Characterization of Communication Mechanisms for Implantable Body Sensor Networks

Master’s thesis on

Characterization of Communication Mechanisms for Implantable Body Sensor Networks

Focusing on Physical layer and Medium Access Control Sub-layer

Vignesh Raja Karuppiah Ramachandran.

In partial fulfillment of the requriements for the degree of

Master of Science

in

Embedded Systems

Faculty of Electrical Engineering, Mathematics and Computer Science (EEMCS) Pervasive systems Research group.

Thesis Committee:

Prof.dr.ir. Paul Havinga Chair, Pervasive systems

Dr. Nirvana Meratnia Associate Professor, Pervasive systems Ir. Bert Molenkamp Lecturer, Computer Architecture for

Embedded Systems

Enschede, August 2014

Abstract

The use of wireless sensor networks (WSN) in health-care has been rapidly increased in the last few years. Miniaturization of sensor nodes, in-body data communication, bio-compatibility are the main outcomes of the on-going extensive research on nano-technology, wireless commu- nication and bio-medical engineering, respectively. Sensing of various life-critical physiological signals such as heart-rate, blood-pressure, blood-glucose level, is made possible with miniaturized implantable wireless bio-sensors. Challenging requirements of WSN in health-care applications have resulted in the advent of a specific type of WSN called Implantable body sensor network (IBSN), in which the sensor nodes are implanted either subcutaneously or by an invasive surgery into the patient’s body. These sensor nodes continuously monitor the physiological signals which is necessary for the patients with life-threatening diseases such as epilepsy. Life-critical implant- able medical devices (IMD) such as pace-makers and neural stimulators, are also connected to the IBSN. A closed control loop of medical devices is envisioned through a network of IMD and bio- sensors using IBSN, in which the IMDs are programmed for different types of therapies through wireless channel, based on the real-time response of the patient by continuously monitoring the medical symptoms with the implantable bio-sensors.

IBSN is less researched compared to the body sensor network (BSN) where the wireless com- munication between sensor nodes takes place on the surface of the body. Extensive research on characterizing the communication mechanisms for IBSN is needed to standardize a reliable RF communication within the human body. In order to create a reliable and energy-efficient sensor network, two main layers of the Open System Interconnection (OSI) network model is required.

The physical layer which is aimed at unification of the hardware requirements in a network to enable the successful transmission of data. The medium access control (MAC) sub-layer that is aimed at controlling the access to the wireless channel, which directly affects the network performance and energy efficiency of the nodes. This master thesis focuses on characterizing the physical layer and MAC sub-layer with different configurations of IBSN by means of software simulation and hardware experimentation. Two main state-of-the-art mechanisms are focused namely Medical Implant Communication Service (MICS) band specifications in the physical layer and wake-up radio integration in the MAC sub-layer which enables globally standardized communication strategies for IBSN and ultra-low power communication with reliable network performance respectively. These mechanisms are studied and characterized for different IBSN scenarios with a bio-medical implant in this thesis work. .

As a result, an optimum configuration of the physical layer and MAC sub-layer for the IBSN is found. The added-value of wake-up radio in MAC layer and the effect of MICS band config- urations in physical layer and MAC layer are identified. The evaluation results will also indicate the potential drawbacks in the existing configurations at physical and MAC layers of BSN, and identify why the existing BSN mechanisms cannot be used for IBSN scenarios. Possible solutions to overcome these drawbacks are suggested for the future research work.

Keywords. Implantable body sensor network, Wake-up radio, Medium access control

iii

Acknowledgement

It was around August 2012 when I started the Master’s education in Embedded systems at the University of Twente. I am always fascinated about wireless sensor networks, which is a part of embedded systems and is also widely applied in different industries for different scenarios. In this regard, I was fortunate to get introduced to Prof. Paul Havinga, by Ir. Andrea Sanchez Ramirez, during one of my courses called ”Energy efficient embedded systems”. Based on my work in EEES course, I was given an opportunity to work as a student assistant in an EU-FP7 project ”WiBRATE”, which involves application of WSN in industrial vibration monitoring. I continued working in the project for my intern-ship and also was given an opportunity to extend the work as a part-time job during my thesis. It was during this tenure as a student assistant I was supervised and guided by Dr. Niravana Meratnia. With highly constructive comments and progressive meetings together with Dr. Paul and Dr. Nirvana, I successfully proposed my idea of research about in-body sensor networks and continued with this thesis work. Although the assistantship work was not related to my thesis, I was given an opportunity to explore my own research interests. With their support, I was able to publish two international articles during my master study and of course a trip to Singapore for presentation. I was able to successfully complete this thesis overcoming all the difficulties. Even though this master thesis is just an exploration and characterization of existing wireless communication mechanisms, I hope with further research I can materialize the closed loop architecture for medical devices. I thank my professors for giving me another opportunity to continue with my research towards a PhD degree.

I must thank Dr. Niels Moseley for his continuous and valuable technical support and Dr.

Berend Jan van der Zwaag, for his constructive feedback at my writing skills. I thank Ir. Kyle zhang who developed the medical implants which were also used for the final hardware char- acterization in this thesis. I thank Ir. Saeid Yazdani for all his support in debugging codes at times. Apart from being thanked, I must acknowledge them for being excellent colleagues for the last one year. I also thank all the people of PS for making me feel comfortable at the office.

I thank my friends, Alex, Frank, Yoppy, Gebremedhin, Anantha, Hasib, Anand, Nolie, Ramesh, Morshed, for being with me at difficult times. Apologies, for not having a whole list of names. I thank all of my friends who supported me either directly or indirectly during my master studies. Without all their support, staying far away from home and focusing on studies would not have been possible

I thank my family and relatives for supporting me financially and emotionally throughout the master education. And a special thanks to my amma, thambees,and logapa family for allowing me to stay abroad for years to complete my education. In-spite of all the difficult times, I thank you all for letting me cherish my dreams.

Last but not the least, I thank God almighty for this wonderful life.

v

Contents

Contents vii

List of Figures xi

List of Tables xiii

1 Introduction 1

1.1 Implantable medical devices and sensors in health-care . . . . 2

1.1.1 Implantable bio-sensors for monitoring physiological and contextual signals in a human body . . . . 3

1.1.2 Drawback of current medical devices in health-care . . . . 6

1.1.3 Challenges of IBSN in a closed loop architecture . . . . 7

1.2 Context of Research . . . . 7

1.2.1 Motivation . . . . 7

1.3 Research question . . . . 8

1.3.1 Research approach . . . . 8

1.4 Outline of thesis . . . . 8

2 Requirements of PHY and MAC layers for IBSN 11 2.1 Communication mechanisms for IBSN . . . . 12

2.1.1 MAC protocols . . . . 12

2.2 Architectural framework for closed loop medical devices using IBSN . . . . 14

2.3 Low power design and Power Scavenging . . . . 14

2.3.1 Wake-up radio . . . . 14

2.3.2 Power Scavenging . . . . 17

2.4 Application scenario and requirements for MAC protocols in IBSN . . . . 18

3 Survey of MAC protocols with and without wake-up radio for implantable sensor network 21 3.1 Features of MAC protocol . . . . 21

