人工林内射频信号传输绕射规律分析

Analysis of propagation and diffraction rule of radiofrequency signal in man-made forest

为解决无线传感器信号在人工林内传输时节点接收不到信号或通信质量差的问题,该文根据人工林的特点,采用4层独立介质简化方法,将人工林内场强模型简化成自由空间模型、衰减屏模型、吸收屏模型3个独立模型,基于波传播的局部性原理和过渡区一致性劈绕射理论,应用Kirchhoff-Huygens近似,推导了吸收屏绕射场强模型。并以白杨木人工林为采样区域采集数据,将90组实测样本平均分为9组,分别用3种模型计算各组的预测值并与实测值进行t检验,结果表明:绕射场强理论预测t值[0.242,2.193]小于t0.05,无显著性差异,可以用理论预测值代替实测值进行信号的分析和计算,距离越近可信度越高,而经验模型和半经验模型存在显著性差异,不适合用于人工林内绕射场强预测。研究结果为无线传感器网络林内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 2.4 GHz signal diffraction which is adopted by WSN（wireless sensor networks） is worse in forests, because the range behind standing tree in forests has blind signal area. Diffraction law of transmission attenuation and clinging trunk of 2.4 GHz radio-frequency signal in forests has been studied. To describe the diffraction effects of the edges of surfaces using a relatively simple mathematical method, the Kirchhoff–Huygens approximation is employed. Based on the uniform geometrical theory of diffraction（UTD）, the region of transition is uniform. To account for the disposal of the sensor, only the illuminated region and the shadow are considered. According to the Kirchhoff approximation, the substitution is used to perform integral transform and the model of wedge diffraction gets simplification. Accounting for the influence of the diffusion factor and according to UDT, the aforementioned approach can also be used to derive the field at the receiver point. The research object is wireless radio-frequency signal transmission in man-made forests. The main contents and conclusions are as follows： 1） The signal transmission characteristics in plantation were studied. Tree trunks are replaced by absorbing screens. The leaves and branches are replaced by partial attenuation screens. The phase and attenuation screen is located directly above the absorbing screen for simplicity. The wireless sensor placed at a breast height of 1.3 m can be further lowered to the basal area in the current paper, so only the effects of the basal area of breast height are considered. Circular area expressions are used for the measurements of the basal area of breast height and standing wood volume, with an average error of ±3%. The configuration characteristics of plantation are combined. Through the modeling methods of hierarchical independent analysis using four-storey independent dielectric layer, the overall propagation model of RF signal in plantation is simplified. The concepts of crown attenuation screen and truck absorbing screen are put forward. And the formula of loss was given equation, when electromagnetic wave was mathematically spreading in plantation medium. 2） The diffraction fields and the shadow of standing tree diffraction are solved using polygons as approximate substitutes for the circle. When shadow results from a single object, such as a standing tree, the attenuation attributable to diffraction can be estimated by considering the obstruction of a diffracting wedge-edge. According to the Kirchhoff approximation, the receiver point can be written as an integral for the complex amplitude over the plane. 3） Taking man-made birch as the example： the frequency of the source point in the base station is 2.4 GHz, the circumference of the standing tree is 800 mm, the radius of standing tree is l/2π, and source and receiver are equal in height, that is, their diameters are both at breast height of 1.3 m. It shows that the propagation loss of the receiver power, transmission height and distance increase with an increase in the distance of the source, i.e., if the propagation loss increases, the free space propagation loss also increases. Application of t-test to test, takes the significance level of 0.05, below which the results are not significantly different, got fired from the receiving point near models more reliable results. 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.