通信工程外文翻譯---蜂窩無(wú)線(xiàn)通信系統(tǒng)的仿真

1、<p>　　SIMULATION OF A CELLULAR RADIO SYSTEM</p><p>　　———taken from《Prentice Hall - Principles Of Communication Systems Simulation With Wireless Aplications》page672-676</p><p>　　1 . Introduc

2、tion</p><p>　　A wide variety of wireless communication systems have been developed to provide access to the communications infrastructure for mobile or fixed users in a myriad of operating environments. Most

3、 of today’s wireless systems are based on the cellular radio concept. Cellular communication systems allow a large number of mobile users to seamlessly and simultaneously communicate to wireless modems at fixed base stat

4、ions using a limited amount of radio frequency (RF) spectrum. The RF transmissions rece</p><p>　　Wireless communication links experience hostile physical channel characteristics, such as time-varying multipa

5、th and shadowing due to large objects in the propagation path. In addition, the performance of wireless cellular systems tends to be limited by interference from other users, and for that reason, it is important to have

6、accurate techniques for modeling interference. These complex channel conditions are difficult to describe with a simple analytical model, although several models do provi</p><p>　　Like wireless links, the sy

7、stem performance of a cellular radio system is most effectively modeled using simulation, due to the difficulty in modeling a large number of random events over time and space. These random events, such as the location o

8、f users, the number of simultaneous users in the system, the propagation conditions, interference and power level settings of each user, and the traffic demands of each user,combine together to impact the overall perform

9、ance seen by a typical user in th</p><p>　　The link performance is a small-scale phenomenon, which deals with the instantaneous changes in the channel over a small local area, or small time duration, over wh

10、ich the average received power is assumed constant . Such assumptions are sensible in the design of error control codes, equalizers, and other components that serve to mitigate the transient effects created by the channe

11、l. However, in order to determine the overall system performance of a large number of users spread over a wide geogr</p><p>　　Cellular systems achieve high capacity (e.g., serve a large number of users) by a

12、llowing the mobile stations to share, or reuse a communication channel in different regions of the geographic service area. Channel reuse leads to co-channel interference among users sharing the same channel, which is re

13、cognized as one of the major limiting factors of performance and capacity of a cellular system. An appropriate understanding of the effects of co-channel interference on the capacity and performance </p><p>

14、　　2 Cellular Radio System</p><p>　　System-Level Description：</p><p>　　Cellular systems provide wireless coverage over a geographic service area by dividing the geographic area into segments call

15、ed cells as shown in Figure 17.1. The available frequency spectrum is also divided into a number of channels with a group of channels assigned to each cell. Base stations located in each cell are equipped with wireless m

16、odems that can communicate with mobile users. Radio frequency channels used in the transmission direction from the base station to the mobile are referred t</p><p>　　High-capacity cellular systems employ fre

17、quency reuse among cells. This requires that co-channel cells (cells sharing the same frequency) are sufficiently far apart from each other to mitigate co-channel interference. Channel reuse is implemented by covering th

18、e geographic service area with clusters of N cells, as shown in Figure 17.2, where N is known as the cluster size.</p><p>　　The RF spectrum available for the geographic service area is assigned to each clust

19、er, such that cells within a cluster do not share any channel . If M channels make up the entire spectrum available for the service area, and if the distribution of users is uniform over the service area, then each cell

20、is assigned M/N channels. As the clusters are replicated over the service area, the reuse of channels leads to tiers of co-channel cells, and co-channel interference will result from the propagatio</p><p>&l

21、t;b>　?。?7.1）</b></p><p>　　where R is the maximum radius of the cell (the hexagon is inscribed within the radius). Therefore, we can immediately see from Figure 17.2 that a small cluster size (small

22、reuse distance ), leads to high interference among co-channel cells.</p><p>　　The level of co-channel interference received within a given cell is also dependent on the number of active co-channel cells at a

23、ny instant of time. As mentioned before, co-channel cells are grouped into tiers with respect to a particular cell of interest. The number of co-channel cells in a given tier depends on the tier order and the geometry ad

24、opted to represent the shape of a cell (e.g., the coverage area of an individual base station). For the classic hexagonal shape, the closest co-channel</p><p>　　Co-channel interference is recognized as one o

25、f the major factors that limits the capacity and link quality of a wireless communications system and plays an important role in the tradeoff between system capacity (large-scale system issue) and link quality (small-sca

