Localization for wireless sensor

International Journal of Computer Networks & Communications (IJCNC) Vol.8, No.1, January 2016
DOI : 10.5121/ijcnc.2016.8105 61
LOCALIZATION FOR WIRELESS SENSOR
NETWORKS: A NEURAL NETWORK APPROACH
Shiu Kumar1,*
, Ronesh Sharma2
and Edwin R. Vans3
1,2,3
Department of Electronics Engineering, Fiji national University, Suva, Fiji
1,2
School of Engineering & Physics, University of the South Pacific, Suva, Fiji
ABSTRACT
As Wireless Sensor Networks are penetrating into the industrial domain, many research opportunities are
emerging. One such essential and challenging application is that of node localization. A feed-forward
neural network based methodology is adopted in this paper. The Received Signal Strength Indicator (RSSI)
values of the anchor node beacons are used. The number of anchor nodes and their configurations has an
impact on the accuracy of the localization system, which is also addressed in this paper. Five different
training algorithms are evaluated to find the training algorithm that gives the best result. The multi-layer
Perceptron (MLP) neural network model was trained using Matlab. In order to evaluate the performance
of the proposed method in real time, the model obtained was then implemented on the Arduino
microcontroller. With four anchor nodes, an average 2D localization error of 0.2953 m has been achieved
with a 12-12-2 neural network structure. The proposed method can also be implemented on any other
embedded microcontroller system.
KEYWORDS
Levenberg-Marquardt (LM) Algorithm, Localization, Neural Network, Received Signal Strength Indicator
(RSSI), Wireless Sensor Network (WSN).
1. INTRODUCTION
Wireless Sensor Networks (WSNs) has gained vast response from academia and industries with
new applications being developing in numerous fields. Some of the prospective applications
include environmental and vegetation monitoring, search and rescue operations [12], object
tracking such as tracking patients and doctors in hospitals, monitoring patients, military
applications [10] and other industrial applications. As a result of advancement in technologies,
sensor nodes that are small in size, are cheaper and consume less power having capabilities such
as sensing, data storage, computing and wireless communication have been developed.
An essential and challenging part of many WSN applications is localization (process of finding
the position of the nodes [8]) amongst the others such as architecture, deployment,
synchronization, calibration, quality of service and security. The sensor data has to be attached
with the measured data to make it significant for localization as this is essentially required in
monitoring and recording a wide-ranging information such as acoustic, thermal, visual, seismic or
any other type of measured observation. For example, a person monitoring a large vegetation
farm with a huge variety of vegetables would require the location or the part of the farm from
which the data is received as different vegetables have different specific requirements. To report
the origin of objects, node localization is necessary so that group querying of sensors, routing,
and questions about the network coverage [1] can be assisted or answered. Over the decade, a
number of solutions have been provided by researches for the problem of node localization (for

