无线宽频网络设计规划研究报告

1、WIFI RF SYSTEM DESIGN无线宽频网络设计规划研究报告7/2/2005IntroductionCoverage, interference, and blocking considerations are the center of the RF system design work. For any point in the service area of the RF system, we must first have enough RSL in both uplink and downlink directions. The required signal is mai

2、nly to combat the noise in the environment. In an analog voice system, this RSL provides the system with enough S/N so that the voice quality is acceptable. In a digital system, the S/N renders a low enough BER so that the amount of retransmission is within limit and the system can operate normally.

3、 Coverage can be improved by raising the transmitter power or by installing more transmitter sites. But doing so will have impact on the other two design considerations let alone 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范围内的每一个地方.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( Downlink )及上行( Uplin

4、k )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 similar effect as the noise

5、. It also deteriorates the communications quality and must be controlled. Raising the power of a transmitter can improve SIR and the communication quality but there are limits to the maximum power a transmitter can have. Besides, raising the power of one transmitter can raise the interference at oth

6、er receivers and adversely impact the performance of the whole system. Consequently, arbitrarily raising transmitter power does not seem to be a viable solution.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

7、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 unavoidable consequence of channel reuse. The channel can be a frequency channel in an FDMA system, a time slot in a TDMA system, or a code in a CDMA system. The amount of co-channel interference is determined by how the channels are

8、 reused spatially. This is the essence of cellular systems. If the channels are used more frequently in an attempt to enhance the system capacity, the co-channel interference tends to increase and the communication quality declines and vice versa. It is the prudent balance of these two conflicting c

9、onsiderations 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time siot的TDMA系统.或是使用code来控制的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

10、 last consideration is the blocking. The density of sites need be roughly proportional to the traffic rate so that blocking can be reduced to a desired level. All there three factors are interrelated. For example, in a rural area we only need to deploy sites sparsely. But because of the coverage con

11、sideration, the transmitter powers need be high, or high tower need be used so that each tower has a wider 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范围.Other factors,

12、 like channel assignment, antenna gains, orientation both in the vertical and horizontal directions, power control, all need be considered in a complete system design. Additionally, RSL contours are not circular because the ground is not flat and there are restrictions as to where a site can be have

13、 further complicate deployment of a real system. In this article, two Wi-Fi RF systems are considered to illustrate some of the design aspects of a real cellular 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内的.除此之外.讯号接受范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

14、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 % and indoor areas are ignored. Mainly, three non-overlapping channels are used when co-channel interference is considered. Partially overlapping channels are also poten

15、tial candidates when spatial separation proves appropriate. Adjacent channel interference is considered in this regard. 在本案例里.室外涵盖被要求达到90%以上.室内涵盖则不在本案例的讨论范围之内.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内.Cover

16、ageOne area of size 1600 m by 1200 m in Taipei basin is used to illustrate the process of coverage calculations. This example is shown in Figure 1 below. The buildups are extracted from the GIS map and are shown in pink. Sixty APs, s1 s60, are considered. They are indicated with red circles. A grid

17、is used to facilitate all calculations. The grid has 20,000 divisions per arc degree. Consequently, the grid cell dimension is about 5 m by 5 m. There are 27,949 outdoor grid cells in this example. RSSIs are calculated according to the following formula:在本案例中.我们选择了台北市大同区一块1600公尺X 1200公尺的范围.( 中山北路以西.

18、环河北路以东.长安西路以北.凉州街以南 )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.(s1s60 ).我们用红色的圆圈来表示它的位置.在每一arc degree划分了20,000个小格子.( cell )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:RSSI = 20 dBm path loss 17 dB(1)The first term is the AP EIRP, 100 mw, in dBm. The second term is the

19、 path loss calculated by TIREM11. The last term is the environmental loss due to building, cluttering, etc. All antennas are omni-directional. The vertical radiation pattern used for all sites is shown in Figure 2 below. The 3-dB width is about 15 degrees. The design results are very sensitive to th

20、is 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TIREM11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.Figure 1 System with 60 APs and buildup area

21、s.Figure 2 Vertical antenna pattern.First, signal level from each AP to each grid cell point is calculated. For each point, if the RSSI of at least one AP is higher than the threshold of -80 dBm, the point has coverage. This is the downlink coverage. The uplink has slightly better coverage than the

22、downlink and is not considered here. Coverage of the whole area when all 60 APs are used is shown in Figure 3.首先计算downlink 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uplink的讯号强度大于 downlink.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.Figure 3 Coverage of 60 APs is 96.61%, with -17dB bu

23、ilding loss and RSSL of 80dBm.For the RSSI calculations, -17 dB loss is used for building and environmental cluttering. The EIRP of the APs is set at 100 mw. The height of all APs is 3.5 m. The height of the mobile antenna is 0.9 m (3 ft). The frequency is 2.4 GHz. Because of the flatness of the ter

