Received Signal Strength Indicator for measuring distance to obtain position of Sensor

1. Chapter 1
Received Signal Strength
Indicator - RSSI
1.1 Intro
In wireless sensor networks (WSN) there are three main principles of mea-
surement that can be used to obtain distances and to measure distance is
necessary to obtain the position of the sensor node.
Three main principles are given below :
• Received Signal Strength Indicator (RSSI)
• Angle of Arrival
• Propogation Time Based. It includes -
– Time of Arrival
– Round Trip - Time - of - Flight
– Time Difference of Arrival
1.2 RSSI Method
Here we will discuss the RSSI in detail in Wireless Sensor network.
1.2.1 Introduction
Some basic characteristics of RSSI are :
• Received signal strength indication is a measurement of the received
signals power.
• Radio frequency signals : Power decreases with the distance.
loss(dB)Loss(db)=20log(d)+20log(f)+32.45
d = km,f = MHz
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2. • We know communication frequency and transmit power. So, if we can
measure received signal’s power. We can find the distance ’d’.
Received power = Transmitted power - Path loss.
• In telecommunication, received signal strength indicator (RSSI) is a
measurement of the power present in a received radio signal.
• RSSI is an acronym for received signal strength indication. It is a
measure of the signal power on the radio link, usually in the units of
dBm while a message being received.
Figure 1.1: RSSI Method
1.2.2 Use of RSSI
Various uses of RSSI are listed below :
• RSSI can be used for estimating node connectivity and node distance.
• Another usage of RSSI is to sample the channel power, when no node
is transmitting to estimate the background noise also called noise floor.
1.2.3 Caution to Use RSSI in TinyOS
Various cautions to be taken while using RSSI in TinyOS are listed below :
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3. • The RSSI data is not provided by the standard platform independent
hardware interface layer.
• RSSI must be accessed by a platform specific HAL interface.
• The RSSI values given by the TinyOS are usually not in dBm units and
should be converted by the platform specific relation to get meaningful
data out of it.
1.2.4 Intercept Base
It is a modified version of the Base station component that provides the
Intercept interface to allow the user application to inspect and modify radio
and serial messages before they are forwarded to the other link and to decide
whether they shall be forwarded or not.
1.2.5 Sending Node
Sending Node is the application to be put in the node that sends the mes-
sage whose RSSI will be ready by the base. It contains a simple logic to
periodically send a RSSI Msg.
1.2.6 RSSI Base
Various Characteristics of RSSI base are :
• It is the application that will be put in a node connected to the serial
port and will effectively read RSSI.
• It uses the Intercept Base component to forward the messages over the
radio but Intercept the RSSIMsg before it is forwarded and also the
RSSI value is included.
• It contains some cryptic macros so that it can work correctly with
chips that use either the CC2420 or CC1000 radio.
• It always forward the messages but modifies the payload by include
the RSSI.
1.2.7 Java Application
A java application called RSSIDEMO was created to see the results on the
computer. The output would be like this :
• RSSI Message received from node 1 : RSSI = -14.
• RSSI Message received from node 1 : RSSI = -1.
• RSSI Message received from node 1 : RSSI = 2.
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4. 1.2.8 Advantages of RSSI Method at Localisation
Various advantages of RSSI Method are :
• No directional sensor like ultrasonic systems.
• Indoor and Outdoor usage.
• Data communication and sensor usage at the same hardware.
1.2.9 Difficulties at RSSI Method
Distance calculation is related RF signal power receiver :
• Multiple Propagation.
• Fading Effects.
• Bad frontend devices
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