac Protocol………………………… …………...13
2.4.8 H-MAC Protocol……………………… …………….13
2.4.9 DTDMA Protocol……………………… ……………14
2.5 Performance Trade-offs made by MAC Protocols………….15
2.5.1 Okundu MAC Protocol………………………………16
2.5.2 Med Mac Protocol……………………………………16
2.5.3 Low Duty Cycle MAC Protocol…………..………...16
ix
2.5.4 B-MAC Protocol……………………………………...16
2.5.5 Ta-MAC Protocol……………………………………..16
2.5.6 S-MAC Protocol………………………………………17
2.5.7 T-Mac Protocol………………………………………..17
2.5.8 H-MAC Protocol………………………………………17
2.5.9 DTDMA Protocol………………..……………,……...17
2.6 MAC Frame Structure……………………………………….18
2.7 Technique for Collision Avoidance for Traffic Control…….19
2.7.1 Okundu MAC Protocol………………………………19
2.7.2 MED MAC Protocol…………………………………20
2.7.3 B-MAC Protocol……………………………………..20
2.7.4 Ta-MAC Protocol……………………………………20
2.7.5 Low Duty Cycle……………………………………...21
2.7.6 IEEE 802.15.4 MAC…………………………………21
2.8 Path Loss Model for WBANs……………………………….23
2.8.1 MAC for In-Body Communication…………………..23
2.8.2 MAC for On-Body Communication……………………………25
2.8.3 MAC for Off-Body Communication…………………26
3. Ubiquitous Healthcare in WBAN – A Survey 28
3.1 Introduction…………………………………………………28
3.2 Related Work….…………………………………………….29
3.3 Most Frequently Used Standards for WBAN
Communication……………………………………………...31
3.3.1 IEEE 802.15.1 Bluetooth………………………… ….31
3.3.2 ZigBee…………………………………………………..31
3.3.3 Medical Implant Communications Service (MIC……32
3.3.4 IEEE 802.15.6 Ultra Wide Band (UWB)…………….32
3.4 Wearable Sensors used for Ubiquitous HealthCare ………32
3.4.1 Wrist watch (eWatch)………………………………..33
3.4.2 Oximeter……………………………………………..33
3.4.3 Chest belt…………………………………………….33
3.4.4 Wearable shirt type (smart shirt/life shirt)…………..33
3.5 General WBAN Architectures…………………………….. 34
3.5.1 Multi Tier Architecture of a WBAN for UHC………34
x
3.5.2 System Based Architecture with Physiological Signal Devices of
UHC…………………………………………………36
3.5.3 Integrated System Architecture of UHC Monitoring Systems...
………………………………………………………………...37
3.5.4 Traffic Based Architecture of WBAN for UHC Monitoring…
………………………………………………………………….38
3.5.5 Components Based System Architecture……………...39
3.5.6 Wearable Smart Shirt Based Architecture for UHC Wearable
Smart Shirt System……………………………………41
3.6 Path Loss in WBAN………………………………………..42
3.6.1 Wireless Body Area Network (WBAN)………………44
3.7 Scenarios of Path Loss in WBAN………………………….46
3.7.1 In-Body Communication……………………………...46
3.7.2 On-Body Communication…………………………….49
4. CONCLUSION 55
BIBLIOGRAPHY 57
xi
List of Figures
2.1 Sources of Power Consumption
………………………………
5
2.2 MAC Frame Structure
………………………………………
19
2.3 Data Traffic Control
…………………………………………
22
2.4 In-Body Communication
……………………………………
24
2.5 On-Body Communication
……………………………………
26
2.6 Off-Body Communication
……………………………………
27
3.1 Ubiquitous health care architecture
…………………………
34
3.2 Deep Tissue Implant to Body surface Path Loss
……………
48
3.3 Amplitude Attenuation in On-Body
…………………………
50
3.4 Phase Distortion in On-Body
………………………………
50
3.5 Channel Output for On-Body Communication
……………
51
3.6 Path Loss vs Distance for On-Body Communication
………
52
3.7 RMS Delay at 15cm Separation
……………………………
53
3.8 RMS Delay at 45cm Separation
……………………………
53
xii
List of Tables
2.1 Comparison between LPL, Schedule Contention, AND TDMA
……………………………………………………………………9
2.2 Qualitative Comparison of MAC Protocols …………………14
2.3 Energy Minimization Techniques and Mechanisms…………15
2.4 Trade-offs of MAC Protocols…………………………………8
2.5 Comparison between IEEE 802.15.4 MAC and Original…22
3.1 Summary of Architecture of WBAN………………………43
3.2 Implant to Body Surface ……………………………………47
3.3 Implant to Implant …………………………………………47
3.4 Summary of In-Body Path Loss in WBAN…………………49
3.5 Summary of On-Body Path Loss in WBAN ………………54
xiii
Abstract
The title of the project “Ubiquitous Healthcare Using MAC Protocols in Wireless
Body Area Sensor Networks (WBASNs)” carries immense significance in our daily
life. Recent advances in wireless communications, system on chip and low power sensor
nodes allow realization of Wireless Body Area Networks (WBANs). WBANs comprise
of tiny sensors, which collect information of a patient’s vital signs and provide a real time
feedback. In addition, WBANs also support many applications including ubiquitous
healthcare, entertainment, gaming, military, etc. Ubiquitous healthcare is required by
elderly people to facilitate them with instant monitoring anywhere they move around. In
this thesis, we provide a survey on different architectures used in WBANs for ubiquitous
healthcare monitoring. Different standards and devices used in these architectures are
also discussed in this thesis. Finally, path loss in WBANs and its impact on
communication is presented with the help of simulations performed for different models
of In-Body communication and different factors (such as, attenuation, frequency, distance
etc) influencing path loss in On-Body communications.
This thesis also presents a survey of energy efficiency of Medium Access Control (MAC)
protocols for Wireless Body Area Sensor Networks (WBASNs). We highlight the
features of MAC protocols along with their advantages and limitations in context of
WBASNs. Comparison of Low Power Listening (LPL), Scheduled Contention and Time
Division Multiple Access (TDMA) is also elaborated. MAC protocols with respect to
different approaches and techniques used for energy minimization; traffic control
mechanisms for collision avoidance are discussed. We also present a survey of path loss
models for In-body, On-body and Off-body communications in WBASNs. These three
path loss scenarios are simulated in MATLAB and results show that path loss is
maximum in In-body communication because of less energy level to take care of tissues
and organs located inside the body.
The main applications of WBANs include In-body and On-body applications, e.g.
Cardiovascular dieses, diabetes, asthma etc.
1
Chapter 1
Practical Perspective of Wireless Body Area Sensor
Networks
1.1 Introduction
Over the last decade there has been a lot of advancement in the field of wireless communication
particularly in the domain of Wireless Body Area Sensor Networks (WBASNs). It consists of
tiny sensors that are attached to the human body or can be implanted in the body, thus the sensor
networks collect vital information like temperature of the body, humidity, glucose level, pulse
rate, ECG, EEG etc, the information is transferred to the personal server (PS) through a wireless
medium. From PS, the information is sent to the desired destination through the use of any
wireless technology e.g. Bluetooth, Zigbee, and AMPs etc. The most important application of
WBAN is in the field of healthcare. Ubiquitous heathcare (UHC) is provided to patients all
around the world through the use of WBAN. The medical staff and doctors get the real time
analysis of the patient and prescribe them with proper medication likewise.
In WBAN nodes share a single medium for combination. Network performance largely depends
upon how efficiently and fairly the nodes can share this common medium. The packet
transmission is directly handled by the MAC layer, compared to the wired medium for the packet
reception. On the other hand WBANs always have restricted power sources.
Nodes of the WBAN have very low energy sources and remain unattended after deployment;
therefore the lifetime entirely depends on energy conservation during communication and the
energy wastage during communication. There are several sources of energy wastage including
packet collisions, over hearing, idle listening, control packet overhead, etc. Major source of
energy inefficiency among the above listed sources is packet collision for WBANs.
1.2 Wireless Body Area Sensor Networks
Due to advances in field of wireless communication over the last decade, development of
networks having low cost, low power, and long battery life is receiving great attention. WBAN
consist of small sensors that are able to sense, process data and communicate with each other
2
over RF channel. WBAN is designed to detect events collect or processed data and transmit the
information to the desired destination.
Event detection in WBAN requires the sensor to be vigilant most of the time as a result of which
sensor lifetime is short. Whereas, data collection generally allow sensors to be turned off
occasionally. Another difference is that data collection requires frequent messaging to report
measurements but event detection requires reporting only on the occurrence of the certain events.
WBAN is made up of small group of sensor nodes; each node has the ability to monitor some
phase of environment and is able to communicate its observation through other modes to a
destination. Recent development in wireless technologies making WBAN smaller and more cost
effective for a growing number of users. The sensors in WBAN communicate with each other
wirelessly and can be extended to hard areas. Sensor node is a complete combination of
computation, communication and sensing. The flexibility, fault tolerance, high sensing fidelity
and low cost makes WBAN application available in remote areas.
1.3 Applications of Wireless Body Area Networks
WBAN is an emerging domain in the field of wireless communication. It has several applications
including remote medical diagnosis, interactive gaming, and military. WBAN include In-body
and On-body applications in the field of healthcare. In-body applications include, monitoring of
pacemakers and implantable cardiac defibrillators, control of bladder function, and restoration of
limb movement. On-body medical healthcare applications include monitoring ECG, blood
pressure, temperature, and respiration. Moreover, on body non-medical applications include
monitoring forgotten things, establishing a social network, and assessing soldier fatigue and
battle readiness etc. Some of the WBAN applications are discussed below:
1.3.1 Cardiovascular diseases
WBAN can be used to prevent the occurrence of myocardial infarction, monitor episodic events
or any other abnormal condition during emergency. WBAN monitors the heart condition of the
patient continuously with the help of tiny sensors and forward data to medical healthcare team
for constant monitoring. In case of any undesired situation, immediate aid is provided to patient's
with the help of WBAN.
3
1.3.2 Diabetes
WBAN can monitor the sugar level in the blood continuously and in case of any disturbance
triggers the patient with the help of an alarm. It also act as a reminder for patients to take the
medicines and insulin injection in time.
1.3.3 Cancer detection
A group of miniaturized sensors capable of monitoring cancer cells can be seamlessly integrated
in WBAN and placed on the body of the patient. This allows physicians' to diagnose tumors
without biopsy.
1.3.4 Asthma
WBAN helps millions of patients around the world suffering from asthma by monitoring allergic
agents in the air and providing real-time feedback to the physician with the help of tiny sensors.
Sensors are incorporated with a GPS device that triggers an alarm in case of any allergic agents
present in the environment.