24、rain, contours come out circular. In general, they can have arbitrary shapes.针对RSSI的计算.外在的环境及建筑物等的讯号衰减参数为-17dB.每个AP的最大输出功率( EIRP )设为100mw.AP的高度为3.5公尺.手机的高度为0.9公尺.( 3ft )使用的频段为2.4GHz.由于地表是不平坦的因素.涵盖的轮廓不会是圆形的.一般来说.它们是个不规则的形状Since the population / traffic distribution is not known at this time, the traf

25、fic is assumed to be evenly distributed. Consequently, area coverage is calculated here. For the 60 APs, the coverage is 96.61%.在本案例里.由于缺少人口及交通量分布的资料.它们被假设为平均分布.经计算的结果.60个AP的涵盖率为96.61%.What we want to accomplish next is to select a minimum number of APs to achieve the required 90 % coverage. This is

26、 a multivariable optimization problem. SA (Simulated Annealing) 3 is used. To do the SA, we need an OF (Objective Function). This OF for the system has two terms as below:接下来的工作是选择出最少的AP数量.又能满足90%涵盖率的要求.这是一种非线性最佳化的问题.我们使用SA ( Simulated Annealing )的技术.首先我们需要一个目标函数OF ( Objective Function ).而这个OF必须要满足系

27、统中的两个条件.OF如下列公式(2).Objective Function = % Coverage + f # of APs removed(2)If the coverage is 92 %, the first term is 92. The second term is the number of APs removed multiplied by an f-factor. This f-factor is a user input and is used to control the amount of APs we intend to remove.如果涵盖被要求为92%.第一个条

28、件就订为92.第二个条件是最多可以减少多少个AP.对AP数量影响的另外一个参数是f.这个参数可由使用者自己来决定.These two terms are conflicting; when one goes up, the other goes down. When we start removing APs from the system, unavoidably, the coverage goes down. The goal here is to remove APs in an optimum way such that the maximum number of APs can b

29、e removed while the coverage stays above 90 %. If we can tolerate a lower coverage, we can set f higher and more APs can be removed. When we set f at 0.66, the optimization result is indicated in Figure 4. Fourty APs remain with coverage of 90.78 %. The removed APs are shown in Green.这两个条件是互相冲突的.当一个

30、数值增加另外一个就会减少.当我们从系统中删减AP时.涵盖范围也跟着减少.我们的目的是要在最好的情况下.删减最多的AP数量.而涵盖率又能维持在90%以上.如果我们能容许这个低的涵盖率.我们就能提高f的值和删减最多的AP.当我们设定f的值为0.66,最佳化的结果我们用图形4来表示.有40个AP被保留下来.系统的涵盖率仍然维持在90.78%.绿色的圆圈表示从系统中删除的AP.Figure 4 Coverage of 40 APs is 90.78%.Channel AssignmentSince the earth is not flat and APs are scattered irregula

31、rly, the regular hexagonal or triangular repetition patterns for channel assignment cannot be employed. To do a complete assignment we must consider co-channel and adjacent channel interference in both uplink and downlink directions.由于地表不是平坦的.所有的AP分布是不规则的.一般六角形或三角形的多次重复的频道使用规划是不适用的.为了要规划一个完整而正确的频道分配

32、.我们必须在uplink及downlink的两种状况下同时考虑同频以及邻频干扰的发生情况.Let us consider first the 3 non-overlapping channels, 1, 6, and 11. We could stagger the 3 channels spatially in a fashion such that adjacent APs use same channel as little as possible. This cannot be done exactly because APs are scattered. More important

33、ly, we have to consider the interference. Since these three channels are non-overlapping, we can ignore the adjacent channel interference for the time being. To assign channels in an optimum fashion, we use SA again.让我们首先考虑这三个没有频谱重叠的频道.第1.6.和11.频道.我们能错开这三个频道的距离.就如同相邻的AP不会使用相同的频道一样.但是由于AP的不规则分布.使得这三个

34、频道不能满足系统频道分配的需求.更重要的是我们还需要考虑频道干扰的问题.由于这三个频道是没有频谱重叠的问题.我们不需要处理邻频干扰的问题.为了要完成一个最完善的频道分配工作.我们再次使用SA的技术.Start with an arbitrary channel assignment. Use the following OF:开始时.我们使用下列的OF (3)并随意的放入一个频道.Objective Function = % of OK cells(3) In order for a cell to be OK, not only must the RSSI be above 80dBm, b

