Bluetooth Positioningusing RSSI and Triangulation

Methods

Speaker : Shih-Chi HuangAdvisor : Dr. Kai-Wei Ke

2013/12/09

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Outline

• Background

• Introduction

• Wireless Positioning Algorithms

• Bluetooth Based Positioning

• Designing The Positioning System

• Positioning Results

• Conclusion

• References

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Background – Bluetooth Origination

• 這項無線技術的名稱取自古代丹麥維京國王 Harald Blaatand 的名字，他以統一了因宗教戰爭和領土爭議而分裂的挪威與丹麥而聞名於世，而 Blaatand  這個名字的英文字面意義便是 Bluetooth

• 命名這項技術的易利信公司認為，藍芽技術的目的就在統一各種數位產品的通訊能力，因此就取了這樣的名字。

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Background – Bluetooth Characteristic

• 成本低

• 效益高

• 操作頻段 2.4GHz ，可及範圍可超過 100m

• 最多可支援八個連接阜 - 1 個 Master ， 7 個 Slave

• 標準是 IEEE 802.11

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Background – Bluetooth Technology

• 跳頻展頻技術 (Frequency Hoping Spread Spectrum ; FHSS)

此 FHSS 的傳輸技術，是將欲傳輸的信號透過一系列不同的頻率範圍廣播出去，而由傳送裝置先傾聽頻道 (Listens Channel) ，若偵測出頻道處於閒置狀態時，信號便由此頻道傳送出去；

使用 79 個頻道 ( 頻寬為 1MHz)

每秒頻道轉換可達 1600 次

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Introduction

Wireless position tracking has been exploited in many areas.

“Satellite Navigation” used in vehicles

Search for their current locations as well as place-of-interest nearby

A classic example is the Global Positioning System (GPS).

However, as the communication between the satellites and GPS receivers requires line-of-sight radio propagation

GPS generally only works well in outdoor environments.

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Introduction (Cont.)

There have been increasing interests in researches for indoor position tracking.

The aim is to develop applications.

low-cost

easy-to-deploy

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Wireless Positioning Algorithms

1. Cell ID methods

2. Triangulation methods

3. Signal Strength methods

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Cell ID methods

• The simplest

• They map the physical location of the mobile terminal to the cell IDs of mobile network, normally the users are proximately located inside a large circle centred by the location of the base station.

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Triangulation methods

Calculate the location based on the distances between specific reference points

Four popular techniques have been developed based on the triangulation method.

a. TOA (Time Of Arrive) / RTOF (round-trip time-of-flight)

b. TDOA (Time Difference Of Arrive)

c. AOA (Angle Of Arrive)

d. RSS (Received Signal Strength)

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Signal Strength methods

• The signal strength measurement can be affected by obstacles and imperfect propagation models used.

• Ex picture, there are three intersections points, the final position estimation of point x will be the average coordinate of intersection points x1, x2 and x3.

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Triangulation methods – TOA/RTOF

• TOA (Time Of Arrive) / RTOF (round-trip time-of-flight)

• the time needed to transmit a signal from base station to mobile terminal (at the speed of light) is recorded and used for calculating the distance.

• A disadvantage is that the device either has to have a big database with the sites’ geographic positions

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Triangulation methods - TDOA

• TDOA (Time Difference Of Arrive)

• the mobile sends positioning signals to surrounding measuring units and the time-difference of arrival of the received signals is evaluated.

• The main benefit of TDOA systems is that it is only necessary to synchronise the measuring units (i.e. the base stations).

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Triangulation methods - AOA

• AOA (Angle Of Arrive)

• the position is calculated using goniometry.

• The measuring units use directional antennas or antenna arrays to measure the angle of incoming signals send by mobile.

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Triangulation methods - RSS

• RSS (Received Signal Strength), also called RSSI (Received Signal Strength Indication)

• Utilise the characteristic of radio propagation over space.

• Using a proper propagation model, we can calculate the distances between a mobile to base stations

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Bluetooth Based Positioning

Bluetooth Compare to WIFI Has several advantages

1. Cost : the cost of Bluetooth chip is lower than WiFi

2. Power consumption : Bluetooth consumes much lower power than WiFi, Bluetooth uses a fifth of the power of WiFi as it requires a lower transmission power and provides a mechanism for automatic power control.

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Bluetooth Based Positioning (Cont.)

Compares to WiFi, there are much fewer Bluetooth based positioning system.