1.3.5 Artificial retina
Retina prosthesis chips can be implanted in the human eye that assists patient with limited or no
vision to see at an adequate level.
1.3.6 Battlefield
WBANs can be used to connect soldiers in a battlefield and report their activities to the
commander on constant basis. The soldiers should have a secure communication channel in order
to prevent ambushes. Furthermore, WBAN can also be used to provide immediate first aid to
soldiers in case of injury.
4
Chapter 2
Energy Efficient MAC Protocols in Wireless Body area
Sensor Networks (WBASNs)
2.1 Introduction
Evolution of wireless, medical and computer networking technology has merged into an
emerging horizon of science and technology called Wireless Body Area Networks (WBANs).
However, applications of WBANs are not limited to medical field only. Miniaturization and
connectivity are notable parameters of this field. WBANs consist of three levels; first level is low
power sensors or nodes which are battery powered and need to be operated for a long time
without repairing and maintenance. These nodes may be placed on the body, around the body or
implanted in the body. Second level is called master node, gateway or coordinator which controls
its child nodes; its power requirements may be less strengthened than nodes due to its
applications and flexibility. Third level is the local or metropolitan or internet network that
serves for monitoring purposes. Energy efficiency or effective power consumption of a system is
one of the basic requirements for WBANs because of limited power of batteries. The most
suitable layer for discussing energy and power issues is MAC Layer. The basic way of saving
power or enhancing energy efficiency is to minimize the energy wastage. There are several
sources of energy wastage including packet collisions, over hearing, idle listening, control packet
overhead, etc. Major source of energy inefficiency among the above listed sources is packet
collision for WBANs. Fig 2.1 best explains that how a node's battery is consumed, in the process
of communication.
Collision avoidance for energy efficiency, minimum latency, high throughput, and
communication reliability, are basic requirements in the design of MAC protocol. The
fundamental way of saving power or enhancing energy efficiency is to minimize the energy
wastage. Simulations are performed in MATLAB for different scenarios to compute path loss.
Results show that path loss is maximum in In-body communication, as compare to On-body and
Off-body communication because human body is composed of tissues and organs in which
5
communication is difficult and thus results in high path loss. On-body and Off-body also show
some variations in results when the source and destination sensors or nodes are placed Line of
Sight (LoS) and Non Line of Sight (NLoS).
In this chapter, we therefore, provide a survey of energy efficient MAC protocols for WBANs.
At first, we elaborate the protocol features and then their advantages and limitations are
discussed. Sources that contribute to the energy inefficiency in a particular protocol is also
identified. Moreover, comparisons of MAC protocols in the context of WBANs are tabulated in
detail.
Rest of the chapter is arranged as follows: Related work is discussed in section II. In section III,
energy efficient MAC protocols are discussed with their advantages and disadvantages, while in
section IV, the MAC frame structure is discussed. Energy minimization techniques are discussed
in section V. In section VI, the traffic control mechanism and in section VII Path loss for
WBANs is described in detail. In last section power model of beacon and CSMA/CA mode for
WBANs is discussed.
MAJOR SOURCES OF ENERGY WASTE
CONTROL
OVERHEAD
POWER
CONSUMPTION IN
SENSING
POWER
CONSUMPTION IN
DATA PROCESSING
POWER CONSUMPTION IN
COMMUNICATION
OVER
EMITTING
IDLE
LISTENING
OVER
HEARING
PACKET
COLLISIONS
NODE POWER
SOURCE (BATTERY)
Fig 2.1 Sources of Power Consumption
6
2.2 Related Work
Gopalan et al. [1] survey MAC protocols for WBANs along with the comparison of four
protocols i.e., Energy Efficient MAC, MedMac, Low Duty Cycle MAC, and Body MAC. Some
key requirements and sources of energy wastage are also discussed. They also discussed some
open research issues in this survey. Still a lot of work has to be done in data link layer, network
layer and cross layer design.
In [2], Shahjahan kutty et al. discuss the design challenges for MAC protocols for WBANs. They
classify data traffic for WBANs into three categories: energy minimization techniques, frame
structures, and network architecture. However the comparison of protocols is not provided by
them.
Sana Ullah et al. in [3] has provided relatively a comprehensive study of MAC protocols for
WBANs. Comparison of the low power listening, scheduled contention and Time Division
Multiple Access(TDMA) is provided. MAC requirements, frame structures and comparison of
different protocols and their trade-offs are discussed in detail.
2.3 Energy Minimization Techniques in MAC Protocols for
WBANs
Low power mechanisms play an important role in performance enhancement of MAC protocol
for WBANs. In this section, different approaches and techniques that provide energy efficiency
in MAC protocols for WBANs are discussed and compared.
Energy efficiency is an important issue because the power of nodes in WBANs is limited
and long duration of operation is expected. The key concept for the energy efficiency is to
minimize the energy consumption in the following sources: sensing, data processing, and
communication.
Most of the energy wastage is caused during communication process because of the
collision of packets, idle listening, over hearing, over-emitting, control packet overhead and
7
traffic fluctuations. Idle listening can be reduced through duty cycling. To reduce energy waste
in order to increase the network's life time and to enhance the performance of MAC protocol,
different wake-up mechanisms are used.
There are three main approaches adopted for the energy saving mechanisms in MAC protocols
for WBANs, which are listed and discussed below:
Low Power Listening (LPL)
Scheduled Contention
Time Division Multiple Access (TDMA)
2.3.1 Low Power Listening
Low power listening (LPL) procedure is that ``node awakes for a very short period to check
activity of channel". If the channel is not idle then the node remains in active state to receive data
and other nodes go back to sleeping mode. This is also termed as channel polling [3]. This
procedure is performed regularly without any synchronization among the nodes. A long
preamble is used by the sender to check polling of the receiver.
LPL is sensitive to traffic rates which results in degradation of performance in the scenario of
highly varying traffic rates. However, it can be optimized effectively for the already known
periodic traffic rates. Wise-MAC [3] is one of the MAC protocols which is based on LPL. This
protocol reduces Idle listening using non-persistent CSMA and preamble sampling technique.
2.3.2 Scheduled Contention
Scheduled Contention is the combination of the scheduling and contention based mechanisms to
effectively cope with the scalability and collision problems. In contention based protocols,
contending nodes try to access the channel for data transmission therefore, ability of collision of
packet is greatly increased. Example of contention based MAC protocol is Carrier Sense
Multiple Access/Collision Avoidance (CSMA/CA) in which Clear Channel Assessment (CCA)
is performed by the nodes before transmitting data. Scheduling or Contention free means that
each node has the schedule of transmission in the form of bandwidth or time slot. TDMA,
CDMA and FDMA schemes are some examples of scheduling mechanisms. However, CDMA
and FDMA are not suitable for WBANs because of high computational overhead and frequency
limitations, respectively.
8
TDMA is the most suitable scheduling scheme, even though it requires extra power
consumption due to its sensitivity for synchronization. The scheduled contention is the
combination of scheduling and contention based mechanisms. In scheduled contention, a
common schedule is adopted by all the nodes to transmit data. This schedule is exchanged
periodically among the nodes to make communication adaptive, flexible and scalable.
Sensor MAC (S-MAC) is one of a MAC protocol based on the scheduled contention. In
this protocol, low duty mode is set as default mode for all the nodes, which assures the
coordinated sleeping among neighboring nodes. The energy wastage due to collision,
overhearing, idle listening etc. is minimized because the node is turned on only for transmission
of data and remains in sleep mode otherwise.
2.3.3 Time Division Multiple Access
In TDMA mechanism, a super frame consists of a fixed number of time slots is used. Time slots
are allocated to the sensor nodes by a central node and are known as master node, cluster head,
coordinator or the Base Station transceiver. Traffic rate is one of the key parameter used by the
coordinator to allocate time for each contending node. This scheme is highly sensitive to clock
drift, which may result in limited throughput. The scheme is power efficient because a node gets
time slot for transmission of data and remains in sleep mode for rest of the time. However, the
synchronization requirements may degrade performance in terms of power consumption.
Preamble-Based TDMA (PB-TDMA) protocol is one of the TDMA based protocol. Other
examples include Body-MAC (B-MAC) [5], Med MAC [3] etc.
These techniques are briefly compared in Table 2.1
9
Table 2.1 Comparison Between LPL, Schedule Contention, AND TDMA
Energy Saving
Mechanisms
LPL
Scheduled
Contention
TDMA
Adaptability to
traffic and delay
Scalable and adaptive
to traffic load, and
low delay
Better delay
performance, due to
sleep schedules
Better end-to-end
reliability, smaller
delays, high reliability
Transmission
latency and
throughput
Flexible, high
throughput, tolerable
latency, and low
power consumption
High transmission
latency, loosely
synchronized, low
throughput
Good for energy
efficiency, prolonged
network's lifetime,
load balancing
Synchronous/
Asynchronous
Asynchronous
Synchronous
Synchronous-Fine
grained time
synchronization
Traffic
heterogeneity
requirements
Low duty cycle nodes
do not accommodate
aperiodic traffic. Very
hard to satisfy the
WBANs traffic
heterogeneity
requirements
Low duty cycle nodes
do not require
frequent
synchronization of
schedules. Hard to
satisfy the WBANs
traffic heterogeneity
requirements
Low duty cycle nodes
do not require
frequent
synchronization at the
beginning of each
superframe. Easy to
satisfy the WBANs
traffic heterogeneity
requirements
Sensitivity
Sensitive to tuning for
neighborhood size
and traffic rate
Sensitive to clock
drift
Very sensitive to
clock drift
Performance with
respect to traffic
rates
Poor performance
when traffic rates
changes
With the increase in
traffic, performance is
improved
Throughput and
number of active
nodes are limited
Cost incurred by
sender and receiver
Receiver and polling
efficiency is gained at
much greater cost of
senders
Similar cost incurred
by sender and
receiver
Require clustering
Extravagant
It doesn't listens for
full contention period
as a result it is less
expensive
Listening for full
contention period
Low duty cycle
Scalability and
adaptability
challenging to adapt
LPL directly to new
radios like IEEE
802.15.4
Scalable, adaptive,
and flexible
Limited scalability
and adaptability to
changes on number of
nodes
10
2.4 Energy Efficient MAC Protocols
In this section, we briefly discuss the energy efficient MAC protocols for WBAN.
2.4.1 Okundu MAC Protocol
An energy efficient MAC protocol for single hop WBANs is proposed by Okundu et al. in [4].
This protocol consists of three main processes: link establishment, wakeup service, and alarm
process. Basic energy saving mechanism of this protocol consists of central control of
wakeup/sleep time and Wakeup Fall back Time (WFT) processes. WFT mechanism is used to
avoid collision due to continuous time slot. This mechanism states that, if a slave node wants to
communicate with a MN and it fails in its task due to MN's other activities, then it goes back to
sleep mode for a specific time computed by WFT. However, data is continuously being buffered
during the sleep time.
