Wireless Sensor Network, 2009, 1, 383-396
doi:10.4236/wsn.2009.15047 Published Online December 2009 (http://www.scirp.org/journal/wsn).
383
Antenna and Base-Station Diversity for
WSN Livestock Monitoring
Konstantinos SASLOGLOU, Ian A. GLOVER, Hock Guan GOH, Kae Hsiang KWONG
Michael P. GILROY, Christos TACHTATZIS, Craig MICHIE, Ivan ANDONOVIC
Department of Electronic and Electrical Engineering
University of Strathclyde, Glasgow, United K i ngdom
Received June 6, 2009; revised August 3, 20 09; accepted August 6, 2009
Abstract
Antenna and base-station diversity have been applied to a wireless sensor network for the monitoring of live-
stock. A field trial has been described and the advantage to be gained in a practical environment has been
assessed.
Keywords: Antenna Diversity, Base Station Diversity, Animal Monitoring, Wireless Sensor Networks,
Distribution, Rayleigh Distribution, Fading
1. Introduction
A wireless sensor network (WSN) is a collection of
spatially dispersed sensors that communicate via a set of
wireless transceivers [1]. Each transceiver forms one
node in the resulting transceiver network. Information
collected by the sensors may be transmitted to a central
base-station, either directly or by relaying it via one or
more intermediate nodes. The network topology and
protocols may be fixed and predetermined, or adaptive
and self-organizing. Recent advances in micro-electro-
mechanical systems, transceiver miniaturisation and
transducer technology have made WSNs flexible, scal-
able and commercially viable. They have found wide and
diverse application in many areas including military [2],
industrial [3], commercial [4] and domestic [5]. Of
particular relevance to the work presented here are
applications to environment and habitat monitoring, agri-
culture and animal husbandry [6–15].
Here we apply antenna and base-station diversity to
star-configured WSNs for animal husbandry in the dairy
and beef industries [16].
The monitoring and/or tracking of mammals usually
involves a single antenna attached to a collar worn around
the animal’s neck. As the animals move, line-of-sight (LOS)
paths between the sensor node and base-station might
become obscured by other animals. This fading mechanism
is likely to be especi ally im porta nt in the context of anim als
with a herding instinct on open grassland. Monitoring
livestock in the dairy and beef industries corresponds
precisely to this case.
The incorporation of antenna diversity at the sensor node
combined with the use of two, widely separated, base-
stations (yielding base-station diversity) dramatically
increases the probability of LOS conditions. The principal
objective of the work reported here is the experimental
assessment of the practical diversity improvement that can
be expected in this little considered, but commercially
important, application.
2. System and Methodology
The transceiver used for the experiment was the MICAz
[17] shown in Figure 1. It operates in the ISM band
between 2.40 GHz and 2.48 GHz.
Figure 1. MICAz module.

K. SASLOGLOU ET AL. 384
The transceiver, with transmit power set to -10 dBm,
was mounted on a PCB, Figure 2. An RF switch was
used to connect the transceiver (both receiver and
transmitter) to two antennas.
Each antenna is an inset-fed microstrip patch with a
ceramic element attached to the top of the radiating
surface. The radiation pattern of the antenna is shown in
Figure 3 for three frequencies, in two orthogonal linear
polarisations and in three orthogonal planes.
The antennas are alternately connected for 1 s to the
transceiver using the RF switch. The switching cycle
therefore has a period of 2 s and the sampling frequency
for a particular antenna is 0.5 Hz. The mobile node
assemblies were attached to animals using collars such
that one antenna was located on the left-hand side of the
animals’ necks, and one on the right-hand side, Figure 4.
The trial area was rectangular in shape, approximately
20 m x 12 m, and enclosed by brick walls and a pitched
metallic roof, Figure 5.
The base-stations are located at the mid point of the
left and right hand side of the trial area (Figure 4) at a
height of 4 m. They comprise of an identical transceiver
to those used at the sensor nodes interfaced to an
MIB600 programming board, Figure 6. The base-station
Figure 2. Assembled PCB.
Figure 3. Antenna radiation patterns.

