Energy-Efficient Design for Mobile Phone-Centered Wireless Body Area Networks

Lin ZhongRice Efficient Computing Group (recg.org)Dept. of Electrical & Computer Engineering

Rice University, Houston, TX

OutlineMotivationsLocal wireless communication

Bluetooth, WibreeGlobal wireless communication

Cellular network, 802.11, WiMaxBattery and human-battery interactionRice Orbit sensor platform

2

3

The “Last-Meter Challenge” of HealthcareCollect information about health

Beyond routine examination“In situ” measurement during daily activities

Deliver health promoting information Beyond diagnosis and consultancy“In situ” guidance to prevent diseases and promote healthy lifestyle

The last-meter situation in healthcare mostly remains unchanged and labor-intensive despite

Information Technology or Moore’s Law

4

Chronic illness care

44.7

45.4

46.2

47

47.7

48.3

44.545.045.546.046.547.047.548.048.5

1995 2000 2005 2010 2015 2020

%

% US population with a chronic condition

Healthcare systems designed for acute diseasesChronic illness care has different requirements

Long-term interaction between healthcare professionals and patientsMore responsibility on the patient side

How to facilitate the interaction and ensure patient responsibility?

Chronic conditions account for >75% national healthcare expenditures

Chronic conditions claims >7 out of 10 deaths

Source [KPT05]

Pervasive mobile phones40% and 80% world population own a mobile phone and reside under cellular network, respectively (2006)

5[PKH+06] (Abowd group, Gatech)

Phone as personal server

6

Medical server

TFA-Rice Mesh Internet

Cellular networkMobile phone with

built in health sensors

Wireless body-area health sensor and

user interfaces

•Data connectivity (wireless)•Energy efficiency•Physical-Cyber integration•User-friendly design•Health data management

Wireless health peripherals

Body-area network: the realityNo networkingPoint to point communication with the phoneSimplicityNot that many sensors

7

Health sensorsAuxiliary user interface

Mobile phone as personal server

Tier 1: wireless body-area network of ultra low-power embedded systems

Tier 3: wireless Internet infrastructure and wall-powered Internet servers

Tier 2: wireless Internet-capable high-performance embedded systems

OrbitECG Health Monitoring System

Internet

Cellular

Wi-Fi

Phone

Bluetooth

Health serverWearable sensor

9

Promoting physical activityPhysical inactivity has been clinically linked to obesity and many cardiovascular diseasesAccelerometers and wrist display

Wrist display to deliver activity

promoting information

Mobile phone analyzes sensor data and composes

health messagesMultiple sensors collect motion and context information

10

Energy efficiency challengeLocation connection

(WBAN)

Global connection (WLAN/WMAN)

Bluetooth

~100mW

8 Joule/MB

WiFi (802.11b)

~1300mW

3-10 Joule/MB

GPRS/EDGE

~725mW

115 Joule/MB

~300mW for staying

connected

~5 Joule for network

association

~25mW for staying

connected

Measurement based on T-Mobile MDA and its data service in late 2006

Phone battery: 16650 Joule (3.7 V,1250 mAh)

Battery lifetime matters

11

12

Heat matters

3 Watt phone: case temperature will be 40 deg C higher

Every one Watt increases surface temperature by about 13 deg C

Fuel cell batteries will double it due to <50% efficiency

Flotherm simulation

43 deg C is “burning” hot

Outline

Local wireless communicationBluetooth, Wibree

13

14

Bluetooth1.2 : 1Mbps (721Kbps useful data)2.0-EDR: 3Mbps (2.1Mbps useful)3.0: 480Mbps (UWB-based)Ultra low-power Bluetooth (Wibree)Three classes

Classes 1, 2, and 3: 100, 10 and 1 metersIntended applications

Cable replacementBody-area communication

15

Network topologyTwo types of devices:

Master and slaves

Source [BS00]

Piconet: Up to 7 active slaves; up to 255 inactive/ parked

Scatternet

16

DiscoverDevice discovery

Inquiry and inquiry scanService discovery

Find out what services a device provideGeneric serial portInput device (HID)Audio gateway (headset), etc.