3.1.1 Attributes of MAC as proposed by IEEE 802.15 TG-6 . . . . 22

3.2 Requirements of MAC protocol for IBSN with different medical devices . . . . . 22

3.3 Taxonomy of MAC protocols for IBSN . . . . 23

3.4 Access mechanisms without wake-up radio . . . . 25

3.4.1 TDMA based MAC protocols for IBSN . . . . 26

3.4.2 CSMA based MAC protocols for IBSN . . . . 26

3.4.3 Hybrid MAC protocols for IBSN . . . . 28

3.4.4 Other Access Mechanisms for IBSN (FDMA, UWB, ALOHA) . . . . 30

Contents vii

3.5 Access mechanisms with wake-up radio . . . . 34

3.6 Discussion . . . . 36

3.7 Conclusions of Literature survey . . . . 40

4 Analysis of wake-up radio based MAC protocols 41 4.0.1 Radio Triggered sensor MAC . . . . 41

4.0.2 OnDemand MAC . . . . 42

4.0.3 SCM MAC . . . . 43

4.1 Simulations and performance evaluation . . . . 44

4.1.1 Simulation results . . . . 44

4.2 Discussion . . . . 49

4.3 Conclusion . . . . 49

5 Characterisation of PHY layer of an implanatable sensor node 51 5.1 Description of test environment . . . . 51

5.1.1 Animal flesh . . . . 51

5.1.2 Different location in muscular tissue for an implant . . . . 52

5.2 CC430 based implant . . . . 54

5.2.1 Pseudo-implementation of wake-up radio and antenna matching circuit . . 55

5.3 Physical layer configurations . . . . 56

5.3.1 Parameters of physical layer configuration . . . . 56

5.3.2 Medical scenarios for different configurations . . . . 57

5.4 Implementation and experimental setup . . . . 58

5.4.1 Implant location in flesh for different scenarios . . . . 58

5.4.2 Collective evaluation of PHY parameters . . . . 58

5.4.3 Set of physical parameters . . . . 60

5.4.4 Set of network parameters . . . . 61

5.5 Results and discussion . . . . 63

5.5.1 Results from evaluating the set of physical parameters . . . . 63

5.5.2 Results from evaluating the set of network parameters . . . . 67

5.6 Conclusion from characterization of implantable sensor node . . . . 70

5.7 Optimum parameters . . . . 70

6 Performance evaluation of wake-up feature based CSMA/CA protocol 71 6.1 CSMA/CA without wake-up radio . . . . 71

6.2 CSMA/CA with wake-up radio . . . . 73

6.3 Performance analysis of CSMA/CA protocol with and without wake-up radio . . 76

6.3.1 Network setup . . . . 76

6.3.2 Results and discussion . . . . 76

6.4 Conclusion . . . . 80

7 Conclusions 81 7.1 Answer to the research question . . . . 82

7.2 Future work . . . . 83

Bibliography 85

Appendix 91

A Animal flesh in SC1 91

viii Contents

CONTENTS

B Animal flesh in SC2 92

Contents ix

List of Figures

1.1 Implantable cardiac pacemaker . . . . 2

1.2 Implantable neural stimulator . . . . 2

1.3 Implantable drug-delivery system (on left is the USB stick for size-comparison) . 3 1.4 Implantable glucose sensor[11] . . . . 4

1.5 Wireless, Battery-less, implantable MEMS sensor for ECG measurement [10] . . 4

1.6 Intra-cranial pressure sensor[28] . . . . 4

1.7 Ingestible endoscopic pill size-compared with one cent coin [32] . . . . 4

1.8 Electro-CardioGram of Human [16] . . . . 5

1.9 Thoracic pressure signal and Photo-PlethysmoGraph of a Human [18] . . . . 5

1.10 SpO ² signal with different contextual measurements . . . . 6

1.11 Representation of physiological signal (ECG) and contextual signal (respiration impedance sensor) . . . . 6

2.1 Different communication strategies of BSN . . . . 11

2.2 Block diagram of sensor nodes with wake-up radio . . . . 15

2.3 Transmitter implementation of 2.4 GHz wake-up radio . . . . 16

2.4 Recevier implementation of 2.4 GHz wake-up radio . . . . 17

4.1 RTM scheme. . . . 42

4.2 On-Demand MAC scheme . . . . 43

4.3 SCM-MAC scheme . . . . 44

4.4 Effect of IPAT on power consumption . . . . 46

4.5 Effect of IPAT on End-to-End delay . . . . 47

4.6 Effect of IPAT on packet delivery ratio . . . . 48

4.7 Effect of IPAT on duty cycle . . . . 48

5.1 A dissected part of pig flesh used for testing the sensor node. . . . 52

5.2 A cross section of skin showing different tissue layers including skin. . . . 52

5.3 A biological safety cabinet used for carrying out the experiments with meat. . . . 53

5.4 Block diagram of radio chip used in the evaluation. . . . 54

5.5 Custom made CC430 based implant. . . . 55

5.6 CC430 based implant enclosed in a paraffin coating. . . . 56

5.7 Flowchart for PHY evaluation . . . . 60

5.8 RSSI vs Distance at 0 degree antenna orientation . . . . 64

5.9 RSSI vs Distance at 90 degree antenna orientation . . . . 64

5.10 RSSI vs Distance at 180 degree antenna orientation . . . . 64

5.11 RSSI vs Frequency at a fixed tx power, distance, orientation . . . . 64

List of Figures xi

5.12 Received signal strength information (RSSI) for in-body to in-body communica-

tion SC1 . . . . 64

5.13 RSSI vs Distance at 0 degree antenna orientation . . . . 65

5.14 RSSI vs Distance at 90 degree antenna orientation . . . . 65

5.15 RSSI vs Distance at 180 degree antenna orientation . . . . 65

5.16 RSSI vs Frequency at a fixed tx power, distance, orientation . . . . 65

5.17 Received signal strength information (RSSI) for in-body to on-body communica- tion SC2 . . . . 65

5.18 RSSI vs Distance at 0 degree antenna orientation . . . . 66

5.19 RSSI vs Distance at 90 degree antenna orientation . . . . 66

5.20 RSSI vs Distance at 180 degree antenna orientation . . . . 66

5.21 RSSI vs Frequency at a fixed tx power, distance, orientation . . . . 66

5.22 Received signal strength information (RSSI) for on-body to on-body communica- tion SC3 . . . . 66

5.23 Tx Rate vs Packet delivery ratio evaluated with fixed packet length. Measured in SC1 . . . . 68

5.24 Tx Rate vs Packet delivery ratio evaluated with fixed packet length. Measured in SC2 . . . . 68

5.25 Tx Rate vs Packet delivery ratio evaluated with fixed packet length. Measured in SC3 . . . . 69

6.1 Packet format of the CSMA/CA protocol. . . . 72

6.2 Flowchart for CSMA/CA . . . . 73

6.3 Wake-up feature of CC430 (ref: CC430 datasheet[45]) . . . . 74

6.4 Flowchart for CSMA/CA with wake-up feature . . . . 75

6.5 The network topology . . . . 76

6.6 Inter packet arrival time vs Duty cycle. Comparison between CSMA/CA with and without Wake up radio . . . . 77

6.7 Inter packet arrival time vs Packet delivery ratio. Comparison between CSMA/CA with and without Wake up radio . . . . 78

