REDUCTION OF INTERFERENCES TO ADJACENT
NETWORKS BY COMBINED LATTICE STRUCTURES AND
SHRUB BARRIERS
Experimental Work done at 5.8 GHz
Iñigo Cuiñas, Paula Gómez, Ana Vázquez Alejos, Manuel García Sánchez
Dept. Teoría do Sinal e Comunicacións, Universidade de Vigo, Vigo, Spain
José E. Acuña
Dept. Telecomunicación, Universidad de la República O. Uruguay, Montevideo, Uruguay
Keywords: Wireless network, Interference, Vegetation.
Abstract: The increasing number of wireless LANs using the same spectrum allocation could induce multiple
interferences and it also could force the active LANs to continuously retransmit data to solve this problem,
overloading the spectrum bands as well as collapsing their own transmission capacity. This upcoming
problem can be mitigated by using different techniques, being site shielding one of them. If radio systems
could be safeguarded against radiation from transmitter out of the specific network, the frequency reuse is
improved and, as a consequence, the number of WLANs sharing the same area may increase maintaining
the required quality standards. The proposal of this paper is the use of combined barriers constructed by
bushes, supported by lattice structures, in order to attenuate signals from other networks and, so that, to
defend the own wireless system from outer interferences. A measurement campaign has been performed in
order to test this application of vegetal elements. This campaign was focused on determining the attenuation
induced in the propagation channel by several configurations that combine six specimen of Ficus elastica
and six structures made of four different materials. The wind effect on the canopies is also considered.
Then, the relation between the induced attenuation and the interference from adjacent networks has been
computed in terms of separation between networks. The network protection against outer unauthorised
access could be also improved by means of the proposed technique.
1 INTRODUCTION
Although wireless standards (IEEE802.16, 2003)
have been designed to solve connection falls, mainly
by retransmission of data, the increasing number of
systems using the same spectrum allocation could
force the active LANs to continuously retransmit
data, overloading the spectrum bands as well as
collapsing their own transmission capacity.
The analysis of interferences on wireless
wideband communication systems is the topic of
different scientific works: considering both narrow
band (Giorgetti et al., 2005) and wideband
interferences (Yang, 2003). Several strategies have
been applied to reduce the interference between
adjacent networks. Among these proposals, the
control of the transmit power appears to be a
successful one (Qiao et al., 2007).
Another problem associated to the wireless
technology is network protection. The users do not
need to be physically connected to the network
nodes. This fact allows external users to utilise the
network for private or even for forbidden purposes.
These upcoming problems can be mitigated by
using different techniques, being site shielding one
of them (Van Dooren et al., 1992). If radio systems
could be safeguarded against radiation from
transmitters out of the specific area of coverage of
the network, the interference could be reduced and,
as a consequence, the number of WLANs sharing
the same area may increase maintaining the required
quality standards.
15
Cuiñas I., Gómez P., Vázquez Alejos A., García Sánchez M. and E. Acuña J. (2009).
REDUCTION OF INTERFERENCES TO ADJACENT NETWORKS BY COMBINED LATTICE STRUCTURES AND SHRUB BARRIERS - Experimental
Work done at 5.8 GHz.
In Proceedings of the International Conference on Wireless Information Networks and Systems, pages 15-20
DOI: 10.5220/0002224200150020
Copyright
c
SciTePress
The proposal of this work is the use of trees or
bushes, supported by a lattice structure, to perform
the barrier to attenuate signals from other networks
and, so that, to defend the own wireless system from
outer interferences. Indoor plants can be used to cut
the line of sight between adjacent radio equipment
of different networks.
The paper consists of four sections. The section 2
contains the description of measurement equipment
and set-up, followed by the procedure used to get the
data, and the vegetal species used during the
experiment. The section 3 is intended to the results
obtained in the measurement campaign, taking into
account the median values, as well as its variability
and confidence. This section is finished by the
evaluation of the improvement of the interference, in
terms of the reduction in the shortest distance
between adjacent networks to maintain the quality of
service. Finally, section 4 contains the conclusions
extracted from these results.
2 MEASUREMENTS
The objective of the measurement campaign was to
determine the attenuation induced by different
barriers in the radio channel at WLAN frequencies.
With this aim, the experimental work that forms the
core of this paper was developed within an anechoic
chamber at the Universidade de Vigo. Different
lattice structures were used to support soft
vegetation elements that configured the barriers,
which were installed in different compositions
Along this section, the measurement setup and
procedure are described, followed by an explanation
of the vegetation elements and the different lattice
structures.
2.1 Measurement Setup
The experimental work has been developed in an
anechoic chamber, with approximated dimensions 6
m long, 3 m width and 3 m height. Within this
controlled environment, no external waves would
disturb the radio link, the multipath propagation is
minimized, and all the measured results could be
attributed to the elements inside the chamber: the
antennas, the supports, the fan, and, of course, the
shrub and lattice structure barrier.
