signal propagation measurements with wireless sensor nodes.pdf
TRANSCRIPT
1
Signal Propagation Measurements with Wireless Sensor Nodes
Joaquim A. R. Azevedo, Filipe Edgar Santos
University of Madeira
Campus da Penteada
9000-390 Funchal
Portugal July 2007
1. Introduction
Several experimental measurements have been done to evaluate the RF signal propagation
inside a laboratory and outdoors. The measurements were realized with a portable spectrum
analyser with different antennas. It was used the Tmote Sky Sensor Mote in the measurements
and Micaz.
Most of the measurements were made to verify the performance of the sensor nodes in several
environments. As it will be presented, the existence of obstacles and indoor reflections affect
severally the link quality. This can influence the topology used for the wireless sensor
network.
Although both the Tomote sensor node and the Micaz have the same radio chip, the
measurements demonstrated that the performances of both systems are different.
2. Radiation pattern of the sensor nodes
The Tmote sensor node incorporates an internal inverted-F antenna, which is a wire monopole
where the top section is folded down to be parallel with the ground plane [1]. The antenna gain is 3.1 dBi [2] and the radio operates at 2.4 GHz (12,5 cm of wavelength) In datasheet it is
referred that the antenna may attain 50 meter indoor and 125 meter outdoor. The sensor node
uses a Chipcon CC2420 radio for wireless communications and the maximum output power
was set to 0 dBm [3].
The first graph of figure 2.1 depicts the antenna pattern, while the Tmote is mounted
horizontally with antennas parallel section aligned to the 0 degree direction. The main null is
24 dB below the maximum of the pattern. The second graph depicts the antenna pattern, while
the Tmote is mounted vertically with antennas parallel section aligned to the 0 degree
direction. The polarization is horizontal. As we can observe, the radiation pattern is not
omnidirectional in any plane. Therefore, the received signal of a sensor node depends on the
antenna orientation of the receiver.
F E D E R
2
Fig. 2.1 – Radiation pattern of the Tmote for horizontal and vertical mounting.
The Micaz sensor node incorporates an external monopole antenna of λ/4 and operates at 2.4 GHz [4]. The theoretic radiation pattern of a monopole is equal to the dipole of half a
wavelength. However, when it is introduced in the sensor node the radiation pattern is
changed. Figure 2.2 presents the measured radiation pattern of the Micaz in an anechoic
chamber with the antenna in the vertical [5]. As we can observe, the pattern is far from
circular. The main null is 9 dB below the maximum of the pattern.
-14
-12
-10
-8
-6
-4
-2
0 0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330°
-14
-12
-10
-8
-6
-4
-2
0 0°
30°
60°
90°
120°
150°
180°
210°
240°
270°
300°
330° (dBm)
Fig. 2.2 – Radiation Pattern of Micaz Mote.
3
3. Measurements of the patterns inside a laboratory
The Tmote and Micaz sensor nodes were used to evaluate its performance in indoor and
outdoor environments. Considering the theory, it was compared the measured results of signal
propagation. The measurement of the received signal strength was realised using the portable
spectrum analyser R&S FSH 3. Another way to measure the received signal is the RSSI
(Received Signal Strength Indicator) parameter of the Tmote and Micaz sensor nodes.
3.1. Tmote sensor node
In order to compare the results obtained with different antennas and to verify the contribution
of reflections inside the laboratory, the Tmote sensor node was placed 5.4 meters apart from
the measurement equipment (figure 3.1). A spectrum analyser was used to make the
measurements. The sensor node and the spectrum analyser antennas were at positions in
distance of 40 cm above the floor. In figure 3.1 it is represented the relative positions of the
transmitter sensor node and reception antenna in the laboratory, with the antennas parallel
section aligned to the 0 degree direction. Figure 3.2 shows the laboratory used for the
experiments.
receptor transmitterreceptor transmitter
Figure 3.1 – Position of the sensor node and of the spectrum analyser.
Figure 3.2 – Used laboratory.
4
Three antennas were constructed for the frequency of interest to deal with the spectrum
analyser: a dipole of half wavelengh dipole, with maximum gain 2.15 dBi and SWR=1.55
(figure 3.3a), a bi-quad antenna with 8.5 of gain and SWR=2.4 (figure 3.3b), and a Yagi
antenna with 11.5 dBi of gain and SWR=1.35 (figure 3.3c).
