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DPNM, POSTECH Master Thesis Defense 1/22
Efficient Energy Scheduling of WBAN Sensors for U-Healthcare
Hyeok Soo Choi
Co-Supervisors: James Won-Ki Hong & Nazim Agoulmine
DPNM Lab.
Department of Computer Science and EngineeringPOSTECH, Korea
June 20, 2011
DPNM, POSTECH Master Thesis Defense 2/22
Outline
IntroductionProblem StatementGeneral Description of the ApproachDetails of the Solution
Mutual Information
Criteria of Sensor Selection
Implementation IssuesSimulation & ResultsConcluding Remarks
DPNM, POSTECH Master Thesis Defense 3/22
Introduction(1/2)
Image source: http://mediax.stanford.edu
Percentage of the population over the age of 65
Aging society and health care problem• Unsustainable health care cost• Health-care cost of elderly is very expensive• Early detection of disease treat disease earlier less expen-
sive
Advancement in low-power electronics, sensor technologies and wireless communication technologies
• Possibility to development small-sized biomedical sensors to monitor more efficiently elderly remotely.
Motivation
(year)
(%)
U-Health Smart HomeWireless Body Area
Network
DPNM, POSTECH Master Thesis Defense 4/22
Introduction(2/2) Wireless Body Area Network (WBAN)
This enable wireless communication between several medical sensor on the human’s body
In WBAN, sensors aims at monitoring human’s health status, activity, motion pattern, etc.
EEG
ECG
CoordinatorSpO2
Temperature
Motion Sensor
WBAN
DPNM, POSTECH Master Thesis Defense 5/22
Problem Statement(1/3)
SizeSensors needs to be as small as possible to be accepted by elderly
Idea of nano sensors !
EnergySmall size small battery
Small battery small life time
Example5mW
1.5V
DPNM, POSTECH Master Thesis Defense 6/22
Problem Statement(2/3)
High energy consumption
Existing WBANs use a Communi-cation based schema
Sensors are configured according to the communication needs (e.g., duty cycle)
Sensed data is regularly sent to the coordinator even though the data is not needed (because there is no anomaly)
1 time/ min
1 time / sec
1 time/ hour
10 times/ min
1 time/ hour
DPNM, POSTECH Master Thesis Defense 7/22
Problem Statement(3/3)Research question
Is it possible to define an alternative communication reducing the energy consumption while not missing important information that are necessary to detect health anomalies?
Define an WBAN communication to detect health anomaly (disease)
With reducing the energy consumption
With not missing important information
Research Goal
DPNM, POSTECH Master Thesis Defense 8/22
General Description of the Solution(1/2)
1
2
Idea: Inspiration from doctors method-ology Disease or
no Disease ?
1. Doctors do not try to check all symp-toms but only the most important ones (heart beat, pressure, tempera-ture)
2. If they’re ok, no further investigation3. Otherwise investigate more symp-
toms to detect a disease
DPNM, POSTECH Master Thesis Defense 9/22
General Description of the Solution(2/2) Information based schema
What is the relation between the symptoms and the diseases ?
Doctors should provide the data
What symptoms to monitor ?
We propose to use the concept of mutual information to identify the symptoms that provide the most information gain to detect particular diseases
Which sensors to activate ?
Identify the sensors that can de-tect these symptoms and ONLY activate them when necessary
What next in case of anomaly ?
Add more sensors (symptoms to detect) to increase the informa-tion gain
What is the sensor that has the highest impact on the coor-
dinator’s knowl-edge?
DPNM, POSTECH Master Thesis Defense 10/22
Mutual Information
Definition of mutual information
Measures the mutual dependence of the two variables
ExampleTwo variables, X and Y have high mutual information if you can predict a lot about one from the other.
If X and Y are independent, then knowing X does not give any information about Y, so their mutual informa-tion is zero
DPNM, POSTECH Master Thesis Defense 11/22
Entropy Linked to Healthcare Disease
An abnormal condition affecting the body of an organism
Construed to be a medical condition associated with spe-cific symptoms and signs
SymptomA departure from normal function or feeling
Indicating the presence of disease or abnormalityInformation Gain
DPNM, POSTECH Master Thesis Defense 12/22
Criteria of Sensor Selection(1/2)
H(D|S) H(S|D)I(D; S)
H(D) H(S)
H(D, S)
DPNM, POSTECH Master Thesis Defense 13/22
Criteria of Sensor Selection(2/2)
IG(ei | ej) =
H(D) H(S1)H(S3) H(S2)
DPNM, POSTECH Master Thesis Defense 14/22
Coordinator-Sensors CommunicationBased on the IEEE 802.15.4
Beacon enabled modeContention Access Period (CAP)
CoordinatorCalculate information gain per every cycleID Pending Address Fields (PAF)Beacon frame broadcast
Medical sensorsDoes PAF contain medical sensor’s ID?
