Place Sticker:
Energy Harvesting Approach for
Low Cost Wi-Fi AP Positioning Infrastructure
Nobuhiko Nishio1, Ryuta Oka1, Tran Xuan Duc1, Ismail Arai2
12

College of Computer Science and Engineering, Ritsumeikan University.
The Research Organization of Science and Technology, Ritsumeikan University.
1-1-1 Noji-Higashi, Kusatsu, Shiga, Japannishio@cs.ritsumei.ac.jp, {okayan, duc, ismail}@ubi.cs.ritsumei.ac.jp

Abstract. In order to realize a feasible and low-cost indoor positioning, we
implemented prototypes of energy-harvesting Wi-Fi AP’s and conductedevaluation experiments at the underground shopping mall in Osaka. Solar panel
driven access points only transmit a unique beacon but do not perform anyother functionality in order to conserve energy. The unique beacon can be used
for Wi-Fi positioning from ordinary Wi-Fi devices. We controlled beacon
transmitting power to adapt positioning precision and obtained average 0.4m at
most and 3.2m at 10m-pitch layout for both stationary status and walking status
in the underground mall.
Keywords: indoor positioning, energy harvesting.

1Introduction

Indoor positioning technology is investigated these days [1]. However, manyapproaches require newly constructed infrastructure and/or mobile or wearable
devices. For example, indoor GPS technology, which is called IMES [2] in Japan,
requires many signal-sending devices act as landmarks as well as rewrite the firmwareof the receivers already on the market. Infrared [3], ultrasonic [4], visible light [5] andso forth also require new devices that should be equipped both in the mobiles andinfrastructure.
Among the conventional positioning technologies, only the Wi-Fi AP positioning
method [6] is good for feasibility and availability both of the mobiles andinfrastructure. Currently, many people already carry Wi-Fi capable mobiles, and WiFi positioning services have already been usable. On the other hand, indoor use of theWi-Fi positioning service seems far from ideal performance for ubiquitous usage. WiFi positioning requires access point (AP) survey war-driving beforehand. Our survey
reports no AP can be heard at nearly half of the 1079 sample points in the biggest
underground shopping mall of Osaka in Japan. Therefore, we have to prepare Wi-Fi
Copyright is held by the author/owner(s)Pervasive 2011, June 12 – 15, San Francisco, CA, USAAP’s proactively and these should be optimized for positioning. In this approach,normal Wi-Fi equipped mobile devices can be used, so that low-cost AP should bedesigned for optimization of positioning services.
In this article, we introduce our prototype implementation of an energy harvesting
Wi-Fi access point, which is optimized only for Wi-Fi positioning by omitting othercommunication functionality. In addition to this, we controlled beacon-transmitting
power so as to obtain more precise positioning compared with conventional outdoor
Wi-Fi positioning, according to the precision requirements of the location-basedservices.

2

Prototyping Place Sticker

Our prototype implementation, named “Place Sticker” (Fig 1), uses the low-power
Wi-Fi chip from GainSpan1 Inc. and we developed the firmware for the device toonly perform transmission of a unique beacon, which means we support passive scanfrom mobile devices, not active scanning. We utilized sleep mode to the full where
only 5.3mA average energy is consumed, while beacon submission requires
100~120mA at its peak consumption.
Solar panels usually produce the best electricity in solar light. However, we use theunder fluorescent lights where only 1/5~1/10 energy would be produced. Therefore,
we adopted a dye sensitized type solar cell (DSC) which has peak performance under
the light frequency of fluorescent lights. A 1-cell DSC module (Fig 2) has dimensionsof 105mm 35mm 2.3mm, weights 19g, and produces up to 0.46V in fluorescentlight. We prepare an energy harvesting module with a series of 4 DSC cells and apower stabilizing circuit, which upgrade the power to 3.3V.

Fig 1. Place Sticker

3Fig 2. DSC (Dye sensitized type Solar Cell)

Positioning Precision Control

We try to put Place Sticker prototype in the best positions as well as known
positions. We assume they are placed next to fluorescent light tubes in the ceilingwith their antennae facing straight toward the ground. This is because only straightwaves with little reflective waves would improve positioning precision. Normal Wi-Fi
1

http://www.gainspan.com/

transmission covers a distance of 100m and is too strong to realize ideal positioning;
this gives many reflective waves, and therefore the variety of signal strength would
become wide, even at the same distance from the access point. As the power control
for each transmitting beacon, we packaged the module with an antenna in a box ofaluminum with a 3.2 cm square hole (Fig 3). Packaging in such a metallic box would
effectively decrease reflection of the beacon waves. This approach results in thesharper figure of RSSI distribution. (Fig 4)

Fig 3. Packaged Place Sticker

Fig 4. The effect of the aluminum shield4

Evaluation Experiment

We conducted an evaluation experiment at an underground shopping mall in Osaka.
The site is shown in the Fig 5. 10 prototypes of Place Sticker are prepared and put influorescent light fixtures attached to the ceiling. We evaluated two types ofpositioning, stationary performance (132 times evaluation at the 10m-pitch layout for
store-wise location and 16 times evaluation at the 2m-pitch layout in quest for the bestprecision), and walking performance (10m-pitch layout, one tracking). To evaluate
the stationary performance, we tapped the correct position on the map of our own
Android application and measured the average error of positioning. In evaluations ofwalking performance, we obtained 29 samples of positioning and their correct
locations from the log data of tracking the straight way at 1.33m/s. We obtained
stationary performances of 3.2m and 1.2m at the 10m-pitch layout and the 2m-pitchlayout, respectively. Also, we obtained a walking performance of 3.3m average
positioning error when server communication delay is eliminated.

Fig 5. Experimental Field

5Conclusion

In order to realize a feasible and low-cost indoor positioning infrastructure, wedeveloped a DSC-based energy harvesting approach for positioning optimized Wi-FiAP. A prototype AP device was developed and we conducted the evaluation
experiment at the underground shopping mall in Osaka. Normal fluorescent lighttubes produced enough power to send Wi-Fi beacons that were used by ordinary WiFi equipped mobile device for positioning.
In the future, we plan more compact packaging and energy conservation in order tomanufacture Place Sticker for actual markets.

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