Due to the antenna SWR, the dipole antenna has less about 0.2 dB in the received power, the
bi-quad less 0.8 dB and the Yagi less 0.09 dB. In reference to the dipole, the Bi-quad has a
gain of 6.35 dBd and the Yagi of 9.35 dBd. In order to confirm the antenna gains, it was made
some measurements in the exterior to minimize the reflections. The received signal strength
varies about ±1 dB due to the outside reflections. The difference between the received signal
obtained by the Bi-quad and the dipole was of 5.8 dBd and for the Yagi was of 9.3 dBd. The
expected value for Bi-quad is (8.5-0.8)-(2.15-0.2)=5.75 dBd and for the Yagi is (11.5-0.09)-
(2.15-0.2)=9.46 dBd. As we can notice the results coincide with the measured values very
well.
a) Dipole b) Bi-quad
c) Yag i
a) Dipole b) Bi-quad
c) Yag i
Figure 3.3 – Used antennas.
To the different orientations of the sensor node, figure 3.4 presents the results measured with
the three antennas. The sensor node is in the horizontal mounting and, therefore, the received
antennas were place horizontally to the ground. Comparing with the radiation pattern of figure
2.1 we can observe the influence of the reflections inside the laboratory. In fact, the nulls of
the pattern are less pronounced in this picture. Moving the sensor node in a small distance
(about half wavelength) towards two different directions, figure 3.5a) and 3.5b) show the
corresponding measured values.
In the radiation pattern of figure 2.1 (horizontal polarization) exits a maximum around 225° and another one (2 dB below) around 135°. The measures suggest a maximum of radiation
around 135°.
5
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,00°
45°
90°
135°
180°
225°
270°
315°
Dipole
Bi-quad
Yagi
Figure 3.4 – Measured received signal strength for different directions of the sensor node.
Dipole
Bi-quad
Yagi
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,00°
45°
90°
135°
180°
225°
270°
315°
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,00°
45°
90°
135°
180°
225°
270°
315°
Dipole
Bi-quad
Yagi
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,00°
45°
90°
135°
180°
225°
270°
315°
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,00°
45°
90°
135°
180°
225°
270°
315°
Figure 3.5 – Dependence of the measured received signal strength with the position.
The fluctuation of the received RF signal strength for each antenna is better perceived in
figure 3.6 for three positions half a wavelength apart. The minimum values are more affected
by laboratory reflections. In small distances the signal have varied several dB. Considering
the average of the measured values, the difference between the mean received power of the
Bi-quad antenna values and the dipole antenna values is of 3.6 dB (standard deviation of 0.8
dB) and the difference between the mean received power of the Yagi antenna values and the
dipole antenna is of 6.7 dB (standard deviation of 2.3 dB). Since the theoretical expected
difference is 5.75 dB and 9.46 dB, respectively, the means are 2.15 dB and 2.76 below these
values. To understand these results, we must take into account that the dipole antenna is
omnidirectional, whilst the bi-quad and the Yagi are directional. Therefore, the dipole can
receive more energy from behind reflections than the other antennas.
6
a)Dipole
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,0
0° 45° 90° 135° 180° 225° 270° 315°
(dB
m)
b) Bi-quad
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,0
0° 45° 90° 135° 180° 225° 270° 315°
(dB
m)
c)Yagi
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
-35,0
0° 45° 90° 135° 180° 225° 270° 315°
(dB
m)
Figure 3.6 – Variation of the received signal in small in nearby distances.
Let us see when the received antennas are in the vertical instead of on horizontal and
maintaining the sensor node in the horizontal polarization. Figure 3.7 presents the results for a
rotation of the sensor node. The received signal strength gives, on average, a value of 10.5 dB
lower using the dipole (standard deviation of 3.8 dB) and of 9.4 using the Yagi antenna
(standard deviation of 4.9 dB). Due to the reflections, the receiver signal strength varies
reasonably.
7
-70
-65
-60
-55
-50
-45
-40
-35
-30
0° 45° 90° 135° 180° 225° 270° 315°
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Dipole - vertical
Dipole - horizontal
Yagi - vertical
Yagi - horizontal
Figure 3.7 – Comparison of the received signal strength for vertical and horizontal polarizations.
3.2. Micaz sensor node
Some measurements were also made with the Micaz sensor node, in order to evaluate the
radiation pattern inside the laboratory. The distance to the reception equipment and the
distance to the ground is similar to the Tmote sensor node measurements. The dipole antenna
and Yagi antennas were used in the measurements with the spectrum analyser.
Figure 3.8 shows the results for the antennas in the vertical position, taking into account the
vertical polarization of the monopole antenna of Micaz. The zero degrees corresponds to the
sensor side where is the monopole. Considering the average of the measured values, the
difference between the mean received power of the Yagi antenna and the dipole antenna is of
9.5 dB (standard deviation of 1.2 dB). The theoretical expected value is of 9.46 dBd, which
coincides with the measured values very well. Comparing the radiation pattern with figure
2.2, we can observe that with the reflections the pattern is more circular.
