通信工程外文翻譯---蜂窩無線通信系統(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>　　（17.1）</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>　　（17.2）</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>　　蜂窩無線通信系統(tǒng)的仿真</p>

36、<p>　　——摘自《通信系統(tǒng)仿真原理與無線應(yīng)用》第672頁-676頁</p><p><b>　　1 、概述</b></p><p>　　人們開發(fā)出了許多無線通信系統(tǒng)，為不同的運行環(huán)境中的固定用戶或移動用戶提供了接入到通信基礎(chǔ)設(shè)施的手段。當今大多數(shù)無線通信系統(tǒng)都是基于蜂窩無線電概念之上的。蜂窩通信系統(tǒng)允許大量移動用戶無縫地、同時地利用有限的射頻（rad

37、io frequency，RF）頻譜與固定基站中的無線調(diào)制解調(diào)器通信。基站接收每一個移動臺發(fā)送來的射頻信號，并把他們轉(zhuǎn)換到基帶或者帶寬微波鏈路，然后傳送到移動交換中心（MSC），再由移動交換中心連入公用交換電話網(wǎng)（PSTN）。同樣的，通信信號也可以從PSTN傳送到基站，再從這里發(fā)送個移動臺。蜂窩系統(tǒng)可以采用頻分多址（FDMA）、時分多址（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>　　無線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，會產(chǎn)生時變多徑和陰影。此外，無線蜂窩系統(tǒng)的性能還會受限于來自其他用戶的干擾，因此，對干擾進行準確的建模就很重要。很難用簡單的解析模型來描述復(fù)雜的信道條件，雖然有集中模型確實易于解析求解并與信道實測數(shù)據(jù)比較相符，不過，即使建立了完美的信道解析模型，再把差錯控制編碼、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，常常需要進行仿真。</p><p>　　跟無線鏈路一樣，對蜂窩無線系統(tǒng)的性能分析使用仿真建模時很有效的，這是由于在時間和空間上對大量的隨機事件進行建模非常困難。這些隨機事件包括用戶的位置、系統(tǒng)中同時通信的用戶個數(shù)、傳播條件、每個用戶的干擾和功率級的設(shè)置（power level setting）、每個用戶的話務(wù)量需求等，這些因素共同作用，對系統(tǒng)中的一個典

40、型用戶的總的性能產(chǎn)生影響。前面提到的變量僅僅是任一時刻決定系統(tǒng)中的某個用戶瞬態(tài)性能的許多關(guān)鍵物理參數(shù)中的一小部分。蜂窩無線系統(tǒng)指的是，在地理上的服務(wù)區(qū)域內(nèi)，移動用戶和基站的全體，而不是將一個用戶連接到一個基站的單個鏈路。為了設(shè)計特定大的系統(tǒng)級性能，比如某個用戶在整個系統(tǒng)中得到滿意服務(wù)的可能性，就得考慮在覆蓋區(qū)域內(nèi)同時使用系統(tǒng)的多個用戶所帶來的復(fù)雜性。因此，需要仿真來考慮多個用戶對基站和移動臺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)或者短的時間間隔內(nèi)信道的順時變化，這種情況下可假設(shè)平均接收功率不變。在設(shè)計差錯控制碼、均衡器和其他用來消除信道所產(chǎn)生的瞬時影響的部件時，這種假設(shè)時合理的。但是，在大量用戶分布在一個廣闊的地理范圍內(nèi)時，為了確定整個系統(tǒng)的性能，有必要引入大尺度效應(yīng)進行分析，比如在大的距離范圍內(nèi)考慮單個用戶受到的干擾和信號電平的統(tǒng)計行為時，忽略瞬時信道特征。我們可以將鏈路級仿真看

42、作通信系統(tǒng)性能的微調(diào)，而將系統(tǒng)級仿真看作時整體質(zhì)量水平粗略但很重要的近似，任何用戶在任何時候都可預(yù)計達到這個水平。</p><p>　　通過讓移動臺在不同的服務(wù)區(qū)內(nèi)共享或者復(fù)用通信信道，蜂窩系統(tǒng)能達到較高的容量（比如，為大量的用戶服務(wù)）。信道復(fù)用會導(dǎo)致公用同一信道的用戶之間產(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)計一個蜂窩系統(tǒng)時，或者在分析和設(shè)計消除同頻干擾負面影響的系統(tǒng)方法時，需要

