外文翻译--天线倾角规划

Lee Kong Haw,华为技术有限公司 天线倾角规划 蜂窝通信中，覆盖理论、频率复用理论、 BSS功能算法理论都是基于一个前提，规则的蜂窝布局。影响蜂窝布局的因素在无线网络规划中主要体现在工程参数的设计上，从无线网络众多基站宏观布局到单个基站位置、天线高度、波瓣宽度、方向、倾角、 EIRP等，形成了一个具体的蜂窝状网络。 通常，天线本身性能指标根据无线组网的特点如基站密集程度、宏观覆盖目标来总的进行选取；基站位置结合组网要求和外界客观条件确定后，一般也很少改变；而天线高度、方向和倾角就需要根据前面已定参数和 单个小区具体覆盖目标来最终确定。 下面从确定天线高度、方向、倾角以及覆盖目标（小区半径 R）入手，分析相互之间关系，并最终给出一定条件下的天线倾角建议值。因无线信号传播与环境密切相关（如高楼密集区损耗，山体、水面或巨型玻璃墙幕反射等会对电波传播产生影响），不一定能适应所有传播环境；但是，在规划时仔细考虑小区蜂窝结构的规则性以及小区覆盖范围和目标，可以为无线网络质量奠定一个十分本质的基 础。 1、天线倾角设计 在设计天线倾角时必须考虑的因素有：天线的高度、方位角、增益、垂直半功率角，以及期望小区覆盖范围。 对于分 布在市区的基站，当天线无倾角或倾角很小时，各小区的服务范围取决于天线高度、方位角、增益、发射功率，以及地形地物等，此时覆盖半径可以采用Okumura-Hata或 COST231公式计算；当天线倾角较大时，因上述公式中没有考虑倾角，无法计算出的覆盖半径（如有比较准确的传播模型和数字地图， ASSET可以计算）。此时可以根据天线垂直半功率角和倾角大小按三角几何公式直接估算，方法如下： 假设所需覆盖半径为 D(m)，天线高度为 H(m)，倾角为 ，垂直半功率角为 ，则天线主瓣波束与地平面的关系如图 4-8所示。 图 4-8 天线主瓣波束与地平面的关系 从上图可以看出，当天线倾角为 0度时天线波束主瓣即主要能量沿水平方向辐射；当天线下倾 度时，主瓣方向的延长线最终必将与地面一点（ A点）相交。由于天线在垂直方向有一定的波束宽度，因此在 A点往 B点方向，仍会有较强的能量辐射到。根据天线技术性能，在半功率角内，天线增益下降缓慢；超过半功率角后，天线增益（特别是上波瓣）迅速下降，因此在考虑天线倾角大小时可以认为半功率角延长线到地平面交点（ B点）内为该天线的实际覆盖范围。 根据上述分析以及三角几何原理，可以推导出天线高度、下倾角、覆盖 距离三者之间的关系为： arctan (H/D)+/2 上式可以用来估算倾角调整后的覆盖距离，在优化现场实际使用效果显示该式具有较强的指导意义。但应用该式时有限制条件：倾角必须大于半功率角之一半；距离 D必须小于无下倾时按公式计算出的距离。式中垂直波束宽度可以查具体天线技术指标或计算得出大致值。 对于垂直波束宽度为 17度，基站天线高度 40米的场合，覆盖距离与天线倾角的关系如图 4-9所示。 图 4-9 覆盖距离与倾角关系（垂直波束宽度 17度，天线高度 40米） 当垂直波束宽度为 6.5度，基站天线高度 40米时，覆盖距离与天 线倾角的关系如图 4-10所示。 图 4-10 覆盖距离与倾角关系（垂直波束宽度 6.5度，天线高度 40米） 从以上两图可以看出，在天线高度和倾角一定时，覆盖距离与天线的垂直波束宽度间的关系。垂直波束宽度越小，覆盖距离越小。因此为了更好地控制越区覆盖，在规划阶段选择天线时应该选择垂直波束宽度小且具有零点填充功能的天线，既控制越区干扰，也改善近处和室内覆盖。 但是垂直波束宽度变小可能引起水平波瓣变宽或增益变大，造成新的越区干扰或相邻小区间交叉覆盖过甚。所以，城区一般选择中等增益的天线，如 GSM900选用 65度 15dBi天线，此时垂直波瓣宽度为 11 15度。必须注意的是：调整倾角后除了可以控制越区覆盖外，还可以改善基站附近的室内覆盖，但远离基站处的覆盖将变差。 2、 实际运用 为了便于实际运用和考虑相邻小区间必要的部分区域重叠，密集市区基站到覆盖目标距离 D可以简化为小区设计半径（长径 R）；天线高度 H指基站与覆盖目标的相对高度，并且本文我们只讨论近似平原地区。 天线下倾分为机械下倾和电气下倾，对覆盖的影响基本一致，由于电气下倾天线价格较贵并且需要定制，我们通常采取机械下倾方式；一般还认为，天线机械下倾在 10度以 内是比较科学的做法，大于 10度时波瓣容易变形而对其他小区造成意想不到的干扰；还有一种结论是，对于机械下倾，其下倾角度不应超过该天线垂直面内的半功率波束宽度，否则覆盖出现畸形，因此机械下倾角不宜大于 10度。 如果仅考虑网络质量控制的便利性，在密集市区组网，我们希望采用电调天线，由于能够在现场进行电气下倾角调节的天线较贵，一般可以采用出厂预设 6 7度（或覆盖区域的平均下倾角度）电气下倾天线，在网络扩容和优化时结合机械下倾，实现 15 20度大下倾角设置。 根据上述讨论，结合我们最常用的 A天线和常见天线高度（ 25 50米），给出在 250、500、 800、 1000米小区半径下的天线倾角建议值。其他情况可以类推。 表 4 4 密集城区天线下倾角参考值 天线型号 天线垂直半功率角 小区半径 R（ m） 天线高度 下倾角 65度，增益 15dBi 12 200 50 20 65度，增益 15dBi 12 250 50 17 65度，增益 15dBi 12 250 40 15 65度，增益 15dBi 12 250 30 13 65度，增益 15dBi 12 250 25 12 65度，增益 15dBi 12 500 50 12 65度，增益 15dBi 12 500 40 11 65度，增益 15dBi 12 500 30 10 65度，增益 15dBi 12 500 25 9 65度，增益 15dBi 12 800 30 8 65度，增益 15dBi 12 1000 30 2 可见，在小区半径过分小时，天线机械下倾也无法保证能够很好控制覆盖范围，此时只能降低天线高度；如果降低高度存在困难，就需要采取天线电气下倾与机械下倾相结合的方式。实际运用中，对于天线高度 40 50米的基站，小区半径最小为250米。一般情况下，密集市区宏蜂窝 理想天线高度为 25 30米，而郊区或指向郊区天线高度为 40 50米。 以上下倾角计算方法主要适合于站距 1200米（即 R=800m）以内的密集基站组网。 当基站距离覆盖目标大于 800米时，大面积覆盖仍是最重要的关注点，估算天线下倾角时不必考虑垂直半功率角的影响，此时下倾角一般为 1 4度；特殊情况下如基站本身已经建在较高位置，此时下倾角也可能较大。 但是，基站周围环境是十分复杂的，下倾角还必须考虑附近山体、水面和高大玻璃墙幕的发射，这种反射容易造成意外的与其他基站同邻频干扰甚至自身时间色散效应；也必须考虑楼房天 面、前方密集建筑群、山坡等对电波的阴影效应。但是实际组网中有时也会结合基站周围地理环境利用大楼或山体等的阻挡来控制覆盖范围，此时需要与下倾角综合考虑。 密集市区组网还必须考虑当天线主瓣正对街道而带来的街道效应和意外越区覆盖。一般情况下，密集市区应避免天线主瓣正对较直的街道。 我们还必须考虑天线后瓣在天线主瓣下倾后的方向情况，因为现在一般的天线前后比只有 20dB左右，信号很强的后瓣容易对高层建筑造成较大干扰；所以在密集市区选用天线时尽量采用电气下倾的方式。同时还需注意上副瓣的影响。 通常，全向天线垂直功率角 是沿水平面上下对称的，倒装和正装效果一样；但是实际工程中还是需要注意具体全向天线的垂直方向图，是否已经具备电气下倾角，此时倒装就要慎重考虑了。 Lee Kong Haw. Huawei Technologies Co., Ltd. 