-80
-75
-70
-65
-60
-55
-500°
45°
90°
135°
180°
225°
270°
315°
Dipole
Yagi
dBm
-80
-75
-70
-65
-60
-55
-500°
45°
90°
135°
180°
225°
270°
315°
Dipole
Yagi
dBm
Figure 3.8 – Measured received signal strength for different orientations of the sensor node.
8
To make a comparison, the received signal strength indicator (RSSI) of the Micaz was also
considered. The Micaz used to receive the signal and connected to the computer was placed in
the same position of the dipole connected to the spectrum analyser. The results are
represented in figure 3.9.
-80
-75
-70
-65
-600°
45°
90°
135°
180°
225°
270°
315°
Stectrum Analyser
RSSIdBm
-80
-75
-70
-65
-600°
45°
90°
135°
180°
225°
270°
315°
Stectrum Analyser
RSSIdBm
Figure 3.9 – Comparison between the received signal strength obtained from the spectrum analyser and RSSI.
3.3 RSSI and received signal
Most of the measurements considered in this work for the signal strength in the reception
were obtained using the spectrum analyser. Another way to measure the received signal could
be the RSSI (Received Signal Strength Indicator) parameter of the Tmote and Micaz sensor
nodes. However, taking into account the datasheet of the radio component CC2420 used in
these sensor nodes [3], there exists an accuracy error of ±6 dB in the RSSI readings.
Therefore, we expect that the readings from the spectrum analyser should be more accurate
for reading the real signal on the received antenna position. Furthermore, the RSSI readings
have a difference of ±3 dB in linearity.
Some comparisons were made inside the laboratory to evaluate the differences between the
RSSI and direct measurements and between sensor nodes.
Considering Tmote sensor nodes, it was showed that the transmitted power is almost the same
for the various sensor nodes. To get this conclusion, several sensor nodes were considered as
transmitters and the signal at reception was obtained using the spectrum analyser. When the
RSSI parameter of the sensor nodes was used, it was found out variations between RSSI
readings in the same position.
For three Tmote sensor nodes were made measurements in four different locations. The three
sensor nodes presented different RSSI readings for the same position. The values have
deferred in ±4 dB. For the previous positions, the measurements made with the spectrum
analyser gave values that can be of –9 dB lower then with RSSI readings. The spectrum
analyser readings have, on average, a value of –4 dB compared with the RSSI readings with a
standard deviation of about 4 dB.
Comparing the measurements obtained by Tmote sensor nodes with Micaz sensor nodes it
gave a mean difference between the RSSI Micaz readings of 14 dB below the RSSI readings
of Tmote. Using the spectrum analyser the difference in the received signal is about 11 dB
below for Micaz comparing with Tmote.
9
Other works have reported that the RSSI obtained from Micaz sensor node did not report the
actual signal strength [6].
3.4 Influence of the distance
Another set of measurements was done to obtain the variation of the received signal strength
with the distance to the Tmote sensor node. The maximum radiation direction of the node
antenna was considered. The sensor node and the spectrum analyser antenna were at positions
in distance of 80 cm above the floor.
Figure 3.10 presents the measured signal using the dipole antenna for several distances from
the sensor node, defined by the continuous line. The distance between measured points is 3
cm. The signal has a decaying behaviour in the distance to the sensor and a great variation due
to the influence of the reflections in the walls, floor and ceiling. In distances of 3 cm the
signal can change around 8 dB. For greater distances, the signal can change 15 dB in small
distances.
A function for the decaying of the signal can be obtained from the model of the path loss [7],
σXd
dndPdP LL +
+=
0
100 log10)()( (3.1)
where n is the path loss exponent and indicates the rate at which the signal attenuates with the
distance (n=2 for free space). PL(d0) is the path loss at a known reference distance d0 which is
in the far field of the transmitting antenna (typically 1 km for large urban mobile systems, 100
m for microcell systems, and 1 m for indoor systems) and Xσ denotes a zero mean Gaussian
random variable (in dB) with standard deviation σ, and reflects the variation in average received power.
From the measurements made we can obtain an estimative for the n parameter, using the
average of results calculated from the formula,
−=
0
10
0
log10
)()(
d
d
dPdPn LL (3.2)
where d0=1 m. The result is n=2.8 for the path loss exponent. This result is in consonance
with those presented in literature. Using this value in (3.1) the result for the path loss is the
represented by the dashed line of figure 3.10. The standard deviation for the difference
between the measured results and this curve is 4.6 dB.
Based on equation (3.1) when the receiver measure a value PL(d), the estimated distance to
the transmitter is
n
dPdP LL
dd 10
)()(
0
0
10
−
×= (3.3)
Since the sensivity of the sensor nodes is of –94 dBm, it will be reached at about 120 meters.
This can be understood if there continues to exist a line of sight between the two antennas.