The reason is that compares to WiFi, there are several technical hurdles to overcome for Bluetooth based positioning systems.

Bluetooth is designed for low cost communication, the Bluetooth standard lacks precise time synchronisation, therefore time based triangulation methods such as TOA, TDOA are difficult to implement.

Bluetooth is designed to be used in low footprint devices. Directional or array antennas are rarely used. There for it is difficult to use angle measurement methods like AOA.

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Bluetooth Based Positioning (Cont.)

RSSI reading is not well defined in Bluetooth standard. The RSSI reading is unreliable and device dependent, especially for early Bluetooth standard.

However, since Bluetooth 2.1, there is a new method to measure RSSI without connection.

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Designing The Positioning System

We found that Android is the perfect platform to implement.

Why ??

1. Android is an open source mobile operation system (OS) that developed by Google.

2. It offers free API for developer to developing their own applications with accessing to different resources including the latest Bluetooth API.

3. The Java-style programming also makes Android an extendable and easy-to-learn developing platform.

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Designing The Positioning System (Cont.)

Use Inquiry_With_RSSI method on mobile phones running android 2.0 or above.

When we click “Start to search”, a list of nearby Bluetooth devices names, MAC addresses and the corresponding RSSI values were retrieved.

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Designing The Positioning System (Cont.)

In indoor situations, as there are many obstacles between transmitter and receiver like walls, furniture and even human bodies. The propagation loss due to radio signal absorption and diffraction by these obstacles cannot be ignored.

Therefore, we have adopted a more sophisticated indoor propagation model for location estimation, it is shown :

n : signal transmission constant

A : received signal strength at 1 meter distance

d : distance from the transmitter node to the receiver node

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Designing The Positioning System (Cont.)

Therefore the environmental factor between two reference nodes can be estimated as :

d : the distance between the two reference nodes and it is known

We can get environmental factors between the two reference nodes i, j.

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Designing The Positioning System (Cont.)

However, as mentioned before, if we get all distances between the mobile and reference nodes, they are not likely intersecting into one point.

In this research we used three different algorithms to calculate the position.

A. Least square estimation

B. Three-border positioning

C. Centroid positioning

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Least square estimation

LSE is widely used in the distance based positioning system.

當 無解時，我們求 最小平方解當 的解

當要求 最小解時，等價於求

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Three-border positioning

We know the coordinates of reference nodes A, B, C are and the distances between these nodes to the mobile . Suppose the coordinates of the mobile are the equations can be established :

Through solving the equation we can get the coordinated of the mobile.

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Centroid positioning

The measured RSSI values from the mobile to reference nodes can yield four distances.

We can draw four arcs using these distances from the corresponding reference nodes and we can get four intersecting point A, B, C and D.

Set the polygon defined by A, B, C and D with coordinates , the coordinates of the mobile (x, y) is the polygon’s centroid:

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Positioning Results

We use three different algorithms to estimate the mobile’s location, here is the first result showing :

However the position calculated by LSE is the most closed point to the real position.

Real / estimated position LSE Three-border Centroid

(300, 400) (297, 400) (296, 384) (269, 364)

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Positioning Results (Cont.)

They are (100,240), (300,300), (450,200), (600,700), (150,300) and (250,600). The estimated positions of the mobile using three different algorithms are recorded.

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Positioning Results (Cont.)

Effect of human body

We then cover the antenna part at the top of the mobile phone, and at same position we collect RSSI for 5 times.

The chart clearly shows that when the human body covers the mobile phone, the signal are weakened and the RSSI readings are dropped by about 6-8 dB .

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Conclusion

In this work, we analysed different indoor wireless positioning methods, especially the RSS based Bluetooth positioning.

After finding a proper propagation model, a RSS and triangulation based positioning scheme was defined and three distance based triangulation algorithms were implemented. Result analyses show that the RSS based triangulation positioning yields very good results.

More work has been planned to extend this research to more complicated scenarios and with more algorithms.

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References

• Yapeng Wang ; Xu Yang ; Yutian Zhao ; Yue Liu ; Laurie Cuthbert, “Bluetooth Positioning using RSSI and Triangulation Methods”, 2013 IEEE 10th Consumer Communications and Networking Conference (CCNC)

• Wikipedia, http://zh.wikipedia.org/wiki/Bluetooth

• Jonas Willaredt, “WiFi and Cell-ID based positioning - Protocols, Standards and Solutions”

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Thanks for listening