iPoint: A Platform-independent Passive InformationKiosk for Cell Phones

Hooman JavaheriCollege of Computer and Information Science

Northeastern UniversityBoston, MA 02115

Email: hooman@ccs.neu.edu

Guevara NoubirCollege of Computer and Information Science

Northeastern UniversityBoston, MA 02115

Email: noubir@ccs.neu.edu

Abstract—We introduce iPoint, a passive device that caninteract and deliver information to virtually any mobile phoneequipped with a WiFi network interface and a camera. TheiPoint does not need any battery but harvests energy fromthe phone WiFi transmissions. The iPoint delivers informationto the mobile phone through a low power LCD display thatcan be captured and processed by a software application. Weintroduce a mechanism of Packet Width Modulation (PWM)to encode the phone requests in the length of WiFi packets.This allows the use of phones not equipped with RFID readers,and still allows the ultralow power microcontroller to decodethe information. In this paper, we describe the architecture ofiPoint, discuss the design choices of each component, and reporton the experimental evaluation of our prototype. Various Radio-Frequency energy harvesters are discussed and a WiFi tailored,modified Greinacher voltage multiplier, a highly efficient parallelfull-wave rectifier, is designed, prototyped and fully character-ized. It features an energy-harvesting efficiency of up to 72%and collecting up to 2.5mW from a phone WiFi transmission.An ultralow-power micro-controller with LCD capability (TIMSP430F417) is optimized, and customized to interface with ourRF Front End (RF-FE). A PWM demodulator is designed, asan integral part of the energy-harvester, and interfaced withthe microcontroller. Finally, the mobile phone application fordecoding the LCD output is presented. The evaluation is basedon actual measurements carried on the third generation ofprototypes we built.

I. INTRODUCTION

Providing information anytime, anywhere, to anybody is achallenging task. Consider an application where we want todeliver information to any person equipped with a standardsmartphone in even remote locations where there is no net-work coverage, and where no source of energy is available.The information should be delivered from a device that canlast decades without maintenance. Examples, of applicationsinclude information delivery to hikers lost in the woods (e.g.,directions, closest points for assistance), caves, and also high-density and interactive information tags (e.g., tourists informa-tion, museums). We explore various technologies, introducenew communication paradigms and build a prototype of sucha system that we call iPoint.

In the process of designing the iPoint, we focussed on fivekey characteristics:

• Universality: the device should be capable of interact-ing with any smartphone, equipped with the commonly

LCD Panel

Camera

Wifi Antenna

Wifi Antenna

Interpretted

Information

to the user

INFORMATION

ENERGY &Information

iPoint

Fig. 1. Outlook of the concept.

available WiFi interface and camera, without hardwaremodifications. Installing a software application should besufficient to enable the ad hoc interaction.

• Longevity: the device should be able to last foreverwithout having to rely on a battery, or other depletableor unpredictable sources of energy (e.g., solar). It shoulddraw all its energy from the interacting smartphone.

• Efficiency: the device should consume as little energy aspossible for both receiving, computing, and transmitting.

• Interactivity: the device should be able to accept, process,and reply to specific requests from the user.

• Sturdiness: the device should be robust to last forever andtherefore should not have moving part or mechanicallyactive components such as interfacing wires or connec-tors.

These defining features led us to a design that introducedinnovative communication paradigms and techniques and theintegration of a set of fairly unrelated technologies. In thefollowing, we review the system hardware and software archi-tecture, the components, and the communication paradigmsand techniques:

• iPoint components: the iPoint consists of an RF-energyharvester optimized for the 2.4GHz WiFi band (col-lects upto 2.5mW power from a WiFi transmission and300µW for a typical use of the iPoint data encodingapplication), an integrated packet width demodulator, anultralow-power microcontroller (TI MSP43F417 consum-ing 40µW when operating at a 8KHz frequency) capable

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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Secon 2010 proceedings.

of driving a multi-segment LCD display with extremelylow energy cost.