The measurement equipment consisted of
separated transmitter and receiver. The transmission
segment was mounted around a signal generator
Rohde & Schwarz SMV-03, feeding a logarithmic-
periodic antenna Electro Metrics EM6952 via a low
losses coaxial cable. The transmitter was placed in a
fixed location pointed following the long axis of the
anechoic chamber.
The reception device was a spectrum analyzer
Rohde & Schwarz FSP40 acquiring the RF signals
by another directive logarithmic-periodic antenna.
The antenna was installed on the top of another
mast. The data capture was PC controlled using
tailor-made software.
Figure 1: Geometry of the experiment setup.
The relative location of the transmitter, receiver,
and obstacle is depicted in figure 1. The distance
from the transmitter to the lattice structure was fixed
at 1,45 m, and from the lattice to the receiver at 2,15
m.
Furthermore, a fan was used to force artificial
wind inside the anechoic chamber. It provides
continuous wind with three selectable speeds: 1.9
m/s (speed 1), 3.9 m/s (speed 2), and 4.7 m/s (speed
3), measured by an anemometer manufactured by
Aandera.
2.2 Measurement Procedure
Measurements were performed in three steps. First
of them consisted of a free space measurement.
Placed the transmitter and the receiver at their fixed
position, 10,000 records of received power were
gotten. Then, the vegetal barriers were installed, and
that measurement procedure was repeated.
Six barrier configurations were considered: the
supporting lattice itself (configuration number 1);
the structure and one line of three shrubs by the
transmitter side (2); the structure and two lines of
shrubs, one at each side (3); the structure with the
line of shrubs by the receiver side (4); just one line
of three shrubs (5); and a double parallel line of
three shrubs (6). A reference is also measured, in
free space conditions, with no barrier between
transmitter and receiver. These configurations can be
WINSYS 2009 - International Conference on Wireless Information Networks and Systems
16
observed at figure 2. The measurements were
performed in horizontal and vertical polarisations.
Figure 2: Obstacle configuration.
All these structure configurations were measured
in calm conditions, as well as in windy conditions,
placing the fan pointing to the obstacle at the
transmitter side of the hurdle, and the at the receiver
side.
2.3 Vegetation Specimen
The six shrubs used during the measurement
campaign are all from the specie Ficus elastica,
commonly known as ficus. The table 1 contains the
dimensions of the shrubs at the time of the
measurement campaign.
Table 1: Shrub dimensions.
Height (m) Canopy
diameter (cm)
Leaf size (cm)
length width
1.70 55 7 3
The shrubs had flexible trunks and light
canopies. The movement of their leaves under the
wind action could be defined as twist and sway.
2.4 Lattice Structures
The lattice structures were selected to check the
effect on the propagation channel of both the shape
of the structure and the material that constructs it.
These elements are designed to be used in gardens or
terraces to construct light walls by vegetation. They
are originally intended to preserve the privacy in
terms of visual frequencies, but they may also be use
to preserve the privacy of wireless networks.We
used structures made by iron cables (one sample),
wood boards (two samples, with different design),
resin (also two samples), and PVC plastic (one
sample). The general design of each supporting
structure is depicted in figure 3.
Figure 3: General design of the lattice structure.
The main differences among the structures, as
well the constitutive material, are the dimensions of
the lattice: how large the holes are, and how width
the solid parts are. At figure 4, the definition of
different describing parameters could be observed,
and its values are summarised at table 2.
Figure 4: Definition of lattice parameters.
Table 2: Supporting structure dimensions.
material structure lattice
Height
(m)
Width
(m)
Depth
(cm)
Solid
(cm)
Hole
side
(cm)
Iron 1.8 1.8 0.5 0.5 9.0
Wood 1 1.8 1.8 1.0 3.0 12.0
Wood 2 1.8 1.8 0.5 6.0 7.0
Resin 1 1.0 2.0 0.5 3.5 6.5
Resin 2 1.0 2.0 0.5 2.5 2.7
PVC 1.0 2.0 0.5 1.5 2.5
3 RESULTS
The large amount of data collected has been
processed in order to allow the extraction of
REDUCTION OF INTERFERENCES TO ADJACENT NETWORKS BY COMBINED LATTICE STRUCTURES AND
SHRUB BARRIERS - Experimental Work done at 5.8 GHz
17
conclusions. The main results are the median
attenuations provided by the barriers at each of the
considered frequencies, with each configuration,
including windy conditions. They are the contents of
the first subsection.
An estimation of the minimum security distance
at which elements from two separated networks
could be installed with no interference problems
could be computed from the median attenuations,
and they are presented in the second subsection. The
third subsection contains the estimation of the
maximum distance of coverage, which could be
useful to control the areas of origin of possible
external attacks, by limiting the coverage of our
network elements.