17

ConnectA master initiates a connection by PagingA slave accepts the initiation by Page Scan

TC

TPC TPS

Time (ms)

Paging/Page Scan

TPSS

All are multiple of 625us

Tradeoff between power and latency

Energy and latency

18

Terminology confusionMaster: initiate a connection, connect to slaveSlave: accept a connectionClient = Master; Server = Slave

Especially Microsoft documentation

19

Power-saving modeHold

Device inactive for a single short periodNegotiate for the next period

SniffDevice inactive except for periodic sniff slots

Sniff subrating: longer periods (multiple seconds)Park

Disconnected, periodically check whether to reconnect (large latency)Still consume significant power

20

WBAN: Bluetooth vs. ZigBeeDesired properties Bluetooth ZigBeePoint-to-point radio No NoLow energy per bit Possible Not reallyHigh and scalable data rate Yes NoLow interference No (FHSS) Yes (DFSS)High availability Yes Not (yet)

Battery lifetime of WBAN members, including sensors

A new, simpler standard is needed. So comes Wibree

21

WBAN: Bluetooth vs. ZigBee (Contd.)First Bluetooth phone/headset: Ericsson, 06/2000

Commercially available in 2001First ZigBee phone: Pantech & Curitel, 12/2004

CNET: with nothing to connect toNothing happened since then

ZigBee is better inPower consumption (not that much)

Bluetooth is better inData rate (multimedia)Energy per bit

Microsoft & Nokia are members of Bluetooth SIGNeither is a member of ZigBee Alliance

WibreeStarted in 2004 as Bluetooth Low End Extension by NokiaReleased in October 2006Became part of Bluetooth in June 2007Ultra low power, 1Mbit/s (physical layer)Short range (10meter, Bluetooth class 3)Digital wristwatch, human interface devicesCan share hardware with Bluetooth

Dual-mode on mobile phones

22Source wibree.com

23

Bluetooth OEM serial adapterKC21 from kcwirefree.com

Status pins

UART

24

Block diagram

Bluetooth adapter can work in two modesData mode: it is transparentCommand mode: it interprets data from UART as commandsCan switch modes

Microcontroller (MSP430)

UART

Radio

Standard Windows socket or virtual serial port connection

Programming Windows MobileVisual Studio 2005/2008

Free 90-day trial from microsoft.comAcademic version for cheapProgram in C++ / C# / …

Windows Mobile 5.0/6.0 SDKFree from microsoft.comPocketPC and/or Smartphone versionsIncludes phone emulator

ActiveSyncEnables Windows XP/Vista to communicate with phone (Installed on lab computers)

Version 4.5+ sometimes incompatible with XP, use lower versions!Version 4.5+ required for Vista!

http://msdn.microsoft.com/windowsmobile/downloads/resourcekit/

25

Bluetooth: Microsoft vs. WIDCOMM stack

WIDCOMM stackFree SDK

http://www.broadcom.com/products/bluetooth_sdk.php

Pros: more direct control Cons: Does not work with smoothly with C# codeHow to tell whether it is Microsoft or WIDCOMM

Connect phone to PC, open it through Windows ExplorerIf BtSdkCE??.dll found under Windows/, it is WIDCOMM

Make sure it displays “protected operating system files”

26

27

Windows Mobile Bluetooth programming

Visual C++Standard Windows socket APIhttp://www.msdn.com

Search Windows CE Bluetooth socketaccept(), bind(), connect(), send(), recv() etc.Example code from the development kit

A good starting point

.NET Bluetooth APINo ready library in .NET Compact FrameworkThird party library: 32Feet.NET

By In The Hand Ltd. (inthehand.com)Shared-source projectMicrosoft Bluetooth stack

28

Download and InstallDownload

http://www.codeplex.com/32feet

Execute the setup file32feet.NET library ready to useDocumentation of the library found in http://inthehand.com/library/

29

Use Bluetooth LibraryInclude the library in project:

Open the project in Visual StudioRight click on “References” in the solution explorerClick on “Add Reference”Select .NET component “InTheHand.Net.Personal”

30

Use Bluetooth Library (Cont.)Add related namespace in .cs souce file

using InTheHand.Net;using InTheHand.Net.Sockets;using InTheHand.Net.Bluetooth;

31

Radio ModeIn BluetoothRadio classGet Bluetooth radio instance

public static BluetoothRadio PrimaryRadio()Set radio mode

public RadioMode ModeExample: BluetoothRadio.PrimaryRadio.Mode = RadioMode.Connectable