• YES senses human body and then transmits sensed data to the co-ordinator
• NO goes to sleep mode
DPNM, POSTECH Master Thesis Defense 15/22
Simulation Environment(1/2) Simulation tool : NS-2 (version 2.31) MAC protocol : IEEE 802.15.4
Beacon enable mode
Routing protocol : NOAH (No Ad-Hoc Routing Agent) The number of sensor nodes: 7 The number of diseases: 5 The number of symptoms: 7 Simulation time : 1 day
DPNM, POSTECH Master Thesis Defense 16/22
Simulation Environment(2/2)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
D1
D2
D3
D4
D5
24(h)
Part 1 Part 2 Part 3
Diseases occurDiseases do not happen
D2 occurs from 3 AM to 4 AM
Simulation Scenario
DPNM, POSTECH Master Thesis Defense 17/22
Simulation Results(1/3)
Total energy consumptionCB’s energy consumption rate is con-
stant
IB’s energy consumption rate changes according to the user’s
health state
Part1 Part2 Part3
DPNM, POSTECH Master Thesis Defense 18/22
Simulation Results(2/3)
Energy consumption per sensors
•Sensors that be-long to CB (S8 ~ S14) have con-stant energy con-sumption rate
•Compact subset (S1, S2) acts like sensors that be-long to CB
DPNM, POSTECH Master Thesis Defense 19/22
Simulation Results(2/3)
Energy consumption per sensors
•Other sensors’ (S3 ~ S7) energy consumption rate changes accord-ing to the user’s health state
DPNM, POSTECH Master Thesis Defense 20/22
Simulation Results(3/3)
Expiration time vs. Latency
Expiration Time (s)
Late
ncy(s
)
DPNM, POSTECH Master Thesis Defense 21/22
Concluding Remarks
We proposed an information based scheduling schema
Information gain model using mutual information
By introducing our solution, medical problem can be detected by medical WBAN on a longer period of time
Future worksFinds compact subset of sensors by defining more feasible information gain
Develops distributed information based communica-tion
DPNM, POSTECH Master Thesis Defense 22/22
Q & A
DPNM, POSTECH Master Thesis Defense 23/22
Simulation Scenario
Dis-ease Symptom
D2
D3
D4
D5
2
2
5
7
4
6
6
1
2
3
5
3
Relationship between disease and symptom
1 42D1 p(D1 | e1) > p(D1 | e2) > p(D1 | e4)
DPNM, POSTECH Master Thesis Defense 24/22
General Description of the Solution
Combining high information gain and energy effi-ciency
Use of a utility function
Combining the information gain and energy consumption
The objective is to choose the sensors which re-flect the larger dependency on the target dis-ease and which have the lower operational cost (including energy consumption).
DPNM, POSTECH Master Thesis Defense 25/22
Details of the Solution(1/2)
The operation cost function is defined as fol-lows
: set of outgoing links at node n
: duty cycle of sensor
: Power gain from transmitter of link k to the receiver of link l
C : total amount of initial energy
DPNM, POSTECH Master Thesis Defense 26/22
Details of the Solution(2/2)
The objective function is augmented with a weighted cost functions as follows
: information utility of including the symptom ej
: communication cost
: relative weight between the information utility and communication cost
Based on the objective function, the criterion for se-lecting the sensors has the following form
DPNM, POSTECH Master Thesis Defense 27/22
Communication Processes
DPNM, POSTECH Master Thesis Defense 28/22
IB Details of the Solution Algorithm (1/3)
The mutual information between diseases and symptoms are calculated off-line.
The coordinator detect and register information about:
List of medical sensorsInitial energy level
Start
Initialization
Send information query
Wait for information
Update knowledge
Knowledge is good enough?
Yes
No
Sensor selection
Anomaly detection
DPNM, POSTECH Master Thesis Defense 29/22
IB Details of the Solution Algorithm (2/3)
Start
Initialization
Send information query
Wait for information
Update knowledge
Knowledge is good enough?
Yes
No
Sensor selection
The coordinator Evaluates the knowledge
The knowledge is updated as measurement as received by the coordinator.
The probability of detection of a particular disease depends on the collected information:
p(D|{ei} i∈B)
Anomaly detection
DPNM, POSTECH Master Thesis Defense 30/22
IB Details of the Solution Algorithm (3/3)
Start
Initialization
Send information query
Wait for information
Update knowledge
Knowledge is good enough?
Yes
No
Sensor selection
The coordinator selects the WBAN sensor which maximizes the infor-mation utility based on the knowledge state, p(D|{ei} i∈B)
sends a request to the se-lected sensor to activate
updates the knowledge state
e.g. p(D|{ei} i∈B ∪ ej)
Anomaly detection
DPNM, POSTECH Master Thesis Defense 31/22
Notation
Superscript t : timeSubscript i ∈ {1, . . . , K} : sensor indexSubscript j ∈ {1, . . . , N} : diagnosis
indexDj
(t) : Diagnosis state at time t
Ei(t) : Measurement of sensor i at time
tE(t) : Measurement history up to time t
E(t) = {e(0), e(1), … e(t)}E(t) : Collection of all sensor mea-
surements at time t E(t) = {e1
(t), e2(t), …
em(t)}
DPNM, POSTECH Master Thesis Defense 32/22
Information Utility MeasureMutual Information
Given two random variables x and y, their mutual in-formation is defined in term of their probabilistic den-sity functions p(x), p(y), and p(x, y)
The information contribution of sensor j with mea-surement ej
(t+1) can be given by the sequential Bayesian estimation
The mutual information reflects the expected amount of change in the posterior knowledge brought by sen-sor j
DPNM, POSTECH Master Thesis Defense 33/22
Demo Room
Light Path forObstacles avoidance
ECG SensorWireless
Sensor Base
Context – U-Health Medical Smart Home @ Postech
Video Control System
Remote AirConditioning Control
Environment Sensors(Light, Temperature,
Humidity)
Remote WindowOpening /Closing
Control
Light Path forObstacles Avoid-
ance