3.1 Attenuation
The attenuation induced in the propagation channel
by the combined barrier (lattice structure and/or
shrubs) was computed for each situation by
comparing the median measured received power in
free space conditions and the median power in
obstructed line of sight (OLoS) conditions.
The results show two main trends. On the one
hand, in most of the configurations, the attenuation
provided by barriers designed as configuration 3
(one line of shrubs at each side of the supporting
structure) appears to be larger than that induced by
the other “lighter” configurations. On the other hand,
the wind effect seems to be negligible in terms of
median values, although the wind could be
important in terms of variability around these
median values.
Besides, the structure itself does not present large
effects in terms of attenuation. Even more,
depending on the electric size of the holes, the
supports could generate diffraction mechanisms and
provide slight gains in the received power, in a
pseudo lens effect.
Tables 3 to 9 summarise the median attenuations
measured at different circumstances: lattice
structure, barrier configuration, location of the fan,
and wind speed, for vertical polarisation. The
configuration 1, which corresponds to the structure
with no vegetation, was only measured in static
conditions, as the wind is expected not to induce any
effect on a rigid element.
In most of the situations, the wind speed seems
to induce almost no effect on the median attenuation.
However, some differences between the static and
windy attenuation measurements appear, but there is
not a clear trend: in some conditions the attenuation
grows in windy compared to static situations, and in
other cases the effect is completely opposite.
Table 10 shows the median attenuations with
only vegetation at the barriers, and without lattice
structures. They could be useful to be compared to
previously presented values.
Table 3: Attenuation (dB) with metallic lattice structure.
Wind configuration
from speed 1 2 3 4
0 -0.44 11.33 15.92 5.83
tx
1 11.02 19.09 5.91
2 11.17 18.99 5.95
3 11.39 18.96 6.20
rx
1 10.73 15.59 5.96
2 10.62 18.37 5.75
3 10.09 18.51 5.57
Table 4: Attenuation (dB) with PVC lattice structure.
Wind configuration
from speed 1 2 3 4
0 -2.55 19.36 32.37 8.24
tx
1 19.56 37.58 8.28
2 19.40 37.58 8.27
3 19.56 35.74 8.21
rx
1 15.83 33.30 8.01
2 15.99 33.71 7.96
3 16.09 33.71 7.94
Table 5: Attenuation (dB) with wood 1 lattice structure.
Wind configuration
Fro
m
speed 1 2 3 4
0 -0.19 24.44 18.85 6.80
Tx
1 24.15 17.88 6.95
2 24.26 17.53 6.95
3 24.38 17.41 6.96
Rx
1 22.92 15.48 6.95
2 22.30 15.67 7.00
3 20.81 15.97 7.06
Table 6: Attenuation (dB) with wood 2 lattice structure.
Wind configuration
from speed 1 2 3 4
0 0.85 19.65 25.99 14.03
tx
1 16.63 26.09 11.25
2 16.35 26.21 11.20
3 16.43 26.23 11.12
rx
1 19.66 24.24 13.95
2 19.57 24.22 12.31
3 19.24 23.92 12.77
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Table 7: Attenuation (dB) with resin 1 lattice structure.
Wind configuration
from speed 1 2 3 4
0 -0.27 14.24 27.89 10.55
tx
1 13.79 29.39 11.75
2 14.31 29.02 11.67
3 14.42 29.52 11.62
rx
1 14.22 28.11 10.56
2 14.39 28.44 10.71
3 14.58 28.55 10.78
Table 8: Attenuation (dB) with resin 2 lattice structure.
Wind configuration
from speed 1 2 3 4
0 -0.80 19.44 19.52 17.01
tx
1 15.14 19.32 17.01
2 14.90 19.13 16.92
3 14.83 19.21 16.32
rx
1 16.43 19.09 16.12
2 16.46 19.13 15.80
3 16.40 19.05 15.52
Table 9: Attenuation (dB) with resin 2 lattice structure.
Wind configuration
from speed 1 2 3 4
0 0.37 16.48 20.06 19.84
tx
1 15.47 19.62 25.54
2 15.21 19.24 25.68
3 15.58 19.27 25.90
rx
1 16.31 20.37 20.89
2 16.36 21.02 21.45
3 16.86 21.62 21.62
Table 10: Attenuation (dB) with no lattice structure.
Wind configuration
from speed 5 6
0 15.34 27.89
tx
1 15.44 27.89
2 15.44 27.89
3 15.44 17.58
rx
1 15.80 26.26
2 15.91 26.17
3 15.99 25.91
The lattice structures that lead to larger
attenuations seem to be the iron one and the closest
among those constructed by the same material.