32

Discover & ConnectIn BluetoothClient classDiscover devices

public BluetoothDeviceInfo[] DiscoverDevices()BluetoothDeviceInfo includes device name, address, services…

Connect devicespublic void Connect(BluetoothAddress , Guid)Guid: service ID

Example: Guid SP_GUID = BluetoothService.SerialPort;33

Send & ReceiveThrough a NetworkStream instanceGet the NetworkStream instance

Public NetworkStream GetStream() in BluetoothClient class

Use the NetworkStream instanceSynchronous R&WAsynchronous R&WMore details found in .NET Framework Class Library

http://msdn2.microsoft.com/en-us/system.net.sockets.networkstream.aspx34

Outline

Global wireless communicationCellular network, 802.11, WiMax

35

Wireless mesh network802.11 based

36

WiMaxWorldwide Interoperability for Microwave Access

Intended to provide wide-area broadband, a “last mile” solution

IEEE 802.16802.16-2004: 75Mbps, tens of kilometers802.16e-2005 (Mobile WiMax) (Wibro in South Korea)Licensed band: 2.3, 2.5, 3.5GHzUnlicensed band: 5.xGHz

Invested by Sprint as the 4G cellular technology ($5b, however, cost the job of CEO)Promoted by Intel

Wi-Fi/WiMax dual-mode module (Echo Peak) for 2008

37

WiMax (Contd.)WiMax complements Wi-Fi

38

Source: Intel.com

WiMax devicesSamsung SPH-P9000 (2006)

LG KC1 (2006)

Nokia promised for 2008

39

Energy considerationStay connectedEstablish connectionTransfer dataTransmit vs. receive

40

41

Global connection impact on battery

148 hours standby battery lifetime

GPRS reporting ECG reduces the battery lifetime to less than half of the original

Wi-Fi is more efficient if ECG data are buffered more than 3

minutes

T-Mobile MDA for ECG monitoring

Energy per bit transfer

Source [OSR+04] 42

Network Conditions in Daily Life14 participants from Rice, 3-4 weeks, 2006

Wi-Fi availability: 49% Cellular availability: 99%

Source [RZ07]43

We should combine their strengths

Complementary Energy ProfilesCellular Wi-Fi

Checking for availability/ Establishing a connection

None* High5 J

Maintaining a connection None*1–6 J/min

High20–60 J/min

Energy per MB transfer

Highupload: 95–125 Jdownload: 40–50

J

Lowupload: 7–11 J

download: 5–7 J

Coverage High99%

Medium49%

* Assume phones are always connected to the cellular network44

Energy-Efficient Data TransferCombining strengths of complementary interfaces

One always-on primary interface (Cellular)One (or more) alternative interfaces (Wi-Fi)

Should the device attempt an alternative interface for data transfer to save energy?

Alternative Network (Wi-Fi) Energy Cost of a Data Transfer

No attempt Cellular transfer

AttemptUnsuccessful Wi-Fi establishment + Cellular transferSuccessful Wi-Fi establishment + Wi-Fi transfer

45

Energy Cost of Data TransferWi-Fi establishment: ~ 5 JCellular / Wi-Fi transfer: depends on size, network conditions

Signal Strength used in our energy model

Cellular signal strength / availability: FREE!Wi-Fi signal strength / availability: COSTLY!

Alternative Network (Wi-Fi) Energy Cost of a Data TransferNo attempt Cellular transfer

AttemptUnsuccessful Wi-Fi establishment + Cellular transferSuccessful Wi-Fi establishment + Wi-Fi transfer

46

Should the Device Attempt a Wi-Fi Connection?

Context-for-Wireless1. Use context information to estimate Wi-Fi

conditions without powering up the interface2. Calculate and compare expected energy costs for

each interface

Naïve: Attempt Wi-Fi for all transfers

Context-for-Wireless: Wi-Fi conditions estimated

with negligible cost

Ideal: Wi-Fi conditions

known free

47

Potential Energy Saving Average energy cost for a transfer

Using network condition traces from TowerLoggerUsing energy model from measurements

11 / 25 48

Simple EstimationUse each person’s average Wi-Fi condition

Large energy saving over cellular-onlyWe use as baseline (0%), compared to Ideal (100%)