35、ut also both uplink and downlink co-channel interference must be lower than the specified limit. For example, SIR need be higher than 10 dB, i.e. the desired signal level must be 10 dB higher than the total interference. Downlink SIR calculation is described first.如何定义一个小格子(cell)是被优化完成.它不但被要求RSSI要大于

36、-80dBm.而且它的 uplink和downlink的同频干扰必须低于最低要求.举例来说.SIR(signal to interference ratio)必需大于10dB.也就是主要讯号强度必须比所有干扰讯号的总和大10dB.For each covered grid cell A, we first find the home AP, which is the AP with the strongest RSSI at cell A. Then we find all other APs using the same channel as the home AP of cell A. T

37、he interference is the sum of signals from all those APs:针对一个小格子(cell A).我们首先应找出它是属于那一个AP 管辖.这个AP在这个格子里的讯号强度RSSI是最强的.我们再找出其它使用相同频道的Ap在这个格子(cell A)讯号强度的总合.这就是cell A的干扰.如下列公式(4)Downlink Interference = RSSI ith AP to cell A(4) Taking the ratio of RSSI from home AP to cell A and the interference, we get

38、 the downlink SIR at cell A:找出指定的AP到cell A的RSSI和干扰的比值.我们就可得到在cell A的downlink SIR.如公式(5)Downlink SIR = RSSI cell A Downlink Interference (5) If the ratio is higher than 10 dB, the downlink SIR at that cell is OK.如果这个比值大于10dB.这个格子(cell A)的干扰就在可接受的范围内了.For the uplink SIR, we find the uplink RSSI at the

39、 home AP for a cell A. Now for every other AP using the same channel, we need to calculate interference as follows. Gather all grid cells served by this AP. Calculate the average uplink RSSI from this area to the home AP of cell A. This is the interference from that AP area to cell A. Sum up all thi

40、s kind of uplink interference RSSIs from all other APs using the same channel as cell A. This is the total uplink interfering signal.关于uplink 的SIR.我们找出在cell A所属AP的uplink SIR.对于其它每一个使用相同频道的AP.我们使用以下的程序来计算它的干扰.在一指定的AP所涵盖的范围内.我们总合所有格子到指定的AP.计算出uplink RSSI的平均值.来和格子A.(cell A)到指定AP的uplink RSSI.这就是这个区域对格子A

41、的干扰.再总合其它使用相同频道的AP所有涵盖范围内的格子.再集合所有这种uplink interference RSSI.这就是全部uplink interference的讯号.计算公式如公式(6)Uplink Interference = RSSI ith area(6) Taking the ratio of the cell A uplink signal and this interfering signal, we get uplink SIR of cell A. If this ratio is higher than 10 dB, cell A is OK.取得了格子A的upl

42、ink signal与其它干扰讯号.我们可得到格子A的uplink SIR.如果这个比值大于10dB.格子A的 uplink干扰就在可接受范围内了.Uplink SIR = RSSI cell A Uplink Interference (7)A cell is considered OK if it has RSSI coverage and both uplink and downlink SIRs are higher than 10 dB. Now, OF in formula (3) can be used to find the best configuration for the

43、 system. That is, what APs to use and what channel to use at each AP. After the calculation is done, OK cells, cells with excessive uplink and/or downlink interference can all be graphically displayed.如果RSSI涵盖以及uplink和downlink的SIR大于10dB.那么A cell就被确定为OK的了.OF的公式(3)就可用来找出系统最佳的涵盖情况.这表示每一个AP都被分配到一个最适当的频道

44、.当计算完成后.那些OK cell以及cell中.uplink和downlink干扰过多的部分都会标示出来.The same example is used for channel assignment demonstration. In order to keep the 90.78 % coverage, we keep the same configuration, i.e. we used the 40 APs suggested by the coverage optimization. After the OF of (3) is optimized, we get a 73.86

45、 % OK rate. That is while 90.78 % of the area has signal coverage, only 73.86 % of the area has adequate communications quality. The result is shown in Figure 5. The white number shows the channel assigned. The green shows OK area. The black cells show area without coverage. The dark gray area is wh

46、ere there is coverage but the downlink co-channel interference is too high, i.e. SIR is less than 10 dB. The light gray cells are where there is coverage, high enough downlink SIR but uplink interference is too high. It can be seen that uplink interference is far more serious than the downlink. Unde

47、r the current conditions, only 73.86 % of the outdoor areas have adequate communications. uplink的干扰远远超过downlink的部相同的例子.我们也可用在频道分配的论证上.在被要求保持90.78%的涵盖条件之下.表示经过涵盖最佳化处理后.被建议保持40个AP.在经过OF(3)的计算之后.系统得到73.86%的cell OK率.那表示在90.78%涵盖率的情况下.只有73.86%的地区通讯品质是良好的.这些结果显示在图5中.白色的数字表示被使用的频道.绿色的地区表示通讯品质良好的位置.黑色表示没有涵盖

48、.深灰色表示有涵盖.但是downlink的同频干扰太多.也就是说SIR低于10dB.浅灰色表示这里有涵盖.downlink的SIR也足够.但是uplink的干扰太多.这可看出份.在这种情况下.室外涵盖地区只有73.86%拥有良好的通讯品质Figure 5 Optimum channel assignment with 3 channels shows that 73.86 % of the area has adequate communications.Using all 60 APs in the optimization process actually makes it worse.