6.8 Inter packet arrival time vs End to End delay. Comparison between CSMA/CA with and without Wake up radio . . . . 79

A.1 Scenario 1, in-body - in-body communication Distance = 16cm . . . . 91

A.2 Scenario 1, in-body - in-body communication, Distance = 6cm . . . . 91

B.1 Scenario 2, in-body 1 (to) in-body2 communication,Distance= 20 cm . . . . 92

B.2 Scenario 2, in-body 1 (to) in-body2 communication,Distance= 160 cm . . . . 92

xii List of Figures

List of Tables

2.1 Requirements for a MAC protocol in IBSN. . . . 18

3.1 Features of MAC protocols as suggested by IEEE 802.15 TG 6 [17] . . . . 22

3.2 Requirements of data communication in implantable medical devices used to com- municated to the base station controller . . . . 23

3.3 Taxonomy of MAC protcols based on Wakeup radio for IBSN . . . . 24

3.4 TDMA-MAC protocols for IBSN . . . . 27

3.5 CSMA-MAC protocols for IBSN . . . . 29

3.6 hybrid-MAC protocols for IBSN . . . . 31

3.7 FDMA, UWB, ALOHA based access mechanisms . . . . 33

3.8 MAC protocols with wakeup-radio for IBSN . . . . 35

3.9 Comparison of MAC protocols in terms of network parameters . . . . 39

4.1 Simulation parameter values . . . . 45

4.2 Strengths and weaknesses of selected MAC protocols . . . . 49

5.1 Evaluation of hardware with two different sets of physical and network parameters. 59 5.2 Set of physical parameters for SC1. . . . 61

5.3 Set of physical parameters for SC2. . . . 61

5.4 Set of physical parameters for SC3. . . . 62

5.5 Set of network parameters. Repeated for SC1, SC2, SC3 . . . . 63

5.6 Optimum values derived from validating the set of physical parameters . . . . 67

5.7 Optimum parameters from hardware evaluation . . . . 69

5.8 Optimum parameters from hardware evaluation . . . . 70

List of Tables xiii

Abbreviations

BSN Body sensor networks CSMA Carrier sense multiple access IBSN Implantable body sensor network TDMA Time division multiple access FDMA Frequency division multiple access MICS Medical implant communication service WBAN Wireless Body Area Network

IEEE Institute for electrical and electronics engineers TG Task group

MAC Medium access control PHY Physical layer

OSI Open systems interconnect QoS Quality of Service

WSN Wireless sensor networks IMD Implantable medical device WuR Wake-up radio

WoR Wake-on timer ECG Electro cardiogram EEG Electroencephalogram EMG Electromyogram PPG Photo-plethysmograph RF Radio frequency RTS Ready to send CTS Clear to send

IPAT Packet inter-arrival time PDR Packer delivery ratio CPU Central processing unit RAM Random access memory kB Kilo bytes

KBPS Kilobytes per second MHz Mega hertz

Tx Transmission

Rx Reception

SC Scenario

UART Universal asynchronous receiver transmitter RSSI Received signal strength information CCA Clear channel assessment

ISM Industrial Scientific and medical band

List of Tables xv

Chapter 1

Introduction

A disease is an abnormal condition that affects the body of an organism. The human-kind is prone to different kind of diseases that are chronic and sometimes fatal. These diseases require continuous monitoring and constant therapies for reducing the negative impact or to prevent any fatal incidents. Most of the elderly patients are easily affected by diseases due to the process of ageing. It is common that the young patients also suffer due to unexpected chronic diseases which directly affects the quality of living. Thankfully, the advent of advanced healthcare systems helps these elderly and young patients who suffer from diseases which in the previous decades were not possible to be medically treated. In the last decades, technology had helped to find relief from certain diseases in neurological disorders [1] [2], cardio-vascular problems[3], chronic pain, and also efficient remedies for syndromes such as Parkinson’s syndrome [4]. The most common diseases, which are found to be treatable with medical devices, are:

Cardio-vascular problem

The Heart is an organ which pumps blood to different parts of the body. The pumping cycle has four distinct operation which is regulated by a muscular node that provides electrical impulse to the heart. On failure of this muscular node, due to various reasons, the pumping action becomes out of phase. A medical device called pace-maker is implanted near to the heart, which acts as the node providing electrical impulses [3]. A Physician programs the pace-maker to pace the heart in a rhythmic fashion. The physiological signals such as Electro Cardiogram(ECG), blood pressure are measured and analyzed before programming the pace of the heart.

Neurological disorders

The human body is composed of nerves, which acts as medium of communication between the brain and other parts of the body. Any dysfunction in the neural system will lead to neurological disorders. Epilepsy is one of the most common neurological disorder which results in seizures in brain which causes random bodily behaviour. Symptoms such as rapid movement of limbs called myoclonic jerks. This is the most common symptom of generalized stroke caused due to epilepsy[1].

Hormonal deficiency

The cases where the naturally secreting hormone of the body fails, causing severe diseases such as diabetes. To overcome this situation, artificial hormone are injected into the body through drug-delivery devices. These device inject drugs resembling the hormone regaining the normal functioning of the body. Programming of these devices as per the requirement of patient is done by physicians.

1

Figure 1.1: Implantable cardiac pacemaker Figure 1.2: Implantable neural stimulator

1.1 Implantable medical devices and sensors in health-care

The medical devices referred to in this study are the devices that are implanted inside the human body, which simulates the natural function of any organ. For example, pace-maker is a medical device that simulates the function of sinus-articular node which is a muscular node on the heart that provides periodic electrical impulses for a proper functioning of the heart. Devices also include nerual stimulators, drug delivery systems which are placed as a replacement for the natural body organs. These devices are typical electronic system, which comprises of a energy source, a small micro-controller, and a communication module for external communication. Some of the devices also contains a sensing part to sense vital bio-signals of the human body which includes heart-rate, blood-pressure, core-temperature of the human body, blood glucose level, blood and tissue oxygen level.

Many medical devices have been found to be useful in treating diseases without knowing the exact mechanism behind the cure. However, most of the devices and its mechanisms are being published in the last few years. For example, the medical reason for curing depression with deep brain stimulator is unknown when it was first used in a patient[5]. Nevertheless, devices with known mechanism of cure are established in the past. Some of such devices are pace-maker, neural stimulator, drug-delivery systems, retinal implant, cochlear implant, semi-functional prosthetic limbs.

2 1.1. Implantable medical devices and sensors in health-care

CHAPTER 1. INTRODUCTION

Figure 1.3: Implantable drug-delivery system (on left is the USB stick for size-comparison)

Pace-maker

The pace-maker is a device which is used to electrically stimulate the heart for normal oper- ation. As shown in 1.1, it has two leads delivering the electrical stimulus. The device itself is implanted near the heart and programmed externally using a telemetry link, usually a magnetic link. Due to the progressing standardization of radio communication inside of human body, latest devices have radio frequency communication link between the implanted device and the external programmer[6]. The pace of electrical stimulus is programmed as per the requirement of the patient. A patient have to visit the doctor frequently in order to reprogram the device.

