Powering Future Mobile Phones Through RFEnergy Harvesting

Ankush Jolly, Mansi Peer, Vivek Ashok Bohara,Wirocomm Research Group, Department of Electronics and Communication, IIIT-Delhi, India

Email: ankush12128@iiitd.ac.in, mansip@iiitd.ac.in, vivek.b@iiitd.ac.in

Abstract—In this paper we present the preliminary measure-ment results of harvesting radio frequency(RF) energy from themobile phones. The aim is to revolutionize the way mobile phonesare being charged and paving a way of charging the future mobilephones through RF energy harvesting. In order to measure theamount of energy that can be harvested, mobile phones from twodifferent manufactures namely Asus and Samsung have beenused. It was shown that depending on the manufacturer it ispossible to harvest 1.53 joules amount of energy per day.

Index Terms—Mobile phone, output DC voltage, RF energyharvester.

I. INTRODUCTION

It has been reported in [1], that the number of mobiledevices used worldwide has outnumbered the total world pop-ulation. The technologies are constantly evolving and demandoperations that are energy-intensive. Hence, the need of thehour is to move towards more efficient sources of energy.Recently, there has been growing impetus on developing RFenergy harvesting as an alternate source of energy which isreadily available and is also non-polluting. With ever increas-ing usage of mobile phone, we believe the best way to powerthe mobile phone can be a scenario where the mobile phoneharvests energy from the radio signals emitted by other mobilephones.

To explore the feasibility of harvesting RF energy frommobile phones, we measured the RF power emitted by themobile phones when the mobile is operating in uplink wherethe frequency band of interest is GSM 900 MHz band.The measured RF power has been demonstrated through thespectrum plot in Fig. 1. It can be observed from Fig. 1 thatthere is a spike in RF power level when a call is initiated bythe mobile phone. It was also observed that the received RFpower from mobile phones usually fluctuate depending on thechannel conditions, distance from cellular towers etc. In thispaper, we have worked with a RF energy harvested circuit,designed and implemented in our communication laboratoryat IIIT-Delhi, that converts the incident RF energy into DCpower 1.

The measurements were conducted by using mobile phonesof some well-known manufacturers such as Asus and Sam-sung. The energy harvesting through the mobile phones hasbeen examined on the basis of its DC output voltage. Theproposed work also highlights the fact that different mobile

1For details of the circuit refer to [2]

(a)

(b)

Fig. 1: (a) No call (b) During call at one time instant

phones have different transmit power which impacts theamount of energy harvested.

II. EXPERIMENTAL RESULTS

Fig. 2 exhibits the measurement setup used to measure theoutput DC voltage when a mobile phone is placed in theclose proximity of the receiving antenna. R&S R©HF907 [3]horn antenna with a gain of upto 14 dBi has been used as areceiving antenna for the RF energy harvesting circuit. Thisantenna has high directivity in the frequency range of interest.As mentioned before, we have used two widely availablesmartphones from two well known manufacturers i.e. Asus

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Fig. 2: Measurement Setup

(a)

(b)

Fig. 3: (a) Asus Zenfone 5 Measurements (b) Samsung GalaxyGrand Max Measurements.

Zenfone 5 from Asus and Samsung Galaxy Grand Max 2. TheDC output voltage of the EH circuit has been measured acrossa 10 kΩ load resistor using a multimeter as shown in Fig. 2.Due to imperfect soldering the load resistance reduced to a

2 Interested readers can go through the specifications of this smart phonesfrom the manufacturers’ website.

value of 8.2 kΩ. For experimental validation of the proposedsetup the handsets were deliberately chosen to operate in 2Gmode so that calls are placed in the desired GSM 900 MHzband. The data was collected over multiple calls from eachhandset. The total call duration was 1 minute which wasdivided into two parts : 30 seconds of call–establishment phase(it includes, connection time, ringing time) and 30 seconds ofcall-conversation phase.