However, even in this case, considering that the signal can fluctuates around ±10 dB, the reception sensor node can lose the signal in about 50 meters.
10
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5-70
-60
-50
-40
-30
-20
Rece
ived signal stren
gth
(dBm)
Distance (m)
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5-70
-60
-50
-40
-30
-20
Rece
ived signal stren
gth
(dBm)
Distance (m)
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5-70
-60
-50
-40
-30
-20
Rece
ived signal stren
gth
(dBm)
Distance (m)
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5-70
-60
-50
-40
-30
-20
Rece
ived signal stren
gth
(dBm)
Distance (m)
Figure 3.10 – Variation of the received signal with the distance to the sensor node.
For a comparison, figure 3.11 presents some positions of the sensor node and measurements
realized with dipole and bi-quad antennas. Once again, the mean received power difference
between results of the bi-quad antenna and of the dipole is of 3.6 dB.
-60
-55
-50
-45
-40
-35
-30
-25
-20
0,25
0,5
0,75
1 1,25
1,5
1,75
2 2,25
2,5
2,75
3 3,25
3,5
3,75
4 4,25
4,5
4,75
5
Distance (m)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Dipole
Bi-quad
Figure 3.11 – Comparison of the variation of the received signal with the distance to the sensor node.
3.5. Influence of the height to the ground
The variation of the received signal strength with the height of the transmitter antenna was
analysed, considering the receptor dipole antenna at 93 cm above the floor and 5.4 m from the
transmitter. Then the Tmote sensor node was varied from 2 cm till 236 cm and the results
were registered, and represented in figure 3.12. Once again, it is clear the effect of the
reflections. We can also observe a great fluctuation of the signal. This is due to the radiation
pattern variation with the distance to the ground for small heights and also the influence of the
ceiling for higher distance to the ground. In fact, at the antenna positions of the experiment,
the theoretical nulls in the pattern are about 0.37 cm apart in the height variable for a perfect
conducting ground.
To confirm the ground influence, for a reflection in a perfect ground we have,
=d
hhF 21sin2
β (3.4)
11
where β=2π/λ, λ is the wavelength, h1 is the height of the transmitter antenna, h1 is the height
of the reception antenna, and d is the distance between antennas. For comparison, the factor
defined by the previous expression is depicted in figure 3.12 by the dashed line, where the
maximums were moved to –45 dB to permit the comparison.
Received signal strength
(dBm)
Height (m)
0.1 0.2 0.3 0.4 0.5 0.60.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4-70
-65
-60
-55
-50
-45
-40
Received signal strength
(dBm)
Height (m)
0.1 0.2 0.3 0.4 0.5 0.60.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4-70
-65
-60
-55
-50
-45
-40
Received signal strength
(dBm)
Height (m)
0.1 0.2 0.3 0.4 0.5 0.60.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4-70
-65
-60
-55
-50
-45
-40
Received signal strength
(dBm)
Height (m)
0.1 0.2 0.3 0.4 0.5 0.60.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4-70
-65
-60
-55
-50
-45
-40
Figure 3.12 – Variation of the received signal with the height of the sensor node.
Another set of measurements was made with the receptor antenna at 100 cm above the floor
and also at a position 5.4 m from the transmitter. The Tmote sensor node was varied from 10
cm to 170 cm in height. Figure 3.13 depicts the results. From the theory, the null in the pattern
obtained by varying the height is about 0.35 cm apart, which can be verified by the figure.
-70
-65
-60
-55
-50
-45
-40
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
Height (cm)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Figure 3.13 – Variation of the received signal with the height of the sensor node for a different position of the
received antenna.
Varying both antennas (reception and emission) related to the floor from 2 cm till 26.5 cm,
the result is the one represented in figure 3.14. The dashed line corresponds to the tendency
line of the measures. At 25 cm the average of the received signal is about 10 dB higher than 2
cm.
12
-70,0
-65,0
-60,0
-55,0
-50,0
-45,0
-40,0
0 5 10 15 20 25 30
Height (cm)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Figure 3.14 – Variation of the received signal with the height of the sensor node and received antenna.
4. Measurements on the distance in a corridor
For greater indoor distances, a set of measurements was made in a corridor with 2.4 m width
and about 40 m long. Figure 4.1 shows the measured received signal using the dipole antenna
for several distances from the Tmote sensor node (continuous line). The distance between
measured points is 30 cm. The sensor node is located 11.15 m from the beginning of the
corridor. The sensor node and the measurement antenna were 40 cm above the floor. The
conclusions for indoor propagation presented previously can be verified for the signal
fluctuation but the signal has a lower decaying compared with the obtained inside the
laboratory.