K. SASLOGLOU ET AL. 385
Figure 4. Antenna configuration.
Figure 5. Trial area.
Figure 6. Base -station.
antennas were vertically polarised and approximately
omnidirectional in the horizontal plane with a gain of 6
dBi [18].
Nine animals were released into the trial area, two
carrying collar mounted sensor nodes. The received
power was recorded for 75 minutes at both base-stations.
The movement of animals was sufficiently slow such
that each 1 s block of contiguous data received from a
given antenna can be assumed to originate from a single
location. The resulting data was smoothed by calculating
the moving average of 15 samples representing a
measurement integration time of 30s.
3. Results
The time-series of signal power received from
base-station 1 (BS1) and base-station 2 (BS2) are shown
in Figures 7(a), (b), (c) and (d) and Figures 8(a), (b), (c)
and (d), respectively. (a) and (b) represent data
originating from antenna 1 (A1) and antenna 2 (A2)
mounted on collar 1 (C1). (c) and (d) represent data
originating from A1 and A2 mounted on C2. The upper
subplots in each subfigure show the raw 0.5 Hz data
samples and the lower subplots show the 15-sample
moving average. The horizontal line in the figures
represents the mean power for each measurement set.
Power fluctuations of up to 20 dB occur in the raw
time-series at both base-stations. The peak-to-peak
variation of received power recorded for each antenna on
each collar at each base-station over the total observation
time is shown in Figure 9.
For diversity advantage to be realised the fluctuation
of received power in the two channels must be
decorrelated. The correlation coefficient ρX,Y between two
random variables X and Y with expected values μX and μY
and standard deviations σX and σY is:
,
(()( )) cov(, )XY
XY
XY XY
EX Y XY 
 

 (1)
where E denotes the statistical expectation and cov
denotes the covariance [19]. The correlation coefficients
between the signals received by antennas mounted on the
same collar are presented in Table 1. The definition of
correlation (Equation 1) excludes any constant (LOS)
component which explains the low values.
The probability distribution of the received signal
from a particular mobile antenna to a particular base-
station might be expected to be close to Ricean as a
result of multipath propagation with a strong LOS
component. The Ricean distribution is given by:
22
222
() (0,0
() 2
0(
r
o
rrA A
eI forAr
pr
for r

 )
0)






(2)
Table 1. Correlation coefficient for signals received by dif-
ferent antennas on a single collar.
Base Station Collar Correlation Coefficient
between A1 and A2
BS 1 Collar 1 0.2407
Collar 2 0.0414
BS 2 Collar 1 –0.0733
Collar 2 0.0158

K. SASLOGLOU ET AL. 386
(a) (b)
(c) (d)
Figure 7. Time-series recorded at base-station 1.
where A denotes the peak amplitude of the dominant
component and I0 is the modified zero order Bessel
function of the first kind [20]. The K-factor of a Ricean
distribution is the ratio between the (constant)
component of signal power due to the LOS path and the
(fluctuating) component of signal power due to all other
paths, i.e.:
2
2
2 
A
K (3)
As the LOS component becomes smaller K-factor
decreases and the Rician probability density function
(pdf) becomes more skewed. As K-factor falls signi-
ficantly below 1.0 the pdf approaches a Rayleigh
distribution. As the LOS component becomes larger,
K-factor increases and the distribution becomes less
skewed. As K-factor tends to infinity the Ricean
distribution tends to a normal distribution. Figure 10
shows the pdfs of the power received at BS1. (a) and (b)
are the pdfs of data obtained from A1 and A2 on C1. (c)
and (d) are the pdfs of data obtain ed from A1 and A2 on
C2.
A normal distribution of power in dBm (i.e. a
log-normal distribution of power in watts) appears to be
the best fit to the data. If fading is due predominantly to
multipath propagation this suggests the presence of a
strong LOS component. An alternative interpretation
would be that the log-normal fading reflects cascaded
independent shadowing processes.
Figure 11 shows the pdf of the power received at BS2.
(a) and (b) represent the data transmitted by C1 (for A1
and A2 respectively) and (c) and (d) represent the data
transmitted from C2 (for A1 and A2 respectively).
Superficially, this distribution appears to be closer to
Rayleigh (in dBm) than normal. The mean signal level is
significantly lower than that for BS1 (due to the larger
distance), however, and is approaching the receiver
sensitivity which is –94 dBm. Since no sign al is record ed
when the received power falls below -94 dBm the pdf is

