Using Bluetooth Low Energy for Location

What is BLE?

Bluetooth Low Energy

Originally designed by Nokia as Wibree (2001)

Bluetooth Smart

Why has BLE been so Successful?

Rapid adoption by vendors

Apple

Samsung

Simple design

Cheap in comparison to Wifi, Zigbee, 3g/4g

Open Specification*

But is it really Bluetooth?

Current specification is 4.1, comprised of

Bluetooth classic

Bluetooth smart (BLE)

Not at all compatible with each other

Network Topology

Broadcasts

Fast and simple to do using advertising packets

No security

Connections

Transmit data in both directions, similar to a socket

Security*

SecurityBLE has in-band key exchange

SIG invented their own key exchange protocol

Has fundamental weaknesses

BLE Location

BLE LocationQuickly determine location within a home

Do this accurately otherwise users will be annoyed

Keep the costs down

How to locate BLE devices?Standard method is to use Received Signal Strength Indicator (RSSI)

A measurement of power present in a received signal

Calculated for each frame

Can be used to estimate distance once calibrated*

The challenges with RSSIThere is no standardised relationship of any particular physical parameter to the RSSI reading.

Vendors and chipset makers provide their own accuracy, granularity, and range for the actual power

Requires testing each receiver to estimate distance..

RSSI based Location in practice

1 m ± 1 m

5 m ± 5m

10 ± 10m

Multipath and RSSIRSSI is easily varied by multipath

Phenomenon which results in radio signals reaching the receiving antenna by two or more paths

Varies building to building

Can be mitigated by antenna diversity

Positioning techniques

Example Space

Bedroom

Bedroom

Combined Living / Kitchen

TrilaterationIs the process of determining absolute or relative locations of points by measurement of distances

Used in global positioning systems

Enables location of BLE

Uses 3 or more fixed way points or beacons

Gather RSSI from these fixed way points to calculate location

Requires lots of way points to work effectively

Fingerprinting

Uses fixed way points

Preferably one per room

Walls or divisions help

Apply a gradient filter for smoothing the RSSI

Enhanced by calibration of a space*

ObservationsRSSI based positioning is hard

Trilateration didn’t work for us

Way to much variance in values

Smoothing didn’t help

Fingerprinting working for room level location*

Requires lots of data and Maths to improve accuracy

Hardware

Important to have a stable and known group of devices reporting RSSI

TI CC2540 / CC2541 (8051)

http://www.ti.com/sensortag

TI Wilink8 which combines wifi/bluetooth/ble

Nordic Semiconductors nRF51822 (ARM)

Future WorkMove from RSSI to Channel State Information (CSI)

Fine Grained metric full of frequency domain information

Requires access to the Baseband Controller

Smarter way points