Considering the reference at 1.5 m, the application of the path loss model gives a value for the
path loss exponent of n=1.9. This value is near the free space propagation. We should taking
into account that the corridor may have some waveguide characteristics. Substituting this
parameter in (3.1), the result is represented by the dashed line of figure 4.1. The standard
deviation for the difference between the measured results and this curve is 4.7 dB.
-80
-70
-60
-50
-40
-30
-20
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
Distance (m)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Figure 4.1 – Variation of the received signal with the distance to the sensor node.
13
Figure 4.2 depicts another set of measurements in the same place but with antennas one meter
above the floor. The mean difference between the two results is not significantly.
-75
-65
-55
-45
-35
-25
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Distance (m)
Rec
eive
d si
gnal
str
engt
h (d
Bm
)
40 cm
100 cm
Figure 4.2 – Comparison of the variation of the received signal with the distance to the sensor node for receiving
antennas at 40 cm and 1 m.
In this test it is intended to compare the performance between of Tmote and Micaz sensor
nodes. Considering the Micaz at positions 1 meter above the ground, in the referred corridor
and for the same positions in distance, figure 4.3 shows the results by the continuous line. The
mean of results obtained from Micaz sensor node is 13 dB lower than the mean of results
determined from Tmote sensor node.
The dashed lines correspond to the curves obtained using (3.1) with the path loss calculated
before (n=1.9). The difference between the two curves is 14 dB below for the Micaz. Once
again, the results demonstrated a difference of about 13 dB between the signals of the two
types of sensor nodes.
-90
-80
-70
-60
-50
-40
-30
-20
0 2 4 6 8 10 12 14 16 18 20 22
Distance (m)
Rec
eive
d si
gnal
str
engt
h (d
Bm
) Tmote
Micaz
Figure 4.3 – Comparison of the variation of the received signal with the distance to the sensor node for Micaz
and Tmote.
14
5. Existence of obstacles in the propagation path
All the tests made till know evolved line of sight between the transmitter and receptor.
However, in a sensor network it is expected communication between two sensor nodes even
when they cannot see each other. The existence of obstacles in the propagation path affects
drastically the communication link.
As a simple example, for the positions of figure 3.1 it was used a Tmote as transmitter and the
spectrum analyser was placed 5.4 m apart to measure the received signal. The antennas were
at 40 m above the ground. For several positions between the two antennas it was placed a
metal plate with 1000×400×1 mm in the transversal section.
Without the obstacle, the received signal is –49 dBm. With the obstacles, figure 5.1 shows the
results for several distances of the plate to the receptor antenna. In central positions of the
plate the received signal is less affected than for positions around the antennas. Although
without line of sight, a lot of signal reaches the receptor antenna due to the reflections. When
the plate approximates to the transmitter or receptor, more reflections are cancelled. The
lowest peak is around 16 dB below the unobstructed propagation value.
-75
-70
-65
-60
-55
-50
-45
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5
Distance (m)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Figure 5.1 – Variation of the received signal due to a metal plate between the sensor node and the receptor.
Another set of measurements was made to evaluate the effect of walls between the transmitter
and receptor. Figure 5.2 shows the test bed. The used laboratories have 8.8 m by 5.7 m and
contain office equipment, which will cause fading in the reception. The referred situations 1,
2 and 3 correspond to different reception positions for a Tmote sensor node placed near the
wall on the horizontal plane, as it is illustrated in the figure. The sensor node is 1.2 m above
the ground and the receiver is 0.75 m from the ground. The situation 1.1 corresponds to the
position of the sensor node referred in situation 1 but with measurements made in another
laboratory. Figure 5.3 depicts the results. The higher curves correspond to the measurements
inside the laboratory were it is the sensor node. The mean difference between the results of
situation 1.1 and situation 1 is –21 dB (standard deviation of 7 dB).
For situation 1, the mean difference related to the free space propagation is –10 dB (standard
deviation of 4.5 dB). For situation 1.1, where a wall exits between the transmitter and the
receptor, the mean difference related to the free space propagation is –23 dB (standard
deviation of 2.5 dB). Therefore, the difference of results is 13 dB.
15
Situation 1
1 4
2 53
6
7
Situation 2
1 4
2 53
6
7
Situation 3
1 4
2 53
6
7
Situation 1.1
1 4
2 53
6
7
Situation 1
1 4
2 53
6
7
Situation 2
1 4
2 53
6
7
Situation 3
1 4
2 53
6
7
Situation 1.1
1 4
2 53
6
7
Figure 5.2 – Positions of the receptor and sensor node.
-95
-90
-85
-80
-75
-70
-65
-60
-55
-50
1 2 3 4 5 6 7
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Situation 1
Situation 2
Situation 3
Situation 1.1
Figure 5.3 – Results for the positions of figure 5.2.
