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| 信号衰减模型 植被衰减模型的发展和实施中的主要问题之一是不同树种的密度。每棵树的枝叶有一个独特的形状和布局，以确定一个参数来比较不同类型的树木，它是非常困难的。需要检查不同类型的树木，并比较它们的参数。 这个项目的目标是调查和提出可以用来描述和量化植被密度的技术。这可以通过设置最佳和最快速的参数来比较不同类型的树来实现。预计该方法将提高现有植被衰减模型的准确性。 无线通信是电信以及人们的革命性突破。无线链路主要用于固定地面、卫星通信和移动市场。提高数据速率的需求，网络容量，需要使用更广泛的带宽要求系统，这将在更高的波段操作。（罗杰斯，ndzi，2003，第2页） 越来越多的消费者，尤其是移动用户需要一个具有更大容量的系统。这是通过放置在高度低于周围的建筑物和树木的天线。（Graham，1998 摘要----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.简介---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.相关工作---Related work 2.1植被衰减---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模式---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 测量---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.伦敦飞机--- 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. 银枫---Silver Maple 3. 问题描述---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. |