To minimize time slot collision, the concept of WFT has been introduced. This concept
helps every slave node to maintain a guaranteed time slot even if it fails to communicate with the
MN. In this protocol, problems like idle listening and over-hearing can be reduced, because of
central management of traffic.
In one cluster, only 8 slave nodes can be connected to MN, which restricts inclusion of
other slave nodes. In link establishment, wakeup service, and alarm processes, communication is
initiated by the MN. Another main problem is that, only one slave node can join network at a
time.
2.4.2 MED MAC Protocol
N. F. Timmons et al. in [5] propose a TDMA-Based MAC protocol for WBANs called Med
MAC that consists of two schemes for the power saving: Adaptive Guard Band Algorithm
(AGBA) and Drift Adjustment Factor (DAF). AGBA along with time stamp is used for
synchronization among coordinator and other nodes. This synchronization is introduced using
Guard Band (GB) between time slots to allow the node to sleep for many beacon periods. DAF is
used to minimize bandwidth. GB is calculated by AGBA and shows the worst cases. However,
practically gaps may be different between time slots depending upon application scenarios. DAF
adjusts GB according to practical situation and avoids overlapping between consecutive slots.
MedMac outperforms IEEE 802.15.4 for Class 0 (lower data rate applications such as
11
health monitoring and fitness) and Class 1 (medium data rate medical applications such as EEG).
Energy waste due to collision is reduced by introducing Guaranteed Time Slot (GTS). Each
device has exclusive use of a channel for a fixed time slot, therefore, synchronization overhead is
also reduced.
This protocol works efficiently for low data rate applications, and work inefficiently for
high data rate applications. However, In-body and On-body applications of WBAN are usually
of higher data rate.
2.4.3 Low Duty Cycle MAC Protocol
Low Duty Cycle MAC protocol for WBANs is designed in [6]. In this protocol, analog to digital
conversion is performed by slave nodes while the other complex tasks such as digital signal
processing is carried out at MN. MNs are supposed to be less power than slave nodes.
This protocol introduces the concept of Guard Time (Tg) to avoid overlapping between
consecutive time slots. After T frames a Network Control (NC) packet is used for general
network information. Power saving is achieved by using effective TDMA strategy.
This protocol is energy efficient because it sends data in short bursts. By using TDMA
strategy, this protocol effectively overcomes the collision problem. This protocol allows
monitoring patient's condition and can reduce the work load on medical staff, while keeping
minimum power usage.
As, TDMA strategy is used, and it is found that TDMA is more suitable for static type of
networks with a limited number of sensors generating data at a fixed rate therefore, this protocol
may not respond well in a dynamic topology.
2.4.4 B-MAC Protocol
Body MAC (B-MAC) protocol achieves energy efficiency by using three bandwidth
management schemes: Burst, Periodic, and Adjust Bandwidth.
Burst bandwidth consists of temporary period of the bandwidth, which includes several
MAC frames and recycled by the gateway (coordinator). Bandwidth is reduced to half if it does
not fully utilized by the nodes, which is also informed about reduction of bandwidth. Periodic
bandwidth is a provision for a node to have access to the channel exclusively within a portion of
each MAC frame or few MAC frames. It is also allocated by the gateway based on node's QoS
12
requirements and current availability of the bandwidth [7]. Adjust bandwidth defines the amount
of bandwidth to be added to or reduced from previous Periodic Bandwidth [7].
Nodes can enter into sleep mode and wake up only when they have to receive and transmit any
data to the gateway, because the nodes and the gateway are synchronized in time. The time slot
allocation in Contention Free Period (CFP) is collision free, which improves packet transmission
and thus, saves energy.
The protocol uses CSMA/CA in the uplink frame of Contention Access Period (CAP)
period, which is not reliable scheme due to its unreliable CCA and collision issues.
2.4.5 Ta-MAC Protocol
Traffic aware MAC (Ta-MAC) utilizes traffic information to enable low-power communication.
It introduces two wakeup mechanisms: a traffic-based wakeup mechanism, and a wakeup radio
mechanism. The first mechanism accommodates normal traffic by exploiting traffic patterns of
nodes, and the second mechanism accommodates emergency and on-demand traffic by using a
wakeup radio signal.
In the traffic-based wakeup mechanism, the operation of each node is based on traffic
patterns. The initial traffic pattern is defined by the coordinator and can be changed later. The
traffic patterns of all nodes are organized into a table called traffic-based wakeup table. In
wakeup radio mechanism, a separate control channel is used to send a wakeup radio signal. The
coordinator and the member node send wakeup radio signal in on-demand and emergency case.
In Ta-MAC, a node wakes up whenever, it has a packet to send/receive. Since the traffic
patterns are pre-defined and known to the coordinator, it does not have to wait for resource
allocation information/beacon. As a result, delay is minimized, as compare to other MAC
protocols. This protocol accommodates normal, emergency, and on-demand traffic in a reliable
manner. To achieve energy efficiency in MAC protocol, the central coordination and resource
allocation is based upon the traffic patterns of the nodes.
As, in this protocol the traffic pattern are defined by the coordinator, in a static topology.
This protocol does not work efficient in dynamic topology, as in dynamic topology, traffic
patterns are changed frequently.
13
2.4.6 S-MAC Protocol
S-MAC [8] is proposed for WBASNs. S-MAC uses fixed duty cycles to solve idle listening
problem. Nodes wakeup after a specific time, as assigned by coordinator, sends data and goes
back to sleep again. As all the nodes are synchronized, therefore, collision can also be easily
avoided. This protocol gives considerably low latency. In this time synchronization overhead
may be prevented due to sleep schedules.
This protocol cannot support fluctuating traffics and no priority is given to the emergency
traffic scenarios. Therefore, it is not a reliable for WBANs. Overhearing and collision may occur
if the packet is not destined to the listening node.
2.4.7 T-Mac Protocol
Mihai et al. [9] suggested Timeout MAC (T-MAC) for WBASNs. It uses flexible duty cycles for
increasing energy efficiency. In T-MAC, the node wakes up after assigned time slot, send
pending messages and if there is no activation event for Time Interval (TA), the node goes back
to sleep mode again. If a node sends Route To Send (RTS) and does not receive Clear To Send
(CTS), then sends RTS two more times before going to sleep. To solve early sleep problem, it
uses future RTS for taking priority on full buffer.
In this protocol packets are sent in burst, as a result delay is minimized. It also
outperforms other MAC protocols under variable load. The main disadvantage in this protocol is that it
suffers from sleeping problems.
2.4.8 H-MAC Protocol
Heartbeat Driven MAC (H-MAC) uses heart beat rhythm information for synchronization of
nodes. This avoids the use of external clock and thus reducing the power consumption. Also
guaranteed time slot (GTS) provision to each node helps to avoid collision.
H-MAC aims to improve BSNs energy efficiency by exploiting heartbeat rhythm
information, instead of using periodic synchronization beacons to perform time synchronization
[3].
Although H-MAC protocol reduces extra energy cost of synchronization, however, it
does not support sporadic events. Since TDMA slots are dedicated and are not traffic adaptive,
H-MAC protocol encounters low spectral/bandwidth efficiency in case of low traffic. The
14
heartbeat rhythm information varies depending on patient's condition. It may not reveal valid
information for synchronization all the time [3].
Table 2.2 Qualitative Comparison of MAC Protocols
2.4.9 DTDMA Protocol
Reservation based dynamic TDMA (DTDMA) protocol uses slotted ALOHA in CAP field of the
super frame to reduce collisions and to enhance power efficiency.
Through the adaptive allocation of the slots in a DTDMA frame, WBAN's coordinator
adjusts the duty cycle adaptively with traffic load. Comparing with IEEE 802.15.4 MAC
protocol, DTDMA provides more dependability in terms of lower packet dropping rate and low
Protocols
Advantages
Disadvantages
Okundu MAC
Minimize time slot collision,
reduce idle listening and
overhearing
Only 8 slave nodes can be
communicated to the MN
MedMAC
Energy waste due to
collision is reduced
Don't support high data rate
applications
Low Duty Cycle
Collision problem is
reduced, allows monitoring
of patients
Not suitable for dynamic
type of networks
B-MAC
Improves packet
transmission hence saves
energy
Uses CSMA/CA in the
uplink frame of CAP period,
which is not a reliable
scheme
Ta-MAC
Accommodate normal,
emergency and on-demand
traffic, energy efficient,
reasonable delay
Not suitable for dynamic
topologies
S-MAC
Simplicity, high latency,
time synchronization
overhead may be prevented
due to sleep schedules
Low throughput, overhearing
and collision may cause if
packet is not destined to
listening node
T-MAC
Packets are sent in burst,
better delay, gives better
result under variable load
Suffers from sleeping
problems
H-MAC
Improves BSN's energy
efficiency, reduce extra
energy cost
Doesnot support sporadic
events, low
spectral/bandwidth
efficiency
DTDMA
Reduce packet dropping rate,
less energy consumption
Doesn't support emergency
and on-demand traffic
15
energy consumption especially for an end device of WBAN [3]. It does not support emergency
and on-demand traffic. Furthermore DTDMA protocol has several limitations when considered
for the Medical Implant Communication Service (MICS) band. The MICS band has ten sub-
channels and each sub-channel has 300 Kbps bandwidth. DTDMA protocol can operate on one
sub-channel, however, cannot operate on ten sub-channels simultaneously [3].
The main purpose of a MAC protocol is to provide energy efficiency, network stability,
and bandwidth utilization and reduce packet collision.
The energy minimization techniques and mechanism in MAC protocols are summarized
in Table 2.3
Protocol
Energy Efficiency Mechanism
Okundu MAC
Wake up fall back time (WFT)
MedMAC
TDMA, Adaptive Guard Band Algorithm (AGBA) And drift Adjustment
factor (DAF)
Low Duty Cycle
TDMA, concept of Guard Time (Tg)
B-MAC
TDMA, Bandwidth mechanism
Ta-MAC
Central coordination according to traffic patterns of the nodes
S-MAC
Scheduled Based, organized in slots and operation is Based on schedules
T-MAC
Have slots and operation is based on schedules
H-MAC
Heart Beat Rhythm information is used for synchronization
DTDMA
TDMA based, use of slotted aloha in CAP field
Table 2.3 Energy Minimization techniques and mechanisms
2.5 Performance Trade-offs made by MAC Protocols
In this section, we discuss the performance of the MAC protocols they achieve and price they
pay. Trade-offs, the MAC protocols have to make.
16
2.5.1 Okundu MAC Protocol
Network's scalability is mainly application dependent, e.g., ECG can support upto maximum of 8
slave nodes because of 8 percent duty cycle. However, in practice this is 6 to allow for possible
retransmissions, here, we have a trade-off, for retransmission, slave nodes attached to the MN are
reduced to attain scalability of network.