Neural stimulator

The neural stimulator is an electronic device which provide electrical impulses to the brain and neural systems to regain the functionality the neural system. The stimulation can be con- structive inducing the activity, or destructive by reducing the activity. The concept is same as that of a pace maker, where two leads delivering the current is connected to a controlling device usually implanted near the collar-cuff below the shoulders[7]. The programming of the stimulator is carried out as per the symptoms and requirements of the patient. Robust mechanism of action in using neural stimulation for many diseases is yet to be known, however a cure was possible in most of the neural-disordered patients. A commercially available neural stimulator is shown in Fig. 1.2

. Drug delivery device

The drug delivery systems are devices which store small quantities of refillable drug capsules, and releases the drug either to either blood stream or to the nerve bundles. In patients with dia- betes, insulin pump in placed near to the vertebral column and the insulin is inject periodically as per the requirement of the patients as shown in Fig. 1.3. The period of the drug release is controlled by the physician based on patient’s medical history and symptoms[8]. Chronic pain, spinal cord injuries, hormonal deficiency are few applications of drug delivery systems. There are literature [8] [5] [3] which witness the introduction of sensing circuitry in the device itself, which can monitor as well as provide the therapy. However the symptoms are not local, but have to be precisely monitored all over the body due to dynamic nature of the human body.

1.1.1 Implantable bio-sensors for monitoring physiological and contex- tual signals in a human body

Successful research on biological, chemical, electrical and mechanical sensor technologies have led to a wide range of wearable and implantable sensors suitable for continuous monitoring. In

1.1. Implantable medical devices and sensors in health-care 3

addition to sensor sensitivity, several factors have to be considered in the design of a pervasive biosensor[9], such as reliability, ease of use, selectivity, sensor packaging, biocompatibility, and power consumption. Bio-sensors are often affected by noise due to bio-fouling, motion artefact, and interference. For example, ECG (Electrocardiogram) sensors are highly sensitive to motion artefact, which can hinder its ubiquitous use. To improve the sensor reliability, multi-sensor or sensor array approaches are commonly used. Sensor fusion techniques can then be applied to fuse information from these sensors. For example, source recovery can be employed to fuse the inform- ation from multiple sensors and infer the intrinsic signal characteristics [10][11]. Although by the introduction of additional sensors can improve the overall system performance, increasing the number of sensors can potentially increase the complexity of the system and affect its practical deployment. To circumvent this problem, minimum number of sensors should be used for differ- ent application scenarios. In fact, selecting only relevant features or sensors not only simplifies the system set-up but also improves the classification accuracy [16]. In practice, feature selection techniques can be employed to identify relevant sensors and their optimum location. The figures 1.4 - 1.7 show different implantable sensor enabled with a wireless communication. Based on the types of commercially available sensors, potential use of them in monitoring physiological and contextual signals are mentioned in the following section.

Figure 1.4: Implantable glucose sensor[11] Figure 1.5: Wireless, Battery-less, implantable MEMS sensor for ECG measurement [10]

Figure 1.6: Intra-cranial pressure sensor[28]

Figure 1.7: Ingestible endoscopic pill size-compared with one cent coin [32]

4 1.1. Implantable medical devices and sensors in health-care

CHAPTER 1. INTRODUCTION

Physiological signals of Human body

Physiological signals of the human body reflect the functioning of the human body and symp- toms of certain diseases can be reflected in the physiological signals. Vital organs of human body such as heart, brain, lungs etc., have unique pattern of electrical signals which can be recorded using the bio-sensors. Apart from vital organs of human body, unique pattern of signals can be sensed from non-vital organs, such as muscles and skin. Some of the physiological signals of human body are shown in the figures 1.8 and 1.9

Figure 1.8: Electro-CardioGram of Human [16] Figure 1.9: Thoracic pressure signal and Photo-PlethysmoGraph of a Human [18]

Contextual signals of Human body

The contextual signals of human body such as chemo-sensory responses in terms of sweat, hormones and enzymes such as adrenalin can be monitored for diagnosing diseases [3]. For ex- ample, a prediction of heart failure can be done if the information from contextual sensor such as anxiety through sweat and adrenalin sensor is available. Monitoring these contextual values will also help in rehabilitation of patients both physically and mentally. It is interesting to note that the mental diseases such as Treatment resistant depression depend on contextual responses of a patient in the therapy. For example, McKeown et al., in [12] presented a way to monitor the health of a patients by monitoring the perception of laughter of a person at different social places.

Deep brain stimulation therapies can be made efficient if the contextual signals of human body is monitored continuously along with physiological responses. Similarly, motion based activity sensors, respiration sensor and hormonal sensor implants can be involved in forming a closed loop medical systems. Agarwal et al., in [13] have presented a novel method of using different con- textual sensors such as sweat sensor, heat loss sensor, number of walking steps using pedometer, skin temperature, vertical acceleration, which are then fed into a Statistical vector machine. As a result Agarwal et al, were able to find the health condition of the patient efficiently. Also, Adolph et al., in [14] have performed experimental trials with chemo-sensors and measured the anxiety of a person. However, in order to use sophisticated implantable sensor network, various aspects of the network should be defined by rigorous research defining dependency on the sensor network and reliability of the medical devices.

1.1. Implantable medical devices and sensors in health-care 5

Figure 1.10: SpO ² signal with different contex- tual measurements

Figure 1.11: Representation of physiological signal (ECG) and contextual signal (respiration

impedance sensor)

1.1.2 Drawback of current medical devices in health-care

Medical environment for critical care patients often involve many medical devices. Physiolo- gical sensors such as pulse rate monitor, oxygen sensor, provide vital information about the fitness of the patient. Medical devices, for example, drug-delivery device, provide therapies for diseases. That is, they stabilize the fitness of the patient, for example, by infusing medication.

Medical systems, considered together with the patient and caregivers, represent an important class of cyber-physical systems. Patient safety is the primary concern in such systems, yet reas- oning about patient safety is very difficult because of insufficient understanding of the dynamics of human body response to treatments. Physicians and care-givers manually program the med- ical devices by monitoring the dynamic response of human body measured by the physiological sensors.Human errors, another important source of patient safety problems, are also difficult to reason about in the framework of conventional medical system development. Traditionally, care- givers perform the role of the controller in such a system. This means that the caregiver needs to continuously monitor all sensor devices and apply an appropriate treatment. The large number of sensor data from different patients and appropriate control of a specific patients, makes the job of the caregiver very difficult. Two simultaneous emergency situation may divert the care-giver’s attention, making him or her miss an important event. As a result, patient safety may suffer and some times the results may be fatal. Multiple such occurrences are documented in the clinical literature[1].

Possible solution to overcome human errors and improve patient safety One of the possible solution to overcome these drawbacks is making the operation of medical device autonomous. The medical device should be able to perform the therapies as prescribed by the physician for different symptoms of a particular disease. The symptoms are continuously mon- itored by the sensors which is given as an input to a closed-loop control system which actuates the medical devices to provide the therapy for a specific symptom of a disease. This mechanism of closed loop operation is possible if three types of technologies combine. Monitoring the symp- toms, which require bio-sensors either invasive or non-invasive to the human body. Processing the sensor-data which is acquired by the bio-sensors and data communication to the implanted

6 1.1. Implantable medical devices and sensors in health-care

CHAPTER 1. INTRODUCTION

medical devices and sensors. Design of bio-sensors and processing the data is an ongoing re- search, where a substantial progress has already been made. IBSN is a suitable candidate to form a network of implantable medical devices and sensors, integrating the autonomous opera- tion with a closed loop operation. Focus on the wireless data communication to and from the implantable medical devices and sensors is needed.

1.1.3 Challenges of IBSN in a closed loop architecture

The need of the closed loop medical devices can be fulfilled by carefully orienting the features of IBSN towards requirements of advanced medical devices. IBSN is highly inter-disciplinary field of research, where communication mechanism depends on the properties of the human body along with other factors related to human body such as movement, noisy RF environment etc.