公司内部资料 Antenna Tilt Planning In cellular communication, coverage theory, frequency multiplexing theory and BSS functional algorithm are all based on regular cellular layout. The design of project parameters is the main factor that affects the cellular layout in radio network planning. In a wireless network system, the macro-BTS layout and the actual location of each base station, antenna height, lobe width, direction, tilt angle, and EIRP together form a specific cellular network. Generally, the performance indexes of the antenna itself are selected according to the radio networking characteristics, such as the base station density and macro coverage goal. Once the location of a base station is determined, it seldom changes. For the antenna height, direction and tilt angle, however, they are finally determined according to the parameters specified previously and the actual coverage goal of a cell. Hereunder is the analysis of the relationship among antenna height, direction, tilt angle, and coverage goal (suppose that the cell radius is R), and the antenna tilt angel is finally recommended according to this analysis. The propagation of radio signals is closely related to the environment. For example, dense buildings and the reflection caused by mountains, water surface, or huge glass walls will affect radio propagation. Therefore, it is not necessarily that all the environments are favorable to radio propagation. However, the regularity of cellular structure and the coverage area and goal of a cell are the foundation for a good network, so they must be carefully considered during network planning. 1、 Antenna Tilt design The following factors must be considered in antenna tilt design: Antenna height Azimuth angle Antenna gain Vertical half power angle Expected coverage area For the base stations distributed in urban areas, when the antenna has no tilt angle or the angle is very small, the serving area of each cell is determined by the antenna height, azimuth angle, antenna gain, transmit power, landforms and ground objects. In this case, the coverage radius can be calculated by the commonly-used propagation module formulas, such as Okumura-Hata and COST231.When the tilt angle of the antenna is large, the coverage radius cannot be calculated out because the angle is not considered in the previous formulas. If accurate propagation module and digital map are provided, however, the coverage radius can be calculated out by planning software. In this case, the antenna vertical half power angle and tilt angle helps to calculate the coverage radius directly based on the triangle geometry formula as follows: If the needed coverage radius is D (m), the antenna height is H (m), the tilt angle is, and the vertical half power angle is, the relationship between the beams of the major antenna lobe and the ground is shown in Figure 4-1. Figure 4-1 Relationship between the beams of the major antenna lobe and the ground As shown in this figure, when the antenna tilt is 0 degrees, the beams of the major antenna lobe, or the major energy, radiate horizontally； when the antenna tilt angle is, the extension line of the major lobe and the ground will ultimately intersect at one point (point A). Because a beam width is present in the vertical direction for the antenna, intense radiation is present in the area form point A to point