12 / 25

Cellular

Naïve

Simple

Ideal

0%

100%

49

Hysteretic Estimation AlgorithmNetwork conditions are related in time

Re-use last measured Wi-Fi conditions up to a specific timeAttempt Wi-Fi for transfer after that time

Simple, no extra hardware

50

History & Cell ID Estimation AlgorithmHistory: People spend days in a predictable fashion

Network conditions related at same time in different daysUse Wi-Fi conditions in 1-hour partitions to train

Cell ID: Network conditions related to location

GPS is power hungry, outdoors onlyGSM localization requires training to ground truthWe directly train based on GSM Cell IDs and Wi-Fi conditions

History and Cell ID Estimation uses bothMore weight for estimation with higher certaintySlightly favor Cell ID

GPS / GSM Location Wi-Fi Conditions

GSM Wi-Fi Conditions

51

Acceleration Estimation AlgorithmNetwork conditions relatively constant at a fixed location

Use motion sensing to detect change in location3-axis accelerometer on Orbit Sensor, 32 Hz, 8 bit, BluetoothSome new devices have built-in accelerometer (for UI)

Re-use last measured Wi-Fi conditions if movement below threshold.

52

Combination AlgorithmsDetermine validity of previous measurement

HystereticAcceleration

Determine conditionsHistory and Cell ID

Re-use last measured network conditions if validUse History and Cell ID if change anticipated

53

Performance EvaluationReal-life network traces from Tower LoggerSimulated ECG reporting application

5 min. transfer interval270 kB data size

54 / 25 54

Field ValidationImplement same ECG reporting application

Upload 270 KB every 5 min., retry failed transfers1. Cellular only mode2. Context-for-Wireless mode

Hysteretic EstimationMeasure battery life with normal phone usage

Two participants, six experiments each

System Battery life: 15.4 h -> 20.8 h (+35%)

55

Outline

Battery and human-battery interaction

56

Battery properties I

0100200300400500600700800900

1992 1993 1994 1995

Year

Ener

gy d

ensi

ty (W

hr/L

)

Energy densityMoore's Law

Volumetric energy density for Li-Ion cells Source [Pow95] [Buc01]

57

Battery properties I (Contd.)

Source [PS05]

58

Battery properties II

Source [CC05]

Voltage drop

59

Battery properties III

Rate dependent Lithium-Ion battery

60Source [RVR03]

Battery properties IV

Temperature dependentLithium-Ion battery

61Source [RVR03]

Battery properties V

Capacity loss (aging)Lithium-Ion battery

62Source [RVR03]

Battery properties VI

Recovery effect

63Source [MS99]

Fuel cell for portable powerProton exchange membrane (PEMFC)Direct methanol (DMFC)

Low operating temperature (60-80 degree C)

64

Source [ESN01]

Fuel cell system

Fuel cell system schematics

40-50% efficiency Source [ESN01]

65

Power consumption trendPower Consumption @ Max. Transmitter Output Power [W]Battery Capacity Index [%]

1W100%

2W200%

3W300%

6150

04020098 06

WCDMAVoice Call

orStand-Alone

Application Space GSM

Voice Call

Multimedia Call Space

Approximate power limit of a handheld device

Battery Capacity Gap or

Reduced Operation

Time

GPRSNokia 6000

SeriesIntroduced

An Enabler of

Size Shrink

Battery Capacity

Source: Sari Tasa, Nokia Research Center66

A hot case: 3-Watt Nokia 3120

Phone case temperature will be 40 deg C higher.

Every One Watt increases surface temperature by about 13 deg C

67

Battery managementAccurate battery capacity monitoring

TI BQ2023 Coulomb (Charge) counting-based monitoringSingle-wire advanced battery monitor IC

68

Battery gas (fuel) gauge

SDQ single-wire serial communications interface

69Source TI.com

Smart battery interfaceSingle wire

Embedded clock

70

Source [Buc01]

Smart battery interface (Contd.)Two-wire SMBus system

One wire for clockTI BQ2040

71

Source [Buc01]

Windows Mobile battery APIBattery is treated as a peripheral device

Battery driver

DWORD GetSystemPowerStatusEx2( PSYSTEM_POWER_STATUS_EX2 pSystemPowerStatusEx2,DWORD dwLen, BOOL fUpdate );