Let us consider a different orientation of the Tmote antenna. In situation 4 (figure 5.4) the
sensor node is placed vertically on the wall but with the antenna in the horizontal. Situation 5
has the sensor node placed on the wall with the antenna in the vertical. Situations 1.4 and 1.5
are similar to these ones but the measurements are made in another laboratory. The results are
represented in figure 5.5. The mean difference between the results of situation 1.4 and
situation 4 is –17 dB (standard deviation of 6 dB) and the mean difference between the results
of situation 1.5 and situation 5 is –20 dB (standard deviation of 4 dB).
For situation 4, the mean difference related to the free space propagation is –7 dB (standard
deviation of 5 dB) and for situation 5 the mean difference is –2.5 dB (standard deviation of 5
dB). For situation 1.4 the mean difference related to the free space propagation is –16.5 dB
16
(standard deviation of 3.5 dB) and for situation 1.5 the mean difference is –15.5 dB (standard
deviation of 1.5 dB). Therefore, the difference of results is –9.5 dB for the first case and –13
dB for the second one.
Situation 4
1 4
2 53
6
7
Situation 5
1 4
2 53
6
7
Situation 4
1 4
2 53
6
7
Situation 5
1 4
2 53
6
7
Figure 5.4 – Point positions for the receptor and sensor node.
-100
-90
-80
-70
-60
-50
-40
1 2 3 4 5 6 7
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Situation 4
Situation 5
Situation 1.4
Situation 1.5
Figure 5.5 – Received signal with and without a wall between sensor node and measurement equipment.
The previous results suggest attenuations introduced by the walls around 12 dB. However, the
received signal strength depends not only of the signal across the wall but also the signal
diffracted around the windows and doors.
6. Outdoors experiments
One of the objectives of the Foresmac project is to work in the forest environment. In this
sense, after the work realised indoors let us make some measurements outdoors to get more
parameters for the sensors deployment. The idea is to obtain an environment with
characteristics near the free space propagation.
For these experiments it was necessary to create measurement facilities appropriated for the
objectives of the work. The flat roof of the University was used in order to minimise the
reflections. The transmitter and the receptor antennas were placed 5 m above the ground and
the distance between antennas was varied from 1 m to 8 m (figure 6.1).
17
Fig 6.1 – Outdoors measurement facilities.
The sensor nodes were placed in a hood connecting rod of 5 m long. To control the direction
of the antenna a small motor controlled by radio was used (figure 6.2). To connect the
reception antenna to the spectrum analyser it was necessary to get a coaxial cable with
reduced attenuation loss. The cable length has 10 m long. The usual employed cable of 50 Ω,
the RG58, has attenuation of 1.06 dB/m for 2.4 GHz. With less attenuation it was utilised the
coaxial cable RG213/U with 0.5 dB/m for 2.4 GHz (5 dB in 10 m). Tests realized with a
signal generator have showed that the attenuation introduced by this cable was 4.7 dB, a value
that was considered in the measurements.
Fig. 6.2 – System to control the sensor node antenna orientation.
3.1. Tmote sensor node
The signal of a Tmote sensor node was measured in the exterior using the dipole and Yagi
antennas for several distances. Both systems were placed at 5 m above the ground. The sensor
node has horizontal polarization and the maximum radiation was used. In reference to the
18
radiation pattern of the Tmote it was measured a difference of 15 dB between the minimum
and the maximum radiation.
The continuous lines of figure 6.3 represent the received signal strength. Applying the path
loss model, from (3.1) we obtain a value for the path loss exponent about n=2.1 for both
antennas. The path loss parameter of the Yagi antenna was calculated using the distance from
the sensor node until the end of the antenna and not to the excited element (difference of 25
cm). If the distance is considered till the excitation element of the Yagi, the path loss
exponent would be n=2.4, which is not an expected result for the free space conditions and
did not fit the measurements.
As it was verified, the obtained results suggest a propagation factor near the free space
conditions. Using the measurements, and taking into account that the dipole gain is around 1,9
dBi, The measured mean of the Tmote gain is 1,4 dBi (standard deviation of 0.6 dB). If the
free space propagation loss is represented including the dipole gain and Tmote gain the result
is the one represented by the dashed lines of figure 6.3. The standard deviation for the
difference between the measured results and these curves is 0.6 dB with a maximum
difference around ±1 dB for the given distances. For comparison, the indoor measurements
gave a standard deviation of 4.6 dB and a maximum difference around ±10 dB. The mean
difference between the Yagi and dipole results is of 8.8 dBd with standard deviation of 0.8 dB
(excited element in the same position). The theoretical result is of 9.46 dBd. The importance
of the Yagi antenna is to extend the limit of the spectrum analyser measurements in 9 dB
when compared with the dipole antenna.