26、le issue). For example, one approach for achieving high capacity (large number of users), without increasing the bandwidth of the RF spectrum allocated to the system, is to reduce the channel reuse distance by reducing t

27、he cluster siz</p><p>　　The level of interference within a cellular system at any time is random and must be simulated by modeling both the RF propagation environment between cells and the position location

28、of the mobile users. In addition, the traffic statistics of each user and the type of channel allocation scheme at the base stations determine the instantaneous interference level and the capacity of the system.</p>

29、;<p>　　The effects of co-channel interference can be estimated by the signal-tointerference ratio (SIR) of the communication link, defined as the ratio of the power of the desired signal S, to the power of the tot

30、al interference signal, I. Since both power levels S and I are random variables due to RF propagation effects, user mobility and traffic variation, the SIR is also a random variable. Consequently, the severity of the eff

31、ects of co-channel interference on system performance is frequently analyz</p><p><b>　?。?7.2）</b></p><p>　　Where is the probability density function (pdf) of the SIR. Note the di

32、stinction between the definition of a link outage probability, that classifies an outage based on a particular bit error rate (BER) or Eb/N0 threshold for acceptable voice performance, and the system outage probability t

33、hat considers a particular SIR threshold for acceptable mobile performance of a typical user.</p><p>　　Analytical approaches for estimating the outage probability in a cellular system, as discussed in Chapte

34、r 11, require tractable models for the RF propagation effects, user mobility, and traffic variation, in order to obtain an expression for . Unfortunately, it is very difficult to use analytical models for these effec

35、ts, due to their complex relationship to the received signal level. Therefore, the estimation of the outage probability in a cellular system usually relies on simulation, which</p><p>　　蜂窩無(wú)線(xiàn)通信系統(tǒng)的仿真</p>

36、<p>　　——摘自《通信系統(tǒng)仿真原理與無(wú)線(xiàn)應(yīng)用》第672頁(yè)-676頁(yè)</p><p><b>　　1 、概述</b></p><p>　　人們開(kāi)發(fā)出了許多無(wú)線(xiàn)通信系統(tǒng)，為不同的運(yùn)行環(huán)境中的固定用戶(hù)或移動(dòng)用戶(hù)提供了接入到通信基礎(chǔ)設(shè)施的手段。當(dāng)今大多數(shù)無(wú)線(xiàn)通信系統(tǒng)都是基于蜂窩無(wú)線(xiàn)電概念之上的。蜂窩通信系統(tǒng)允許大量移動(dòng)用戶(hù)無(wú)縫地、同時(shí)地利用有限的射頻（rad

37、io frequency，RF）頻譜與固定基站中的無(wú)線(xiàn)調(diào)制解調(diào)器通信。基站接收每一個(gè)移動(dòng)臺(tái)發(fā)送來(lái)的射頻信號(hào)，并把他們轉(zhuǎn)換到基帶或者帶寬微波鏈路，然后傳送到移動(dòng)交換中心（MSC），再由移動(dòng)交換中心連入公用交換電話(huà)網(wǎng)（PSTN）。同樣的，通信信號(hào)也可以從PSTN傳送到基站，再?gòu)倪@里發(fā)送個(gè)移動(dòng)臺(tái)。蜂窩系統(tǒng)可以采用頻分多址（FDMA）、時(shí)分多址（TDMA）、碼分多址（CDMA）或者空分多址（SDMA）中的任何一種技術(shù)。</p>&

38、lt;p>　　無(wú)線(xiàn)通信鏈路具有惡劣的物理信道特征，比如由于傳播途徑中有再大的障礙物，會(huì)產(chǎn)生時(shí)變多徑和陰影。此外，無(wú)線(xiàn)蜂窩系統(tǒng)的性能還會(huì)受限于來(lái)自其他用戶(hù)的干擾，因此，對(duì)干擾進(jìn)行準(zhǔn)確的建模就很重要。很難用簡(jiǎn)單的解析模型來(lái)描述復(fù)雜的信道條件，雖然有集中模型確實(shí)易于解析求解并與信道實(shí)測(cè)數(shù)據(jù)比較相符，不過(guò)，即使建立了完美的信道解析模型，再把差錯(cuò)控制編碼、均衡器、分集及網(wǎng)絡(luò)模型等因素都考慮再鏈路中之后，要得出鏈路性能的解析在絕大多數(shù)情況下任