72

Human-battery interaction (HBI)Battery is a usability constraint on phones

Mobile users do care about battery lifetime

80% ever worriedMany take measures to improve battery lifetime

19% turn off phones, 16% stop using some features, 13% get new batteries

More than 50% recharges happen when there are more than 50% battery left

Help users deal with limited battery lifetimeTo better utilize the battery energy

73Source

[BRC+07]]

User Types/Strategies in HBIType A

Charge regularly, regardless of battery levelContext driven (time/location)

Type BCharge based on battery level indicatorFeedback driven

74Source [RQZ07]

User Types/Strategies in HBI (Contd.)Physical vs. mental effort

A mobile user can be either type, depending on the context but we did not observed thisType A

Poor battery indicator design failed Type B users50% battery remains for Type B40% battery remains for Type A

75 75

Outline

Rice Orbit sensor platform

76

The Rice Orbit Platform

User input

Orbit Watch

Commands

GPRS / WiFi

Orbit Sensors

Orbit Edu

Applications:Health monitoringMultimedia experience recordingRemote, ambient phone interactionEducational

Bluetooth

Bluetooth Bluetooth

Phone

77

78

The Orbit Edu BoardTI MSP430 F1612 Microcontroller

16-bit, 8 MHz, Ultra-low-power (~1uA in standby)55 KB Flash ROM, 5 KB RAM2 timers (16-bit)8 ADC inputs (12-bit)2 DAC outputs (12-bit)2 UART ports48 General Purpose IO (GPIO)

MSP430 microcontroller

Open-source and open-hardware http://www.recg.org/orbit.htm

78

79

The Orbit Edu BoardKCWireFree KC21 Bluetooth 1.2 Module

Small, power efficient, built in antennaSeparate regulator, microcontroller controllableSerial communication with MSP430LED indicators on board for BT power, BT enabled, BT radioClass 2, up to 20m range

1 unused UART port

KC21 Bluetooth module

BT power

BT enabled

BT radio

On module antenna

79

80

The Orbit Edu BoardKionix KXM52 3-axis accelerometer (motion sensor)

3 analog outputs for each directionEnable pin from MSP430

National Semiconductor LM20 temperature sensorAnalog outputPowered by MSP430 (P6.5)

2 unused ADC inputs / DAC outputs

KXM52 3-axis accelerometer

LM20 temp sensor

80

81

The Orbit Edu BoardLCD connector

Contrast potentiometerUnregulated power

Standard TI JTAG programming connectorConnectors for every pin on the MSP430 & BT module8 touch sensor pads4 pushbuttonsProcessor controllable indicator LED (P5.3)

Programming (JTAG) connector

Pushbutton

Pad (for touch sensor)

LCD connector

Indicator LED

Contrast pot.

81

82

The Orbit Edu BoardPower from battery or power supply (DIP switch 1)

Li-Ion battery (40mAh, 3.7v)4 – 6 volt external supplyRegulated to 3.3 volt for MSP430, BluetoothCurrent sense resistor in series (reminder: V=IR)Power LED

DIP-switch operation

DIP switches

Current sense resistor

Battery

External power connector

Regulator for MSP430

Regulator for BT module

Power LED

82

The Orbit MiniSimilar to Orbit Edu

Amplified analog inputMicroSD card

LCD connector

Power Switch

LEDs

BatteryMicroSD cardUniversal connector

Programming (JTAG) connector

Push buttons

83

Orbit Platform Applications

Orbit Watchwith motion gesture

recognition

Orbit ECGhealth monitoring

system

84

The MSP430 Microcontroller64 pinsEach pin can be configured:

Example: Pin 5

P6.6: bit 6 of port 6 (GPIO), inputP6.6: bit 6 of port 6 (GPIO), outputA6: ADC input 6DAC0: DAC output 0

Must be configured in programsUnused pins should be configured as GPIO outputs

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?familyId=911&sectionId=95&tabId=1527&family=mcu

MSP430 F1611

85

The MSP430 MicrocontrollerLow frequency crystal oscillator

Optional, present on board Accurate real-time clockLonger sleep intervals -> lower power usage

High frequency internal oscillator (VCO)Optional crystal can be soldered on board

Alternative clock sourceCrystal: accurateInternal: software controllable

86

Programming the MSP430IAR Embedded Workbench

C compiler & debuggerFree version (4kb limit) available from ti.com

USB ProgrammerDIP 3: board powered by programmerDIP 4: board powered independently