B. According to the technical performance of the antenna, the antenna gain decreases slowly within half power angle, but it decreases sharply beyond the half power angle, especially for the upper lobe. Therefore, when the antenna tilt angle is considered, the scope between the extension line of the half power angle to intersection point (point B) can be taken as the actual coverage area of the antenna. Based on the previous analysis and the principles of triangle geometry, the relationship between the antenna height, tilt angle and coverage distance can be obtained as follows: arctan (H/D) + /2 This formula can calculate the coverage distance after the adjustment for tilt angle. Actual results of on-site optimization projects show that this formula is of great significance. However, the application of this formula meets limited conditions. It can be applied when the tilt angle is 1.5 times greater than the half power angle, and the distance (D) must be less than the distance calculated by the previous formula when no tilt angle is present. For the width of vertical beams in the previous formula, it is provided in the specific antenna technical indexes. Figure 4-2 shows the relationship between coverage distance and antenna tilt angle when the vertical beam width of the antenna is 17 degrees. (The antenna height is 40 meters.) Figure 4-2 Relationship between coverage distance and tilt angle (The width of the vertical beam is 17 degrees, and the antenna height is 40 meters.) Figure 4-3 shows the relationship between coverage distance and antenna tilt angle when the vertical beam width of the antenna is 6.5 degrees. (The antenna height is 40 meters.) Figure 4-3 Relationship between coverage distance and tilt angle (The width of the vertical beam is 17 degrees, and the antenna height is 40 meters.) The previous two figures shows the relationship between the coverage distance and the width of the antenna vertical beams when antenna height and tilt angle are certain. The smaller the width of the vertical beam, the shorter the coverage distance is. Therefore, if the cross coverage are effectively controlled, the antennas with smaller vertical beam width and with the zero point filling function should be selected during the planning phase. In this case, the cross interference can be controlled and the indoor coverage near the base station. However, if the vertical beam width grows smaller, the horizontal lobe will grow wider or the antenna gain will grows larger. In this case, new cross interference is caused and the cross coverage area between neighbor cells is too large. Therefore, the antennas of medium gain are often selected in urban areas. For example, if the antenna of 65 degrees and 15dBi is selected for a GSM 900MHz base station, the vertical beam width is about 13 degrees to 15 degrees. 