39、然是很困難的甚至是不可能的。因此，在分析蜂窩通信鏈路的性能時(shí)，常常需要進(jìn)行仿真。</p><p>　　跟無(wú)線(xiàn)鏈路一樣，對(duì)蜂窩無(wú)線(xiàn)系統(tǒng)的性能分析使用仿真建模時(shí)很有效的，這是由于在時(shí)間和空間上對(duì)大量的隨機(jī)事件進(jìn)行建模非常困難。這些隨機(jī)事件包括用戶(hù)的位置、系統(tǒng)中同時(shí)通信的用戶(hù)個(gè)數(shù)、傳播條件、每個(gè)用戶(hù)的干擾和功率級(jí)的設(shè)置（power level setting）、每個(gè)用戶(hù)的話(huà)務(wù)量需求等，這些因素共同作用，對(duì)系統(tǒng)中的一個(gè)典

40、型用戶(hù)的總的性能產(chǎn)生影響。前面提到的變量?jī)H僅是任一時(shí)刻決定系統(tǒng)中的某個(gè)用戶(hù)瞬態(tài)性能的許多關(guān)鍵物理參數(shù)中的一小部分。蜂窩無(wú)線(xiàn)系統(tǒng)指的是，在地理上的服務(wù)區(qū)域內(nèi)，移動(dòng)用戶(hù)和基站的全體，而不是將一個(gè)用戶(hù)連接到一個(gè)基站的單個(gè)鏈路。為了設(shè)計(jì)特定大的系統(tǒng)級(jí)性能，比如某個(gè)用戶(hù)在整個(gè)系統(tǒng)中得到滿(mǎn)意服務(wù)的可能性，就得考慮在覆蓋區(qū)域內(nèi)同時(shí)使用系統(tǒng)的多個(gè)用戶(hù)所帶來(lái)的復(fù)雜性。因此，需要仿真來(lái)考慮多個(gè)用戶(hù)對(duì)基站和移動(dòng)臺(tái)之間任何一條鏈路所產(chǎn)生的影響。</p&g

41、t;<p>　　鏈路性能是一個(gè)小尺度現(xiàn)象，它處理的是小的局部區(qū)域內(nèi)或者短的時(shí)間間隔內(nèi)信道的順時(shí)變化，這種情況下可假設(shè)平均接收功率不變。在設(shè)計(jì)差錯(cuò)控制碼、均衡器和其他用來(lái)消除信道所產(chǎn)生的瞬時(shí)影響的部件時(shí)，這種假設(shè)時(shí)合理的。但是，在大量用戶(hù)分布在一個(gè)廣闊的地理范圍內(nèi)時(shí)，為了確定整個(gè)系統(tǒng)的性能，有必要引入大尺度效應(yīng)進(jìn)行分析，比如在大的距離范圍內(nèi)考慮單個(gè)用戶(hù)受到的干擾和信號(hào)電平的統(tǒng)計(jì)行為時(shí)，忽略瞬時(shí)信道特征。我們可以將鏈路級(jí)仿真看

42、作通信系統(tǒng)性能的微調(diào)，而將系統(tǒng)級(jí)仿真看作時(shí)整體質(zhì)量水平粗略但很重要的近似，任何用戶(hù)在任何時(shí)候都可預(yù)計(jì)達(dá)到這個(gè)水平。</p><p>　　通過(guò)讓移動(dòng)臺(tái)在不同的服務(wù)區(qū)內(nèi)共享或者復(fù)用通信信道，蜂窩系統(tǒng)能達(dá)到較高的容量（比如，為大量的用戶(hù)服務(wù)）。信道復(fù)用會(huì)導(dǎo)致公用同一信道的用戶(hù)之間產(chǎn)生同頻干擾，這是影響蜂窩系統(tǒng)容量和性能的主要制約因素之一。因此，在設(shè)計(jì)一個(gè)蜂窩系統(tǒng)時(shí)，或者在分析和設(shè)計(jì)消除同頻干擾負(fù)面影響的系統(tǒng)方法時(shí)，需要