From figure 6.3 we can also observe that the fluctuation around the tendency curve increases
for higher distances from the sensor, reflecting the influence of the ground.
-60
-55
-50
-45
-40
-35
-30
-25
-20
1 2 3 4 5 6 7 8
Distance (m)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Measured w ith dipole
Free space+gains
Measured w ith Yagi
Figure 6.3 – Variation of the received signal with the distance with horizontal polarization for the Tmote.
The Tmote sensor node was positioned with the antenna in the vertical. From the
measurements, the antenna gain for this polarization is around -6.4 dBi. Figure 6.4 shows the
received signal strength for several distances between from the transmitter and the free space
propagation including the antenna gains. Once again, the signal follows the free space
19
propagation curve and the influence of the ground is more obvious for higher distances.
Comparing with the results of figure 6.3, the received signal has a mean difference of –7.5 dB
with 0.8 dB of standard deviation. Thus, the sensor node has a better reception signal for the
horizontal position in an environment with minimal reflections.
-70
-65
-60
-55
-50
-45
-40
1 2 3 4 5 6 7 8
Distance (m)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
) Measured w ith dipole
Free space+gains
Figure 6.4 – Variation of the received signal with the distance with vertical polarization for the Tmote.
3.2. Micaz sensor node
As realized for the Tmote, the received signal strength from a Micaz sensor node was
measured outside using the dipole and Yagi antennas for several distances from the
transmitter. Figure 6.4 presents the results through the continuous lines. Applying the path
loss model to the measurements of the dipole antenna, the path loss exponent is n=2.0. The
standard deviation for the difference between the measurements and these curves is 0.8 dB
with a maximum difference around ±1.5 dB for the considered distances. The mean difference
between the Yagi and dipole results is of 9.1 dBd with standard deviation of 1.4 dB (the
theoretical value is 9.46 dBd).
Comparing with the Tmote sensor node, Micaz has a mean received signal that is –13 dB
below the signal received of Tmote. This result was also obtained in previous tests. From the
measurements, the suggested gain of Micaz is -11.8 dBi (standard deviation of 0.8 dB). The
gain difference between Tmote and Micaz is 13.1 dB. Other tests to minimize the reflection
on the ground gave similar conclusions for the Tmote antenna gain.
20
-75
-70
-65
-60
-55
-50
-45
-40
-35
-30
1 2 3 4 5 6 7 8
Distance (m)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
)
Measured w ith dipole
Free space+gains
Measured w ith Yagi
Figure 6.5 – Variation of the received signal with the distance with horizontal polarization for the Micaz.
If the Micaz sensor node is positioned with the antenna in the vertical, the received signal
strength obtained is the one of figure 6.6. Comparing with the horizontal polarization, the
received signal has almost the same amplitude.
-75
-70
-65
-60
-55
-50
-45
1 2 3 4 5 6 7 8
Distance (m)
Rec
eive
d s
ign
al s
tren
gth
(d
Bm
) Vertical polarization
Horizontal polarization
Figure 6.6 – Received signal of Micaz for vertical and horizontal polarizations.
3.3. Mica2 sensor node
The Tmote and Micaz sensor nodes operate at 2.4 GHz whilst the Mica2 operate at 900 MHz
band. This mote has a monopole antenna and Micaz. To have an approximation for the gain
some measurements was realized in outdoor. To use the spectrum analyser, a half wavelength
dipole was constructed. For a SWR of 1.35 and antenna gain is about 2.1 dB. The figure 6.7
21
shows the received signal for vertical and horizontal polarizations. From the measurements
the Mica2 gain is -8 dBi for vertical polarization. The horizontal polarization suffered more
the environment influence.
-60
-55
-50
-45
-40
-35
-30
1 2 3 4 5 6 7 8 9
Distance (m)
Rec
eive
d S
ign
al S
tren
gh
t (d
Bm
)
Vertical polarization
Free space+gains
Horizontal polarization
Figure 6.7 – Received signal of Mica2 for vertical and horizontal polarizations.
3.4. Influence of the height to the ground
The variation of the received signal strength with the height to the ground was analysed,
considering the Tmote at 5.4 m from the dipole. The values were obtained moving both
antennas from 0.2 cm to 4.75 m, in steps of 5 cm. The results are presented in figure 6.8. As
we can observe, the received signal has a great variation due to the ground reflection. For
higher distances from the ground, the signal can varies around the average of ±2 dB.
-62
-60
-58
-56
-54
-52
-50
-48
-46
-44
0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0 3,2 3,4 3,6 3,8 4,0 4,2 4,4 4,6
Eight (m)
Rec
eive
d s
ign
al s
tren
gth
(dB
m)
Figure 6.8 – Variation of the received signal with the height of the sensor node.
