系统无线宽频网络设计规划研究报告

WIFI RFSYSTEM DESIGN

无线宽频网络设计规划研究报告

7/2/2005

A Introduction

Coverage, interference,and blocking considerations are the centerof the RF system design work. For any point in the service area ofthe RF system,we must first have enough RSL in both upl ink anddownl ink directions. The required signal is mainly to combat thenoise in the environment. In an analog voice system, this RSLprovides the system with enough S/N so that the voice qual ity isacceptable. In a digital system, the S/N renders a low enough BERso that the amount of retransmission is within l imit and the systemcan operate normal ly.Coverage can be improved by raising thetransmitter power or by instal l ing more transmitter sites. But doingso wi l l have impact on the other two design considerations let aloneDOC版本

the cost factor.

含盖,干扰,以及通讯阻塞等问题的思考.是R/F系统设计工作的中心议题.在无线电通讯系统含盖围的每一个地方.我们必须要提供足够的讯号.足够的讯号应定义为同时满足下行(Downl ink)及上行(Upl ink)两种含盖的需求.这种足够的讯号同时被要求能克服外在环境中噪声的影响.在复频式的通讯系统中.最低接受讯号强度(RSL)应该满足系统中主讯号与噪声的比值(S/N).在这种条件下.通话的品质是可以被接受的.在数字式的通讯系统中.主讯号与噪声的比值.可提供RF系统工程师计算出最低的误码率.使得讯号重复传输的情况减少到最小.使得系统能正常的运作.工程师可借着这些资料来提供基地台的增减及天线输出功率大小的参数.含盖设计的好坏也影响到干扰及频道阻塞的问题思考以及系统的成本因素.

The co-channel interference has simi lar effect as the noise. It alsodeteriorates the communications qual ity and must be control led.Raising the power of a transmitter can improve SIR and thecommunication qual ity but there are l imits to the maximum poweratransmitter can have.Besides, raising the power of one transmittercan raise the interference at other receivers and adversely impactthe performance of the whole system. Consequently, arbitrari lyraising transmitter powerdoes not seem to be aviable solution.DOC版本

同频干扰和噪声对R/F系统影响的效果是相同的.它同样可以使通讯品质恶化.所以同频干扰也必需要加以控制.提高输出功率可以加强讯号和干扰的比值( SIR)并改善通讯的品质.但是输出功率有一定的限制.此外,输出功率的提高.同时也提高了干扰的强度.这对系统的整体通讯品质.会产生负面的影响.而且任意提高输出功率.并不是系统最佳的解决方案.

The co-channel interference comes as an unavoidableconsequence of channel reuse.The channel can be a frequencychannel in an FDMA system, a time slot in a TDMA system, or acode in a CDMA system.The amount of co-channel interference isdetermined by how the channels are reused spatial ly.This is theessence of cel lular systems. If the channels are used morefrequently in an attempt to enhance the system capacity, theco-channel interference tends to increase and the communicationqual ity decl ines and vice versa. It is the prudent balance of thesetwo confl icting considerations that produce an optimum system.由于无线电通讯系统频道有限而必须重复使用的时候.同频干扰就是不可避免而必须面对的问题.不论它是使用frequency channel的FDMA系统.或是使用timesiot的TDMA系统.或是使用code来控制DOC版本

的CDMA系统.同频干扰的情况都是由频道重复使用的次数及空间来决定的.这就是蜂巢通讯系统的基本精神.如果系统企图增加通讯的容量而频繁的重复使用频道.同频干扰就不可避免的增加.系统的通讯品质则下降.反之亦然.系统规划的重要课题就是如何仅慎小心的取得两者之间的平衡.来造就一个最优良的通讯系统.

The last consideration is the blocking.The density of sites need beroughly proportional to the traffic rate so that blocking can bereduced to a desired level .Al l there three factors are interrelated.For example, in a rural area we only need to deploy sites sparsely.But because of the coverage consideration, the transmitter powersneed be high,or high tower need be used so that each tower has awider coverage area.

最后一个问题是频道阻塞.基地台设立的密度是视通话量的多寡而来决定的.如此频道阻塞的情形就可以降低到一个理想的水准.但是这三个因素在R/F系统的设计里.是会相互影响的.举例来说.在乡下人口密度较小的地区.我们只需要建置较少的基地台.但是在考量含盖因素的问题之下.我们需要调高天线的输出功率.或增加天线的高度.以加大基地台含盖的围.