In order to establish a wireless communication in IBSN to operate in a closed loop control, the following technical challenges have to be addressed.

• establishing reliable link in different positions of the body

• sensor nodes operated at ultra-low power consumption

• ability to operate in link-failures

• achieving zero-latency in emergency conditions

• providing QoS requirement for medical device inter-communication

1.2 Context of Research

IBSN and BSN are suitable candidates to continuously monitor the vital signs of human body and control the medical devices based on the physiological response from the patient. As mentioned earlier,the characteristics of IBSN is not known compared to BSN. Hence a thorough characterization and evaluation of IBSN is required. The context of this research is to characterize the PHY and MAC layer in an IBSN.

1.2.1 Motivation

The communication mechanism in an in-body environment is not completely standardized.

Many physical layer parameters and MAC layer parameters are unknown for a given hardware.

In order to understand the effect of communication mechanism, an evaluation of physical layer and data link layer is necessary. Moreover, the behaviour of real hardware inside the body of living organism depends on various parameters of wireless communication such as transmitted frequency, power of transmission, modulation format, and orientation of antenna. Knowledge of these parameters and effect of hardware design of the implant is an important starting point for the IBSN. Having known the physical layer parameters, for a given hardware design it is important to evaluate the data link layer. MAC sub-layer is important layer of the network stack of a sensor network.

The MAC protocols are one of the important aspects of the communication strategies to reduce power consumption, increase reliability and throughput. It is also known from literature (refer Chapter 3), that the wakeup-radio can be incorporated in the MAC protocols in order to provide a high reliability by reducing the collision and reduce power consumption by overcom- ing the problem of over-hearing and idle-listening. The benefits of wake-up radio based MAC protocols in BSN is enormous, but the effect of the same in different physical medium such as

1.2. Context of Research 7

in IBSN is still unknown. Hence, characterizing the existing MAC protocols with and without wake-up radio in an IBSN scenario is required. A thorough characterization of PHY and MAC layers is required to design the infrastructure for IBSN.

1.3 Research question

Wireless communication to the implanted sensor node is a challenging task to achieve due to inter-dependency of power consumption, network performance and dynamic nature of human body. Hence, research is needed in the direction of characterizing the communication strategies for IBSN to establish a robust wireless link within the human body. This research will be a carried out by characterizing the RF communication mechanisms inside the human body.. The following question will be answered as a result of this research

Can a wake-up radio integrated with MAC protocol, meet the QoS requirements and power constraints of an IBSN while operating inside the human body ?

1.3.1 Research approach

The objectives of the research is focused on the IBSN to address the reliability, accuracy and power efficiency. The main aim of the IBSN is to increase the reliability of the medical devices and reduce the power consumption. Four steps are carried out to characterize the physical layer and MAC layer of the IBSN,

• Investigate the existing MAC protocols and the use of wake-up radio in MAC protocols.

• Verify the findings of existing MAC protocols in an IBSN scenario using software simula- tions.

• Characterize the physical layer parameters depending on different medical scenarios of implantable sensor networks using a implantable sensor node implanted inside the animal tissue.

• Evaluate the network performance by choosing optimum physical layer parameters for the implantable sensor network.

1.4 Outline of thesis

This thesis is organized as follows,

• Chapter 1 presents the general need for Implantable sensor network and its applications.

• Chapter 2 describes the general requirements for an implantable sensor network.

• Chapter 3 presents an elaborate survey of existing MAC protocols with and without wake- up radio and their applicability in IBSN.

• Chapter 4 describes a detailed analysis of different kinds of access mechanisms in a MAC protocol with and without wake-up radio in an IBSN scenario by software simulation.

• Chapter 5 presents a detailed characterization of the PHY layer in an implantable sensor node and provides optimum parameters for evaluating the network performance.

8 1.3. Research question

CHAPTER 1. INTRODUCTION

• Chapter 6 analysis the advantages and disadvantages of the wake-up radio with carrier sense multiple access mechanism in a network of sensor nodes implanted inside the animal tissue

• Chapter 7 concludes the results from characterization and evaluation of PHY and MAC layers of implantable sensor node and some of the open research questions are presented.

1.4. Outline of thesis 9

Chapter 2

Requirements of PHY and MAC layers for IBSN

In the last few decades, wireless body sensor network have been established in three different scenarios namely, Off-body communication, On-body communication and In-body communica- tion. Off-body communication is the communication from the base station to the transceiver on human side. On-body communication is the communication with on-body sensor nodes and wearable system. In-body communication is the communication between invasive or implantable devices and external base station. Monitoring in-body functions and the ability to communicate with an implanted therapeutic device, such as a pacemaker, are essential for its best use. Out of the three scenarios, most of the research was mainly on the first and second scenarios ruling out in-body sensor network due to its complex nature[15][16]. Technologies enabling the first two scenarios were already existing, which resulted in establishment of Body Area Networks and Personal Area Networks. However, the in-body sensor networks demands critical requirements since the sensor nodes are placed inside the body.

Figure 2.1: Different communication strategies of BSN

11

2.1 Communication mechanisms for IBSN

IBSN sensors are either implanted into patients or worn by the patients, low power radio has to be used to minimize the radiation. In addition, as sensitive physiological data is being transmitted in a IBSN, reliable and secure wireless links are essential. Due to the growing demand of the medical devices, ITU have allocated 402-405 MHz under the citizen band radio services.

This frequency have to be shared with the satellite communication. MICS is one of five Citizens Band Radio Services. The others are the Citizens Band Radio Service at 27 MHz, the Wireless Medical Telemetry Service (WMTS) at 216-217 MHz, the Low Power Radio Service (LPRS) at 216-217 MHz, and the Family Radio Service (FRS) at 460 MHz.The 402-405 MHz frequencies have propagation characteristics conducive to the transmission of radio signals within the human body. In addition, equipment designed to operate in the 402-405 MHz band can fully satisfy the requirements of the MICS with respect to size, power, antenna performance, and receiver design. Further, the use of the 402-405 MHz band for the MICS is compatible with international frequency allocations. Finally, the use of the 402-405 MHz frequency band for the MICS does not pose a significant risk of interference to other radio operations in that band. MICS systems consist of the transmitters connected to medical implant devices, and programming, monitoring and control equipment. International regulation of the frequency band for life-critical medical devices will benefit the user, the wireless medical industry, and regulators. It will also impact positively on cost-saving, quality, reliability and delivery of healthcare. MICS is accepted globally for the use of medical devices and the work-group for standardization is set.

Why not ISM band for medical devices?

The ISM band also has the same properties as MICS band in terms of electrical design.

However, due to the threat of interference from other users which can significantly increase the noise floor and cause unwanted and unpredicted behaviour to the medical devices operating in the same band. The interference can even occur from BMW’s Comfort Access and MB’s Keyless Go which uses the same 433 MHz ISM band for security operations in auto-mobiles. Also, patients travel internationally and often carry and use medical devices across national borders, thus global harmonization of rules and standards is essential. The frequency bands, service status (level of protection), technical standards and certification requirements are not efficiently controlled and predicted in the ISM band. The introduction of IBSN and the use of dedicated spectrum for some medical applications (e.g. telehealth) need to be standardized and harmonized internationally which is not possible in the case of ISM band. Thus use of ISM band in the medical devices is not a suitable candidate for the medical devices which are implanted inside the body.