62
example, [2]-[6], [9], [11], [13]-[16], [18]-[21], [23], [25]-[26], [33]-[34], [36]). In general,
mostly there is a trade-off between the accuracy of localization, the computational complexity,
and energy efficiency (for example, [24], [28-32]) of the techniques depending on their
application requirements.
A WSN usually comprises of a huge number of spatially distributed sensor nodes making it
difficult and impractical to record the sensor node locations when the nodes are being deployed.
There is also a possibility that the sensor node might later be moved from its original location to
some other location. Therefore, an algorithm that will autonomously determine the location of the
nodes is required. A novel neural network based node localization scheme is proposed in this
research work. The Received Signal Strength Indicator (RSSI) has been used for estimating the
coordinates or position of wireless sensor nodes in order to tackle the problem of determining the
location of the node. The following contributions are made:
• Is RSSI a suitable component to be used for node localization in WSNs? We employ the
use of RSSI value to determine the location of the sensor nodes and explain whether
RSSI is suitable to be used for localization purposes (Section 4). The need for additional
hardware will not arise as sensor nodes are freely equipped with RF modules for wireless
communication; therefore no extra hardware will be required. However, multipath fading
and noise will contaminate the RF signals, thus affecting the RSSI value by corrupting
the signal to noise ratio.
• The choice of the evaluation model is also of paramount importance. In this research, a
2D indoor environment is considered, that is the location is determined by the x and y
coordinates. A single anchor node will require steerable directional antennas, thus
accounting for a direct increase in cost and power consumption of the sensor node.
Therefore, two or more anchor nodes with different configurations (Section 3.2) have
been employed and the best configuration to be used for the anchor nodes is explained
(Section 4).
• We introduce a novel neural network based 2D indoor localization scheme. Global
Positioning System (GPS: [4], [19]) can be used for accurate localization. However, use
of GPS requires an external hardware to be attached to the sensor nodes leading to an
increase in cost and power consumption. Moreover, line-of-sight is required for GPS and
thus is not applicable for indoor environments. The method of indoor localization
proposed uses a feed-forward neural network (See Section 2). A number of neural
network structures trained using Levenberg-Marquardt (LM), Bayesian Regularization
(BR), Resilient Back-propagation (RP), Scaled Conjugate Gradient (SCG) and Gradient
Descent (GD), have been evaluated and the best neural network employed (refer to
Section 4).
1.1. PROBLEM DEFINITION
Consider the case when N number of sensor nodes is deployed in a sensor network with locations
L = {L1, L2, ..., LN}. Let Lxi, Lyi, Lzi, denote the x, y and z coordinates of the ith
sensor node
respectively. The problem of node localization involves determining these locations and by
letting Lzi = 0, problem of 2 dimension is obtained. Sensor nodes having knowledge of their
positions are known as anchors or beacons. All nodes in the network with unknown position
localize themselves with the aid of these anchor nodes. Therefore, mathematically the node
localization problem is stated as follows: for a given a multi-hop WSN, represented by a graph G
= (V, E) with A anchor nodes having positions {xa, ya} for all a ɛ A, we want to determine the
position {xu, yu} for all nodes with unknown positions u ɛ U.

63
2. THE PROPOSED NEURAL NETWORK METHOD
A practical method of learning discrete valued and real valued functions form given examples is
provided by artificial neural network (ANN, also referred to as neural network). Supervised
learning is used by ANN. The inputs and the outputs are provided to the ANN in order for it to
learn and form a suitable model. It is generally used to tackle the problem of classification and
regression. Usually, a multilayer neural network has three layers of interconnected “artificial
neurons”; the input layer, hidden layer(s) and the output layer (see figure 1).
Figure 1. A general feed-forward neural network structure with 4 inputs and 3 outputs.
Feed-forward neural networks and feedback neural networks are the two groups of neural
network. In a feed-forward neural network, the outputs from one layer of neurons is fed to the
next layer and in this process no layers are skipped and there is no feedback to the system. The
three main types of neural networks used in localization [3] are MLP, Recurrent Neural Network
(RNN) and Radial Basic Function (RBF).
It must be noted that the RSSI values obtained are highly unstable and turn to vary under
environmental noise and mobility of sensor nodes. A neural network offers the advantage that
prior knowledge of the environment and noise distribution is not necessary. Moreover, higher
accuracies are achieved by neural networks compared to other techniques such as the Kalman
filter [3]. The trade-off between the accuracy and memory requirements of the MLP neural
network is the best when compared with other types of neural networks, thus it has been chosen
to be used in this research.
Matlab has been used for the implementation of the MLP neural network (a feed-forward ANN)
with three layers. The best solution chosen (refer to Section 4) is shown in figure 2 comprising of
three inputs, twelve nodes in the first and second (hidden) layers, and two nodes in the output
layer. RSSI values acquired from the 3 anchor nodes are fed as the input to the system, while the
output generates the estimate of the x and y coordinates of the mobile node.
Figure 2. The proposed MPL neural network with 3 inputs and 2 outputs.