• Smartphone: virtually any smartphone with a WiFi net-work interface and an integrated camera. The only con-straint is that the smartphone should allow the downloadof applications which is the case of most phones today(e.g., android, iphone, j2me capable phones).

• Energy-provisioning: the smartphone can transfer energyto the iPoint through electro-magnetic radiation using theWiFi interface. This is somewhat similar to the operationof an RFID, without requiring a hardware modificationto the phone to integrate an RFID reader. However,this results in severe constraints such as the limitedefficiency of energy transfer at higher frequencies, andthe limited transmission power of smartphones (typicallyin the milliwatts using a small antenna instead of severalwatts of an RFID reader using a relatively larger antenna).

• Multimodal communication: we propose two novel com-munication mechanisms to circumvent the severe energyasymmetry and constraints. 1) The information from thesmartphone to the iPoint is encoded in the WiFi packetwidth, this drastically reduces the complexity and energycost of information demodulation at the expense of asignificantly lower datarate. 2) The information from theiPoint to the smartphone is encoded on an multi-segmentLCD display requiring significantly less energy from theiPoint than a traditional RFID.

In Section III, we present the architecture of iPoint, keycomponents, and communication techniques. In Section IV, wepresent and discuss the performance of our iPoint prototype.In Section V, we discuss direction for future work.

II. RELATED WORK

RFID tags have the potential to deliver informationanytime, anywhere [1], [2]. However, RFID tags havesignificant limitations making them impractical for delivering asubstantial amount of information to commodity smartphones.First of all, equipping smartphones with an RFID readeris a significant and challenging modification to the phonehardware. Secondly, among the three types of RFIDs (i.e.,passive, active, and semi-active), only the passive ones donot require a battery and therefore satisfy the longevityconstraints of the iPoint. However, passive RFIDs require thereaders to transmit at high power (in the order of watts), withlarge antennas. Furthermore, such RFIDs are only capable ofstoring a very limited amount of information (e.g., 128 bytes)and are not capable of sophisticated interactions.

Recently, a clever alternative solution to RFID tagsand traditional barcodes, called bokode, was developed todeliver information from a dot of 3 millimeters diameterencapsulating a high density Data Matrix code [3]. Theinformation is revealed by putting an off-the-shelf camerain an out of focus mode. This solution has the advantage toreduce the size of the tag and increase the information densitybut still keeping the tag passive. However, bokode still lacks a

two-way communication capability and requires sophisticateddigital cameras (10Megapixel with a large lens) with in/outfocus capability. In the future, if smartphones becomeequipped with controllable focus cameras, the envisionediPoint system might benefit from integrating a bokode-basedLCD display to deliver information to a smartphone at alower energy cost.

Several RF-energy harvesting techniques and prototypeswere explored over the last few years. The WISP platformand its variants harvest energy from RFID reader [4], TV radiostations [5], and are capable of powering a similar ultralow-power microcontroller as the one used in our iPoint prototype.The WISP was also used as a batteryless sensor node tocommunicate with a traditional RFID reader [6]. It relies ona high energy sources (30dBm) operating at a medium RFfrequency 915MHz. The constraint of the iPoint to operate onthe low RF energy from smartphones (few dBm) and higherWiFi frequency (i.e., 2.4GHz) requires more advanced RF-energy harvesting mechanisms that we present in the nextsections. Other platforms for wireless power transfer exist butrequire either high transmission power on 915MHz band [7],or require highly customized transmitters and receivers suchas in wireless power transfer via strongly coupled magneticresonances [8].

III. SYSTEM ARCHITECTURE

In this section, we discuss the architecture of the proposedsystem. We start with detailed description of the systemcomponents including the required features, important pa-rameters, and trade-offs. Different design choices along withtheir advantages and disadvantages are discussed. At the end,we elaborate the interfaces and communication mechanismsbetween each components.

A. iPoint Components

We break down the design of the iPoint device into thefollowing components: An RF energy harvester front endresponsible for receiving wireless information and providingthe required energy to the device, and an ultra low powercomputing core with a display to process the information andshow the outputs.