2.1.1 MAC protocols

The development of an affordable IBSN induces a number of issues and challenges such as interoperability, scalability, Quality of Service (QoS), and energy efficient communication. There are various low-power techniques to ensure energy efficient communication in a wireless sensor network such as fixed duty cycling technique in SMAC[17] and wake-up slots in TDMA[18].

However, they are not energy efficient in case of a heterogeneous IBSN. Unlike SMAC, the traffic characteristics in a IBSN vary from periodic to non-periodic and vice versa[18]. The concept of fixed duty cycling technique gives limited answer when it comes to the heterogeneous behaviour of autonomous sensor nodes in a IBSN. The dynamic nature of these nodes does not urge synchronized periodic wakeup periods. Some nodes, e.g., electrocardiogram (ECG), may send data at 1/hour rate to the coordinator, while other may send data twice in a week. These nodes should also have the capabilities to sense and transmit emergency information. The data is

12 2.1. Communication mechanisms for IBSN

CHAPTER 2. REQUIREMENTS OF PHY AND MAC LAYERS FOR IBSN

classified into three categories, i.e., Normal Traffic, On-Demand Traffic, and Emergency Traffic.

The IBSN MAC protocol is required to accommodate the entire traffic classification in a soph- isticated manner. Most of the well-known low-power MAC protocols such as IEEE 802.15.4 [17], SMAC, TMAC, and WiseMAC cannot accommodate these diverse traffic requirements[18].

They give limited answers in terms of energy efficiency and reliability. Furthermore, they cannot handle both medical and non-medical IBSN applications. Medical data usually needs high pri- ority and reliability than non-medical data. Time critical event needs highest reliability. IEEE 802.15.4 [17] Guaranteed Time Slot (GTS) can be utilized to handle time critical events but they expire in case of low traffic[21].

The IEEE 802.15.6 aims to provide low-power in-body and on-body wireless communication standard for medical and non-medical applications. The standardization committee has sugges- ted four options to design MAC and PHY layer for a IBSN:

• To define MAC and PHY standard for on-body communication to serve immediate market needs. A slight modification to the existing MAC standard with an alternative PHY layer for in-body communication is also suggested.

• To define MAC and PHY only for on-body communication to serve immediate market needs

• To define MAC and PHY only for in-body communication

• To define MAC and PHY for in-body and on-body communication simultaneously regard- less of their effects on the availability of specification

Due to the demanding market needs and the option to serve immediately, more focus was given to the on-body sensor networks, rather than in-body sensor networks. However, it is important to note that use of in-body sensor networks is essential in life-critical situations where the implanted sensor has to be used for continuous monitoring of physiological signals without any external interferences.

Design challenges of In-Body MAC

The most challenging task in developing a low-power IBSN-MAC protocol is to accommodate in-body sensor nodes in an energy-efficient way. In-body nodes are implanted under human skin and have critical power requirements. They are totally different than on-body nodes in terms of power efficiency and data transmission rate (10kbps for medical and up to 10Mbps for non-medical applications). Moreover, they need to send emergency data in less than 1 second to the coordinator. This is a hot issue in the design and implementation of an in-body MAC.

The nodes are required to be self-triggered when exceeds a predefine threshold for emergency situation. Critical data requires low latency and high reliability [19]. The solution is to adjust initial back-off windows for critical and non-critical traffics [22]. Non-critical traffic nodes must have larger initial back-off window than critical traffic nodes. The smaller initial back-off window for the critical nodes results in lower latency.

According to Zhen et all, the use of CSMA/CA for in-body communication does not provide reliable solution [20]. The main reason is that the path loss inside human body due to muscular tissues is much higher than the path loss in free space. The in-body nodes cannot perform Clear Channel Assessment (CCA) in a favourable way. Alternatively, Ullah et all proposed a

2.1. Communication mechanisms for IBSN 13

TDMA-Based solution to accommodate the entire traffic classification of in-body nodes. The communication is based on pre-defined wake-up patterns, stored in a Pattern-Based wake-up table.

2.2 Architectural framework for closed loop medical devices using IBSN

The sensor nodes for measuring the physiological data is available off the shelf. However, for a closed loop operation it is important that the location of sensor nodes and the topology of connection between this sensor nodes, location of central control unit and communication to the external control unit have to be optimized for reliable operation. This has to be defied under an architectural framework of body sensor networks which defines its operation.

An architectural framework of communication strategies in the IBSN will define,

• Connection topology of sensor network

• Communication mechanism that is used under the topology

• Data handling in different scenarios

• Number of nodes and the duty cycle of the sensor node

• Communication to the top level protocols in OSI layer

The foreseen framework consists of different technologies fused together. Off-the-shelf sensors and chip level electronics for processing, radio communication, power scavenging, control and medical devices will be used. Communication strategies of the on-body sensor network and off- body sensor network are well established [30] [32]. Different approaches to improve the reliability factor of on-body sensor network have been published [20] [24].

2.3 Low power design and Power Scavenging

Power source is one of the key elements for IBSN. It often dominates the size and lifetime of the sensor nodes. Thus far, battery remains the main source of energy for sensor nodes.

There are different ways of using the power source efficiently. One of the ways is to reduce the power wastage in wireless radio. Wakeup radio is a key technology that is emerging as a solution to reduce the power consumption of the sensor nodes. There are also other ways such as on-node processing of data, there by reducing the amount of data transmitted. Since the power consumed by the processors now a days is much lesser than the power consumed by the wireless radio. Energy harvesting is also a key researched area, where the power required for the operation of a sensor node can be harvested from various sources. The following sections discuss in brief about the low power design and power harvesting techniques involved in IBSN.

2.3.1 Wake-up radio

The wireless communication is the most power hungry part of a sensor network apart from processing and sensing. The power consumption of the wireless radio is controlled and reduced by software mechanisms such as duty-cycling using MAC protocols as discussed in the previous

14 2.2. Architectural framework for closed loop medical devices using IBSN

CHAPTER 2. REQUIREMENTS OF PHY AND MAC LAYERS FOR IBSN

section. In the last decade, an interesting concept of wake-up radio is thoroughly being investig- ated. A wake-up radio is a transceiver radio which consumes very small amount of energy in few nW (nano-watts), than the main transceiver radio which operates usually in orders of µwatts (micro-watts). The main features of wake-up radio to consider it as a suitable candidate for the IBSN are

• Low power consumption.

• Operable only in smaller range of network.

• Reliable performance in smaller network size.

• Ability to operate outside MICS band.

• Less complex hardware required.