64
The nodes in the first and second layer use the hyperbolic tangent sigmoid (“tansig”) activation
function, while the third layer uses a linear (“purelin”) activation function. Bayesian
Regularization (BR) algorithm was used for training of the network since optimal results were
obtained using this training algorithm (refer Section 4). The network training time of the BR
algorithm was high; however it was still employed as only offline training was required. The
structure of the matrix containing the RSSI values and the output coordinates is as follows:














=
mmmnmjm
iiiniji
nj
YXRRR
YXRRR
YXRRR
data
L
MMMOMM
L
L
1
1
111111
(1)
Where Rij signifies the RSSI values of the signal obtained from the jth
anchor, at the ith
reference
point while Xi and Yi signify the x and y coordinates of the ith
reference point. For this research,
the total number of reference points (m) is 94 and the number of anchors (n) used is three. The
neural network obtained is represented by the equation as follows:
( ) kkjjii bWbWbWR
y
x
332211 )tanh(tanh +•+•+•=





(2)
Where R is the input row vector of length 3, which consists of the RSSI values acquired from the
3 anchor nodes, Wmn is the weight vector of nth
node at the mth
layer, and bmn is the bias vector of
nth
node at mth
layer. Equation 3 was incorporated on a mobile node equipped with an Arduino
microcontroller for node localization. However, this method can also be used to implement neural
network on other programming platforms.
3. DATA COLLECTION
3.1. MEASURING THE RSSI VALUES
For experimental purposes, the wireless sensor node comprising of Arduino UNO in conjunction
with the XBee series 2 module that is compliant with the IEEE 802.15.4 standard called ZigBee
have been used for the mobile node. The anchor nodes only comprise of the XBee series 2
modules on its own. For the wireless communication between the nodes, the standard ZigBee
communication is employed with an air data rate of 256 kbps operating at the ISM (Industrial,
Scientific & Medical) 2.4 GHz frequency band. The XBee has a receiver sensitivity of -96 dBm
and a communication range of 40m for indoor/urban environment.
For RSSI computation, XBee Series 2 modules special IEEE 802.15.4-2003 hardware support
such as has been utilized. The XBee modules have an RSSI pin, which outputs a PWM signal to
represent the RSSI value. This pin value can be read by the microcontroller and converted
appropriately to reflect the RSSI value. However, for this research the mobile node with XBee in
API mode and the anchor nodes in the AT mode was used. In API mode, packets are constructed
to command the XBee explicitly, as opposed to just sending the data serially in the AT or the
“Transparent Mode”. The DB command which returns the RSSI value of the latest packet
received was used.
The mobile node sends a signal to the anchor nodes requesting to send localization beacon, while
the anchor nodes respond by sending a beacon to the mobile node. A single instance of
measurement consists of one RSSI value for each anchor node signal received on the mobile node

65
(see figure 3). The RSSI measurements between the mobile and the anchor nodes are measured
and recorded for further analysis using the Docklight (version 1.9) software running on Windows
7, which reads the serial data from the node and writes it to a file.
Mobile NodeAnchor Node
Beacon Request
Localization Beacon
Figure 3. Message flow between mobile and anchor nodes for RSSI measurements.
3.2. Experimental Setup
Training and testing of a neural network requires a set of data [15], [26]. The dataset in this
research is prepared by collecting the RSSI measurements for each mobile node coordinates, ri =
[xi, yi] denoting the position of the ith
reference point. For indoor localization, the structure of the
room or experimental environment is an important factor [27]. Measurements were carried out in
a research laboratory containing furniture’s and equipment’s such as tables, chairs and computers.
The structure of the measured training data is 9 x 11 measurement points (black points) as shown
in figure 4. The distance between the grid points is 0.45 m. Measurements from the training
positions and from unknown positions were used for testing. The unknown positions were chosen
between the training measurement points as indicated in green in figure 4.
Anchor 1
Anchor 2
Anchor 3
Anchor 4
Anchor 5
Unkonwn
Positions
Figure 4. Experimental setup layout.
The anchor node configurations used for this research to determine the best anchor node
configuration are 2 anchors (consisting of: (a) anchor 2 & 4. (b) anchor 1 & 4), 3 anchors
(consisting of: (a) anchor 1, 2 & 4. (b) anchor 1, 3 & 5), 4 anchors (consisting of anchor 1, 2, 4 &
5) and 5 anchors.