1) RF energy harvester front end: As its main feature,the RF energy harvester front end efficiently harvests theenergy of the received WiFi signals from the Smartphone. Inaddition, the front end will perform a demodulation on thereceived signal and sends the information to the computingcore.

Rectifier: To collect the energy, the received WiFi signalfrom the antenna is rectified. The rectification process takesan alternating current/voltage (in this case a 2.4GHz wave)and converts it into a direct current/voltage that can beaccumulated into capacitors. Several types of rectifier circuitshave been designed to perform such task, each of whichpresents its own advantages and disadvantages. In our system,

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Secon 2010 proceedings.

Use

r In

terf

ace

Battery

An

ten

na

An

ten

na

Cam

era

WiFi chip

Memory (information)

LCD

Dis

pla

y

Rectifier

ProcessorLow-Power

Microcontroller

SmartphoneiPoint

(information)

RF Energy

Optical Channel

WiFi Channel(information)

integrated demodulator

RF Energy Havester Front End

Fig. 2. Detailed diagram of the system.

the input signal of the front end is a very low power WiFisignal with an amplitude much lower than minimum supplyvoltages for today’s active chips (i.e. 1V ∼ 5V ). Therefore,in order to build a large enough voltage to run the activecomponents, using a multi-stage rectifier (voltage multipliers)is unavoidable. Extremely small input power calls for ahigh efficiency in the rectification procedure. Using diodeswith low forward voltage such as Schottky and low leakagecapacitors significantly improves the efficiency of the rectifier.On the other hand, while half-wave rectifiers are simple andcompact, they do not show a good efficiency or ripple.Full-Wave rectifiers bring the desirable high efficiency andsmaller output ripple by operating on both phases of the inputsignal. However, a full-wave rectifier requires a differentialoutput design. Considering the characteristics of the system,a multi-stage full-wave rectifier such as Greinacher multiplieris a reasonable choice [9].

Integrated Demodulator: Having investigated differentmechanisms for demodulating the information in the receivedsignal, we found out that the output of the early stages inthe multi-stage rectifiers represents the envelope of the inputsignal provided the intermediate capacitance of the rectifieris small. Assume the input signal is a train of WiFi signalbursts with information encoded in length of the bursts (SeeFigure 3). Therefore, the envelope signal can be used asa partially demodulated information fed to the computingcore of the system. A passive external circuit will adjust theamplitude level of the signal to make it readable by chipspowered by the rectifier. This is discussed in detail in SectionIV.

Matching: The antenna and the rectifier should be carefullymatched in the frequency of the operation which in the caseof iPoint is one of the standard WiFi channels. This is doneby using a matching network between the antenna and therectifier. Normally, the values for the matching network

are calculated experimentally. Note that there is always atrade-off between having a better match and the bandwidth ofthe system. Hence, the matching network should be optimizedto get the maximum power from the WiFi signal with thebandwidth of approximately 22MHz.

Regulatory Circuit: In some cases, a rectifier can buildan excessively large output voltage. This may be destructiveto other iPoint components. In order to avoid any possibleharm, the output voltage of the rectifier is connected to avoltage regulator.

In our prototyped device, we used a modified 10-stageGreinacher multiplier tuned on WiFi channel 1 (2.412GHz).The front end shows an efficiency up to 82%. We providemore details on the design and its performance in Section IV.

2) Computing Core: The Computing Core of the iPointreceives the demodulated request from the integrateddemodulator of the front end, processes it, and sends thecorrespondent output to the display. This component isthe active part of the device. The required power for theoperations is obtained from RF Energy harvester front end.Due to the very restricted budget of the energy, the coremust function in ultralow power fashion while providingmoderate computing capabilities. This makes the use ofmicrocontrollers (MCUs) a preferred design choice in thiscase. Ultralow power MCUs such as TI MSP430 series aredesigned and optimized for low power embedded devices andhave the desired computing power including LCD driver. Welowered the clock frequency of the MCU in order to reducethe power consumption even further.