2. Application For the purpose of application and necessary overlaps of adjacent cells, the distance (D) from the base station in populated urban areas to the target coverage area can be simplified as the designed cell radius (R). The antenna height (H) refers to the relative height from the base station and target coverage area. This textbook introduces the application of antenna tilt planning in the areas similar to plains. Antenna tilt can be divided into mechanical tilt and electrical tilt, and their effect on coverage is almost the same. Because electrical tilt antennas are expensive, mechanical antennas are more often used. Emulation shows that if the mechanical tilt is greater than 10 degrees, the lobes are distorted, which will cause unexpected interference against other cells. Therefore, it is better to keep the mechanic tilt within 10 degrees. If only for the convenience of controlling network quality, the adaptation of the electrical adjustment antenna will win more advantages. Because the electrical adjustment antennas are expensive, electrical antennas with a certain preset tilt angle (for example, 6 degrees to 7 degrees) are more often used in actual networking. When the network needs to be expanded and optimized, the electrical tilt antenna and the mechanical tilt antenna work together to set the tilt angles greater than 10 degrees. According to the previous analysis and in combination with the common antenna height (25 meters to 50 meters), the reference tilt angles can be provided for the cells whose radius is 250 meters, 500 meters, 800 meters, and 1000 meters in populated urban areas. The case is the same for other situations. Table 4-1 lists the reference tilt angles for antennas in populated urban areas. Table 4-1 Reference tilt angles for antennas in populated urban areas Antenna model Vertical half power angle Cell radius R(m) Antenna height (m) Tilt angle (degree) 65 degrees, a gain of 15 dBi 15 200 25 15 65 degrees, a gain of 15 dBi 15 200 25 13 65 degrees, a gain of 15 dBi 15 250 30 14 65 degrees, a gain of 15 dBi 15 250 35 15 65 degrees, a gain of 15 dBi 15 250 40 17 65 degrees, a gain of 15 dBi 15 500 25 10 65 degrees, a gain of 15 dBi 15 500 30 11 65 degrees, a gain of 15 dBi 15 500 35 12 65 degrees, a gain of 15 dBi 15 500 40 12 65 degrees, a gain of 15 dBi 15 800 30 10 65 degrees, a gain of 15 dBi 15 1000 30 2 According to the table, when the cell radius is small, the coverage area cannot be effectively controlled even through mechanically tilting the antenna. In this case, the coverage area can be controlled through lowering the antenna height only. If it is hard for the antenna height to be lowered, the antenna electrical tilt together with the antenna mechanical tilt must be used. The previous methods for calculating tilt angles are mainly applicable for the dense base station networking with the distance within 1200 meters (that is, R = 800 meters) between stations. When the distance from the base station to the coverage target is greater than 800 meters, large area coverage is still being emphasized. In this case, it is unnecessary for you to consider the effect of the vertical half power angle when estimating the antenna tilt angle. Generally, the tilt angle now is 1 degree to 4 degrees. In special cases, such as the base station has already been installed at a high position, the tilt angle may also be large. However, because the environment around the base station is rather complicated, the reflection caused the nearby mountains, water surface, huge glass walls has an effect on antenna tilt angle. The reflection of this kind will easily cause unexpected interference against the neighbor frequencies and time dispersion effect. In addition, the shadow effect caused by building roofs, front den