K. SASLOGLOU ET AL. 387
(a) (b)
(c) (d)
Figure 8. Time-series recorded at base-station 2.
effectively truncated at this level. It seems likely,
therefore, that the pdf of the underlying signal is normal
(in dBm) even though the pdf of the recorded (truncated)
signal is skewed.
The corresponding cum ul ative distribution functions (cdfs)
are presented i n Figures 12 and 13 . The best-fit norm al curves
along with 95% confidence intervals are superimposed.
The close fit of the normal distribution for the data
logged at BS1 is apparent. The fit is less good for the
data obtained from BS2. Figures 14 and 15 show similar
plots for received voltage.
Figure 16 represents similar data to that presented in
Figures 7 and 8 but of shorter time duration (approxi-
mately 40 minutes). The advantage of the use of
base-station diversity is especially apparent in this data.
Figures 16(a) and (b) represent the power received at
BS1 and BS2, respectively, from the signal transmitted
from A1 on C1. Figures 16(c) and (d) represent the
power received at BS1 and BS2, respectively, from the
signal transmitted from A2 on C1.
Figure 9. Peak-to-peak variation of received power at each
individual antenna. (From left to right, BS1C1A1,
BS1C1A2, BS1C2A1, BS1C2A2, BS2C1A1, BS2C1A2,
BS2C2A1, BS2C2A2.)

K. SASLOGLOU ET AL. 388
(a) (b)
(c) (d)
Figure 10. Pdfs of power (dBm) at BS1 for (a) A1 on C1, (b) A2 on C1, (c) A1 on C2 and (d) A2 on C2. (Smooth curve repre-
sents the best-fit normal distribution.)
(a) (b)
(c) (d)
Figure 11. Pdfs of power (dBm) at BS2 for (a) A1 on C1, (b) A2 on C1, (c) A1 on C2 and (d) A2 on C2. (Smooth curve repre-
sents the best-fit normal distribution.)

K. SASLOGLOU ET AL.389
(a) (b)
(c) (d)
Figure 12. Cdfs of power (dBm) at BS1 for (a) A1 on C1, (b) A2 on C1, (c) A1 on C2 and (d) A2 on C2. (Smooth curve repre-
sents the best-fit normal distribution.).
(a) (b)
(c) (d)
Figure 13. Cdfs of power (dBm) at BS2 for (a) A1 on C1, (b) A2 on C1, (c) A1 on C2 and (d) A2 on C2. (Smooth curve repre-
sents the best-fit normal distribution.)

K. SASLOGLOU ET AL. 390
(a) (b)
(c) (d)
Figure 14. CDFs of detected voltage at BS1 for (a) A1 on C1, (b) A2 on C1, (c) A1 on C2 and (d) A2 on C2. (Smooth curve
represents the best fit log-normal distribution.).
(a) (b)
(c) (d)
Figure 15. CDFs of detected voltage at BS2 for (a) A1 on C1, (b) A2 on C1, (c) A1 on C2 and (d) A2 on C2. (Smooth curve
represents the best fit log-normal distribution.).

K. SASLOGLOU ET AL. 391
Table 2. Correlation coefficient of signals received by dif-
ferent base-stations.
Collar Antenna Correlation Coefficient
between BS1 and BS2
C1 A1 –0.2646
A2 –0.3062
C2 A1 –0.0212
A2 –0.2217
The mean received power for each measurement is
indicated by a horizontal line in Figure 16. The correla-
tion coefficients between BS1 and BS2 signals are
presented in Table 2.
Base-station diversity clearly offers advantage. The
consistently small negative correlation is interpreted as
being due to essentially zero short-term correlation due
to the physically independent multipath propagation
structure experienced by the base-stations, and a
longer-term negative correlation due to the changes in
distance between collar and base-stations as the animals
move.
4. Diversity Gain
Two types of diversity gain have been evaluated.
Antenna diversity relates to the advantage obtained by
having two antennas on one collar. Base-station diversity
relates to the advantage obtained by having two base-
stations.
4.1. Antenna Diversity
Since there are two collars each with two antennas and
two base-stations, the trial contains four independent
instances of antenna diversity. These are: (i) diversity
collar 1 to BS1, (ii) diversity collar 2 to BS1, (iii)
diversity collar 1 to BS2 and (iv) diversity collar 2 to
BS2.
Figure 17 shows the cdfs corresponding to each of
these antenna diversity instances. In each sub-figure,
there are four curves: the base-station signal received
from A1, the base-station signal received from A2, the
mean base-station signal received (calculated using A1
and A2), and the maximum base-station signal received
(selected from A1 and A2). The mean base-station signal
is adopted as the reference with which to calculate
diversity gain. This is because either of the diversity
antennas could be adopted as the reference. Taking the
mean therefore reduces statistical noise to give a better
estimate of expected value. The mean value is calculated
from the received powers in dBm. This results in a final
diversity gain (in dB) corresponding to the geometric
mean of the diversity gains expressed as ratios obtained
using A1 and A2 re fer ences.
(a) (b)
(c) (d)
Figure 16. Data received by A1 on C1 at (a) BS1 and (b) BS2, and data received by A2 on C1 at (c) BS1 and (d) BS2.