2.5.2 Med Mac Protocol
The low data rate applications of Class 0 medical devices include monitoring of respiration
system, temperature of human body, pulse monitoring etc. Power consumed by respiration
transceiver is slightly high in MedMAC protocol with respect to other protocols, while
temperature and pulse node show much lower power consumption compared to other protocols.
MedMAC trade-offs power consumption of respiration for less power of other two applications.
2.5.3 Low Duty Cycle MAC Protocol
The number of extra slots needed for protocol robustness is dependent on Packet Error Rate
(PER) and Packet Loss Ratio (PLR). When PER is high, it will increase PLR. However, PLR,
may be reduced by using extra slots in the time frame. Therefore, this protocol can trade-offs
extra slots for less PLR.
2.5.4 B-MAC Protocol
B-MAC trade-offs idle listening for a reduced time to transmit and reception of data. As, we
know that reducing duty cycle increases sleep time which in turn reduces idle listening. Another
trade-off is between idle listening and packet length, because this overhead dominates the energy
consumption.
2.5.5 Ta-MAC Protocol
Ta-MAC uses two wakeup mechanisms one for handling data traffic and other for emergency
traffic. By using these two mechanisms this protocol outperforms all other protocols in terms of
power consumption because problems like idle listening, collision, overhearing are reduced.
However, by sending frequent control messages to the nodes increases node's overhead, which is
a trade-off. The initial traffic patterns of all the nodes are defined by the coordinator, as a result
delay is also slightly increased.
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2.5.6 S-MAC Protocol
For transmission and reception of data in S-MAC, an extremely low duty cycle is used. When
throughput increases SAC's duty cycle also increases , which further increases the overhead of
SYNC period, as a result, power consumption is increase linearly. S-MAC can trade-offs
throughput for energy, also it can trade-offs energy for latency.
2.5.7 T-Mac Protocol
T-MAC uses adaptive duty cycle, implemented as a time out after the last event. At lower
transmission rates, throughput increases because probability of packet loss is much less than
received packet, however, the latency is increased between source and destination node.
2.5.8 H-MAC Protocol
In H-MAC a Guard Band is introduced in time slots to avoid collision by overlapping of data,
however, when time slots are completely aligned then there will be no data transmission in
Guard Band, therefore, it reduces bandwidth utilization. The coordinator of BSN then uses this
GB for synchronization, by sending re-synchronization control packets, hence achieving energy
efficiency. Thus making a trade-offs between energy efficiency and bandwidth utilization
efficiency.
2.5.9 DTDMA Protocol
DTDMA is a TDMA based protocol, which uses time slots for data transmission and as a result
less power is consumed, however, TDMA requires synchronization between nodes and the
coordinator, as a result overhead is increased. This overhead is a trade-off for energy.
The trade-offs of each of the protocol is summarized and given in Table 2.4
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Protocol
Trade-Offs
Okundu MAC
Trade-offs number of slave nodes attached to the MN are reduced for
scalability of network
MedMAC
Trade-offs idle listening for a reduced time to transmit and reception of
data
Low Duty Cycle
Can trade-off extra slots for less PLR
B-MAC
Trade-off is between idle listening and packet length
Ta-MAC
Trade-off delay for less power consumption
S-MAC
Can trade-off energy for latency
T-MAC
Can trade-off latency for high throughput
H-MAC
Trade-offs between energy efficiency and bandwidth utilization
efficiency
DTDMA
Trade-offs overhead for a less power consumption
Table 2. 4 TRADE-OFFs of MAC Protocols
2.6 MAC Frame Structure
MAC frame structure consists of control portion or control packet and data portion. Control
portion is responsible for the management and control messages (beacon period, request period,
topology management period) to control and manage dynamic topology and varying data rate
traffic. Data portion consist of two sub parts: Contention Access Period (CAP) and Contention
Free Period (CFP). CAP consists of CSMA/CA while the nodes contend in CAP transmit MAC
control packets. Similarly, small size data packets can also be transmitted in CAP.
In [7], the allocation of time slots is controlled by the coordinator. The coordinator
arrange the duration of control and data packet on the basis of current traffic of topology that is
why the slots allocated to CFP is collision free. In each frame, bandwidth allocation in CFP can
be changed.
In [5], the Guard Band is used to maintain synchronization among devices even if a node
is sleeping for many beacon periods.
In [4][10][11], MAC protocol use slotted ALOHA in its frame structure to divide a slot
into 4 equal mini slots.
In [6], the Guard Time (Tg) is introduced in its frame structure to reduce overlapping
19
between the two following nodes.
Fig 2.2 MAC Frame Structure
2.7 Technique for Collision Avoidance for Traffic Control
The main schemes of MAC protocol for WBANs are divided into two groups: contention based
i.e., (CSMA) and contention free i.e., (TDMA). Most of the traffic is interrelated in WBANs,
therefore, contention based solutions are not suitable for it. For example, if a patient is suffering
from fever, the body temperature increases which increases blood pressure, hence, the sensor
sensing temperature variation and the sensor that senses blood pressure variation, both become
active. Along with them other respiration sensors also become active at the same time and try to
access the channel/coordinator. However in this situation, collision occurs in CSMA. In TDMA
each node is communicated to the MN according to the assigned pattern by the coordinator. As a
result, collision in data traffic is low as compared to CSMA.
2.7.1 Okundu MAC Protocol
This protocol controls traffic using centrally controlled wakeup/sleep time. Slots are assigned to
20
sensors change every time when coordinator detects any change in traffic pattern. Assignment of
different time slots, decreases collision between the nodes. It makes the system to handle
fluctuating traffic. The sensor nodes established link with the coordinator after listening to the
Radio Frequency (RF)-channel for a fixed time period. MN sends request to the sensor node for
information by setting and communicating the next wakeup time after establishing the link.
2.7.2 MED MAC Protocol
MedMac reduces the collision by using the AGBA. AGBA allows the sensor nodes to sleep for
a GB time period between each time slot. Each node has specific time slot to communicate with
master node/coordinator, which means there is no collision and thus minimizes the
synchronization overhead.
2.7.3 B-MAC Protocol
B-MAC uses downlink and uplink schemes and sleeping mode for data traffic control. Downlink
is only used by MN therefore, traffic and data load on downlink is reduced. Uplink is divided
into CAP and CFP. MN allocates time slots to CFP according to data traffic which makes CFP
collision free. In case, when nodes have no data to transmit or receive then they go to sleeping
mode.
2.7.4 Ta-MAC Protocol
Ta-MAC protocol uses two channel access mechanisms for traffic control i.e., traffic based
wakeup mechanism for normal traffic, and wakeup radio mechanism for on demand and
emergency traffic. In traffic-based wakeup mechanism, all nodes have traffic pattern that is
assigned by the coordinator. The initial patterns are defined and updated by the coordinator. The
traffic patterns of all nodes are synchronized and arranged in a specific table, called Traffic
Based Wakeup Table . Node's ID and its respective traffic patterns are stored in this table.
Normally, all the nodes become active/wakeup according to their traffic patterns. If two or more
nodes have same wakeup pattern, then the node with high priority is treated first by the
coordinator, as shown in Fig 2.3 By assigning these patterns, load at the coordinator is
minimized, and it also reduces the chances of collision.
2.7.5 Low Duty Cycle
21
Low duty cycle MAC protocol is based on Time Division Multiple Access (TDMA). In TDMA,
time slots are assigned to the sensor nodes by the coordinator. To avoid collision between the
data traffic Tg is introduced. Use of Tg between every consecutive slots prevents the
transmission overlaps and controls data traffic.
2.7.6 IEEE 802.15.4 MAC
The basic requirement of QoS is to minimize delay and maximize the probability of successful
transmission. CFP scheme is used to control data traffic to guarantee the QoS. If a node wants to
send data, first it listens for the network beacon. After node finds the beacon that is sent by the
coordinator, the node synchronizes to the super frame structure. IEEE 802.15.4 supports up to
250 Kbps data rate with possible coverage of 10 meters. This data rate is not enough to support
the required rates of WBANs that is up to 10 Mbps. According to IEEE 802.15.4, packets are
transmitted in the contention period, which may result longer delays in real time critical
applications. When traffic is increased, the nodes compete for the contention based slots,
resulting in long delays and the actual size of the network is almost doubled [7]. In [10], to
satisfy the requirements of WBANs including QoS, network scalability, support for multiple
PHY's and multiple application traffics, IEEE 802.15.4 MAC is proposed, which is the modified
version of original IEEE 802.15.4. QoS means to decrease the packet latency and increase the
probability of successful transmission of data packets without collision and loss of data. In
original IEEE 802.15.4, GTS mechanism is provide to support the emergency data. GTS is very
effective for data transfer however, inherently the limit of GTS in a super frame is seven. As a
result, it cannot support more than seven devices simultaneously in CFP. Whereas, in IEEE
802.15.4 MAC the coordinator may allocate more than seven GTS simultaneously to the sensor
devices.
IEEE 802.15.4 MAC and 802.15.4 original is compared briefly in Table 2.5
22
IEEE 802.15.4 MAC
Original IEEE 802.15.4
Low Power consumption
High power consumption
Higher Data rate
Low Data rate
Higher flexibility
Low flexibility
TDMA based
Contention based
Collision Free
Greater Collisions
Sleep Mode
Idle listening
Table 2.5 Comparison between IEEE 802.15.4 MAC and Original
Fig 2.3 Data Traffic Control
23
2.8 Path Loss Model for WBANs
Energy efficiency of MAC protocols with respect to path loss is discussed in this section.
Path loss in WBANs is also of great importance. It is due to reflection, diffraction, scattering and
shadowing of signals from, tissues and organs located inside the human body and objects in the
surroundings. Large scale fading of signal is also known as path loss. Reduction in path loss will
ensure energy efficiency of the protocols. Channel modeling plays an important role for the
optimization of communication system for WBANs. In [12], path loss is defined as:
(1)
Where,
(2)
whereas, d is the distance between transmitting and receiving node, and is variable. On the other
hand, do denotes the reference distance between a transmitting and receiving node, which is
fixed. WBANs can be classified into three different scenarios, and these scenarios works well
when the path loss is minimum. These three scenarios are discussed below:
2.8.1 MAC for In-Body Communication
Developing a power-efficient MAC for In-body sensor networks is the most challenging task. In-
body sensor nodes are implanted under human skin, where the signal propagation is affected by
the electrical properties of the body that varies from person to person. Human body poses many
wireless transmission challenges. Several components in a human body i.e., thickness of tissues,
their conductivity, permeability etc, differ in each human body. The main purpose of the In-body
sensor nodes is to monitor In-body parameters to communicate with the other implanted devices,
such as pacemakers etc [13].