Lots of sample/driver codeFrom ti.comFrom RECG.ORG

LCD driver and libraryBT communication / power management

87

Orbit Edu Energy ConservationBluetooth module is the biggest power consumer

Power down when not transferring dataUse power saving modes if absolutely necessary to maintain connection

Peripherals (accelerometer, temp sensor, LCD, …)Disable / power down when not in use

MSP430Write efficient, interrupt driven codeNothing to do? Go into low power statesMinimize interrupts, minimize time CPU running

Use low frequency osc. instead of full speed osc. for timekeepingUse internal hardware frequency dividers (vs. software)

88

Bluetooth CommunicationBluetooth Serial Port Profile (SPP)Orbit Edu Bluetooth module

Connected to MSP430 UART 0 No flow controlBT powered by DIP switch 2 or MSP430 P5.62 LED indicator outputs also connected to MSP430

BT_EN: Module enabled (P3.2)BT_radio: Radio activity / connection (P3.3)

Phone Bluetooth programmingIndependent socket connection to each Bluetooth deviceSample code & introduction available on course page

BT_EN BT_radio

89

90

Orbit Bluetooth energy reductionPrinciple

Remain power-off as long as possibleConnect/disconnect as fast as possible

Scheduled communicationEvery time a peripheral connected with the Orbit center, the two agree on the next time for connection before disconnect

11696

8 0.53 70

20406080

100120140

AC

TIVE

CO

NN

ECTE

D-Id

le

SNIF

F

DEE

P SL

EEP

Page

sca

n

Power

91

Connection establishment

Timing of Bluetooth page-scan sessions on Orbit adaptors

Tcs and Tpss are multiple of 625us

Orbit adaptors are connectable

Orbit adaptors are NOT connectable

92

Connection establishment: Latencies

Source [YZJ07]

93

Energy cost for connection establishment

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400 500 600 700Tcs (ms)

Ener

gy c

onsu

mpt

ion

(J)

Master: Mobile system

Slave: Health sensor

Connection establishment: Energy tradeoffs

Longer latencies

More energy consumed by mobile system

Less energy consumed by

health sensors

Tpss = 40msSource [YZJ07]

94

Orbit sensor power profile<1mA@3.3V for >90% time (Bluetooth off)Power trace for one connection

BT powered off

BT powered up

BT PENDING

Establishing connection

Connected

Rcv.data cmd.Sending sensor data cmd.

Rcv. POWER cmd.

BT powered off again

Source [ZSB06]

95

Orbit sensor average power

1mW: One week battery lifetime

Most health monitor sensors

Sensors for contextawareness

0

5

10

15

20

25

30

35

0 100 200 300 400 500 600

Communication interval (s)

Aver

age

pow

er (m

W)

Avg. phone powerOriginal phone powerAvg. sensor power

Caveats for WM phone developmentNot all Windows Mobile API are supported

Not all hardware (e.g. Bluetooth) services are supported

96

Example: BT power-saving modesWindows Mobile 5.0 API never works for us

BthEnterSniffModeBthEnterHoldModeBthExitSniffModeBthEnterParkMode

97

Example: Internet connectivityAn early version of the ROM (ROM 1.x) does not allow turning on/off Wi-Fi interface through software

If Wi-Fi/USB is connected, no DATA or connection for GPRS/EDGE

98

Example: Event-driven mechanismIn situ survey triggered by rechargingWindows CE in theory

UserNotificationTriggerCeSetUserNotificationExCeRunAppAtEventNOTIFICATION_EVENT_ON_AC_POWERNOTIFICATION_EVENT_OFF_AC_POWER

Didn’t work on HTC wizardEnd up polling

99

Example: No Bluetooth SPPMoto Q CDMA

Serial Port Profile disabled

100

Caveat: Bluetooth power-saving modePower-saving modes only works with peer KCFree Bluetooth modules

101

Acknowledgments

102

ReferencesMotivations

[KPT05] R. Kane, R. Priester, and A. M. Totten. Meeting the Challenge of Chronic Illness. The Johns Hopkins University Press, November 16, 2005.[PKH+06] Shwetak Patel, Julie Kientz, Gillian Hayes, Sooraj Bhat, Gregory Abowd. Farther Than You May Think: An Empirical Investigation of the Proximity of Users to their Mobile Phones. In Proceedings of the International Conference on Ubiquitous Computing 2006. (pdf)