Other factors, l ike channel assignment, antenna gains, orientationDOC版本

both in the vertical and horizontal directions,power control ,al l needbe considered in a complete system design.Additional ly, RSLcontours are not circular because the ground is not flat and thereare restrictions as to where a site can be have further compl icatedeployment of a real system. In this article, two Wi-Fi RF systemsare considered to i l lustrate some of the design aspects of a realcel lular system.

有关其它可能影响到R/F系统设计的因素.如频道的安排.天线增益.天线水平和垂直方向的定位.输出功率的控制等.在一个完整的系统设计里.这些因素都应该被考虑在的.除此之外.讯号接受围的外形因为地形的不规则.使得它的轮廓并不是一个圆形的.由于外在环境的限制.使得正确的系统天线选址及布建.变得更为复杂.在这篇文章里.我们举出了两个Wi-Fi RF系统的设计.来论述蜂巢无线电通讯系统在R/F设计上.基本的观念和方法

In this article, outdoor areas need be covered up to 90%andindoor areas are ignored.Mainly, three non-overlapping channelsare used when co-channel interference is considered. Partial lyoverlapping channels are also potential candidates when spatialseparation proves appropriate.Adjacent channel interference isconsidered in this regard.

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在本案例里.室外涵盖被要求达到90%以上.室涵盖则不在本案例的讨论围之.一般的无线局域网络(WiFi)在频道使用上.只选择了三个没有频谱重叠的频道.这种设计使得频道的重复使用变得非常困难.造成同频干扰变得更加严重.所以当同频干扰被处理的同时.其它有频谱重叠的频道也可以一并被考虑使用在系统中.当然.邻频干扰的问题也一并列入系统设计的考虑条件.

B Coverage

One area of size 1600 m by 1200 m in Taipei basin is used to i l lustrate the process of coveragecalculations.This example is shown in Figure 1 below.The bui ldups are extracted from the GIS mapand are shown in pink.Sixty AP’s,s1 –s60,are considered.They are indicated with red circles.A gridis used to faci l itate al l calculations.The grid has 20,000 divisions per arc degree.Consequently, thegrid cel l dimension is about 5 m by 5 m. There are 27,949 outdoorgrid cel ls in this example.RSSI’sare calculated according to the fol lowing formula:

在本案例中.我们选择了台北市区一块1600公尺X1200公尺的围.(北路以西.环路以东.长安西路以北.街以南)来处理涵盖及干扰问题的最佳化计算.我们从GIS的地图中取得本区的建筑物资料.用粉红色显示出来.在这区域里共选取了 60个AP的位置.(s1—s60 ).我们用红色的圆圈来表示它的位置.在每一arcdegree划分了20,000个小格子.( cel l )每个格子的面积为5公尺平方.本区室外部分共有27,949个格子.RSSI (最低接收功率)的计算公式如下:

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RSSI=20 dBm–path loss–17 dB (1)

The first term is the AP EIRP, 100 mw, in dBm. The second term is the path loss calculated byTIREM1 1 .The last term is the environmental loss due to bui lding, cluttering, etc.Al l antennas areomni-directional .The vertical radiation pattern used for al l sites is shown in Figure 2 below.The 3-dBwidth is about 15 degrees.The design results are very sensitive to this pattern. In the real design work,one has to make sure that the correct patterns are used.

第一个要件是AP的EIRP.(最大输出功率)为100mw,我们用dBm为单位.第二个要件.我们用TIREM1 1来计算讯号传递衰减.最后的要件是外界的环境影响.如建筑物等.全部的天线是用omni-directional .各项天线参数如图2所示.天线参数对设计的结果是非常重要的.在正式的设计前.必须取得完整而正确的天线参数数据.

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Figure 1 Systemwith 60 AP’s and bui ldup areas.DOC版本

Figure 2 Vertical antenna pattern.

First, signal level from each AP to each grid cel l point is calculated. For each point, if the RSSI of atleast one AP is higher than the threshold of -80 dBm, the point has coverage.This is the downl inkcoverage.The upl ink has sl ightly better coverage than the downl ink and is not considered here.Coverageof the whole area when al l 60 AP’s are used is shown in Figure3.

首先计算downl ink coverage,从每一个AP到每一个格子的讯号强度.最少要有一个AP的讯号强度大于-80dBm,这个格子就被认为有涵盖了.由于upl ink的讯号强度大于downl ink.在本案例中不列入考虑.全部60个AP所形成的涵盖情况请参考图3所显示的资料.

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