Development process of a power-efficient MAC protocol for In-body sensor networks is
affected by the diverse nature of In-body nodes and with electrical properties of human body.
Pacemaker and Capsular endoscope are two of the examples of In-body networks and their data
rate varies from few Kbps to several Mbps. In-body sensor network requires, critical traffic, low
24
latency and high reliability [13]. The use of CSMA/CA does not provide reliable solution in this
scenario due to high path loss inside the human body [14]. In In-body sensor networks heating
effect caused by electromagnetic wave should also be considered. Nagamine et al. [15] discussed
the thermal influence of the BAN nodes using different MAC protocols.
Figure 2.4 shows path loss between two implanted sensor nodes for varying distance and
frequencies. It shows that path loss increases with increasing distance between sensor nodes. In
In-body communication, path loss is maximum, as compared to the scenarios discussed below
because human body is composed of tissues and organs, which increase the path loss.
Simulations are performed by taking different frequencies ranging from 800MHz to 2800MHz.
The simulation results show that path loss between the two sensor nodes increases when
frequency is increased.
Fig 2.4 IN-BODY Communication
25
2.8.2 MAC for On-Body Communication
On-body sensor networks comprise of miniaturized and non-invasive sensor nodes that are used
for various applications, ranging from medical to interactive gaming and entertainment
applications. Wireless Medical Telemetry Services (WMTS), unlicensed industrial, scientific and
medical (ISM), and Ultra-wide band (UWB) are some of the bands used for data transmission in
On-body networks. WMTS is a licensed band designated for medical telemetry system. Due to
fewer interfering sources Federal Communication Commission (FCC) urges the use of WMTS
for medical applications. However, only authorized users such as physicians and trained
technicians are eligible to use this band. Furthermore, restricted WMTS (14 MHz) band cannot
support video and voice transmission. The alternative band for medical applications is 2.4 GHz.
The band includes guard bands to protect adjacent channel interference [13].
The design and implementation of a power-efficient MAC protocol for On-body sensor
networks have been an emerging research topic for the last few years. H-MAC, a novel TDMA
protocol for On-body sensor network exploits bio signal features to perform TDMA
synchronization and improves energy efficiency [16].
Figure 2.5 shows the simulation results of ``On body communication", for varying
distances and frequencies. Sensors are placed on human body. Simulation result depicts that
increase in distance between two nodes consequently increase path loss. Path loss is less in On-
body communication as compared to In-body communication because On-body sensors
communicate in air and path loss is minimum in air medium. It is also observed in Fig 2.5 that
by increasing frequency, path loss is also increased.
26
Fig 2.5 ON-BODY Communication
2.8.3 MAC for Off-Body Communication
This model describes variations of the channel with respect to the following three aspects:
distance between body and access points (receiver) is denoted by , body orientation angel
and transmitter based azimuth angel .
Human body is taken as a cylinder of average size. The model uses two types of the co-
ordinates i.e., Principle Coordinates (PCS) which includes the cartesian coordinates, and BCS
which are the body cartesian coordinates.
Path loss is defined in [17] as:
(3)
27
where, In the equation above, denotes the first breakpoint angle, observed in the lit region of
the transmitter. Similarly, , denotes the shadow-region breakpoint angle which is observed in
the shadow region of the transmitter.
, is the azimuth decay coefficient.
represents the channel loss,
and = refrence path loss.
The path loss along the coordinate is given as:
(4)
Due to the symmetry of the human body with respect to z-axis
(5)
If the transmitter is in the Line of Sight LoS with the sensor at the body then the path loss is low;
with the movement of body the rotation angel varies and path loss increases.
Figure 2.6 shows the relationship between the distance, path loss and frequency in Off-
body communication. We consider that the sensors are LoS. Path loss is low when the sensors are
LoS and high when the sensors are NLoS. In general, path loss is minimum when compared with
the scenarios discussed above.
28
Fig 2.6 OFF-BODY Communication
Chapter 3
Ubiquitous Healthcare in Wireless Body Area Sensor
Network - A Survey
3.1 Introduction
With an increasing population around the world, specially the elderly people who are more
fragile to health diseases, require a comprehensive healthcare system. A system fulfilling needs
of elderly people provides them with proper healthcare facilities wherever, they move around.
Wireless Body Area Network (WBAN) [18] is gaining attention worldwide for providing
healthcare infrastructure. This system consists of several devices including tiny sensors which
are placed in or around the body in close proximity to monitor a patient. As a result, elderly
people are monitored everywhere and treated well intime in case of any emergency. The patients
specially elderly people face problems in moving around and cannot frequently visit doctor(s),
indeed require Ubiquitous HealthCare (UHC) [19]. Besides having applications in healthcare.
WBAN is also used in entertainment, gaming, military etc.
WBAN comprises of tiny sensors that monitor patients everywhere and reduce the number of
visits to doctors. The sensors may be placed on or implanted in the body for constant monitoring.
Different standards for WABN are defined which provide efficient means of data transfer and
communication (such as, Bluetooth, ZigBee, MICS and UWB) [20]. Similarly, different devices
are used to collect patients' vital signs information and transfer it to remote healthcare personnel.
These devices include wearable watch, oximeter, wearable shirt, chest belt type etc. A
comprehensive and analytical survey is provided about these standards and devices in this thesis.
Since each architecture has its own applications, therefore, different architectures of WBAN are
discussed. Several types of antennas are designed for BAN (i.e., electrical or dipole antenna and
magnetic or loop antenna). Depending on the scenario of body communication (i.e., In-Body or
On-Body), selection of antenna is very important, therefore, it has a direct effect on
communication resulting in path loss.
29
Data collected by the sensors and devices are transferred through wireless medium to remote
destination, path loss is probable to occur. Path loss for In-Body and On-Body communications
are different. It depends on frequency of operations as well as distance between transmitter and
receiver. A simple path loss model for WBAN is proposed in [21].
In this thesis, a comprehensive and analytical survey is provided about the standards and
devices discussed above. A detailed overview of UHC architectures in WBAN is provided. Also,
we simulate In-Body path loss models proposed in [22] using MATLAB. In simulations, we
considered four path loss models; deep tissue implant to implant, near surface implant to implant,
deep implant to implant, and near surface implant to implant. Further, we performed simulations
on different parameters effecting (i.e., attenuation, phase distortion, RMS delay etc)
communication in On-Body networks [23].
Rest of the thesis is organized as follows: Section 2 discusses related work in WBAN specially
in UHC. Section 3 describes the standards used in WBANs and the standard best suited for
different architectures in UHC. Wearable devices used in ubiquitous health care are briefly
discussed in Section 4. Section 5 surveys the general architectures of WBAN used for UHC in
detail. In Section 6 path loss in WBAN and its effect in degradation of performance in
communication are discussed in detail. Section 7 describes different scenarios of path loss in
WBAN. Section 8 discusses the channel model and evaluation of different M-ary modulation
schemes along with MATLAB simulation.
In next section, we briefly discuss the related work and motivation that paved way for this
survey.
3.2 Related Work
A multi-tier system architecture for UHC monitoring is proposed in [18]. This system
provides remote supervision of chronic patient's disease like heart attack. To achieve this
purpose a multi-tier system is adopted. Emergency medical services are accessed simultaneously
using this system architecture.
Signal monitoring and health consulting UHC system are presented by authors in [19]. They
introduce different systems for various types of applications, for example, device provider,
system provides installation program to the mobile system etc.
30
Authors in [20], describe an integrated system architecture for UHC system. This architecture
is based on hierarchical networks. The authors introduce a sensor platform that integrates
different corresponding sensors on a single chip or a single platform with respect to relative
application. This makes an efficient use of resources in WBAN. Different application dependent
standards are also discussed in this work.
Path loss models for medical implant and communication channels are presented by authors
in [21]. They investigated statistical path loss model in Medical Implant Communication Service
(MICS) channels. A work relating to path loss for On-body communications is provided by
authors in [24].
Another architecture based on traffic is discussed in [25]. In this architecture, network
coordinator contains a wake up circuit to accommodate life critical events depending on traffic
application. Authors also discussed other applications besides UHC.
In [26], sensor devices and server based architecture for UHC monitoring system is proposed.
Introduction of wireless sensor devices and server part is given in this architecture.
Communication between sensor and server is done via Base Station Transceiver (BST), which is
connected to a server PC.
To provide services for the elderly people, components based system architecture of UHC
monitoring is designed in [27]. A prototype system that monitors location and health status using
Bluetooth as WBAN and smart phone with accelerometer as Intelligent Central Node (ICN) is
used in this architecture. This architecture provides accessibility to family members or medical
authorities to identify real time position and health status of patients via internet. ZigBee is used
for small data rate applications because it consumes less power then Bluetooth.
WBAN architectures using wearable devices are proposed in [28]. One of them is wearable
smart shirt, which is based on UHC and activity monitoring. This architecture comprises of smart
shirt with multi-hop sensor network and server PC. Communication between smart shirt and
server PC is done by BST. A device with two PCB mounted on each other is used in this
architecture to reduce the size of integrated wearable sensor node along with Universal Serial
Bus (USB) programming board as a separate module. It is needed only when nodes are
connected to server PC.
31
Recently, a lot of work is going on in the field of health care and telemedicine. Wireless Body
Sensors are being introduced, which provide efficient uses of resources. A survey on UHC is
carried out in this thesis. Sensors used in WBAN are lightweight, small in size, provide ultra-low
power and are used for intelligent monitoring. These sensors continuously monitor human vital
signs, physical activities and actions. Different architectures of WBAN are discussed and
analytically compared in detail along with the effects of path loss specially in UHC. There is
increasing demand of UHC systems, which consume less power and provide longer battery
lifetime. The system completely fulfills this demand. High data rate communication can be
achieved by making body sensors compatible with underlying technologies. Different system
architectures discussed below provide continuous monitoring, in-time medical support, reduced
mobility, low power consumption and high data rate for communication, for patients.
3.3 Most Frequently Used Standards for WBAN Communication
There is a number of standards that are adopted for communication in WBAN. Microscopic
chips, which are typically used in wearable devices, depends on these standards. We briefly
discuss the standards, Bluetooth, ZigBee, MICS, and Ultra Wide Band (UWB) IEEE (802.15.6)
[20].
3.3.1 IEEE 802.15.1 Bluetooth
Bluetooth is a short range communication standard with data rate of 3 Mbps and range of about
10m. It is adopted in UHC due to high bandwidth and low latency. It also supports many mobile
platforms. However, in UHC monitoring application, use of this standard is avoided because of
high power consumption. It is suitable for latency and bandwidth sensitive scenarios [20].
