1、WIFI RF SYSTEM DESIGN無線寬頻網(wǎng)絡(luò)設(shè)計(jì)規(guī)劃研究報(bào)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.含蓋,干擾,以及通訊阻塞等問題的思考.是R/F系統(tǒng)設(shè)計(jì)工作的中心議題.在無線電通訊系統(tǒng)含蓋范圍內(nèi)的每一個(gè)地方.我們必須要提供足夠的訊號(hào).足夠的訊號(hào)應(yīng)定義為同時(shí)滿足下行( Downlink )及上行( Uplin

4、k )兩種含蓋的需求.這種足夠的訊號(hào)同時(shí)被要求能克服外在環(huán)境中噪聲的影響.在復(fù)頻式的通訊系統(tǒng)中.最低接受訊號(hào)強(qiáng)度( RSL )應(yīng)該滿足系統(tǒng)中主訊號(hào)與噪聲的比值( S/N ).在這種條件下.通話的品質(zhì)是可以被接受的.在數(shù)字式的通訊系統(tǒng)中.主訊號(hào)與噪聲的比值.可提供RF系統(tǒng)工程師計(jì)算出最低的誤碼率.使得訊號(hào)重復(fù)傳輸?shù)那闆r減少到最小.使得系統(tǒng)能正常的運(yùn)作.工程師可借著這些資料來提供基地臺(tái)的增減及天線輸出功率大小的參數(shù).含蓋設(shè)計(jì)的好壞也影響到干擾及頻道阻塞的問題思考以及系統(tǒng)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.同頻干擾和噪聲對(duì)R/F系統(tǒng)影響的效果是相同的.它同樣可以使通訊品質(zhì)惡化.所以同頻干擾也必需要加以控制.提高輸出功率可以加強(qiáng)訊號(hào)和干擾的比值( SIR )并改善通訊的品質(zhì).但是輸出功率有一定的限制.此外,輸出功率的提高.同時(shí)也提高了干擾的強(qiáng)度.這對(duì)系統(tǒng)的整體通訊品質(zhì).會(huì)產(chǎn)生負(fù)面的

7、影響.而且任意提高輸出功率.并不是系統(tǒng)最佳的解決方案.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.由于無線電通訊系統(tǒng)頻道有限而必須重復(fù)使用的時(shí)候.同頻干擾就是不可避免而必須面對(duì)的問題.不論它是使用frequency channel 的FDMA系統(tǒng).或是使用time siot的TDMA系統(tǒng).或是使用code來控制的CDMA系統(tǒng).同頻干擾的情況都是由頻道重復(fù)使用的次數(shù)及空間來決定的.這就是蜂巢通訊系統(tǒng)的基本精神.如果系統(tǒng)企圖增加通訊的容量而頻繁的重復(fù)使用頻道.同頻干擾就不可避免的增加.系統(tǒng)的通訊品質(zhì)則下降.反之亦然.系統(tǒng)規(guī)劃的重要課題就是如何僅慎小心的取得兩者之間的平衡.來造就一個(gè)最優(yōu)良的通訊系統(tǒng).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.最后一個(gè)問題是頻道阻塞.基地臺(tái)設(shè)立的密度是視通話量的多寡而來決定的.如此頻道阻塞的情形就可以降低到一個(gè)理想的水準(zhǔn).但是這三個(gè)因素在R/F系統(tǒng)的設(shè)計(jì)里.是會(huì)相互影響的.舉例來說.在鄉(xiāng)下人口密度較小的地區(qū).我們只需要建置較少的基地臺(tái).但是在考量含蓋因素的問題之下.我們需要調(diào)高天線的輸出功率.或增加天線的高度.以加大基地臺(tái)含蓋的范圍.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.有關(guān)其它可能影響到R/F系統(tǒng)設(shè)計(jì)的因素.如頻道的安排.天線增益.天線水平和垂直方向的定位.輸出功率的控制等.在一個(gè)完整的系統(tǒng)設(shè)計(jì)里.這些因素都應(yīng)該被考慮在內(nèi)的.除此之外.訊號(hào)接受范圍的外形因?yàn)榈匦蔚牟灰?guī)則.使得它的輪廓并不是一個(gè)圓形的.由于外在環(huán)境的限制.使得正