66
4. RESULTS AND DISCUSSION
A neural network requires supervised learning and for this research five different training
algorithms namely LM, BR, RP, SCG and GD have been used for training the neural network. A
number of tests were conducted using various activation functions with varying number of layers
and nodes. A network with three layers having a 12-12-2 structure gave optimal results. It
employed 12 nodes in the first and second layer (which are the hidden layers) and 2 nodes in the
output layer. Hyperbolic tangent sigmoid activation function has been used for the hidden layers
while a pure line function has been used in the output layer. This structure was used with different
number of inputs that is with different number of anchors to evaluate the performance of different
anchor node configurations that follows.
An experiment was carried out to determine the best training algorithm to be used for training the
neural network and was performed using 4 anchors. The data structure used for training is
mentioned in Section 2 and consists of 2350 data sets of which 80 percent was used for training
and validation while 20 percent was used for testing. The network was then further tested for its
performance using 105 data sets obtained from 7 unknown positions (see figure 4). The
performance of the network is evaluated based on the error in the distance between the estimated
and exact distance. Equation 3 is used for calculating the average error.
∑=
−+−=
n
i
iiii yyxx
n
e
1
2
o
2
o )()(
1 (3)
Where n is the total number of test sets, (xoi,yoi) is the exact position and (xi,yi) is the estimated
position of the mobile node at the ith
test data set. Figure 5 shows the average localization error
obtained in this phase. The time taken to train the network using various training algorithms is
shown in figure 6.
Figure 5. Localization error of different methods with 4 anchors.

67
Figure 6. Time taken to train the neural network for different training algorithms with 4 anchors.
The maximum error, percentage of error less than 0.8m, average error for the test set of known
positions and average error for test set of unknown positions together with the time taken to train
the neural network for each method has been utilized to evaluate the performance. It can be noted
that the percentage of time the error is less than 0.8 m is quite low for RP, SCG and GD training
algorithms while they also have higher maximum error compared to that of LM and BR training
algorithms. The training time for BR and GD training algorithms is noticeably high compared to
other methods. However, for this research, offline training of the neural network is performed and
the neural network is than implemented on the mobile node. Therefore, the LM method and BR
method were chosen to be used for training the neural network for evaluation of other mentioned
anchor node configurations.
Figure 7 shows the localization errors of the neural network for the different anchor node
configurations, where the BR and LM training algorithms have been used to train the neural
network. It can be noted that increasing the number of anchor nodes increases the localization
accuracy. The lowest average error of all the configurations evaluated was obtained with five
anchor nodes. The maximum error for neural network obtained using LM training algorithm is
almost same when three, four or five anchors are used. For the two different configurations with
three anchors evaluated, the second configuration with anchors one, three and five produced a
slightly better result compared to that of when anchors one, two and four are used.
Figure 7. Localization error using LM and BR training algorithms for different anchor configurations.
TrainingTime(s)

68
From the results obtained it can be concluded that RSSI values can be used for localization where
accuracy of less than a few meters is required. However, for applications requiring precise
localization accuracy for indoor localization, additional sensors such as ultrasonic sensors [36]
can be employed.
4.1. COMPARISON WITH OTHER RELATED LITERATURE
Various localization algorithms have been reported over the years with the goal of achieving
higher localization accuracy at a low computational and hardware cost. Several concepts such as
lateration, angulation, trilateration [25], multilateration and triangulation are used in localization.
The various localization schemes can be categorised in one of the following groups: GPS based
or GPS free, anchor based or anchor free, range based [7] or range free [2], [5]-[6], [23], fine
grained or coarse grained, and stationary or mobile sensor nodes [22]. Several range based
algorithms that have been proposed by a number of researchers include Time-Of-Arrival (TOA),
Time Difference of Arrival (TDOA) [34], Angle Of Arrival (AOA) [9], [11] and Received Signal
Strength Indicator (RSSI) [35]. As mentioned in Section 1, GPS localization schemes require
external hardware, are costly and consume lot of energy, which is not suitable for WSNs. Use of
directional antennas [21] have also been explored for the problem of node localization, with one
or more nodes being used to measure the angle of arrival of the signal. The angle obtained is then
used to compute the position of the node whose position is unknown. However, it must be noted
that directional antennas are costly and require larger amount of power that leads to a WSN
localization system which is not energy efficient.
The authors in [17] proposed a 2D and 3D localization algorithm using weighted centroid (WCL)
technique. The number of anchors involved in this method is controlled by use of an optimized
threshold. A range free WCL for 3D WSN using Mamdani & Sugano Fuzzy Inference System
(FIS) is presented in [20]. The method involves computing the edge weights from the RSSI
values by making use of the Mamdani & Sugano inference system where 121 anchor node were
employed. The high-resolution range-independent localization (HiRLoc) [16] and modified
HiRLoc [33] scheme used omni directional antennas. The average localization error, number of
anchor nodes used and the implementation environment for some of the localization algorithms
are given in table 1.
Table 1. Comparison of different localization algorithms.
Localization Algorithm
Average
Localization
Error (m)
Number of
anchor nodes
Implementation
Environment
2D-WCL [17] > 3.000 100 Simulation
Mamdani and Sugano FIS [20] 3.0000 121 Simulation
HiRLoc Scheme [16] 3.6000 Simulation
Modified HiRLoc Scheme [33] 3.5100 Simulation
Sequence-Based Localization [14] 5.0000 10 Simulation
Neural Network (3D) [18] 0.4855 4 Simulation
Proposed Method
0.6887 3
Real time0.2953 4
0.3435 5