3.3.2 ZigBee
ZigBee standard is the most commonly used standard. It has the capability to handle complex
communication in low power communication devices (such as, nodes) with collision avoidance
schemes. It consumes less power (nearly 60 mW) and provides low data rate (250 kbps).
Hardware support with encryption is featured by many ZigBee controllers to provide effective
protection for communication in WBAN [20].
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3.3.3 Medical Implant Communications Service (MICS)
This band is specially designed for communication in WBAN. It is a short distance standard and
is used to gather signals from different sensors on the body in a multi-hop structure. As
compared to UWB, MICS has very low power radiation and thus is most suitable for the sensors
used in UHC monitoring system [20].
3.3.4 IEEE 802.15.6 Ultra Wide Band (UWB)
It provides very high bandwidth and data rate for communication. It is used for localization of
transmitters. When very high bandwidth is required in any application, UWB is the best choice.
For example, when emergency or critical situation occurs, UWB with GPS (global positioning
system) provides the best, short and traffic free route to the medical centre without any
interference. User localization is usually important in hospitals or whenever a emergency
situation takes place. The advantage of UWB is that it is the only reliable method of localization.
The drawback is receiver's complexity because of which it is not suitable for wearable
applications in health monitoring [20].
3.4 Wearable Sensors used for Ubiquitous HealthCare
In this section, we briefly discuss wearable sensors used for UHC. Several types of tiny sensors
are used in WBANs, which are attached to the body of a person to measure vital signs such as
glucose level, Electrocardiogram (ECG), Electroencephalograph (EEG), detection of cancer cells
etc., and surrounding parameters like temperature, atmospheric pressure, humidity etc. The size,
shape and material of these sensors are of great importance. Moreover, these sensors must be
compatible with the human body and their precise placement on the body, since these sensors are
very sensitive and can harm human body. Therefore, these sensors are designed to be easily
weared and provide comfort to patients. The sensors discussed in this section are normal clothing
elements for the patients. Following is the discussion of sensors such as: wrist watch like an
eWatch, wrist oximeter, chest belt, shoulder, necklace and wearable shirt type like a smart
shirt/life shirt.
33
3.4.1 Wrist watch (eWatch)
This device is just like a wrist watch, with a wrist body and a band attached to it. The major
components of this device are: two Polydimethylsiloxane (PDMS) electrodes for ECG, a ribbon
type temperature sensor, reflective flat type Pulse Oximeter Oxygen Saturation (SpO2) sensor,
three printed circuit boards for analog and digital circuitry and other additional sensors. The
device has a size of 60 x 65 x 15mm and weighs about 160g including one lithium polymer
battery. Simple software is developed for this device to facilitate the users which are mostly
elderly people. It consumes very low power since it is designed to be of very small in size.
3.4.2 Oximeter
Wrist pulse oximeter is a device that continuously monitors the patients pulse. It is small in size,
light in weight, attached to the wrist and the sensor is placed on the finger. It is attached on the
wrist using probe, which contains Light Emitting Diodes (LEDs) and photodiode to measure
SpO2 Value. The wrist oximeter performs data acquisition 5 bytes/sec containing
Photoplethysmographic (PPG) data sampled at 75 Hz and SpO2 values [26].
3.4.3 Chest belt
A wearable chest sensor belt has following essential components: main body of the belt,
conductive fabric electrodes, an ECG sensor, an accelerometer sensor, double-layer PCB board,
a wearable USN node, here, a conductive fabric electrode is used for obtaining ECG signals from
the body.
3.4.4 Wearable shirt type (smart shirt/life shirt)
Wearable shirt; also known as smart shirt/life shirt is another useful device for measuring the
physiological parameters as well as physical activities help to improve patient's diagnosis. It is
such a comfortable device that a patient does not feel the presence of any sensor or other
components in the shirt. The device ensures a wide range of mobility.
The wearable sensor nodes are designed to be tiny in size due to which they have limited
power and computing capability. The designed wearable sensor node for
UHC features an ultra low power Texas Instrument MSP430 micro-controller with 10KB RAM,
48KB flash memory and 12-bit A/D converter [28].
34
3.5 General WBAN Architectures
3.5.1 Multi Tier Architecture of a WBAN for UHC
WBAN is used in health monitoring. The system discussed in [18], consists of three tiers as
shown in Fig. 3.1:
1) TIER1 Wireless Body Area Sensor Networks (WBASN)
2) TIER2 Personal Server (PS)
3) TIER3 Medical Server (MS)
Fig. 3.1 Ubiquitous Health Care Architecture
35
Tier1 WBASN
It is the most predominant part of telemedical system and comprises of many intelligent nodes.
Each node has the ability to sense, sample, process and communicate different physiological
signals. For example, heart activity is monitored by ECG sensor, muscle activity is monitored by
EMG sensor, brain electrical activity is monitored by EEG sensor, blood pressure is monitored
by a blood pressure sensor, while differentiation of the user's status and estimation of his/her
activity level is done by motion sensors.
PS sends initialization commands to each sensor and it also responds to queries from
server. WBAN nodes must be able to meet the requirements for low power consumption.
Because this ability is necessary to enable prolong continuous monitoring, small size, minimum
weight, consistent integration into a WBAN, standards based interface protocols, and calibration
and customization specific to patients.
Sensor nodes can be patched to the clothes or shoes to constantly collect the information,
store them locally and eventually transmit the
information to the PS after necessary processing. In case of an indication of emergency or critical
situation during process of local analysis of the data, the PS can send request for transmission of
raw signals to MS.
In short, each sensor node gets initialization command from PS and also responds to its
queries. In order to ensure confidentiality of patient's information, data transfer at all tiers in
UHC system must be encrypted. However, in critical cases, PS can also get direct connection
with emergency medical services if the user desires to use this service.
TIER2 Personal Server (PS)
PS takes the information from sensor nodes about health status and transfer it to MS through
WLAN or any internet service. A PS can be a laptop computer, handheld pocket PC or home PC,
PDA, cell phone etc. WBAN nodes are interfaced by a PS through a network coordinator, which
is responsible for implementing ZigBee (802.15.4) or Bluetooth connectivity. PS is ideal for
elderly patients, who are home-bound, and it is used for communicating with MS. Interface of PS
to WBAN comprises of a network configuration and management that includes various tasks.
These tasks are node registration (e.g., type and number of sensors), initialization (e.g., state
sampling frequency and operation mode), customization (e.g., run user specific adjustment), and
36
setup of a secure communication (key exchange). On successful configuration of WBAN,
network is handled by PS, which also ensures time synchronization, channel sharing, data
recovery, data processing, and coalition of the data.
PS is also responsible for patient's authentication information. To interface with the MS,
PS is configured with MS IP address. In order to ensure complete user mobility with secure and
near real time health information provisioning, PS functions in such a manner that the
information/data of the patient is transmitted when the link with MS is available. However,
when the link is not available, PS stores the data locally and transmits it after the availability of
communication channel or link.
TIER3 Medical Server (MS)
The telemedical system is spread over a network that comprises individual monitoring systems
connected to the MS tier through internet. MS is optimally used for provision of services to a
large number of individual users. Besides, this tier also provides service to a complex network
comprising of interconnected services, medical personnel and healthcare professionals.
Keeping the records of registered users, variations in their health status and informs
medical caregivers in case of emergency are main functions of MS. MS also forwards the
instructions from physician regarding prescribed medicines and exercises to the users. The
physician in this way can monitor patient's vital information while sitting in his/her office
through internet and ensures that patient follows the prescribed medicine and exercise.
A Server Agent
Uploaded data and patient's record are examined by a server agent. It also creates an alarm in
case of any critical medical situation. Large amount of data deposited through these services can
be used for creating awareness among the patients.
3.5.2 System Based Architecture with Physiological Signal Devices of UHC
UHC system architecture is shown in Fig 3.1. This architecture consists of: Physiological Signal
Devices (PSDs), a mobile system, a device provider system, a healthcare service provider
system, a physician system, a healthcare personal system.
37
Physiological Signal Devices (PSDs)
Physiological signals of the patient are measured by PSDs and then the data is transferred to
mobile system using ZigBee (802.15.4).
Mobile System
It can display the physiological data from PSDs and with the help of Wireless Local Area
Network (WLAN) or Code Division Multiple Access (CDMA) transfers the data to health care
service provider system.
Device Provider System
Device installation data is supplied to the mobile system with the help of this system.
Healthcare Service Provider System
This system is like a portal where all the tasks regarding health care are performed.
Physician System
The patient's history is stored in health care service provider system which can be analyzed and
scrutinized by the physician in this system.
Healthcare Personal System
The patient can examine and observe his own vital parameters and physiological signals after
making a log in this system.
3.5.3 Integrated System Architecture of UHC Monitoring Systems
The most efficient architecture of UHC monitoring system is shown in Fig. 3.1, which consists
of a hierarchical network of WBAN. The number of sensor platforms in WBAN, their
packaging, size, shape, material and placement on human body depends on precise arrangement
of integrated sensor nodes. The number of sensor nodes/platforms also depends on type of
application being used in WBAN. The sensor platform consists of accelerometer and gyroscope
sensors which are small in size. Accelerometer sensors are used to measure patient's position,
posture and movement while gyroscope measures the level of patient's activity (e.g. running fast,
38
walking, slow walking). WBAN consists of tiny sensors which are either planted or attached to
the human body as patches or knitted to the clothes or implanted below the skin. The system
architecture contains several networks that are hierarchically organized. This system is discussed
as follows:
Sensor Area Network (SAN)
Several sensors, are combined into a single sensor platform, , with the help of wired or
wireless interface. The sensors can be wired to or located on the sensor platform itself.
Sensor platform can be seen in Fig 3.1. Combination of two sensors: accelerometer and
gyroscope is used by motion sensor platform for wearable monitoring applications.
Body Area Network (BAN)
BAN integrates sensor platforms into a single monitoring system that is controlled by the PS
whereas, PS acts as a network coordinator that gathers the information from all sensors and
transfers it to BST. The communication in BAN can be wired in clothing or implemented using
short range wireless communication standards such as Bluetooth, Zigbee, UWB and MICS [20]
etc. Integrating information from individual sensors to users by home/gateways is used in
WBAN systems to assist the medical staff in the hospitals.
Wide Area Network (WAN)
Integration of multiple monitoring systems into a mobile health system through a cellular
network is done by WAN. For connectivity, ubiquitous monitoring systems count on WAN using
LANs such as WiFi when accessible.
Server Cloud
Multiple servers such as MS containing patient's medical history, information servers, social
network servers and other servers are incorporated by server cloud.
