Modeling of Signal Attenuation

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Modeling of Signal Attenuation due to Vegetation

Abstract

One of the main problems in the development and implementation of vegetation attenuation models is the densities of different trees. The branches and leaves of each tree have a unique shape and placement and it is very difficult to determine a parameter to compare different types of trees. It is needed to examine different types of trees and to compare their parameters.

The objective of this project is to investigate and propose a technique that can be used to characterize and quantify vegetation density. This can be achieved by setting the parameter in which the best and most quickly will be able to compare different types of trees. It is anticipated that this method will improve the accuracy of existing vegetation attenuation models.

1. Introduction

Wireless communications are a revolutionary breakthrough in telecommunications as well as for people. Wireless links are used mainly in fixed terrestrial, satellite communication and cellular mobile markets. Increasing data rate demands, network capacity and need to use wider bandwidth require development of systems that will operate at frequencies in higher bands. (Rogers, Ndzi, 2003, p.2)

Growing number of consumers, especially mobile users require a system with greater capacity. This is achieved by placing the antennas at heights lower than surrounding buildings and trees. (Graham, 1998

In wireless fixed access or land mobile systems, surrounding trees affect the radio signals as they reflect or absorb it. After conducting experimental studies and research, has concluded that individual or group of trees have the following effects on the radio waves distribution and level:

a. Directly-as to free space attenuation is provided an additional attenuation

b. Indirectly by scattering

c. Depolarization of the incident wave (Shukla. A.K., Seville. A., Ndzi. D., Richter. J., Eden D.,2002, p2)

The consequences of the influence of vegetation should be very well researched and studied. The purpose of this is to achieve more accurate mobile communication and to avoid the influence of vegetation. (Richter, Al-Nuaimi, Caldeirinha, 2002, p.1)

It was created generic mode, incorporating direct signal (through vegetation), ground reflection and edge diffraction and working in the frequency range 1-60 GHz. This model was created using the Radiative Energy Transfer (RET) theory. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, pviii)

2. Related work

2.1 Attenuation due to vegetation

In distribution in various communication channels the radio waves are subjected to different types of attenuation. Attenuation is objectionable because it leads to reduced signal in the receiver. (Anttalainen, Tarmo, 2003, p126) Attenuation due to vegetation is one of the main problems in mobile communications.

“Vegetation is a multi-component structure consisting of free water and the actual vegetation itself, which is a mixture of bounded water and air.” (Kozlov, A. I., Ligthart, L. P., Logvin, A. I., 2001, p159) In the literature several models have been proposed (empirical, semi-empirical and analytical), which describe the effects of vegetation on the distribution of radio waves.(Shukla. A.K., Seville. A., Ndzi. D., Richter. J., Eden D.,2002, p2)

a. Empirical Models

The Empirical Models refuse to refer to natural processes and are strictly dependent on the specific measurement data. Their main advantage is that the math they use is simple and easy to work with them. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, p107)

b. Semi-Empirical Models

They do not cope very well with highlighting the underlying mechanisms, but by definition give the best fit to measured data and are easy to implement. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, p109)

c. Analytical Models

They require the use of numerical analysis methods to provide solutions to the intractable mathematical formulations. Analytical Models are dealing with the physical processes of reproduction of vegetation.(Shukla. A.K., Seville. A., Ndzi. D., Richter. J., Eden D., 2002, p2)

2.2 RET Model

At frequencies above 1 GHz, RET is the model that can be applied a highly effective vegetation attenuation and scatter prediction. RET model stands for Radiative Energy Transfer model. This model determines the vegetation absorption and scattering and was created using the Radiative Energy Transfer (RET) theory. For characterization of the vegetation density and structure was used measurements of tree spacing, heights and leaf dimensions and computerized analysis of hemispherical-aperture photographs of the canopy. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, pviii) This model works with the following input parameters:

* α-scatter ratio forward/total

* β-beamwidth

* στ-absorbtion and scatter cross section

* w-albedo

and these parameters depend on the frequency of radio waves, foliation state, types of trees, density and shape of leaves. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, p13)

2.3 Measurements

Measurements made on frequencies 1GHz, 2GHz and 11GHz of the following trees London Plane, Silver Maple, Common Lime, Horse Chestnut and Sycamore. These measurements include finding the delay spread and excess attenuation in-leaf and out-of-leaf of the trees. (Shukla. A.K., Seville. A., Ndzi. D., Richter. J., Eden D., 2002, p3) Measurements of the Sycamore trees were made during the wind. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, p73)

Table-1: Delay spread and excess attenuation for selected foliage depths at 1 GHz

Table-2: Delay spread and excess attenuation for selected foliage depths at 2 GHz

Table-3: Delay spread and excess attenuation for selected foliage depths at 11 GHz

Tables indicate measurements given at 5, 15, 30 and 50 metres foliage depth. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, p74)

After the measurement was concluded that the attenuation at 11GHz signal is greater in the in-leaf, than in out-of-leaf irrespective of the tree type, shape or measurement geometry. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, p75)

2.4

a. London Plane

In London plane the size of the leaves are large and comparable to the wavelength at 2GHz.In a wavelength at 11.647GHz are comparable to the branches of the tree. Although the location of the leaves and branches of the trees are random, they have little effect on the broadcasting waves. This is because the leaves had a physical dimension relative to the
wavelength at L-band. (NC Rogers, A Seville, J Richter, D Ndzi, N Savage, RFS Caldeirinha,AK Shukla, MO Al-Nuaimi, K Craig, E Vilar and J Austin, 2002, p51)

b. Silver Maple

3. Problem description

In order to characterized and quantified the vegetation density and the computation of attenuation to be developed should have several objectives.

1. Finding the conductivity of branches and leaves for different types of tree

2. Finding the relation between branches and leaves conductivity with attenuation of vegetation.

3. Finding parameter against which to compare the reflective and absorption properties of wood.

4. Finding an optimal method for measuring vegetation density

Finding solutions to these problems and will help to build on the existing vegetation attenuation models.

4. Aims and objectives

The main object of this research is to look into ways of how to characterize the vegetation. Important parameter to characterize the attenuation due to vegetation is the conductivity of leaves and tree branches. The relationship between the branches and leaves conductivity with attenuation of vegetation must be detected and characterized. Also it is very important to quantify the vegetation and to find a parameter for optimal comparing of different types of trees. These will help in the calculation of vegetation density. In this way the accuracy of existing models may be optimized.

5. Proposed solution

6. Research plan

7. Evaluation plan

8. Conclusion

References

Kozlov, A. I.; Ligthart, L. P.; Logvin, A. I.. Mathematical and Physical Modelling of Microwave Scattering and Polarimetric Remote Sensing. Monitoring the Earth's Environment Using Polarimetric Radar: Formulation and Potential Applications. ISBN: 9781402001222

Secaucus, NJ, USA: Kluwer Academic Publishers, 2001. p 182. http://site.ebrary.com/lib/portsmouth/Doc?id=10067346&ppg=182

Copyright © 2001. Kluwer Academic Publishers. All rights reserved.

Anttalainen, Tarmo. Introduction to Telecommunications Network Engineering (Second Edition).

Norwood, MA, USA: Artech House, Incorporated, 2003. p 147. http://site.ebrary.com/lib/portsmouth/Doc?id=10081930&ppg=147

Copyright © 2003. Artech House, Incorporated. All rights reserved. ISBN: 9781580535007 9781580536165

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