22
Considering a typical permeability (εr=5) and conductivity (σ=5×01-3 S/m) of the brick
material to calculate the coefficient of reflection on the ground,
+=
=
−
−+
−
−−
=Γ
Γ+=−
d
hh
j
j
eF
r
r
h
d
hhj
h
21
2
0
2
0
2
arctan
2
)(cos)(sen
)(cos)(sen
)cos(121
ψ
λπβ
ψωεσεψ
ψωεσεψ
ψβ
(3.5)
with λ the wavelength, and h1 and h2 the height of emitter and receiver antennas, respectively,
the theoretical curve for the received signal strength is shown in figure 6.9 (continuous line).
The cos(ψ) term in F represents the radiation pattern of the reception antenna. For this
calculation it was taken into account the radiation pattern of the dipole antenna and the
previous results. From the curve, we can observe other influences in the received signal, such
as the influence of the measure system.
Receivedsignalstrength(dBm)
-62
-60
-58
-56
-54
-52
-50
-48
-46
-44
-42
Height (m)
0.5 1 1.5 2 2.5 3 3.5 4 4.5-64
Receivedsignalstrength(dBm)
-62
-60
-58
-56
-54
-52
-50
-48
-46
-44
-42
Height (m)
0.5 1 1.5 2 2.5 3 3.5 4 4.5-64
Figure 6.9 – Comparison between the theoretical variation and the received signal with the height.
3.5. Different polarizations for the Tmote
It was analysed the radiation of the Tmote sensor node for different polarizations and
orientations. Figure 6.10 shows the three main orientations of the sensor node.
23
Horizontal mounting and
horizontal pola rization
0 °
0 ° 0 °
Vertica l mounting and
horizontal pola rization
Vertica l mounting and
vertical pola rization
Figure 6.10 – Different positions of the sensor node.
The measures were made with the Tmote at 5 m above the ground. The dipole antenna used
with the spectrum analyser was 4 m of distance and at the same height of the sensor node.
Figure 6.11 presents the results. It was noticed some influence of the reflections in the lower
values of the pattern. As we can see, the best results are obtained with the sensor node in
horizontal mounting and horizontal polarization. As it was observed indoor, the maximum
radiation is around 135°. An approximation for the outdoor radiation pattern is presented in
figure 6.11, for the horizontal mounting and horizontal polarization. The antennas were
placed 5.4 m apart and 5 m above the ground. We can compare this graph with the one of
figure 2.1. The surrounding environment is noticed in the results.
-75
-70
-65
-60
-55
-50
0° 45° 90° 135° 180° 225° 270° 315°
Vertical mounting andhorizontal polarization
Vertical polarization
Horizontal mounting andpolarization
Figure 6.11 – Results for the different orientations of the Tmote sensor node.
24
-90
-85
-80
-75
-70
-65
-60
-55
-500°
10°20°
30°
40°
50°
60°
70°
80°
90°
100°
110°
120°
130°
140°
150°160°
170°180°
190°200°
210°
220°
230°
240°
250°
260°
270°
280°
290°
300°
310°
320°
330°340°
350°
7. References [1] Moteiv Corporation, 2006. Moteiv. "Tmote Sky: Ultra Low Power IEEE 802.15.4 Compliant Wireless Sensor
Module." 2006. Available from http://www.moteiv.com/products/docs/tmote-sky-datasheet.pdf.
[2] Raman, B., Chebrolu, K., Madabhushi, N., Go, D. Y., Valiveti, P. K.k and Jain, D., “Implications of link
range and (In)stability on sensor network architecture”, Proceedings of the 12th annual international
conference on Mobile computing and networking , Los Angeles, CA, USA, pp. 65-72, 2006.
[3] CC2420 Datasheet, Chipcon . Available from http://www.chipcon.com/files/CC2420_Data_Sheet_1_3.pdf.
[4] Crossbow Technology Inc. "MICAz Wireless Measurement System." 2005. Available from
http://www.xbow.com/Products/Product_pdf_files/Wireless_pdf/MICAz_Datasheet.pdf.
[5] Tan, E. B., Lim, J. G., Seah, W. K., and Rao, S. V., “On the Practical Issues in Hop Localization of Sensors
in a Multihop Network”, Vehicular Technology Conference, VTC 2006-Spring. IEEE63rd, pp. 358-362,
2006.
[6] Scott, T., Wu, K, and Hoffman, D., “Radio propagation patterns in wireless sensor networks: new
experimental results”, Proceeding of the 2006 International Conference on Communications and Mobile
Computing, Vancouver, Canada, pp. 857-862, July 2006.
[7] Andersen, J. B., Rappaport, T. S., Yoshida, S., “Propagation Measurements and Models for Wireless
Communications Channels”, IEEE Communications Magazine, vol. 33, pp. 42-49, 1995.