3.5.4 Traffic Based Architecture of WBAN for UHC Monitoring
WBAN traffic based architecture for ubiquitous health monitoring is classified into three types:
(1) On-Demand Traffic
(2) Emergency Traffic
39
(3) Normal Traffic
On-Demand Traffic
This type of traffic is activated by the doctor or the physician whenever, he/she needs any
information for diagnostic recommendation. This traffic is further divided into following forms:
Continuous
In case of surgical events, On-Demand traffic is continuous.
Discontinuous
When casual information is required, On-Demand traffic is discontinuous.
Emergency Traffic
When WBAN nodes get beyond a predefined threshold, emergency traffic is initiated. Priority
must be given to this type of traffic and it must be accommodated in less than one second. This
type of traffic is not generated at regular intervals and is initiated in case of emergency or critical
health condition.
Normal Traffic
As its name signifies, this kind of traffic takes place during normal condition when there is no
emergency, time criticality and any event warranting on-demand traffic. It is basically a routine
traffic that contains unnoticeable and routine health monitoring information about the patient and
treatment of many diseases like gastrointestinal tract, cancer detection, neurological disorders
and heart disease in normal time. This kind of normal data is collected and processed by the
Network Coordinator (NC) which is activated by a radio circuit as per the application
requirements. The wake up radio circuit has the ability to quickly respond to life critical events.
With the objective of obtaining concerned recommendations, the NC is also linked to
telemedicine and MSs. In short, a well integrated WBAN in the health care system caters for not
only avoiding the occurrence of myocardial infarction and other life threatening diseases.
Besides, it is also very handy for non-medical applications i.e gaming and entertaining
applications etc.
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3.5.5 Components Based System Architecture
This architecture of UHC, as shown in Fig 2.1, comprises of three components which are
discussed as under:
Wearable Wireless Body Area Network (WWBAN)
In this type of network, sensor(s) are attached to patient's body which collect and transmit
suitable data to ICN through Bluetooth communication protocol. Adoption of centralized
architecture of star topology in WWBAN contributes to concentration of system intelligence on a
central node, as shown in Fig 3.1.
Intelligent Central Node (ICN)
ICN is nothing but a smart phone with operating system that remains in communication with ICS
through GPRS technology. The ICN is connected to the ICS and it performs the function of
collecting and processing the data generated from different nodes in WWBAN. The collected
data includes information pertaining to location area identifications such as: Mobile Country
Code (MCC), Mobile Network Code (MNC), Location Area Code (LAC) and Cell Identification
(CI) from GSM network. This information helps in determining the location of elderly patients
with the help ICS. In the event of a change in collected parameters/information, ICN uses a
comparison algorithm to decide whether to send information to ICS prior to transmit data from
the sensors.
Smartphone, as a central node and body gateway are one of the best options for meeting
the requirement of constantly increasing processing power. The use of smart phone as ICN has
an inherent advantage of determining the mobility and location through GPS because of its
integration with the sensors. In this way, an integrated wearable mobility system is formed. For
example, accelerometer helps in determining the mobility and GPS provides the exact position.
Intelligent Central Server (ICS)
The purpose of ICS is to receive sensor data from all ICNs. The data received by the the server is
stored in database such that it can be analyzed independently without any human intervention.
The latest uploaded patient's information on the server is compared with the already existing
41
information to cure disease. In addition, it also provides information regarding
prescription/recommendation of doctor or health care professional.
ICS uses logistic regression technique to store data and anticipate any health risk from
information of patient's mobility, location and bio-signal sensor data. Web application helps to
inform family members or medical staff with a real time health status and position after
processing patient's data. During critical situation, an alert signal is transmitted to medical
professional for provision of patient's current health status and his current location in case of an
emergency.
3.5.6 Wearable Smart Shirt Based Architecture for UHC Wearable Smart
Shirt System
As shown in Fig 3.1, this system contains a shirt with integrated WSNs, a BST and a server PC
for distant monitoring. Then Smart Shirt is used to provide the individual physiological data.
Then this data is transmitted in ad-hoc wireless communication for further processing using a
wireless link. This is possible because Smart Shirt is compatible with wireless sensor network.
Wearable Sensor Node
The functionality of wearable sensor nodes is to get physiological data from human body and
forward it to base station which in turn transmits data through a wireless link to the medical staff.
A sensor node is small in size and consumes less power. It has low computation and
communication capability and provides longer battery life for WBAN. To minimize the size of
the sensor nodes, a board known as Universal Serial Bus (USB) programming board is designed
as a separate module. This module is required only when the nodes are connected to a server PC
for downloading an application or when the node itself acts as a BST for data transmission.
Sensor Board
The wearable sensor devices board which connects different sensors for various applications, is
small in size, consumes low power and provide longer battery life. In ECG, different electrodes
are attached to the human body which measure heart's electrical signal and record potential
difference between them. Three axis- accelerometer is a device which is used to monitor patient's
42
behavior and physical activity in daily life. This is done with the help of fall detection system.
Accelerometer signals can be collected by fall detection system and system determines that
whether person has fallen or not.
Architecture of Wearable Smart Shirt with Integrated Sensors
The measured accelerometer data and ECG data is transmitted to PS in WSN. ECG is used to
monitor the heart status of the patient while accelerometer is used to measure his physical
activity. If both signals and data can be measured simultaneously, the determination of patient's
disease can be improved.
To reduce the size of the wearable sensor node, a structure of two PCB stories is used,
which comprises a wireless sensor node plate for communication with Wireless Sensor Network
(WSN) and a sensor board plate with ECG interface and accelerometer. In this architecture, to
reduce the size of sensor node, two round shape PCB boards are combined. In which, one PCB
board is wireless sensor node and second is sensor board.
Wireless sensor node is placed at the top position of two PCB stories, and the sensor
board with ECG interface and accelerometer is placed at bottom position. This two stories
structure of the sensor node reduces the wideness of a normal single story structure with sensors
and provides convenience to wear it with two AAA size batteries. Two conductive electrodes
knitted to smart shirt are extended from interface circuit of sensor board to get physiological
ECG data from Smart Shirt.
Summary of Architectures of WBAN is given in Table 3.1.
3.6 Path Loss in WBAN
WBAN is greatly influenced by the amount of path loss that occurs due to different impairments.
Devices for WBAN are generally placed inside or on the body surface, so, losses between these
devices would affect the communication and can degrade the performance monitoring in UHC.
In the following sections, we study in detail about the WBAN communication and path loss that
occurs in it and how it affects the performance of UHC.
Reduction in power density of an electromagnetic wave introduces path loss [21] [27].
Path loss is mainly caused by free space impairments of propagating signal like refraction,
43
attenuation, absorption and reflection etc. It also depends on the distance between transmitter and
receiver antennas, the height and location of the antennas, propagation medium such as moist or
dry air etc, and environment around the antennas like rural and urban etc [29]. Path loss for
WBAN is different from traditional wireless communication because it depends on both distance
and frequency. Frequency is catered because body tissues are greatly affected by the frequency
Table 3. 1 Summary of Architecture of WBAN
44
on which sensor device is working.
The path loss model in between the transmitting and the receiving antennas as a function of
the distance is computed by [24] [30] as:
(1)
where, is the path loss at a reference distance , is the path loss exponent, and is the
standard deviation.
Path loss in WBAN is of great importance. UHC in WBAN works well when the path
loss between the transmitter and receiver is at its minimum. Path loss in WBAN occurs due to
many factors such as reflection, diffraction and refraction etc, from the body parts which may
distort the signal and can cause interference at receiver located at a distant location. So, data may
face distortion due to path loss which causes difficulty for medical team located at far distance to
correctly retrieve data. Path loss in UHC will decrease the efficiency of monitoring different vital
signs in human body at patient's level as well as at medical team's level. The main focus of this
section is to minimize the path loss that occurs at different stages in WBAN. This increases the
efficiency of UHC monitoring in BAN which is our main goal. Path loss dependence on distance
as well as frequency is given in Eq. (2) and Eq. (3):
(2)
where, is the path loss in decibels, denotes wavelength and specifies distance between
transmitter and receiver [29].
As we know that:
; So, the above Eq., can be rewritten as under:
(3)
3.6.1 Wireless Body Area Network (WBAN)
In development of WBAN, one of the consideration is the characterization of the electromagnetic
wave propagation from devices embedded inside the human body or close to it. A simple path
loss model for WBAN is difficult to be driven in view of complex nature of human tissue
structure and body shape. Since the antennas for the WBAN application need to be placed on or
inside the body, channel model has to take into consideration the effect of the body on radio
45
propagation. To calculate the path loss in WBAN, three types of nodes are defined as under:
a) Implant Node
This type of node is embedded inside the body either below the skin or deeper.
b) Body Surface node
This type of node is placed on the surface of human skin or maximum 2cm away.
c) External node
This type of node is kept away from the body by a few centimeters upto a maximum of 5
meters.
For body surface communication, it is also important to consider the distance between the
transmitter and receiver around the body. If these are not placed on the same side in a straight
line, then it allows the creeping wave diffraction to be also taken into account. As mentioned
earlier, for external node communication, the distance between transmitter and receiver from the
body vicinity is normally 3 meters away. However, in some cases, the maximum range of
medical device can go upto 5 meters [31].
Effect of WBAN Antennas
In case of antennas placed on the surface or inside the body, it is influenced by its surroundings.
It is therefore essential to understand the changes in the antenna patterns and other characteristics
must also be taken into account in the scenarios requiring propagation measurements It is
noticeable that the form factor of antenna is dependent on the requirements of applications.
Different types of antennas are suitable for different applications e.g., for MICS, a circular
antenna is used for pacemaker implant, while a helix antenna is most appropriate for a heart stent
or urinary implant. Antenna's performance is greatly influenced by the form factor, which in turn
effects the overall system performance. Antennas which take into account the characteristics of
human body (such as, change in body tissues etc), are designed for measurements of channel
model [32]. Antennas used in WBAN communication are categorized into following two types
[33]:
Electrical antennas, such as dipole
Electrical antennas are generally used for On-Body communications. They are avoided for In-
Body communications because electrical activity of these antennas is harmful for tissues and
muscles of body. On-Body communications, through these antennas do not make any direct
46
contact with the body tissues and muscles. Thus, not resulting in the heating of tissues.
Magnetic antennas, such as loop
Magnetic antennas are mostly used for In-Body and Implant communications. Magnetic antennas
do not overheat the body tissues and is not dangerous to human body unlike electrical antennas.
A loop of magnetic field is formed in magnetic antennas, which is within the defined range of the
antenna, thus, these can communicate within this range not interfering with the body.
Characteristics of Human Body
For wireless communication in WBAN, human body is not considered an ideal medium for the
propagation of signal. Human body consists of materials which contain different dielectric
constants, thickness and impedance which may not be ideal for communication. Depending on
the frequency of operations, human body may encounter many impairments and losses such as
absorption, attenuation and diffraction etc. Therefore, the characteristics of human body should
be kept in mind before designing the path loss model for WBAN [34].