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In [18], a neural network approach for 3D localization of nodes using 4 anchor nodes is
presented. Average localization error of 0.4855 m for 2D movement was reported. For 95% of the
mobile nodes, a localization error of less than 0.8 m was achieved. A 10-10-3 neural network
structure having four inputs was employed. The first layer used a hyperbolic tangent sigmoid
activation function; the second layer used a log sigmoid activation function while the output used
a liner activation function. However, for this research an average localization error of 0.2953 m
was obtained using neural network with 4 anchor nodes, which is comparably lower than that of
[18]. On the other hand, the authors in [18] presented their result based on simulation whereas in
this research the actual implementation and testing has been carried out in real time environment.
5. CONCLUSIONS
An efficient 2D localization algorithm for WSN has been proposed by utilizing ANN. Different
anchor node configurations have been evaluated that can be used by researchers in selecting the
number of anchor nodes to use depending on their application environment. The input to the
proposed method is the RSSI values of the signal received from the anchor nodes. As all sensor
nodes are equipped with RF modules for wireless communication, no external hardware is
required. The proposed method is thus energy efficient and uses only a two way message to
obtain the inputs for the localization. However, it is recommended that the RSSI value of the
localization beacon request signals received by the anchor nodes also be utilized that can result in
a further reduction of the localization error.
For obtaining the best neural network for localization, the BR training algorithm is evaluated to
give the best result. However, the training time for this algorithm is quite high compare to other
methods such as LM, RP and SCG. Therefore, it is recommended to use the BR method when
offline training is performed as in this case. For applications requiring online training, the LM
method is recommended.
ACKNOWLEDGEMENTS
This research work was fully supported by the College Research Committee of Fiji National
University.
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AUTHORS
Shiu Kumar received Bachelor of Engineering Technology and Postgraduate Diploma in
Electrical and Electronics Engineering from the University of the South Pacific (USP) in
year 2009 and 2012 respectively. He received his Masters in Electronics Engineering from
Mokpo National University, South Korea. Currently he is pursuing his PhD in Machine
Learning (Signal Processing) from USP. His research interests include Automation and
Control, Wireless Sensor Networks, Embedded Microprocessor Applications, Artificial
Intelligence and Signal Processing. He is a member of IEEE, IAENG and IACSIT.
Ronesh Sharma received the BTech degree from the University of the South Pacific
(USP), Suva, Fiji, in 2007 and MEng degree from Mokpo National University, South
Korea. He is now pursuing his PhD degree in Engineering at University of the South
Pacific, Suva, Fiji. His research interests include Bioinformatics, protein secondary, fold
and structural class prediction problems, protein subcellular localization prediction
problems, intrinsically disordered protein related problems, data mining, and pattern
recognition. He is a member of IEEE.
Edwin Vans was born in Ba, Fiji, on October 26, 1987. He received his Bachelor of
Engineering Technology degree, Postgraduate Diploma in electrical and electronics
engineering and MSc in engineering degree from the University of the South Pacific, Fiji
in 2009, 2011 and 2015 respectively. He is currently a Lecturer in electronics engineering
at School of Electrical and Electronic Engineering in Fiji National University. He is a
current member of IEEE and IEEE Robotics and Automation Society. His research
interests are in machine learning, intelligent robotics, and computer vision.

Localization for wireless sensor

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Localization for wireless sensor