14、確的系統(tǒng)天線選址及布建.變得更為復(fù)雜.在這篇文章里.我們舉出了兩個(gè)Wi-Fi RF系統(tǒng)的設(shè)計(jì).來論述蜂巢無線電通訊系統(tǒng)在R/F設(shè)計(jì)上.基本的觀念和方法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.室外涵蓋被要求達(dá)到90%以上.室內(nèi)涵蓋則不在本案例的討論范圍之內(nèi).一般的無線局域網(wǎng)絡(luò)(WiFi)在頻道使用上.只選擇了三個(gè)沒有頻譜重疊的頻道.這種設(shè)計(jì)使得頻道的重復(fù)使用變得非常困難.造成同頻干擾變得更加嚴(yán)重.所以當(dāng)同頻干擾被處理的同時(shí).其它有頻譜重疊的頻道也可以一并被考慮使用在系統(tǒng)中.當(dāng)然.鄰頻干擾的問題也一并列入系統(tǒng)設(shè)計(jì)的考慮條件內(nèi).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.我們選擇了臺(tái)北市大同區(qū)一塊1600公尺X 1200公尺的范圍.( 中山北路以西.

18、環(huán)河北路以東.長安西路以北.涼州街以南 )來處理涵蓋及干擾問題的最佳化計(jì)算.我們從GIS的地圖中取得本區(qū)的建筑物資料.用粉紅色顯示出來.在這區(qū)域里共選取了60個(gè)AP的位置.(s1s60 ).我們用紅色的圓圈來表示它的位置.在每一arc degree劃分了20,000個(gè)小格子.( cell )每個(gè)格子的面積為5公尺平方.本區(qū)室外部分共有27,949個(gè)格子.RSSI (最低接收功率)的計(jì)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.第一個(gè)要件是AP的EIRP.(最大輸出功率)為100mw,我們用dBm為單位.第二個(gè)要件.我們用TIREM11來計(jì)算訊號(hào)傳遞衰減.最后的要件是外界的環(huán)境影響.如建筑物等.全部的天線是用omni-directional.各項(xiàng)天線參數(shù)如圖2所示.天線參數(shù)對(duì)設(shè)計(jì)的結(jié)果是非常重要的.在正式的設(shè)計(jì)前.必須取得完整而正確的天線參數(shù)數(shù)據(jù).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.首先計(jì)算downlink coverage,從每一個(gè)AP到每一個(gè)格子的訊號(hào)強(qiáng)度.最少要有一個(gè)AP的訊號(hào)強(qiáng)度大于-80dBm,這個(gè)格子就被認(rèn)為有涵蓋了.由于uplink的訊號(hào)強(qiáng)度大于 downlink.在本案例中不列入考慮.全部60個(gè)AP所形成的涵蓋情況請(qǐng)參考圖3所顯示的資料.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.針對(duì)RSSI的計(jì)算.外在的環(huán)境及建筑物等的訊號(hào)衰減參數(shù)為-17dB.每個(gè)AP的最大輸出功率( EIRP )設(shè)為100mw.AP的高度為3.5公尺.手機(jī)的高度為0.9公尺.( 3ft )使用的頻段為2.4GHz.由于地表是不平坦的因素.涵蓋的輪廓不會(huì)是圓形的.一般來說.它們是個(gè)不規(guī)則的形狀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資料.它們被假設(shè)為平均分布.經(jīng)計(jì)算的結(jié)果.60個(gè)AP的涵蓋率為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:接下來的工作是選擇出最少的AP數(shù)量.又能滿足90%涵蓋率的要求.這是一種非線性最佳化的問題.我們使用SA ( Simulated Annealing )的技術(shù).首先我們需要一個(gè)目標(biāo)函數(shù)OF ( Objective Function ).而這個(gè)OF必須要滿足系

27、統(tǒng)中的兩個(gè)條件.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.如果涵蓋被要求為92%.第一個(gè)條

28、件就訂為92.第二個(gè)條件是最多可以減少多少個(gè)AP.對(duì)AP數(shù)量影響的另外一個(gè)參數(shù)是f.這個(gè)參數(shù)可由使用者自己來決定.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.這兩個(gè)條件是互相沖突的.當(dāng)一個(gè)