The display power consumption should be minimizedat the same time. LCD displays require very little energyconsumption compared to other display panels like LED thuswould be a very good candidate for the design. The mainfactor in LCD energy consumption is the size of the LCD.Larger LCD panels need higher voltages for providing the

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same contrast on the panel. This translates into a highervoltage requirement for the MCU LCD driver, and a highersupply voltage, which results in higher power consumptionof the core.

In the prototyped system, we used a TI MSP430F417MCU with the capability of driving 96 segments of LCD.We underclocked the MCU to 8KHz to minimize the powerconsumption of the core (< 50µW while driving the LCDdisplay). This is well below the capabilities of the RF energyharvester. More details about the performance of the designare provided in section IV.

B. Smartphone component

In order to achieve the desired universality for the system,we kept this part as simple as possible. The iPoint systemworks with any smartphone equipped with a camera and aWiFi Interface. The only component of the system addedto the smartphone is the application installed on the phone.Obviousely the phone should allow downloading applications.A suitable API is necessary to give the application a moderatecontrol over the WiFi interface if needed. There will be nochange to the hardware of the phone. Installed application al-lows the user to send different requests to the iPoint, processesand interprets the information received from the device, andinteracts with the user by phone’s UI (Keyboard, touch screen,etc.).

C. Communication between components

Due to severe energy constraints, the components of thesystem should perform their task and also communicate withone another consuming minimum amount of energy. One pos-sibility to reduce the energy consumption is to keep each mod-ule as simple as possible. This demands novel communicationmechanisms between different components. In our system, weintroduce two different communication channels: Smartphoneto iPoint (WiFi), and iPoint to smartphone (Optical).

1) Smartphone to iPoint (WiFi) channel: In the WiFichannel, information is encoded in and transferred by WiFipackets. Because of the limited energy budget of the iPointdevice, implementing an active WiFi demodulator is notfeasible. We propose Packet Width Modulation (PWM), anovel modulation scheme that allows us to demodulate theinformation with the integrated demodulator in the front end.This significantly reduces the complexity of the demodulatorand energy consumption of the decoding processing at theprice of lower rate of communication. PWM also makes theencoding feasible from any smartphone with a socket networkAPI.

PWM Encoding: In PWM, information is encoded in thelength of the transmitted WiFi packets by the smartphone.Each length represents a specific symbol in the code. Amessage is defined as a sequence of WiFi packets. In WiFicommunication, packet width is determined by the sizeof the packet, rate of communication, and fragmentation.

Wifi Signal

to demodulator

Envelope signal

to MCU’s PWM port

MCU Sampling

Data saved to memory

011111000111111111111111100011111111111100 M0 M3 M2

Bits saved in the memory, to be software decoded by the MCU

M0 M3 M2

Fig. 3. PWM decoding.

To simplify the encoding process, we configure the WiFiinterface of the smartphone to send out packets at a fixedrate. Furthermore, the WiFi fragmentation threshold is setto the maximum possible to prevent any interference withthe PWM encoding process. Since the receiver side (i.e.,iPoint) does not have the transmission capability, it sends noacknowledgement packets back. This applies both for the linklayer (WiFi) and transport layer. We therefore use broadcastpacket to prevent WiFi retransmissions. We also limit ourselfto UDP packets since the one-way nature of this channelrules out using TCP in the transport layer but also to be ableto control the timing for packets transmissions. Assumingthe PWM mechanism uses M different packet lengths, eachpacket encodes log2 M bits of information.

PWM Decoding: It is possible to retrieve the length ofthe received packets simply by detecting the envelope ofthe signal. Note that a single stage of the RF front end’srectifier is essentially an envelope detector circuit. Becauseof the small capacitance of the intermediate capacitors, wecan obtain a quite accurate envelope signal. The first stagepresents the best ripple characteristics as it sees the largestamount of the load. Therefore, we use the output of thefirst stage of the rectifier as the partially demodulated inputsignal. The partially demodulated signal is connected to theMCU after passing a level-adjustment external circuit. Thelevel-adjustment circuit is a passive voltage shifter that setsthe 0 and 1 level of the signal in the readable range for theMCU. MCU samples the PWM signal with an appropriatefrequency and processes them after the message is finished.For the MCU to be able to recognize the beginning andthe end of the message, we use predefined start and stop Flags.