3.2 Reduction in Interference Free
Distance
The measured attenuation values could be used to
compare the influence of the hurdles in the control
of interference. The standard IEEE 802.16a defines
different minimum signal to noise ratios (SNR) to
maintain its various modulation schemes: QPSK, 16-
QAM and 64-QAM (IEEE802.16, 2003), which are
summarized in table 11.
Table 11: Minimum SNR to maintain each modulation
scheme, IEEE 802.16a.
modulation coding rate SNR (dB) at receiver
QPSK
1/2 9.4
3/4 11.2
16-QAM
1/2 16.4
3/4 18.2
64-QAM
2/3 22.7
3/4 24.4
A WLAN element could receive signals from
other elements at its own network, and from other at
different networks. Considering the co-channel
interference as a kind of noise, and applying the
Friis equation with the limited SNR values assuming
all the network transmitters are emitting the same
power, we can compute the improvement provided
by the vegetal barrier in terms of interference
reduction: comparing the minimum distance to
assure the interference would not degrade the
performance of the network with and without the
hurdle. Thus, a reduction in this distance indicates
how much the hurdle improves the network security:
it allows installing two networks closer than without
the hurdle, and maintaining their performances.
Taking into account the mean of the measured
attenuations, the minimum security distance have
been computed, and the results are summarized in
table 12.
Table 12: Minimum distance to be free from interference.
modulation
Distance (m)
no hurdle hurdle
QPSK
2.95 0.13
16-QAM
6.6 0.29
64-QAM
13.65 0.59
3.3 Protection Against External
Attacks
The presence of the vegetation barrier provides an
additional attenuation to the propagation channel:
this means that the maximum physical distance to be
connected to the network is shortened than when the
fence is absent. Thus, uncontrolled accesses could be
reduced compared to the open coverage situation
with the proposed method.
The sensitivity of the receivers at network
elements could be used to compute the performance
REDUCTION OF INTERFERENCES TO ADJACENT NETWORKS BY COMBINED LATTICE STRUCTURES AND
SHRUB BARRIERS - Experimental Work done at 5.8 GHz
19
of the fence in terms of protection against external
attacks. The IEEE 802.11 standard defines these
sensitivities to be -80 dBm for a bit rate of 1 Mbps,
or -75 dBm for 2 Mbps. With these values, we could
compute the coverage distances in free space (LoS)
and obstructed by a shrub/structure line (OLoS)
conditions, knowing that the maximum transmitting
power is defined to be 30 dBm. The typical
transmitting power is also known, being 13 dBm.
Tables 13 and 14 contain the coverage distances
with and without combined structure and vegetation
fences when using the maximum and the typical
transmitting powers, respectively. Values at both
tables indicate a very significant reduction of the
coverage distance when the line of sight is only
obstructed by a shrub fence. Obviously, actual
situations involve one or more walls between the
network element inside a building and the possible
hacker in the street. This means that the actual
distances of coverage would be strongly shorter than
those provided at tables 13 and 14. Nevertheless, the
coverage distances with typical transmission appear
to be enough to avoid attacks from outside the parcel
of most corporative buildings.
Table 13: Maximum coverage distances, assuming
maximum transmitting power.
transmission
rate
distance
LoS OLoS
1 Mbps 2.60 km 92 m
2 Mbps 1.46 km 52 m
Table 14: Maximum coverage distances, assuming typical
transmitting power.
transmission
rate
distance
LoS OLoS
1 Mbps 368 m 13 m
2 Mbps 207 m 7 m
4 CONCLUSIONS
The use of barriers constructed by shrubs supported
by lattice structures is proposed to reduce the
interference between adjacent wireless networks at
5.8 GHz. The results of an exhaustive measurement
campaign are presented along this work, involving
six specimen of Ficus elastica and six structures
made of four different materials. The wind effect on
the canopies has been also considered.
The attenuation results show two main trends.
On the one hand, the attenuation provided by hard
barriers appears to be larger than that induced by the
other “lighter” configurations. On the other hand, the
wind effect seems to be negligible in terms of
median values, although the wind could be
important in terms of variability around these
median values.
Minimum distance to produce interference and
maximum coverage distance have been also
computed from the attenuation data. The estimated
distances appear to confirm the performance of the
vegetation/supporting structure barriers and to
validate the thesis of this paper. The minimum
interference-free coverage could be reduced to very
short values, less than 60 cm, whereas the maximum
distance to perform an unauthorised access could
move from 368 m to 13 m. Both situations represent
interesting improvements in network performance.
ACKNOWLEDGEMENTS
This work was supported by the Autonomic
Government of Galicia (Xunta de Galicia), Spain,
under Projects PGIDIT 05TAM32201PR and
08MRU002400PR, and by the Spanish Ministry of
Education and Science, under project TEC2008-
06736-C03.
The authors would also like to acknowledge Mr.
Rodrigo Cao and Mr. Antonio Mariño for their help
during the measurement campaign.
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