43、正確理解同屏干擾對(duì)容量和性能的影響。這些影響主要取決于通信系統(tǒng)的狀況，如共享信道的用戶(hù)數(shù)和他們的位置。其他與傳播信道條件關(guān)系更密切的方面，如路徑損耗、陰影衰落（或叫陰影）、天線(xiàn)輻射模式等對(duì)系統(tǒng)性能的影響也很重要，因?yàn)檫@些影響也歲特定用戶(hù)的位置而改變。本章我們將討論在同頻干擾情況下，包括一個(gè)典型系統(tǒng)中的天線(xiàn)和傳播的影響。盡管本章考慮的例子比較簡(jiǎn)單，但提出的分析方法可以容易地進(jìn)行擴(kuò)展，以包括蜂窩系統(tǒng)的其他特征。</p><

44、;p><b>　　2、蜂窩無(wú)線(xiàn)系統(tǒng)</b></p><p><b>　　系統(tǒng)級(jí)描述：</b></p><p>　　如圖17-1所示，通過(guò)把地理區(qū)域分成一個(gè)個(gè)稱(chēng)為小區(qū)的部分，蜂窩系統(tǒng)可以在這個(gè)區(qū)域內(nèi)提供無(wú)線(xiàn)覆蓋。把可用的頻譜也分成很多信道，每個(gè)小區(qū)分配一組信道，每個(gè)小區(qū)中的基站都配備了可以同移動(dòng)用戶(hù)進(jìn)行通信的無(wú)線(xiàn)調(diào)制解調(diào)器。從基站到移動(dòng)臺(tái)這個(gè)

45、發(fā)送方向使用的射頻信道稱(chēng)為前向信道，而從移動(dòng)臺(tái)到</p><p>　　基站這個(gè)發(fā)送方向使用的信道稱(chēng)為反向信道。前向信道和反向信道共同構(gòu)成了雙工蜂窩信道。當(dāng)使用頻分雙工（FDD,frequency division duplex）時(shí)，前向信道和反向信道使用不同的頻率；當(dāng)使用時(shí)分雙工時(shí)（TDD,time division duplex）時(shí)，前向信道和反向信道占用相同的頻率，但使用不同的時(shí)隙進(jìn)行傳送。</p>

46、;<p>　　高容量的蜂窩系統(tǒng)在小區(qū)間進(jìn)行頻率復(fù)用，同頻小區(qū)（共用相同頻率的小區(qū)）之間要離開(kāi)足夠的距離以減輕同頻干擾。如圖17-2所示，N個(gè)小區(qū)構(gòu)成一個(gè)簇（cluster，又叫“區(qū)群”），覆蓋地理上的服務(wù)區(qū)，以實(shí)現(xiàn)信道復(fù)用，N是簇的大小。</p><p>　　把服務(wù)區(qū)內(nèi)可用的無(wú)線(xiàn)頻譜都分配給每一個(gè)簇，使同一個(gè)簇內(nèi)的小區(qū)不共用相同的信道。如果服務(wù)區(qū)內(nèi)的可用頻譜由M個(gè)信道構(gòu)成，用戶(hù)均勻分布在服務(wù)區(qū)內(nèi)，則

47、每個(gè)小區(qū)可以分得M/N個(gè)信道。因?yàn)榇卦诜?wù)區(qū)內(nèi)復(fù)制，復(fù)用信道將導(dǎo)致同頻小區(qū)的層狀結(jié)構(gòu)（tier）。同頻基站和移動(dòng)臺(tái)之間的射頻能量傳播，會(huì)引起同頻干擾。例如，如果一個(gè)移動(dòng)臺(tái)同時(shí)接收來(lái)自本地小區(qū)基站的信號(hào)和鄰近層的同頻小區(qū)基站產(chǎn)生的信號(hào)，就會(huì)產(chǎn)生同頻干擾。本例中，其中一個(gè)同頻前向鏈路信號(hào)（基站到移動(dòng)臺(tái)的傳輸）是我們的有用信號(hào)，移動(dòng)臺(tái)接收到的其他同頻信號(hào)就構(gòu)成了對(duì)接機(jī)的同頻干擾，同頻干擾的功率級(jí)與同頻小區(qū)之間的分隔距離密切相關(guān)。如果小區(qū)建模為

48、如圖17-2所示的六邊形。兩個(gè)同頻小區(qū)中心之間的最小距離（叫做復(fù)用距離）等于</p><p>　?。?7.1）式中R式小區(qū)的最大半徑（這個(gè)六邊形內(nèi)接在半徑為R的圓中）。因此，我們馬上可以從圖17-2看出，小簇（小復(fù)用距離）會(huì)引起同頻小區(qū)間的大干擾。</p><p>　　在一個(gè)指定小區(qū)中接收到的同頻干擾的電平，還取決于任一時(shí)刻活躍的同頻小區(qū)的數(shù)量。如前所述，在我們感興趣的那個(gè)特定小區(qū)周?chē)?，?/p>