Obviously, the smartphone needs to send WiFi packetsafter the end of the message in order to provide the energyrequired for the iPoint to process the information and sendthe reply message back through the optical channel.

PWM transmission rate analysis: Let M denote thenumber of the packet lengths in the PWM. Assume thetransmitter sends the packets at the rate, R. Let Smin bethe smallest packet size. Smin is determined by the MCUclock to guarantee packet detection and length estimation,and by the energy harvester efficiency and MCU energy

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requirements. We consider packets of size multiples of Smin,Si = i×Smin. Hence, the average size of the packet would beSavg = Smin+Smax

2 = (M+1)2 Smin. Further assume a message

is a sequence of packets separated by idle periods of the lengthequal to Sidle. We can calculate the time needed to send apacket, Tp as following:

Tp =Savg + Sidle

R=

(M + 1)Smin + 2Sidle

2R

Each packet encodes logM2 bits of information. Therefore,

RPWM =logM

2

Tp=

2 logM2 .R

(M + 1)Smin + 2Sidle

The values of Smin and Sidle are determined by themaximum sampling rate of the MCU at iPoint side. AssumingFMCU is the sampling frequency of the MCU, we have,

Smin, Sidle >2

fMCU(Nyquist theorem)

Hence,

RPWM (max) <2R logM

2

(M + 1)( 2fMCU

) + 2( 2fMCU

)

=logM

2 .R.fMCU

M + 3There are a few more constraints in the design of the system.

One might have noticed that during idle periods, iPoint isnot receiving power from the smartphone. This lowers theoutput voltage of the rectifier which may result in unwantedshut-down of the system. To maintain the harvested levelabove the desired threshold, Smin needs to be larger thanSidle. We define the duty cycle of the system as the Smin

Sidle

ratio. The minimum duty cycle that allows the system tooperate continuously, depends on the implementation and maybe evaluated experimentally. Moreover, Smax = M × Smin

should be smaller than the fragmentation threshold in thesmartphone’s WiFi interface.

2) iPoint to smartphone (Optical) channel: Thesmartphone’s transmitting power is much lower than aconventional RFID reader (few milliwatts for the smartphoneversus few watts for the RFID reader). Therefore, using asimilar scheme as passive RFID tags (i.e. back scattering)is not practical. Instead, iPoint uses a low cost way ofsending information to the smartphone taking advantage ofthe imaging and computing capability of the phone. Afterprocessing the request sent from the user, iPoint sendsthe corresponding information via the optical channel. Theinformation is encoded in series of LCD segment patterns.The smartphone captures the sequence of the patterns with thecamera, recognizes the patterns, interprets the information,and finally sends the interpreted data to the user through itsown UI. It is worth noting that all the expensive operationsare done on the smartphone side. This leads to a significantdrop in the encoder/transmitter complexity of the iPoint.This also proves to be a very energy efficient method asdisplaying information on the LCD panel requires far less

{{{

M MM

LCD Patterns(Encoded Information)

Bit stream

The Output MessageDisplay Time

Fig. 4. LCD Pixel Encoding for a M -segment LCD panel.

energy compared to conventional back scattering scheme usedin passive RFID tags. To get an intuition about the energyefficiency of LCD displays, one can think of the lifespan ofwrist watches with LCD display; they run for years on a tinybutton cell holding a small amount of charge.

LCD Pattern Encoding (LPE): Let a LCD pattern denotea combination of the LCD display’s segments where asegment can be ON or OFF. Upon receiving a request, theMCU computes the output message for the LPE, which isdefined as a sequence of distinct LCD patterns. The MCU isprogrammed in advance to run the computation instructions.Finally, the MCU displays the output message pattern bypattern on the LCD panel. An output message which consistsof n patterns on a M -segment LCD panel, encodes n × Mbits of information.

