摘要
林内无线传感器网络采用的2.4GHz信号绕射能力较差,在林内立木后方较近的范围内存在信号盲区,研究2.4GHz射频信号在林内的传输衰减与遇树干时的绕射规律,建立场强模型和活立木绕射盲区计算方法是林内立木胸径无线传感器网络优化布置策略的理论依据。
本文在充分分析和借鉴己有的国内外关于林内信号传输衰减、电磁波传播理论及其他相关学科领域研究成果基础上,以人工林内无线射频信号传播为研究对象,对绕射场强和盲区进行了研究,研究内容和研究结果如下:
1.论文分析了信号传播的环境及信号发射的硬件设施。
2.研究了信号在人工林内的总传输特性;结合人工林的配置特点,以四层独立介质层分层独立分析建模的方法对射频信号在人工林内的总体传播模型进行了简化,提出树冠衰减屏和树干吸收屏的概念,并给出了电磁波在人工林介质中传播损耗计算公式L=L(Z)+L(S)+L(X).
3.应用一致性几何绕射理论(UTD)以多边形近似法简化了活立木模型,研究了模型的信息存储,应用反向射线跟踪法解决了人工林内信号的路径搜索及传输寻迹问题,求出了接收点已知和未知两种情况下绕射路径的通用解Q(t)=p1+t(p2-p1).
4.在总体传输模型及射线追踪研究结果的基础上,对吸收屏(林内树干部分)进行了重点研究:
(1)提出了活立木垂直入射和斜入射两种条件下的绕射模型;
(2)应用电磁波理论、Fresnel-Kirchhoff原理和UTD理论推导了活立木绕射场求解公式,En(S)=EN-1(Q)D(θ)A(s)exp(-JKS SINγ)exp(-jkz COSγ);
(3)对近似计算造成的阴影边界处过渡区的不连续,采用了过渡区修正,得出了修正绕射系数表达式D(1)=-exp(-jπ/4)×cotγ(1)F(2kLn2sin2γ(1))/2n√2πκ。
(4)分析了绕射盲区的影响因素,在计算焦散距离的同时得出了盲区计算公式-(κΡ/2)1/3+π/2≤θ≤(κΡ/2)1/3+π/2;
(5)以白杨木人工林为采样区域,进行活立木绕射场仿真模拟,验证了模型的正确性。
本文的研究结果为无线传感器网络优化布置策略提供了一个新思路,成果也可为林区的其他射频信号通讯技术开发提供理论支撑。
The ability of 2.4GHz signal diffraction which is adopted by WSN(Wireless Sensor Networks) is worse in forests, because of the range behind standing tree in forests exists blind signal area. Diffraction law of transmission attenuation and clinging trunk of 2.4GHz radio-frequency signal in forests has been studied. Establishment of field intensity model and computing method of blind signal area of alive standing tree can serve as the theoretical basis of optimum arrangement strategy, which is in standing tree DBH(Diameter at Breast Height) of forests of WSN.
Based on the analysis and learning from signal transmission attenuation in forests, the theory of electromagnetic propagation and other related research achievements at home and abroad. The research object is wireless RF signal transmission in man-made forests. Diffraction field strength and blind area have been studied. The main contents and conclusions as follows:
1. In this paper, signal propagation environment and transmission facilities was analyzed.
2.The signal transmission characteristics in plantation was studied. The configuration characteristic of plantation was combined. Through the modeling methods of hierarchical independent analysis at using four-storey independent dielectric layer, to simplify the overall propagation model of RF signal in plantation. The concept of crown attenuation screen and truck absorbing screen were put forward. And the formula of loss(L= L(Z)+L(S)+L(X)) was given, when electromagnetic wave mathematical were spreading in plantation medium.
3.The UTD was used and the alive standing tree model was simplified by polygonal approximation. Information storage of model was studied and through the reverse ray tracing method to solve the problem of path search and transmission tracing of signal in plantation. The general solution(Q(t)= p1+t(p2-P1)) of receiving point was found under the cases between known and unknown.
4. On the basis of overall transmission model and results of ray tracing, the of absorption screen (trunk in trees) was research primarily:
(1) The diffraction models of alive standing tree was proposed under vertical incidence and oblique incidence.
(2) The diffraction field solution formula of standing tree was deduced through the Electromagnetic Wave Theory, the law of Fresnel-Kirchhoff and the theory of UTD, which is given by En(s)= En-1(Q)D(θ)A(s)exp(-jks sinγ)exp(-jkzcosy).
(3) The transition zone of shadows boundary was not continuous by approximate calculation, the transition zone correction was used to solve and the expression of modified diffraction coefficient was obtained, which is given by D=-exp(-jπ/4) x cotγ(1) F (2κLn2 sin2γ(1)/2n√2πκ
(4) The influence factors of diffraction blind area were analyzed. When the caustics distance was calculated, the formula of blind area was obtained, which is given by一(κΡc/2)1/3+π/2≤θ≤(κΡc/2)1/3+π/2.
(5) To take the white poplar as the sampling area, the diffraction field of alive standing was carried analogue simulation. The correctness of the model was tested and verified.
In this article, the research result provides a new thread for network optimization decoration strategy of WSN. Meanwhile it can provide theoretical support for communication technology development of other radio-frequency signal.
引文
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