The output DC voltage of the calling sessions has beenplotted for each handset. Apart from the instantaneous values,an average DC voltage corresponding to the first 30 secondsand an average corresponding to last 30 seconds has alsobeen plotted to demonstrate the difference in RF power levelsbetween the two phases. Fig. 3(a) and Fig. 3(b) show theoutput voltage measurements for Asus Zenfone 5 and SamsungGalaxy Grand Max respectively. The voltage readings for AsusZenfone 5 show a regular fluctuation however the averagevoltage for first 30 seconds is 2.2 V while for next 30 secondsis 0.4 V. For Samsung Galaxy Grand Max, the plot is almostmonotonically decreasing with an average DC voltage for first30 seconds is more than 0.5 V and next thirty seconds isapproximately 80 mV and 0.26 V respectively.

One common observation from Fig. 3 is that output DCvoltage has dropped by 4.9 dB between the call establishmentphase and call-conversation phase. We also observed thatduring the call conversation phase output voltage is on a higherside when voice activity is higher as compared to low periodsof voice activity.

The average DC voltage obtained over the two call phasesfor the Asus handset can be easily found to be :

Average DC voltage =2.2 V + 0.4 V

2= 1.3 V. (1)

Hence the amount of power harvested can be calculated as

Power harvested =V 2

R= (1.3 V)2/(8.2 kΩ) = 0.2 mW.

(2)EH circuit can be further connected to a energy accumulator,which can store energy for future use. Statistical analyticsgiven in [4] show that a person uses a smartphone on anaverage for 2.5 hours per day. So, the energy that can beharvested for this time duration can be determined as below

Energy stored per day = ζ × 0.2 mW × 2.5 hr (3)

where ζ is the efficiency of the battery charger 3. A goodbattery charger has an efficiency, ζ = 0.85 [5]. Hence,

Energy stored per day = 0.85 × 0.2 mW × 2.5 hr = 1.53 J(4)

This amount of energy can be used to operate the future mobilephones which have an inbuilt energy harvester circuit hencelowering the dependency of mobile phones on the regularbatteries.

3Similarly, amount of energy stored can be calculated for other mobilephones.

III. CONCLUSION AND FUTURE VISION

This paper explored the feasibility of self-powering thefuture mobiles phones through RF Energy harvesting. In thisrespect, measurements results were taken for two differentmobile phones operating on GSM 900MHz band. Output DCvoltages were measured for two call duration phases, i.e. call-establishment phase and call-conversation phase. Although theoutput DC voltage fluctuates over a period of time, howeverwe were able to harvest 0.2 mW of power or approximately1.5 J of energy per day. The measurement results obtainedthrough the designed experimental RF EH prototype showsthat it is possible to harvest RF energy (albeit small amount)from the mobile phones. This motivates us to design an EHcircuit in future that can be integrated with the mobile phonesto power the mobile handset itself.

REFERENCES

[1] “More mobile devices in the world than people - how manydo you have?” [Online]. Available: https://www.connected-uk.com/more-mobile-devices-in-the-world-than-people-how-many-do-you-have/

[2] “Rf energy harvesting.” [Online]. Available: https://www.iiitd.edu.in/∼wirocomm/resources/RF Energy Harvesting.pdf

[3] “R&s R©hf907 antenna.” [Online]. Available: https://www.rohde-schwarz.com/us/product/hf907-productstartpage 63493-7982.html.

[4] “Mobile marketing statistics compilation.” [On-line]. Available: http://www.smartinsights.com/mobile-marketing/mobile-marketing-analytics/mobile-marketing-statistics/.

[5] “Energy, work and the storage battery.” [Online]. Available: http://http://evbatterymonitoring.com/WebHelp/Battery Book.htm#Section 1.htm.

[6] M. Peer, N. Jain, and V. A. Bohara, “A hybrid spectrum sharingprotocol for energy harvesting wireless sensor nodes,” in 2016 IEEE17th International Workshop on Signal Processing Advances in WirelessCommunications (SPAWC), July 2016, pp. 1–6.

[7] “Energy harvesting could be the future of mobile power.”[Online]. Available: https://www.theguardian.com/media-network/2015/jun/04/energy-harvesting-future-mobile-charging.