LCD Pattern Decoding (LPD): At the other end of thechannel, the LPE message is captured by the smartphone’scamera. This can be done by recording a video or taking aseries of pictures at a specific rate. The phone decodes thecaptured message by running a simple pattern recognitionalgorithm on each frame. Lastly, the interpreted data is sent tothe user via UI or used in the next sessions of communication.

LPD transmission rate analysis: The LCD pattern updaterate can go up to 200Hz. However, our experiments indicatethat the deciding factor on an error free decoding is thesampling rate of the camera. Let Rc denote the maximumsampling rate of the smartphone’s camera. Applying Nyquisttheorem we have,

Rmax <Rc

2fps.

Therefore, for a M -segment LCD display, we have theupper bound of RcM

2 bps. For most of today’s commercialsmartphone cameras, Rc = 25fps.

In the prototype, we utilize a simple pattern recognitionalgorithm based on applying a detection mask on each cap-tured frame. To simplify and speed-up the detection process,the mask-frame alignment is done by the user. Section IVdiscusses the design in more detail.

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Fig. 5. The iPoint prototype boards: A) Computing core upper layer (LCDdisplay is shown) B) Computing core lower layer (MSP430 is shown) C) RFenergy-harvester front end, 10-stage Greinacher voltage multiplier.

Fig. 6. The realization of the iPoint.

PERFORMANCE EVALUATION

We prototyped a version of the iPoint based on the designdescribed in Section III. The device is shown in figures 5and 6. In this section, we explain the implementation ofthe components in detail. Several experiments are carriedout in order to accurately characterize the prototyped deviceand prove the functionality of the design components. Thissection presents the detailed description of the testbed andexperimental results.

A. Smartphone

The smartphone used in the experiments is the HTC Dreamalso known as T-Mobile G1 with Android mobile deviceplatform Ver. 1.6. This 3G phone is equipped with a 528 MHzQualcomm ARM11 Processor, 192 MB of DDR SDRAM,320 × 480 pixel LCD Display with 180 ppi, 3.2 megapixelcamera with auto-focus capability, and a WiFi (802.11 b/g)wireless interface [10].

B. Software application

A user application was developed for the prototype.This program sends the different requests modulated withthe proposed PWM scheme. The WiFi interface of thesmartphone was configured to send the packets at a fixed

START

START

AlignmentGuides

LCD Display

iPoint

[ ]1 0 0 1 0 0 0 1 0 0 0 00 0 0 1 0 10 0 0 1 0 0

RecognizedPattern

Applying Mask

Fig. 7. The pattern recognition, Sampling of the frame is performed onlyon the intersections of mask grid-lines (Narrow dashed lines).

rate of 1Mbps, the lowest rate for WiFi communication.This has the advantage of preventing the WiFi auto-ratefunctionality from interfering with the PWM mechanism. Italso minimizes the idle time, therefore delivering more energyto the energy-harvester. Ideally, the application should createan ad hoc network and send the broadcast UDP packets.However, the Android platform support for the ad hoc modeis currently limited. As a temporary alternate solution, a WiFinetwork created by an external Access Point was used bythe smartphone to send the information. Note, that is not aroadblock to the iPoint concept, since other mobile operatingsystems provide adequate support for the ad hoc mode (e.g.,Windows Mobile) and Android will most probably do thesame in the near future.

A session of the program can be described as following: Forthe LPD, the program applies a very simple pattern recognitionscheme. The application shows the live capture of the cameraon the screen. As illustrated in figure 7, there is a virtualguide on the screen that helps user to align the camera andLCD display of the phone. Having aligned the LCD panelcapture with the virtual guide, the user starts the applicationand chooses to send a request. This triggers the PWM encoderthat encodes the request with PWM mechanism and sends itover the WiFi interface. At the same time, the camera startsrecording the capture and saves the frames. Since the useralready aligned the LCD panel, the LPD decoder samples apredefined points on each capture and decodes the informationbased on ON segments on each frame. Finally, the interpretedresults are shown on the screen.

