Factors effecting positional accuracy of iBeacons For more information: contact chris@codepilots.com Chris Thomson (chris@codepilots.com)

Background

Indoor localization ? Where am I

iBeacons Small (50x30mm) Cheap (£8-30 per beacon) Range of up to 75m Battery powered Supported by Apple (unlike RFID tags) Battery powered is a very important issue, as it means they can be placed anywhere without the expense of routing power or network connections. So Ideal for quick deployments, and use in locations inside and outside where we don’t want to make changes – e.g. heritage locations. Battery’s last up to 2 years, at which point the unit is replaced.

Distance to a iBeacon These graphs show the measured power at distances from a typical iBeaco, we can use power information to estimate the distance of an receiver. Anon (2014) 'Adjust beacon range with Estimote’s new App and change UUID using Estimote’s new SDK', Reality matters [Online]. 28/1/2014. Available at http://blog.estimote.com/post/74816977799/estimote-app-v1-2 (Accessed 19/3/2014).

iBeacon localization Estimote iBeacons, from http://estimote.com If we have at least 3 beacons we can, using geometry work out the unknown location of the receiver, based on the known location of the transmitters and their distance from the receiver. Unfortunately finding the distance is quite difficult! Estimote iBeacons, from http://estimote.com

The only maths: distance The d, is the distance we want to know, the d0 a measure of distance for a known power output, the n is the way the signal power degrades over distance, and squiggle the environmental effect, the environmental effect is actually quite large! Qiu, T, Zhou, Y, Xia, F, Jin, N, & Feng, L 2012, 'A localization strategy based on n-times trilateral centroid with weight', International Journal Of Communication Systems, 25, 9, pp. 1160-1177, Academic Search Complete, EBSCOhost, viewed 18 March 2014. Bulusu, N, Heidemann, J, & Estrin, D n.d., 'GPS-less low-cost outdoor localization for very small devices', Ieee Personal Communications, 7, 5, pp. 28-34, Science Citation Index, EBSCOhost, viewed 18 March 2014.

Overcoming Environmental factors Bayesian learning (Öktem & Aydin, 2010) Real-time reference measurements (Hyo-Sung & Wonpil 2009) Clever uses of the radio (Wu et al, 2013) A fair bit of research in measuring distance in this way has been done, lots of it is to do with wireless sensor networks, three approaches seem common, these are as listed on the slide. But for iBeacons, we are limited to only the top approach as real time measurements are two power hungry, and we don’t have access to the raw radio in iOS to do anything clever. Öktem R, & Aydin, E 2010, 'An RFID based indoor tracking method for navigating visually impaired people', Turkish Journal Of Electrical Engineering & Computer Sciences, 18, 2, pp. 185-196, Academic Search Complete, EBSCOhost, viewed 18 March 2014. Hyo-Sung, A, & Wonpil, Y 2009, 'Environmental-Adaptive RSSI-Based Indoor Localization', IEEE Transactions On Automation Science & Engineering, 6, 4, pp. 626-633, Business Source Complete, EBSCOhost, viewed 18 March 2014. Wu, K, Xiao, J, Yi, Y, Chen, D, Luo, X, & Ni, L 2013, 'CSI-Based Indoor Localization', IEEE Transactions On Parallel & Distributed Systems, 24, 7, pp. 1300-1309, Business Source Complete, EBSCOhost, viewed 18 March 2014.

Practical issues to consider Differences in devices: Tablets, Smart phones and iBeacons Noisy environments Objects in the environment Cost of deployment and maintenance Limited access to raw hardware The main issues and limitations are as listed on the slide, the questions we can actually investigate are underlined. I only have one hardware device for testing at the moment, which limits this initial investigation. How the signal is received and transmitted Now noisy is the environment, and its effect Humans and other objects

Solutions?

Experimental setup 4mx3m Test ‘Lab’ IPad Air (2013 wi-fi only model) 3 Estimote pre-production iBeacons (early 2014) Core Location SDK and Estimote SDK Trilateration algorithm (wwnick &Austin, 2010) Its worth mentioning here, that at this stage, all of the above is basically standard practice available on the market, wwnick, Austin, J. (2010) 'Trilateration using 3 latitude and longitude points, and 3 distances', Geographic Information Systems Stack Exchange [Online]. 26/6/2012. Available at http://gis.stackexchange.com/a/415 (Accessed 18/3/2014).

Technical stuff Estimotes x 3 iPad Air Hardware: D3.2, Software: A1.9 Power: 4 dBm, Interval 200ms Estimote API dated 19/02/2014 Elevation: 210cm (wall) or 240cm (celling) iPad Air Elevation: 130cm (held) or 70cm (on stool) Held flat, home button to left of operator, hands under the device. If you want to reproduce the results, this information will probably be important. As radio reflection may also be an issue, the room has a concrete floor, breeze block walls, and a plasterboard ceiling. The presences of 2 doors and a window may also set up interesting interference patterns.

Estimote Uniformity Measured power levels (typical) at 1 meter. Receiver iPad Air, home button towards Estimote spot. Typical RSSI values: -65 dBm, -71 dBm, -66 dBm Levels fluctuated approximately ±5 dBM Stayed within this range with other WiFi and Bluetooth devices disabled. I need to do a statistical analysis of this variation, but have not yet done so. However the variance does not seem great.

Test ‘lab’ Beacon and reported distance Operator and facing direction Trilateration location

Test ‘lab’ obstructions Mac with WIFI Shelf below beacon Beacon on bookshelf, obstructed to left Window These are the main high level, and radio obstructions which could cause a problem. Door Lamp shade WiFi access point Door

Operator Rotation The way the iPad is held is very important, the receiver seems most sensitive to being held at the front bottom of the iPad near the home button. Doh! So the orientation of the iPad is rather important. In this experiment the ipad was held.

iPad rotation In this experiment the ipad was held, operator standing looking towards the top of each graph, and the ipad rotated in front of the operator.

No Operator In this case the ipad was placed onto a stool. Something interesting going on with the bottom right beacon, why is it so far out?

Rotation of iBeacon Clearly I need to refine the experimental protocol here, as the changes in the top right beacon’s distance are rather interesting. Bottom right beacon rotated only, stuck to wall, vertical orientation facing iPad, operator stood in front of iPad, iPad at 70cm elevation on stool.

Rotation of iBeacon Bottom right beacon rotated only, stuck to wall, Horizontal orientation, base facing iPad

Rotation of iBeacon Bottom right beacon rotated only, stuck to celling, horizontal orientation facing iPad, operator stood in front of iPad

Recommendations Ceiling mounting of Estimotes reduces variance and may also help with blocking by the operator Signal transmission and reception are dependent on the orientation of both the transmitter and receiver. So this should be built into positioning models. Radio absorption by the operator is significant in determining distance. Again models should take account of the direction of the operator. Other radio interference does not seem significant in practice.

Future research Does other hardware suffer from the same directionality issues? Improve experimental protocol to eliminate operator issues, and take more accurate measurements. Collect further data on the variation in signal strength, to check for statistical significance of effects observed. Experiment with a predictive model to take observed effects into account.