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.

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

 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

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

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

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

1 11.02 19.09 5.91

2 11.17 18.99 5.95

3 11.39 18.96 6.20

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

1 19.56 37.58 8.28

2 19.40 37.58 8.27

3 19.56 35.74 8.21

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

speed 1 2 3 4

0 -0.19 24.44 18.85 6.80

1 24.15 17.88 6.95

2 24.26 17.53 6.95

3 24.38 17.41 6.96

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

1 16.63 26.09 11.25

2 16.35 26.21 11.20

3 16.43 26.23 11.12

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

1 13.79 29.39 11.75

2 14.31 29.02 11.67

3 14.42 29.52 11.62

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

1 15.14 19.32 17.01

2 14.90 19.13 16.92

3 14.83 19.21 16.32

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

1 15.47 19.62 25.54

2 15.21 19.24 25.68

3 15.58 19.27 25.90

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

1 15.44 27.89

2 15.44 27.89

3 15.44 17.58

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

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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