The main purpose of the wake-up radio is to reduce the power wastage of the main radio. The power wastage of the main radio is due to the idle-listening, over-hearing, data collision and state-switching (on state to off state and vice-versa). The wake-up radio is used to turn-on the main radio only for useful data communication. By doing so, the power consumed by the main radio for idle listening is eliminated, along with the over-hearing problems preventing the data collision to occur. Unwanted state switching is eliminated, since the main radio is turned on only for useful communication. This process will not only increase the reliability of the low power wireless communication, but also increase the energy efficiency. The wake-up radio operated in a different frequency band than the normal radio is used to send wake-up signals to the node, which on positive verification will turn on the main radio. The data communication is then initiated and completed using the main radio, reducing the total amount of time that the main radio is turned-on. The main radio is used only for useful data communication instead of idle-listening and waiting for a slot to communicate. A normal schematic of a wake-up radio [21] is shown in the Fig. 2.2

Figure 2.2: Block diagram of sensor nodes with wake-up radio

The wake-up radio can be implemented using a wake up transmitter circuit [22] as shown in the 2.3 and wake-up receiver circuit [23] as shown in Fig. 2.4. It is certain that the wake-up radio will occupy more space on the hardware of the implant. However due to the miniaturization of

2.3. Low power design and Power Scavenging 15

electronic component, the implementation of wake-up radio can be done in the chip level. One such example is the Microsemi ZL70120 [23] which is a implant module comprising of both main radio and wake-up radio in a package of 80mm × 80mm area [22]. The Fig. 2.4 and 2.3 shows the block diagram of the chip-level implementation of wake-up radio. The CC2550 from ChipCon Ltd., is implemented in ZL70120[24] as a die along with the main radio on the same module. The module from Microsemi also comprises of the matching circuit for the antenna in MICS band and 2.4 Ghz band. Thus, a wake-up radio implementation is feasible for the medical devices, meeting the requirement of the size constraints and the power reduction is highly possible by careful implementation of software. An analysis of MAC protocols using the wake-up radio is presented in chapter 3.

Practicality of MAC protocols with WUR in IBSN

Wake-up radio typically introduces additional hardware, leading to energy overhead, perform- ance overhead and chip-area overhead. With the advancement in nano-electronics and advanced chip-fabrication technologies, it is possible to have two radio transceivers operating at different frequency bands in a single chip package. Also, as shown in [25] a wake-up radio with OOK modulation can operate in the nanowatt range. The radio hardware should also have very low transition time from off-state to on-state, coping with the state transition request from the MAC protocol. The practicality of MAC protocols with WUR in IBSN can be stated as the capab- ility to communicate using dual-band radio transceivers implanted inside a human body with a bio-compatible package, meeting the requirements of different health-care services. For the hardware to be practical for the implementation in implants [26][27][28], a power-efficient yet performance-oriented control is required over the wireless medium. The power consumption and performance of MAC protocols with WUR are not traded off as it is in the case of normal MAC protocols. So clearly, the MAC layer has to be modified for a practical use of hardware available.

Figure 2.3: Transmitter implementation of 2.4 GHz wake-up radio

16 2.3. Low power design and Power Scavenging

CHAPTER 2. REQUIREMENTS OF PHY AND MAC LAYERS FOR IBSN

Figure 2.4: Recevier implementation of 2.4 GHz wake-up radio

Operation of wake-up radio

The basic operation of wake-up radio takes place in four steps as shown in fig 2.2. There can be different software to control the wake-up radio, however the steps below explain the simple methodology implemented in the Microsemi implant module.

Step 1 : Start up base station

Step 2 : Send 2.4 GHz wake-up message

Step 3 : Implant node (IMD) receives 2.4 GHz message

Step 4 : Implant node (IMD) send wake-up response in 400 MHz using main radio.

Each steps use different access mechanisms, duty cycling and power level. The efficiency of the hardware depends on the software implemented on the module. In chapter 3, different software methodology for wake-up radio is discussed. In a nutshell, wake-up radio is a suitable candidate to increase energy efficiency of the sensor nodes by eliminating the conventional energy issues faced in the main radio.

2.3.2 Power Scavenging

In parallel to power reduction, perpetual energy supply with power scavenging can prolong the lifetime of the sensor and enable long term monitoring of the patient. A number of power scaven- ging sources have currently been proposed, which include motion, vibration, air flow, temperature difference, ambient electromagnetic fields, light and infra-red radiation. For instance, Mitcheson et al. developed a vibration based generator designed for wearable/implantable devices, which is capable of delivering 2uJ/cycle [29]. Similar vibration based thin film piezoelectric energy scavenging systems was proposed by Reilly et al. [28]. A thermal micro-power generator has been developed by IMEC, which can convert thermal energy to 4uW power at 5’C temperature difference on the thermopile [29]

2.3. Low power design and Power Scavenging 17

2.4 Application scenario and requirements for MAC pro- tocols in IBSN

In order to characterize and evaluate the PHY and MAC layers of IBSN, an application scenario with implantable medical devices and sensors is explained. Consider an epileptic patient implanted with a deep-brain stimulator which stimulates the brain with electrical impulses in the event of seizure occurrences. It is possible to detect the occurrence of a seizure beforehand with symptoms that can be measured with implantable sensors such as an EEG sensor, blood flow sensor, and external sensors such as inertial accelerometers. To predict the onset of a seizure, the data from these sensors have to be processed in real time at a base station, and control signals have to be sent to the implanted stimulator to suppress the seizure. The whole process including the processing and communication has to be in real time in order to predict and suppress the seizure on time. The wireless communication between these sensor nodes and medical devices should be highly reliable with negligible delay in order to handle the situation flawlessly. Also, the occurrence of the symptoms is completely random, which means sensing should be done continuously, and occurrence of an event should be predicted locally by the sensor node. Processing the signal locally is out of the scope of this paper, but wireless communication should be efficient not only in terms of performance but also in terms of energy consumption to ensure a long-time operation. As an indication, the battery life time of implantable deep-brain stimulators is typically two to three years [30]. Few off-the-shelf stimulators are equipped with a rechargeable battery that can be recharged via an inductive link [30].

Having short-range communication such as a magnetic-induction system will not serve the purpose of communication with the base station, hence a radio frequency (RF) link with a coverage of at most ten meters and at least two meters is required. To ensure reliable RF com- munication, the underlying MAC protocol is crucial in terms of reliability and energy efficiency.

In order to standardize in-body and on-body communication, IEEE 805.14 task group 6 was set.

Network parameter Requirement Frequency of operation 402-405 Mhz

Bandwidth 3Mhz

No. of Channels 10, each channel is 300 Khz bandwidth Power of operation 25 µW isotropic radiation power

Interference Accepted Topology Star, P2P Network size 20 nodes max.

Duty cycle 0.1% for non emergency data communication Latency upto 60 ms

Throughput upto 100 Kbps

Table 2.1: Requirements for a MAC protocol in IBSN.

This task group has defined the physical layer properties of body sensor networks. Four types of communication links are foreseen: in-body to in-body communication (Scenario 1 (SC1)), in-body to on-body communication (SC2), on-body to on-body communication (SC3), and on- body to external nodes communication (SC4). Only the first two channel models are considered, where in-body communication is involved, focusing only on the IBSN scenario. In this analysis the characteristics of the proposed Physical (PHY) and MAC layer constraints set by the task

18 2.4. Application scenario and requirements for MAC protocols in IBSN

CHAPTER 2. REQUIREMENTS OF PHY AND MAC LAYERS FOR IBSN

group are considered. RF links do not propagate well inside the human body which has to be taken into account at the MAC layer for packet-loss and fading. For in-body communication, a dedicated RF band has been allocated called the Medical Implant Communication Service (MICS) band operating at 402-405 MHz, containing ten channels of 300 KHz bandwidth each with effective isotropically radiated power (EIRP) of 25 µW [31]. It is observed that, inside the human body, the MICS band can propagate with less loss and fading than the other frequency bands. The recommended network topology is a star network with a central network controller.

However, in a complex application scenario as explained earlier, the need of peer-to-peer (P2P) communication is optimal if the processing can be done locally on the sensor nodes. Scalability is not an issue, since the number of nodes is typically less than fifteen [28] [32] [33]. The data rate of the network can vary for different sensor nodes depending on the type of sensed data.