49、Only 68.71 % of the area is OK in this case as shown in Figure 6 below:如果60个AP都被使用.经过最佳化的处理程序后.所得的结果只有68.71%的地区保持良好的通讯品质.这个结论我们用图6来显示.Figure 6 Optimum channel assignment with 60 APs has worse result. Only 68.71 % area is OK.Overlapping ChannelsWe have only used the non-overlapping channels, 1, 6, an

50、d 11 in above discussions. Since in the frequency spectrum, the other channels are partially overlapping with channels 1, 6, and 11, and with each other, there is a potential that these channels can be used when the spatial factor proves appropriate. In other words, when the spatial separation is no

51、t enough for a channel to be reused because its spectrum overlaps fully with itself, there is the possibility that a partially overlapping channel can be used. Also, there are spectrum gaps between these three channels. For example, if we put half of the APs already on channel 1 on channel 2, we red

52、uce the co-channel interference because some fully overlapping APs are partially overlapping now.在以上的讨论中.我们只使用了没有频谱重叠的频道,即频道1.6.和11.等三个频道.由于在频率的频谱中其它的频道对频道1.6.11.都会有部分的重叠.相对于其它的每一个频道当空间因素被证明是适合的时候.则每一个频道都有被使用的可能.从另外一个角度来看.当空间距离不足以让相同的频道再重复使用时.那些有部分频谱重叠的频道就有可能被拿来使用了.同时在这三个频道中间也有一些频谱空间的浪费.举例来说.如果我们将一半

53、的AP放入频道1和一半的AP放入频道2.我们就能减少一些同频干扰.因为那些全部重叠的频道已变更为部分重叠了This situation opens up the possibility of using the other channels and thus enhances the capacity and/or coverage of the RF system. Since it depends on the configuration and geography of the system, the results cannot be ascertained in general. F

54、or the example on hand we get the following, if we utilize all eleven channels.这种情形增加了其它频道使用的可能性.也将RF系统的通讯容量及涵盖扩张到可能的最大.由于系统的布建要视地形地物的因素而决定.而这些结果也不能用常规来确定.在本案例中.我们就使用了全部的11个频道.Figure 6 Optimum channel assignment using all 11 channels boosts OK area to 81.89 %.We can now boost the OK area from 73.86

55、% to 81.89 %. The amount of improvement depends on the system configuration as mentioned before. Here for better viewing, dark gray is replaced by blue and light gray is replaced by orange. Following are some zoomed in views for Figure 5 and Figure 6.现在我们能提高OK的通讯区域从73.86%到81.89%.正如前面所述.系统结构的良窳.是可以用数

56、字来证明和决定的.上图7.让我们更容易观察.深灰色用蓝色来取代.浅灰色用橘色来取代.图8.9.是图7.做部分地区的放大.Figure 7 Zoomed in view of Figure 5.Figure 8 Zoomed in view of Figure 6.We will consider another example with smaller area below. The system as shown in Figure 9 has a dimension of 230 m by 340 m. Thirty-seven APs are included. Some sites a

57、re on top of each other but have different elevations, as they are on different floors. All parameters are same as the previous example. The major difference is that the area of this example is much smaller.下面的例子.我们选择了一块面积较小的区域来作为规划的样本.这个系统的分布情况我们用图9来显示.本区的面积大约为230公尺乘340公尺.有37个AP分布在此区域内.有一些Ap是重叠在一起.

58、但是它们的高度不同.因为这些AP被分别设在不同的楼层.所有的参数被设定和上一个例子相同.这两个案例主要的不同在于案例(2)的面积比案例(1)小了许多.The coverage of all APs is shown in Figure 10. As can be seen in Figure 11, three APs can cover the whole area 100 percent. Since there are three non-overlapping channels available, there will be no co-channel interference if only two APs are used.参与含盖计算的AP都显示在图10里.我们可以从图10中很容易的看出.如果正确的选择AP的位置.则全区只须要两个AP.就能达成百分之百的含盖率.由于802.11的协议.Wi-Fi系统拥有3个没有频谱重叠的频道.在只使用两个AP的情况下.是不会产生同频干扰的Figure 9 Example of 37 ApsFigure 10 Coverage of the 37-AP ExampleFigu