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Combined Human, Antenna Orientation in Elevation Direction and Ground Effect on RSSI in Wireless Sensor Networks

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Abstract

In this paper, we experimentally investigate the combined effect of human, antenna orientation in elevation direction and the ground effect on the Received Signal Strength Indicator (RSSI) parameter in the Wireless Sensor Network (WSN). In experiment, we use MICAz motes and consider different scenarios where antenna of the transmitter node is tilted in elevation direction. The motes were placed on the ground to take into account the ground effect on the RSSI. The effect of one, two and four persons on the RSSI is recorded. For one and two persons, different walking paces e.g. slow, medium and fast pace, are analysed. However, in case of four persons, random movement is carried out between the pair of motes. The experimental results show that some antenna orientation angles have drastic effect on the RSSI, even without any human activity. The fluctuation count and range of RSSI in different scenarios with same walking pace are completely different. Therefore, an efficient human activity algorithm is need that effectively takes into count the antenna elevation and other parameters to accurately detect the human activity in the WSN deployment region.
Combined Human, Antenna Orientation in Elevation
Direction and Ground Effect on RSSI in Wireless
Sensor Networks
Syed Hassan Ahmed , Safdar H. Bouk, N. Javaid
Department of Electrical Engineering,
Comsats Institute of Information Technology,
Islamabad, Pakistan.
Sani9585@gmail.com, {bouk,
nadeemjavaid}@comsats.edu.pk
Iwao Sasase
Department of Information and Computer Science
Keio University,
Yokohama, Japan.
sasase@ics.keio.ac.jp
Abstract— In this paper, we experimentally investigate the
combined effect of human, antenna orientation in elevation
direction and the ground effect on the Received Signal Strength
Indicator (RSSI) parameter in the Wireless Sensor Network
(WSN). In experiment, we use MICAz motes and consider
different scenarios where antenna of the transmitter node is tilted
in elevation direction. The motes were placed on the ground to
take into account the ground effect on the RSSI. The effect of
one, two and four persons on the RSSI is recorded. For one and
two persons, different walking paces e.g. slow, medium and fast
pace, are analysed. However, in case of four persons, random
movement is carried out between the pair of motes. The
experimental results show that some antenna orientation angles
have drastic effect on the RSSI, even without any human activity.
The fluctuation count and range of RSSI in different scenarios
with same walking pace are completely different. Therefore, an
efficient human activity algorithm is need that effectively takes
into count the antenna elevation and other parameters to
accurately detect the human activity in the WSN deployment
region.
Keywords— MICAz, antenna orientation, WSN, RSSI, ground
effect
I. INTRODUCTION
Wireless Sensor Networks (WSN) has been investigated
and researched extensively due to their large application
domain. Recently, the parameter that has been considered in
experiments of WSN deployment, localization, distance
estimation etc is the variations in the received signal strength,
called Received Signal Strength Indicator (RSSI). RSSI is an
indication of power or power level present in the received
signal strength at the receiving antenna. It is a very general
feature or metric in most of the low-power radio enabled
devices, i.e. Wireless Sensor Mote (WSM) [1-2].
In an ideal scenario, the power level of the received signal
does not fluctuate. However, in practical scenario the RSSI is
affected by different factors: e.g. physical distance, reflections
of objects, environmental parameters, movement of objects or
change in the environment, antenna position and polarization
etc.
In this paper we experimentally analyze the combined
effect of human activity, antenna orientation in elevation
direction and ground effect on the RSSI of the MICAz WSM.
The results show that even without human mobility, RSSI level
is highly infected by the antenna orientation. The results also
show that RSSI is immensely affected when all the above
factors are collectively considered during the experiment.
Remaining part of the paper is organized as follows: Previous
work is discussed in Section-II. The experimental setup and
results are explained in Section-III and IV, respectively. In last,
Section-V concludes the paper.
II. PREVIOUS WORK
Several papers have been published in the past that
investigate the effect of human or object movement, antenna
orientation on RSSI or channel characteristics of WSM [3-9].
In [3], the channel propagation of MICAz mote with different
antennas is analysed in the Anechoic Chamber and simulated
as well. It has been observed that the radio pattern of the
MICAz mote is not circular when a mote’s antenna is tilted at
orientations.
The authors in [4] have experimentally evaluated the impact
of human movement on the RSSI in the indoor environment.
The sensors were deployed inside the researchers’ offices
above the ground level and impact of human activity in
investigated. It shows that there is significant effect of human
activity on the RSSI of the mote.
In [5], the impact of antenna orientation on WSN
performance has been experimentally investigated. It has been
observed that when two motes’ antennas are tilted in elevation
direction in opposite direction has large effect on the RSSI
compared to when two motes’ antennas are parallel to each
other. The signal propagation of T-mote Sky and MICAz mote
is also investigated in [6].
The authors in [7], the sensor motes have been deployed on
the ceiling where antenna is inverted and pointed to the floor
and effect of human activity on the RSSI have measured.
The effect of human activity on RSSI of TelosB mote is
experimentally investigated in [8]. The motes were placed at
1m to 3m high from the ground during the experiment. The
human activity algorithm detects the presence of the human
motion if the number of fluctuations in RSSI is less than the
predefined threshold that is 60%. The RSSI fluctuations are
also experimentally investigated in [9], where results of RSSI
fluctuations of a moving node are compared with the
accelerometer.