K. SASLOGLOU ET AL. 392
(a) (b)
(c) (d)
Figure 17. Received signal power (dBm) for antenna 1 and antenna 2, mean recei ved signal (dBm), and selection diversity
signal (dBm) for (a) C1 to BS1, (b) C2 to BS1, (c) C1 to BS2 and (d) C2 to BS2.
Figure 18 shows the cdfs of the expected diversity
gain for (a) C1 to BS1, (b) C2 to BS1, (c) C1 to BS2 and
(d) C2 to BS2.
The expected diversity gain (dB) is the difference
between the selected diversity signal power (dBm) and
the mean signal (dBm).
The median diversity gain averaged over all four
instances is 4.4 dB. The 10% and 90% diversity gain
exceedances averaged over all four instances are 5.7 dB
and 0.5 dB, respectively.
4.2. Base-Station Diversity
There are four instances of base-station diversity gain.
These are A1 on C1 to BS1 and BS2, A2 on C1 to BS1
and BS2, A1 on C2 to BS1 and BS2, and A2 on C2 to
BS1 and BS2. The base-station diversity cumulative
distributions are calculated in an identical way to the
antenna diversity cumulative distributions. The results
are shown in Figures 19 and 20.
The median diversity gain averaged over all four
instances is 4.5 dB. The 10% and 90% diversity gain
exceedances averaged over all four instances are 8.5 dB
and 1.1 dB, respectively.
4.3. Overall Diversity
There are four instances of antenna and base-station
diversity gain with regards to each collar. These are A1
to BS1, A2 to BS1, A1 to BS2 and A2 to BS2.
Figure 21 shows the four curves for each instance,
themean received signal (dBm) and the maximum signal
received for the first collar.
Figure 22 shows the four curves for each instance, the
mean received signal (dBm) and the maximum signal
received for the second collar.
The overall diversity gain (i.e. the diversity gain
available from the combined antenna diversity and

K. SASLOGLOU ET AL. 393
(a) (b)
(c) (d)
Figure 18. Antenna diversity gain for each instance: (a) C1 to BS1, (b) C2 to BS1, (c) C1 to BS2 and (d) C2 to BS2.
(a) (b)
(c) (d)
Figure 19. Received signal power (dBm) for BS1 and BS2, mean received signal (dBm), and selection diversity signal (dBm)
for (a) A1 of C1 to BS1 and BS2, (b) A2 of C1 to BS1 and BS2, (c) A1 of C2 to BS1 and BS2 and (d) A2 of C2 to BS1 and BS2.

K. SASLOGLOU ET AL. 394
(a) (b)
(c) (d)
Figure 20. Base-station diversity gain for each instance: (a) A1 of C1 to BS1 and BS2, (b) A2 of C1 to BS1 and BS2, (c) A1 of
C2 to BS1 and BS2 and (d) A2 of C2 to BS1 and BS2.
Figure 21. Overall selection and mean diversity for the first
collar (two antennas and two base-stations). Figure 22. Overall selection and mean diversity for the sec- ond collar (two antennas and two base-stations).

K. SASLOGLOU ET AL.395
Figure 23. Overall diversity gain for the first collar.
Figure 24. Overall diversity gain for the second collar.
Figure 25. Mean overall diversity gain for the two collars.
base-station diversity) for the first and second collar is
shown in Figure 23 and 24, r espectively.
The median overall diversity gain for the first and
second collar is 8.1 dB and 7 dB, respectively. The 10%
exceedances are 14.3 and 11.2 dB for the first and
second collar, respectively. The 90% exceedances are 4.1
dB for the first and 3.5 dB for the second collar,
respectively.
The mean overall diversity gain is shown in Figure 25.
This is the mean value calculated from the two individual
collar diversity gains. The median overall mean div ersity
is 7.9 dB. The 10% and 90% diversity gain exceedances
are 11.6 dB and 5. 1 dB , respectively.
5. Conclusions
Antenna and base-station diversity has been applied to
the wireless monitoring of farm animals. The statistical
distributions of received signals and antenna/base-station
signal correlations have been summarised. The advan-
tage offered by selection diversity has been evaluated.
The overall (antenna and base-station) diversity gain
offered at each collar has also been studied.
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