49、頻小區(qū)組成一個(gè)個(gè)的層。在一個(gè)給定層中，同頻小區(qū)的數(shù)量取決于層的階次和用來(lái)表示小區(qū)的幾何形狀（如一個(gè)基站覆蓋的面積）。對(duì)于典型的六邊形，最近的同頻小區(qū)在第一層，有六個(gè)同頻小區(qū)，第二層有12個(gè)，第三層有18個(gè)，以此類(lèi)推。因此，總的同頻干擾時(shí)從所有層的全部同頻小區(qū)發(fā)送出的同頻干擾信號(hào)的總和。但是第一層的同頻小區(qū)對(duì)總的干擾時(shí)從所有層的全部同頻小區(qū)發(fā)送出的同頻干擾信號(hào)的總和。但是第一層的同頻小區(qū)對(duì)總的干擾有較強(qiáng)的影響，因?yàn)樗鼈兏拷鼫y(cè)量干擾的小區(qū)

50、。</p><p>　　人們認(rèn)識(shí)到同頻干擾時(shí)制約無(wú)線(xiàn)通信系統(tǒng)的容量和鏈路質(zhì)量的主要因素之一。在系統(tǒng)容量（大尺度系統(tǒng)問(wèn)題）和鏈路質(zhì)量（小尺度系統(tǒng)問(wèn)題）之間作折中時(shí)，它起到舉足輕重的作用。例如，在不增加分配給系統(tǒng)的無(wú)線(xiàn)頻譜帶寬的前提下，得到高容量（大量的用戶(hù)）的一種措施是，通過(guò)減小蜂窩系統(tǒng)簇的大小N，來(lái)縮短信道復(fù)用距離。然而，減少簇大小又增加了同頻干擾，這會(huì)降低鏈路質(zhì)量。</p><p>　　

51、蜂窩系統(tǒng)中的干擾電平在任何時(shí)候都是隨機(jī)的，必須通過(guò)對(duì)蜂窩之間的射頻傳播環(huán)境和移動(dòng)用戶(hù)的位置進(jìn)行建模才能仿真。另外，每個(gè)用戶(hù)話(huà)務(wù)量的統(tǒng)計(jì)特性以及基站中信道分配方案的類(lèi)型決定了瞬時(shí)干擾電平和系統(tǒng)的容量。</p><p>　　同頻干擾的影響可以用通信鏈路的信干比（SIR）來(lái)估計(jì)，這里信干比定義為有用信號(hào)的功率S與總干擾信號(hào)的功率I之比。由于無(wú)線(xiàn)傳播影響，用戶(hù)移動(dòng)性以及話(huà)務(wù)量的變化，功率級(jí)S和I都是隨機(jī)變量，SIR也是一

52、個(gè)隨機(jī)變量。因此，同頻干擾對(duì)系統(tǒng)性能產(chǎn)生影響的嚴(yán)重程度，通常用系統(tǒng)的中斷概率來(lái)進(jìn)行分析。在這個(gè)特定場(chǎng)合下，中斷概率定義為SIR低于給定閾值的概率，即</p><p><b>　　（17.2）</b></p><p>　　其中 是SIR的概率密度函數(shù)。要注意鏈路中斷概率和系統(tǒng)中斷概率之間的區(qū)別，前者是根據(jù)可接受的聲音性能所需的特定誤比特率（BER）或者Eb/

53、N0閾值，確定是否為中斷，而后者考慮的是一個(gè)典型用戶(hù)可接受的移動(dòng)性能所需的SIR閾值。</p><p>　　如第11章所述，用來(lái)估計(jì)蜂窩系統(tǒng)中斷概率的解析方法，需要已知射頻傳播影響、用戶(hù)移動(dòng)性和話(huà)務(wù)量變化等隨機(jī)量的易于處理的模型，以求得 的解析表達(dá)式。然而，由于這些影響和接受信號(hào)電平間的復(fù)雜關(guān)系，很難對(duì)這些影響采用解析模型。因此，主要靠仿真來(lái)估計(jì)蜂窩系統(tǒng)的中斷概率，仿真還為分析提供了靈活性。本章我們