C. iPoint Hardware

1) RF energy harvester Front end performance evaluation:As depicted in Figure 8, the implemented rectifier is a10-stage modified Greinacher voltage multiplier. This,basically is a full-wave rectifier built from two mirroredhalf-wave rectifiers connected to the same RF input. This

IV. PROTOTYPE AND

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Impe

danc

e M

atch

ing

Net

wor

k

GN D

R egulated Voltage

Vol

tage

Reg

ulat

or

Antenna

+

-

Envelope signal(The output of integrated demodulator)

+ -

Level Adjusting CircuitTo MCU’s sampling port

Fig. 8. RF Energy Harvester Front end including integrated demodulator.

design gives us a double output voltage and improves theripple characteristics of the device. Since the rectifier andthe computing core do not need to share a common ground,there is no need for a differential design for the interface.We simply ground the negative output of the rectifier. Tomaximize the performance of the rectifier, we use highquality low-leakage RF capacitors (Cap names here) andlow-threshold Schottky diodes (HSMS-282 series from AvagoTechnologies). These Schottky diodes have the lowest forwardvoltage (150 ∼ 200mV ) among all bulk diodes available inthe market and are optimized to work in the frequencies above1.5GHz [11]. The intermediate capacitors are chosen small(few picofarads) to maximize the output voltage level. incontrast, a very large load capacitance in the order of 10µF isinstalled to reduce the ripple and big voltage drop on the load.

The rectifier was matched on WiFi channel 1 (2.412GHz)experimentally (using a network analyzer) with an LCmatching network. In our approach, the rectifier is tuneduntil the output voltage is maximized in the frequency ofoperation. Note that the parallel RF input will reduce theinput impedance of the circuit and makes the matchingprocedure easier to deal with. The frequency response of thecircuit obtained from the network analyzer before and aftermatching is illustrated in Figure 9.

In order to characterize the efficiency of the energy-harvester, a MXG Vector Signal Generator was used tofeed the rectifier and the output voltage level of the energy-harvester was measured. The signal generator was connectedto the energy harvester via a 0.5 feet coaxial cable. Theenergy harvester was fed with a WiFi signal in a widerange of input power, from −20dBm to 15dBm. The outputvoltage and efficiency were measured without a load andwith a load of 140KΩ. This 150KΩ load was chosen becauseit corresponds to the MCU impedance in active mode. Theoutput voltage is presented in Figures 10 and 11.

Integrated PWM demodulator: In order to test the

2.2 2.25 2.3 2.35 2.4 2.45 2.5 2.55 2.61

1.5

2

2.5

3

3.5

4

4.5

5

5.5

Frequency (GHz)

Sta

ndin

g W

ave

Rat

io (

SW

R)

MatchedUnmatched

Fig. 9. SWR values for the rectifier circuit before and after matching. Thedata obtained from a E5062A ENA Series network analyzer from AgilentTechnologies.

−25 −20 −15 −10 −5 0 5 10 1510

0

101

102

103

104

105

Input Power (dBm)

Rec

tifie

rs’s

Out

put V

olta

ge (

mW

)

140 K ΩNo Load

Fig. 10. Performance of the energy harvester unit.

−20 −15 −10 −5 0 5 10 15 2020

30

40

50

60

70

80

Input power (dBm)

Rec

tifie

r E

ffici

necy

Fig. 11. Energy-harvester efficiency as a function of input power.

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1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.0

50

100

150

200

250

Voltage (V)

Inpu

t cur

rent

(μA

)

8 KHz

16 KHz

64 KHz

128 KHz

Clock Frequency

Fig. 13. Power consumption of MCU for different clock frequencies.

functionality of the integrated demodulator of the front end,packets with different sizes were sent over the WiFi channelby the smartphone while the output of the energy harvesterand integrated demodulator being measured. The rate ofcommunication was fixed to 1Mbps. The result is shown inFigure 12.