3.7 Scenarios of Path Loss in WBAN
There are different scenarios of path loss which can take place in WBAN, since sensor nodes can
be implanted inside the human body either planted on the surface of the body or atmost 2mm
away from the body surface. Since path loss is dependent on distance as well as frequency,
therefore, the variations of these parameters in these scenarios will affect the path loss model.
The simulation study is performed in MATLAB. A detailed discussion of these simulations is
given as under.
3.7.1 In-Body Communication
In order to study propagation characteristics inside human body, simulations are carried out
using a 3D visualization scheme [22]. The reason of using this scheme is that the study of
physical parameters and their measurements are not feasible inside the human body. The antenna
used in this study is a multi-thread magnetic loop antenna [35]. Like in [35], we consider four
models for our simulation which include:
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1) Deep Implant to On-Body
2) Near Surface Implant to On-body
3) Deep Implant to Implant
4) Near Surface Implant to Implant
The path loss formula is same as [24] [30], given in Eq. (1) with a reference distance
and frequency of 402-405 Mhz. The path loss exponent and standard deviation values for
implant to body surface models are given in Table 3.2.
Models
Path Loss in
()
Deep Tissue Implant
to Body Surface
46.14
4.86
7.25
Near Surface Implant
to Body Surface
47.81
4.532
6.23
Table 3. 2 Implant to Body Surface
Models
Path Loss in
()
Deep Tissue Implant
to Implant
35.55
5.71
8.36
Near Surface Implant
to Implant
41.25
5.12
8.95
Table 3. 3 Implant to Body Surface
From Table 3.2 we conclude that the path loss at a reference distance for deep tissue implant to
body surface is less than that of near surface implant to body surface because of high distance.
The path loss exponent and standard deviation values for implant to implant models is given in
Table 3.3.
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Fig 3.2 Path Loss Models
Figure 3. 2 describes the simulation results of path loss from deep tissue implant node to body
surface node communication (Model 1). The simulation is carried out between no. of observation
points between the deep tissue implant node and reference node placed at some distance, and
path loss at each point of observation. The graph shows fluctuations in path loss at each
observation point. The no. of observations are fixed at for each model in the simulation. Deep
Tissue Implant to Body Surface (Model 1) curve in Fig 3.2 is taken as reference for other three
models. Since, no. of observation points are fixed at , therefore, the fluctuations are same for
all models. However, increase or decrease in the path loss value is dependent upon on the model
which is being used. For near surface implant to body surface path loss model (Model 2), there is
an increase of in path loss at each observation point from the reference model. For deep
implant to implant and near surface implant to implant models, increase of and in
path loss, respectively is noticed in Fig 3.2. For deep tissue implant to implant path loss model
(Model 3), a decrease of in path loss occurs at each observation point from our reference
model (Model 1). The decrease is evident because distance between the nodes are smaller in this
model (i.e., Model 3) from the reference model (Model 1). With comparison to near surface
49
implant to body surface path loss model (Model 2), a decrease of in path loss is observed.
This is because of further lessening of distance between the nodes, and decrease of in path
loss from near surface implant to implant model (Model 3). For near surface implant to implant
path loss model (Model 4), a decrease of in path loss is obtained at each of the observation
points from reference model (Model 1), for near surface implant to body surface path loss
model (Model 2), while an increase of in path loss for deep tissue implant to implant path
loss model (Model 3). If we further increase the no. of observation points, the fluctuations in
path loss will be more sudden. This is due to different impairment factors such as refraction,
diffraction, reflection etc. The summary of the models and their path loss with respect to the
reference model is presented in Table 3.4.
Models
Results
Deep Tissue Implant to
Body Surface (Model 1)
Reference Model
Near Surface Implant to
Body Surface (Model 2)
Increase of 4dB, 15dB and 9dB in Path Loss
from Model 1, 3 and 4 respectively
Deep Implant to Implant
(Model 3)
Decrease of 11dB, 15dB and 6dB in Path
Loss fron Model 1, 2 and 4 respectively
Near Surface Implant to
Implant (Model 4)
Decrease of 5dB and 9dB in Path Loss from
Model 1 and 2 respectively. Increase of 6dB
in Path Loss from Model 3
Table 3. 4 Summary of In-Body Path Loss in WBAN
3.7.2 On-Body Communication
For On-Body communications in WBAN, placement of sensors and actuators on the body
surface is of great importance. Simple path loss model that takes into account the placement of
sensors on the body and their communication with respect to body postures and movements is
required. Channel response output of the On-Body communication as well as the frequency
response can be easily found out. UHC monitoring in WBANs depends on both In-Body and On-
Body communications of sensor nodes [23].
50
Fig 3.3 Amplitude Attenuation in On-Body
Amplitude attenuation of the signal with respect to frequency for On-Body communication is
depicted in Fig 3.3. As frequency increases, attenuation of amplitude also increases since the
channel undergoes impairments. Thus, these impairments degrades the intensity of signal, as it
travels from transmitter to the receiver node planted on the human body.
Fig 3.4 Phase Distortion in On-Body
51
Phase distortion of the signal with respect operating frequency for On-Body communication is
obvious from Fig 3.4. The direct relationship exists here as well; with the increase in frequency
the phase distortion of the signal increases in a linear fashion and vice versa. Each component of
the signal is distorted in phase and if relationship is not linear then there will be different phase
distortion at different frequencies. From UHC point of view, both amplitude attenuation and
phase distortion of the signal for On-Body communication should be eradicated to achieve better
monitoring results.
Fig 3.5 Channel Output for On-Body Communication
Figure 3.5 describes the channel output of On-Body communication with respect to the
signalling data. The signalling data is uni-polar Non Return to Ground (NRG) stream of ones and
zeros, respectively. Depending on this data, the channel output fluctuates with it having a higher
output when the signalling data stream of more ones than zeros exists and lower output when the
signalling data stream consists of more zeros than ones.
52
Fig 3.6 Path Loss vs Distance for On-Body Communication
As, discussed earlier that path loss depends on the distance between the transmitting and
receiving antenna/node as well as frequency of operation. The simulation results shown in Fig
3.6 are carried out for two different frequencies (i.e., and ), as, it is obvious
from Eq. (3) of path loss model. Since, direct relationship exists between path loss and distance
between transmitter and receiver, therefore, by increasing distance, path loss increases linearly.
Also, path loss has a direct relationship with frequency, thus, at the path loss curve is
higher then that at .
Minimum delay spread is another requirement of On-Body communication and is
analytically compared in our simulations. Delay spread or Root Mean Square (RMS) delay
spread is a part of power delay profile. RMS delay spread as well as frequency dispersion of the
signals operating at two different frequencies are determined by power delay profile. RMS delay
spread is the standard deviation value of the delay of reflections, weighted proportional to the
energy of the signal [29].
53
Fig 3.7 RMS Delay at 15cm Separation
Fig 3.8 RMS Delay at 45cm Separation
Figure 3.7 and 3.8 show the results of RMS delay spread measured at node separation of
and respectively. Cumulative Distribution Function (CDF) is used to measure RMS delay
spread and determines the probability of delay spread which occurs at a specified value. Thus,
CDF value for probability of delay spread lies between to . Graph shows that at , RMS
delay spread is having a slight linear curve, as compared to at which the delay spread is
54
more straight. Fig 3.8 portrays that if, we increase the distance then a straight relationship
occurring at is changed into a curve similar to that is noticed at . However, the
delay spread at is higher than that of , because of correlation between the two
frequencies.
The impairment factors and their results with respect to distance and frequency are summarized
in Table 3.5.
Parameters
Results
Amplitude Attenuation
Increases with increase in frequency
Phase Distortion
Increases with increase in frequency
Path Loss
Increases with increase in distance and frequency
RMS Delay Spread (15cm
Separation)
RMS delay spread increases initially for 2.4Ghz, however,
in 900MHz RMS it is initially at 0 and then increases
sharply
RMS Delay Spread (45cm
Separation)
RMS delay spread increases initially for 900MHz and in
2.4GHz it takes some time
Table 3.5 Summary of On-Body Path Loss in WBAN
55
Chapter 4
CONCLUSION
WBAN is becoming a popular technology in the world in the field of healthcare and
entertainment applications. A new set of challenges have emerged in terms of energy efficiency
of a system, antenna design, QoS, co-existence with other technologies, interference control and
security through the use of WBAN. For the MAC layer different low power mechanisms have
discussed with respect to WBAN and the conclusion have been drawn that TDMA mechanism is
suited and most appropriated in WBAN. Different low power MAC protocols for WBAN were
also the part of discussion along with their advantages and disadvantages.
WBAN is an emerging domain in the field of wireless communication. It comprises of
many tin
information and transfer it to medical personnel for diagnosis. WBAN has many applications,
most important of which is in ubiquitous UHC. With UHC, patients are not required to visit
doctor frequently. They can get diagnosis of their disease while sitting at home. Nowadays, lot of
work is going on to make low power sensors and devices that can be used in UHC. In this thesis,
an analytical survey is done on different architectures of WBAN used in UHC. These
architectures are suited for different applications. Wearable devices and standards used in
WBAN are also discussed. Different standards are used depending on the type of application.
Path loss in WBAN and its effects are also discussed in detail. Simulations for In-Body and On-
Body communication are also performed. The results for On-Body communications show that
path loss increases between transmitter and receiver with increase in distance and frequency.
Similarly, phase distortion and attenuation also increases with frequency. Moreover, path loss in
different models of In-Body communication is also carried out. The summarization of
architectures and path loss in On-Body and In-Body communications is presented in Tables.
Finally, the channel model for WBAN is discussed with different modulation schemes suited for
WBAN healthcare environment with their MATLAB simulations
In the project we study MAC protocols for WBAN, we present a survey of different
protocols with respect to energy efficiency and their advantages and disadvantages. Low power
listening, scheduled contention and time division multiple access (TDMA) is also compared. It is
56
shown that TDMA is more power efficient however, suffers with synchronization sensitivity.
Techniques for collision avoidance of different MAC protocols are comparatively analyzed. Path
loss model for In-body, On-body and Off-body communication in WBANs is also described.
Simulation results show that path loss is maximum for In-body communication, as compare to
On-body and Off-body communication because human body is composed of tissues and organs
in which communication is difficult and thus results in high path loss. On-body and Off-body
also show some variations in results when the source and destination sensors or nodes are LoS
and NLoS. Path loss increases considerably when the sensors are NLoS. Moreover, off-body
communication describes the variations of the channel with respect to the following three
aspects: distance between body and Access points (receiver) denoted by , body orientation
angel , and transmitter based azimuth angel . It is suggested that the experimental results
may be more accurate if the human body is considered spherical instead of cylindrical.
57
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