30、數(shù)值增加另外一個(gè)就會(huì)減少.當(dāng)我們從系統(tǒng)中刪減AP時(shí).涵蓋范圍也跟著減少.我們的目的是要在最好的情況下.刪減最多的AP數(shù)量.而涵蓋率又能維持在90%以上.如果我們能容許這個(gè)低的涵蓋率.我們就能提高f的值和刪減最多的AP.當(dāng)我們?cè)O(shè)定f的值為0.66,最佳化的結(jié)果我們用圖形4來表示.有40個(gè)AP被保留下來.系統(tǒng)的涵蓋率仍然維持在90.78%.綠色的圓圈表示從系統(tǒng)中刪除的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分布是不規(guī)則的.一般六角形或三角形的多次重復(fù)的頻道使用規(guī)劃是不適用的.為了要規(guī)劃一個(gè)完整而正確的頻道分配

32、.我們必須在uplink及downlink的兩種狀況下同時(shí)考慮同頻以及鄰頻干擾的發(fā)生情況.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.讓我們首先考慮這三個(gè)沒有頻譜重疊的頻道.第1.6.和11.頻道.我們能錯(cuò)開這三個(gè)頻道的距離.就如同相鄰的AP不會(huì)使用相同的頻道一樣.但是由于AP的不規(guī)則分布.使得這三個(gè)

34、頻道不能滿足系統(tǒng)頻道分配的需求.更重要的是我們還需要考慮頻道干擾的問題.由于這三個(gè)頻道是沒有頻譜重疊的問題.我們不需要處理鄰頻干擾的問題.為了要完成一個(gè)最完善的頻道分配工作.我們?cè)俅问褂肧A的技術(shù).Start with an arbitrary channel assignment. Use the following OF:開始時(shí).我們使用下列的OF (3)并隨意的放入一個(gè)頻道.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.如何定義一個(gè)小格子(cell)是被優(yōu)化完成.它不但被要求RSSI要大于

36、-80dBm.而且它的 uplink和downlink的同頻干擾必須低于最低要求.舉例來說.SIR(signal to interference ratio)必需大于10dB.也就是主要訊號(hào)強(qiáng)度必須比所有干擾訊號(hào)的總和大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:針對(duì)一個(gè)小格子(cell A).我們首先應(yīng)找出它是屬于那一個(gè)AP 管轄.這個(gè)AP在這個(gè)格子里的訊號(hào)強(qiáng)度RSSI是最強(qiáng)的.我們?cè)僬页銎渌褂孟嗤l道的Ap在這個(gè)格子(cell A)訊號(hào)強(qiáng)度的總合.這就是cell A的干擾.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和干擾的比值.我們就可得到在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.如果這個(gè)比值大于10dB.這個(gè)格子(cell A)的干擾就在可接受的范圍內(nèi)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.關(guān)于uplink 的SIR.我們找出在cell A所屬AP的uplink SIR.對(duì)于其它每一個(gè)使用相同頻道的AP.我們使用以下的程序來計(jì)算它的干擾.在一指定的AP所涵蓋的范圍內(nèi).我們總合所有格子到指定的AP.計(jì)算出uplink RSSI的平均值.來和格子A.(cell A)到指定AP的uplink RSSI.這就是這個(gè)區(qū)域?qū)Ω褡覣

41、的干擾.再總合其它使用相同頻道的AP所有涵蓋范圍內(nèi)的格子.再集合所有這種uplink interference RSSI.這就是全部uplink interference的訊號(hào).計(jì)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與其它干擾訊號(hào).我們可得到格子A的uplink SIR.如果這個(gè)比值大于10dB.格子A的 uplink干擾就在可接受范圍內(nèi)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涵蓋以及uplink和downlink的SIR大于10dB.那么A cell就被確定為OK的了.OF的公式(3)就可用來找出系統(tǒng)最佳的涵蓋情況.這表示每一個(gè)AP都被分配到一個(gè)最適當(dāng)?shù)念l道