2) Computation core performance evaluation: For thecommunication core, we embed a TI MSP430F417, anultralow power microcontroller from Texas Instruments. This16-bit flash MCU provides our desired computing capabilitiesat low power consumption. It features 32KB + 256B offlash memory, 1KB of RAM, Low supply voltage of 1.8 V,integrated LCD driver for 96 segments, on-chip comparatorthat can be used for finalizing the PWM signal demodulation,and very low active power consumption of 200µA at 1MHz,which makes it a preferred choice for the iPoint prototype.

The power budget of the energy harvester is extremelylimited due to the small transmission power of the smartphone.Meeting the requirements of this very low power scenariorequires the MCU to reach its minimum power consumption.One possibility to reduce the power consumption of the MCUis to lower the clock frequency. Since the operations of theiPoint’s computing core do not demand a very fast clock, itis possible to reduce the clock frequency to a few kilohertz.This is an area that differentiates the iPoint design fromother approaches in low-power embedded computing whichrequire that the MCU operates at much higher frequencies(i.e., few megahertz) [4]. Figure IV-C2 illustrates the resultsof our measurements of the power consumption of the MCUrunning the same instructions at different clock frequencies.

Minimum duty cycle as one of the important characteristicsof the iPoint system was discussed in section III. An

0 200 400 600 800 1000 12000

0.5

1

1.5

2

2.5

Size of the packets (bytes)

Max

imum

rec

tifie

d po

wer

(m

W)

for

150

KΩ

load

6 ms20 ms100 ms

Sidle

Fig. 14. The rectified output voltage as a function of the packet length fordifferent idle times Sidle (therefore duty cycles).

experiment was performed to evaluate the minimum dutycycle of the prototyped iPoint device. The results of theexperiment are summarized in figure 14.

LCD Display: The LCD panel selected for this generationof prototyped device is a 26 segment watch LCD display.For this selection, the power consumption of two LCD panelswith different sizes were measured. Panel 1 (1.5cm×3cm, 24segment), and Panel 2 (1cm × 1.8cm, 26 Segments). A testimage was taken from the LCD panels at the same distanceand under the same light environment while the same patternwere displayed on both panels. The contrast of the panelswere compared digitally in Photoshop. It was shown that thelarger LCD needs 2.9V to create the same contrast as LCDpanel 2 does being driven by 1.9V . The input current of theboards were measured 49µA and 21µA respectively.

V. IMPROVEMENTS AND FUTURE WORK

In the short term, we will investigate the channel capacity,and BER for both the Packet Width Modulation and Opticalfeedback. We will propose error correction codes speciallydesigned for these two channels. We also plan to investigatethe adaptation of the bokode concept to utilise an ultralowpower LCD display with high information density.

VI. CONCLUSION

We introduced iPoint, a device that can interact with com-modity smartphones, equipped with a WiFi network interfaceand camera, therefore enabling ad hoc and universal commu-nication. At the core of iPoint lies an ultralow power micro-controller. iPoint draws the integrality of its energy from thesmartphone transmissions, through an RF energy-harverster,making the use of batteries unnecessary and guaranteeing itslongevity. Two new communication paradigms are introduced:1) Packet Width Modulation allows the smartphone to encodeinformation in the width of the WiFi packet and making

This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Secon 2010 proceedings.

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idle period.

Envelope voltage measureddirectly from integrateddemodulator

Vcc

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200 Bytes Packet400 Bytes Packet

800 Bytes Packet

Fig. 12. The outputs of Energy harvester (Vcc and integrated demodulator (envelope signal). Note that the size of the packet is easily detectable. The WiFicommunication rate was fixed to 1 Mbps. harvester’s output level is fairly smaller than the peaks of envelope signal. That reason is the long idle timesbetween packet transmissions. The data is captured by a Infinium MSO8104 oscilloscope from Agilent Technologies. The plot is regenerated in MATLAB.

demodulation extremely energy efficient, 2) LCD informationencoding and camera decoding. We discussed several designpossibilities and built a prototype of the iPoint. We reportedon the performance of our system for various transmissionpowers, operation frequencies of the microcontroller, packetsizes and duty cycles.

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This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE Secon 2010 proceedings.