Local communication[BS00] J. Bray and C. Sturman. Bluetooth: Connect Without Cables. 1st. Prentice Hall PTR, 2000.Wibree.com. Ultra-low power radio technology for small devices. (http://www.wibree.com/technology/)

Global communicationIntel WiMax. Mobilize Your Internet. (PDF)

103

References (Contd.)Energy-efficient wireless

[OSR+04] I. Oppermann, L. Stoica, A. Rabbachin, Z. Shelby, and J. Haapola, “UWB wireless sensor networks: UWEN - a practical example," IEEE Communications Magazine, vol.42, no.12, pp. S27-S32, Dec. 2004[RZ07a] Ahmad Rahmati and Lin Zhong. Context for Wireless: Context-sensitive energy-efficient wireless data transfer. In Proc. ACM/USENIX Int. Conf. Mobile Systems, Applications, and Services (MobiSys), June 2007.[LZ08] Jiayang Liu and Lin Zhong, "Micro power management of active 802.11 interfaces,“toappear in ACM/USENIX Int. Conf. Mobile Systems, Applications, and Services (MobiSys), June 2008.[YZJ06] Le Yan, Lin Zhong, and Niraj Jha, "Energy comparison and optimization of wireless body-area network technologies," in Proc. Int. Conf. Body Area Networks (BodyNets), June 2007. (PDF)[ZSB06] Lin Zhong, Mike Sinclair, and Ray Bittner, "A Phone-centered body sensor network platform: Cost, energy efficiency & user interface," in Proc. IEEE Int. Wkshp. Body Sensor Network, Apr. 2006. (PDF)

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References (Contd.)Energy-efficient user interfaces

[ZJ05] Lin Zhong and Niraj K. Jha, "Energy efficiency of handheld computer interfaces: Limits, characterization, and practice," in Proc. USENIX/ACM Int. Conf. Mobile Systems, Applications, and Services (MobiSys), June, 2005. [ZBS06] Lin Zhong*, Bin Wei*, and Mike Sinclair, “SMERT: Energy-efficient design of a multimedia messaging system for mobile devices,” in Proc. ACM/IEEE Design Automation Conf. (DAC), July, 2006. (*Equal contribution)[VZJ06] K. S. Vallerio, Lin Zhong and N. K. Jha, "Energy-efficient graphical user interface design," in IEEE Trans. on Mobile Computing, July 2006.

105

References (Contd.)Battery

[Pow95] R. Powers, “Batteries for low power electronics,” Proceedings of IEEE, vol. 83, no. 4, April 1995.[ESN01] M. W. Ellis, M.R. Von Spakovsky, and D. J. Nelson, "Fuel cell systems: efficient, flexible energy conversion for the 21st century," Proceedings of the IEEE , vol.89, no.12, pp.1808-1818, Dec 2001[Buc01] I. Buchman. Batteries in a Portable World: A Handbook on Rechargeable Batteries for Non-Engineers, Second Edition, Cadex Electronics Inc, 2001.[RVR03] R. Rao, S. Vrudhula, and D.N. Rakhmatov, “Battery modeling for energy aware system design,” Computer, 36, 12 (2003), 77-87.[CC05] R. Casas and O. Casas, “Battery sensing for energy-aware system design,” Computer 38, 11 (Nov. 2005), 48-54.[PS05] J. Paradiso and T. Starner, "Energy scavenging for mobile and wireless electronics", IEEE Pervasive Computing, Volume 4(1), 2005, pp. 18-27. [MS99] T. Martin and D. Siewiorek, "Non-Ideal battery behavior and its impact on power performance trade-offs in wearable computing," in Proc. International Symposium on Wearable Computers, San Francisco, CA, October 18-19, 1999; pp. 101-106. [RQZ07] Ahmad Rahmati, Angela Qian, and Lin Zhong. Understanding human-battery interaction on mobile phones. In Proc. ACM Int. Conf. Human Computer Interaction with Mobile Devices and Services (MobileHCI), September 2007.[BRC+07] Nilanjan Banerjee, Ahmad Rahmati, Mark Corner, Sami Rollins, and Lin Zhong, "Users and batteries: interactions and adaptive energy management in mobile systems," in Proc. Int. Conf. Ubiquitous Computing (Ubicomp), September 2007. (PDF)

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