44、.當(dāng)計(jì)算完成后.那些OK cell以及cell中.uplink和downlink干擾過多的部分都會(huì)標(biāo)示出來.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的干擾遠(yuǎn)遠(yuǎn)超過downlink的部相同的例子.我們也可用在頻道分配的論證上.在被要求保持90.78%的涵蓋條件之下.表示經(jīng)過涵蓋最佳化處理后.被建議保持40個(gè)AP.在經(jīng)過OF(3)的計(jì)算之后.系統(tǒng)得到73.86%的cell OK率.那表示在90.78%涵蓋率的情況下.只有73.86%的地區(qū)通訊品質(zhì)是良好的.這些結(jié)果顯示在圖5中.白色的數(shù)字表示被使用的頻道.綠色的地區(qū)表示通訊品質(zhì)良好的位置.黑色表示沒有涵蓋

48、.深灰色表示有涵蓋.但是downlink的同頻干擾太多.也就是說SIR低于10dB.淺灰色表示這里有涵蓋.downlink的SIR也足夠.但是uplink的干擾太多.這可看出份.在這種情況下.室外涵蓋地區(qū)只有73.86%擁有良好的通訊品質(zhì)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個(gè)AP都被使用.經(jīng)過最佳化的處理程序后.所得的結(jié)果只有68.71%的地區(qū)保持良好的通訊品質(zhì).這個(gè)結(jié)論我們用圖6來顯示.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.在以上的討論中.我們只使用了沒有頻譜重疊的頻道,即頻道1.6.和11.等三個(gè)頻道.由于在頻率的頻譜中其它的頻道對(duì)頻道1.6.11.都會(huì)有部分的重疊.相對(duì)于其它的每一個(gè)頻道當(dāng)空間因素被證明是適合的時(shí)候.則每一個(gè)頻道都有被使用的可能.從另外一個(gè)角度來看.當(dāng)空間距離不足以讓相同的頻道再重復(fù)使用時(shí).那些有部分頻譜重疊的頻道就有可能被拿來使用了.同時(shí)在這三個(gè)頻道中間也有一些頻譜空間的浪費(fèi).舉例來說.如果我們將一半

53、的AP放入頻道1和一半的AP放入頻道2.我們就能減少一些同頻干擾.因?yàn)槟切┤恐丿B的頻道已變更為部分重疊了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.這種情形增加了其它頻道使用的可能性.也將RF系統(tǒng)的通訊容量及涵蓋擴(kuò)張到可能的最大.由于系統(tǒng)的布建要視地形地物的因素而決定.而這些結(jié)果也不能用常規(guī)來確定.在本案例中.我們就使用了全部的11個(gè)頻道.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.現(xiàn)在我們能提高OK的通訊區(qū)域從73.86%到81.89%.正如前面所述.系統(tǒng)結(jié)構(gòu)的良窳.是可以用數(shù)

56、字來證明和決定的.上圖7.讓我們更容易觀察.深灰色用藍(lán)色來取代.淺灰色用橘色來取代.圖8.9.是圖7.做部分地區(qū)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.我們選擇了一塊面積較小的區(qū)域來作為規(guī)劃的樣本.這個(gè)系統(tǒng)的分布情況我們用圖9來顯示.本區(qū)的面積大約為230公尺乘340公尺.有37個(gè)AP分布在此區(qū)域內(nèi).有一些Ap是重疊在一起.

58、但是它們的高度不同.因?yàn)檫@些AP被分別設(shè)在不同的樓層.所有的參數(shù)被設(shè)定和上一個(gè)例子相同.這兩個(gè)案例主要的不同在于案例(2)的面積比案例(1)小了許多.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.參與含蓋計(jì)算的AP都顯示在圖10里.我們可以從圖10中很容易的看出.如果正確的選擇AP的位置.則全區(qū)只須要兩個(gè)AP.就能達(dá)成百分之百的含蓋率.由于802.11的協(xié)議.Wi-Fi系統(tǒng)擁有3個(gè)沒有頻譜重疊的頻道.在只使用兩個(gè)AP的情況下.是不會(huì)產(chǎn)生同頻干擾的Figure 9 Example of 37 ApsFigure 10 Coverage of the 37-AP ExampleFigu