However, in our application scenario, there is no need for high-bandwidth data transmission such as video or audio. For the scenario explained above, a data rate of 20 Kbps is sufficient for reliable transmission of data from stimulator communication, blood flow sensor, inertial sensor, and EEG sensor. The requirements are set based on the recommendations from task group 6 and to meet the applications similar to the scenario explained in this section. Table 2.1 lists the requirements of IBSN.

2.4. Application scenario and requirements for MAC protocols in IBSN 19

Chapter 3

Survey of MAC protocols with and without wake-up radio for implantable sensor network

In an IBSN, sensor nodes have limited resources such as energy, size of the components (including the sensor, processor and radio), and range of communication. Despite the limited resources available, IBSN applications impose strict requirements for the wireless network in terms of communication reliability, delay, throughput, energy-efficiency and in some applications even Quality of Service(QoS). MAC protocols in wireless networks, aim for minimum delay, maximum throughput and an increased network life-time by controlling the main sources of energy waste such as idle-listening, collisions, over-hearing, and packet lost. There are a number of MAC protocols available for wireless sensor networks, even some are focussed on the IBSN applications. The following section presents a survey on the existing MAC protocols that are optimized for IBSN applications and attempts to find the potential problems in MAC protocols that needs to be solved by further research.

3.1 Features of MAC protocol

The IBSN is a special type of WSN, which varies from WSN in various features such as scalability, reliability, latency and energy-efficiency. As explained in section 2.1, IBSN has three types of communications namely, In-body communication, On-body communication, and Off- body communication. This work focusses on MAC protocols that are available for In-body and On-body sensor networks. The fundamental task of MAC protocol is to avoid collision of data packets and to prevent simultaneous transmissions while preserving maximum throughput, min- imum latency, communication reliability and maximum energy-efficiency [34]. QoS is also an important factor of good MAC protocol. In medical applications a latency of 125 ms of is only allowed, whereas in consumer electronics latency can be less than 250ms [17].Other important features include adaptability to changes in network topology, maximum achievable throughput in different network scenarios, least jitter in heterogeneous traffic, efficient bandwidth utilization with high payload, safety and security. The following table presents the expected values for different features of IBSN as per the IEEE 802.15.6 [17].

As a summary, a good IBSN-MAC should have energy-efficiency, reliability even in heterogen- eous traffic, safety and security in addition to QoS. [34]

21

Feature of good IBSN MAC Acceptable value for Implanted medical devices Throughput upto 200 KBPS for medical devices

upto 4Mbps for non-medical devices Latency upto 100ms in life critical implants

upto 2 seconds in monitoring medical devices

Bandwidth 300KHz MICS band

100MHz in 2.4GHz ISM band 1.74 MHz in 433 MHz ISM band

Duty cycling less than 0.1 % in MICS band medical devices no restriction if Listen before talk is incorporated Interference mitigation CRC, FEC, frequency agility are recommended for

safety purposes.

Table 3.1: Features of MAC protocols as suggested by IEEE 802.15 TG 6 [17]

3.1.1 Attributes of MAC as proposed by IEEE 802.15 TG-6

Wireless Body Area Network (WBAN) has attracted many researchers in academia and industry, because of its great potential to revolutionize the technology for healthcare. Due to its growing requirements, a task group have been set to standardize in-body, on-body and off-body communication [35]. The purpose of the task group is to define new physical and Medium Access Control (MAC) layers optimized for low power in-body/on-body nodes (not limited to humans) to serve a variety of medical and non medical applications. This section will briefly explain the attributes of MAC layer set for in-body and on-body by the task-group and in compliance with MICS band regulations.

IEEE 802.15.6 specification

The IEEE 802.15 task-group 6 [17] suggests that the nodes should be organized into one-hop or two-hop star network. In the case of single-hop star network a single co-ordinator controls the entire operation of the network whereas in the case of two hop star network a relay-capable node may be used to exchange data between hub and the destination-node. The entire physical channel (in time-axis) is divided into super-frame structures. Each super-frame is usually bounded by a beacon period of equal length. In MICS band regulation, the transmission of beacons bounding the super-frame is prohibited. For such non-beacon modes, where beacons are not used, the super-frame boundaries are defined by polling frames.

3.2 Requirements of MAC protocol for IBSN with differ- ent medical devices

The features of MAC protocol specified by the IEEE 8025.15 TG-6 are generalized for the in- body and on-body BSN, however the features do not exactly suit the requirements of closed-loop architecture. The focus of the TG-6 specifications of MAC protocol aim at general monitor- ing of vital signals, periodic transfer of data to the base station through inter-networking, and

22 3.2. Requirements of MAC protocol for IBSN with different medical devices

CHAPTER 3. SURVEY OF MAC PROTOCOLS WITH AND WITHOUT WAKE-UP RADIO FOR IMPLANTABLE SENSOR NETWORK

emergency event handling in case of life-critical events. The closed-loop functioning of medical devices does not only require these features but also additional features such as ability to in- telligent medium access for emergency events, reliable and low latency communication between nodes and medical devices, ability to respond quickly within the a specific time-frame for a given medical devices. It is important to know the requirements of the wireless communication in case of closed loop access. The requirements are derived for different medical devices based on the literature and medical case histories.

Network parameter

Requirement of implantable medical devices

Pace-maker Neural Stimulators Drug-delivery systems Retinal implants

• Throughput upto 100 KBPS upto 100 KBPS upto 150 KBPS upto 150 KBPS

• Latency upto 10 ms upto 30ms upto 60 ms upto 20ms

• Payload 40 KBPS 60 KBPS 30 KBPS 80 KBPS

• Duty cycling (MICS band) 0.1% 0.1% 0.25% 0.1%

Table 3.2: Requirements of data communication in implantable medical devices used to communicated to the base station controller

3.3 Taxonomy of MAC protocols for IBSN

The requirements of the existing medical devices are studied in the previous section. In order to meet the needs of the medical devices and introduce closed loop mechanism, a sophisticated MAC protocol for reliable wireless communication is needed. IBSN is a relatively new field when compared to the BAN, BSN. The closed loop medical devices fall in the category of the IBSN.

IBSN being relatively new, is not completely a different wireless domain. Many characteristics of the BSN and WSN also applies to IBSN. In order to understand the existing literature with a focus of IBSN, good classification of the state-of-the-art MAC protocols is needed. Table 3.3 shows the classification of existing MAC protocols based on wake-up radio, for the IBSN.

From the literature it is clear that use of Wake-up radio will eliminate the power wastage of the main radio. This study is focused on the MAC protocols with low energy consumption.

Moreover, impact of Wakeup radio in MAC protocols is also the focus of this study. Hence, it is wise to classify the existing MAC protocols that are developed with and without wake-up radio.

Not many literature is available on IBSN MAC protocols. Hence, features of existing MAC protocols are extracted and presented, such that these features will match the requirements of IBSN.

Time Division Multiple Access (TDMA), Carrier Sense Multiple Access with Collision Avoid- ance (CSMA/CA), Frequency Division Multiple Access (FDMA), Slotted ALOHA are the mostly used multiple access schemes in body sensor networks. Each of the access mechanism has its own advantages and drawbacks. The following sections will explain about each specific access methods, and other researches that have been done in different mechanisms.

3.3. Taxonomy of MAC protocols for IBSN 23