We note in the previous discussion that most of the
previous work was focused on different factors that affect
RSSI, i.e. antenna orientation and human movement. The
collective influence of human activity, ground effect and
antenna orientation on the received signal level are not
investigated. In this paper, we propose an experimental
approach to investigate the combined effect of human antenna
orientation and ground effect on the RSSI of the MICAz
motes.
III. EXPERIMENTAL SETUP
The experimental setup to measure the combined effect of
human activity, antenna orientation and ground effect is briefly
discussed in this section. MICAz OEM Edition motes
(MPR2600J) equipped with Atmega128L processor, CC2420
RF Chip and a half-wave external monopole antenna are used
in this experiment [10]. The pair of MICAz motes is placed on
the ground in a corridor of 5m wide at the distance of 3m apart.
In the pair of motes, one mote works as a sender and other
mote works as a receiver. The sender mote’s antenna is tilted in
elevation direction in reference to the receiver mote’s antenna
at varying angles, as shown in Figure 1.
Figure 1. Experimental setup where a pair of MICAz motes is placed on the
ground and antenna of sender mote is tilted in elevation direction with
reference to the receiver mote.
RSSI readings of the successfully received packets are
recoded for different scenarios, e.g. without human motion,
with one and two persons moving between the nodes at slow,
medium and fast pace and 4 persons moving at the medium
pace.
IV. EXPERIMENTAL RESULTS
The results of the experimental setup discussed in previous
section are discussed in this section. Figure 2 shows the RSSI
readings of the antenna orientation at different angles and
without the human movement. It is observed in that
illustration that signals strength (RSSI) is highly affected
when the antenna of the sender node is tilted in elevation
direction and nodes are placed on the ground. The percentage
of constant RSSI reading (Frequency %) is very wide for some
angles e.g. RSSI values for 90° elevation angle lie between
-75dbm to -85dbm.
Figure 2. Frequency percentage vs RSSI at varying elevation angles with no
movement
Figure 3. Frequency percentage vs RSSI at varying elevation angles with
single person moving at slow pace
Figure 4. Frequency percentage vs RSSI at varying elevation angles with
single person moving at medium pace
Figure 5. Frequency percentage vs RSSI at varying elevation angles with
single person moving at fast pace
Figure 3, 4 and 5 show the effect of one person moving
between nodes at slow, medium and fast pace, respectively. It
is obvious from the figures that due to human mobility,
frequency percentage of RSSI readings widens the range of
RSSI values. The reason is that fluctuations in the RSSI values
increase due to the human movement. If a person moves at
slow, medium and fast pace, he/she obstructs the radio
frequency signal for long, medium and short time respectively.
This is evident from the figures that in case of slow pace
movement, the RSSI value fluctuates between much wider
range compared to the fast pace movement, refer Figure 3 and
Figure 5.
The standard deviation of no movement, slow, medium and
fast pace movement is shown in Figure 6. It shows that RSSI
deviation is very less in case of no movement and deviation of
slow movement is much higher for slower paces compared to
faster paces at some angles where signal strength is stronger,
i.e. 0°, 90° etc.
Figure 6. Standar deviation vs antenna orientation angles with single person
moving at varying pace
The RSSI frequency ratios of a two persons slow and fast
pace scenarios are shown in Figure 7 and Figure 8,
respectively. It is obvious from those illustrations that the
slower movement of two persons have relative high impact on
the RSSI fluctuations compared to fast movement. The range
of RSSI values is much wider in slow pace compared to the
fast pace movement, refer Figure 7 and 8.
Figure 7. Frequency percentage vs RSSI at varying elevation angles with
two persons moving at slow pace
Figure 8. Frequency percentage vs RSSI at varying elevation angles with
two persons moving at fast pace
The standard deviation of two person movement paces
versus elevation angles is depicted in Figure 9. The RSSI
deviation is with no movement is much smaller compared to
the movement case. However, the interesting point in this
figure and Figure 6 is that RSSI at 90° with no movement has
larger deviation in no movement scenario compared to other
angles.
Figure 9. Standar deviation vs antenna orientation angles with two person
moving at varying pace
Figure 10. Frequency percentage vs RSSI at varying elevation angles with
four persons moving at medium pace
Figure 10 shows RSSI frequency rate of the four persons
medium pace movement case. It is observed that four persons
movement between pair of nodes have really very high impact
on the RSSI compared to the two and one-person slow pace
movement scenarios, refer Figure 4 and 10.
It has been experimentally observed that antenna
orientation, human movement with different movement paces
and number of persons and ground effect have really high
impact on the signal strength of the sensor mote radio
frequency. In this case, it is very difficult to select a single
RSSI fluctuation rate threshold point to detect the human
activity in the WSN deployment area. Hence, the human
activity detection algorithm based on the RSSI fluctuations
must efficient and adaptive.
V. CONCLUSIONS
In this paper, we experimentally examined the combined
effect of human movement, antenna orientation in elevation
direction and ground effect on the RSSI. We found that, even
without the human movement, signal strength and its
fluctuation are different at varying antenna angles. It is also
observed that, human movement from slower to faster paces
increases the fluctuations in the RSSI. Therefore, the human
mobility detection in the WSN deployment area must be
adaptive and consider all these factor to properly detect the
human activity with minimum false counts.
VI. ACKNOWLEDGMENT
This work was partly supported by Keio University 21st
Century Centre of Excellence Program on “Optical and
Electronic Device Technology for Access Network” and
Fujitsu Laboratories.
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