43、正確理解同屏干擾對容量和性能的影響。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）、天線輻射模式等對系統(tǒng)性能的影響也很重要，因為這些影響也歲特定用戶的位置而改變。本章我們將討論在同頻干擾情況下，包括一個典型系統(tǒng)中的天線和傳播的影響。盡管本章考慮的例子比較簡單，但提出的分析方法可以容易地進行擴展，以包括蜂窩系統(tǒng)的其他特征。</p><

44、;p><b>　　2、蜂窩無線系統(tǒng)</b></p><p><b>　　系統(tǒng)級描述：</b></p><p>　　如圖17-1所示，通過把地理區(qū)域分成一個個稱為小區(qū)的部分，蜂窩系統(tǒng)可以在這個區(qū)域內(nèi)提供無線覆蓋。把可用的頻譜也分成很多信道，每個小區(qū)分配一組信道，每個小區(qū)中的基站都配備了可以同移動用戶進行通信的無線調(diào)制解調(diào)器。從基站到移動臺這個

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

46、;<p>　　高容量的蜂窩系統(tǒng)在小區(qū)間進行頻率復(fù)用，同頻小區(qū)（共用相同頻率的小區(qū)）之間要離開足夠的距離以減輕同頻干擾。如圖17-2所示，N個小區(qū)構(gòu)成一個簇（cluster，又叫“區(qū)群”），覆蓋地理上的服務(wù)區(qū)，以實現(xiàn)信道復(fù)用，N是簇的大小。</p><p>　　把服務(wù)區(qū)內(nèi)可用的無線頻譜都分配給每一個簇，使同一個簇內(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個信道構(gòu)成，用戶均勻分布在服務(wù)區(qū)內(nèi)，則

47、每個小區(qū)可以分得M/N個信道。因為簇在服務(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）。同頻基站和移動臺之間的射頻能量傳播，會引起同頻干擾。例如，如果一個移動臺同時接收來自本地小區(qū)基站的信號和鄰近層的同頻小區(qū)基站產(chǎn)生的信號，就會產(chǎn)生同頻干擾。本例中，其中一個同頻前向鏈路信號（基站到移動臺的傳輸）是我們的有用信號，移動臺接收到的其他同頻信號就構(gòu)成了對接機的同頻干擾，同頻干擾的功率級與同頻小區(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>　　（17.1）式中R式小區(qū)的最大半徑（這個六邊形內(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）會引起同頻小區(qū)間的大干擾。</p><p>　　在一個指定小區(qū)中接收到的同頻干擾的電平，還取決于任一時刻活躍的同頻小區(qū)的數(shù)量。如前所述，在我們感興趣的那個特定小區(qū)周圍，同

49、頻小區(qū)組成一個個的層。在一個給定層中，同頻小區(qū)的數(shù)量取決于層的階次和用來表示小區(qū)的幾何形狀（如一個基站覆蓋的面積）。對于典型的六邊形，最近的同頻小區(qū)在第一層，有六個同頻小區(qū)，第二層有12個，第三層有18個，以此類推。因此，總的同頻干擾時從所有層的全部同頻小區(qū)發(fā)送出的同頻干擾信號的總和。但是第一層的同頻小區(qū)對總的干擾時從所有層的全部同頻小區(qū)發(fā)送出的同頻干擾信號的總和。但是第一層的同頻小區(qū)對總的干擾有較強的影響，因為它們更靠近測量干擾的小區(qū)

50、。</p><p>　　人們認識到同頻干擾時制約無線通信系統(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)問題）和鏈路質(zhì)量（小尺度系統(tǒng)問題）之間作折中時，它起到舉足輕重的作用。例如，在不增加分配給系統(tǒng)的無線頻譜帶寬的前提下，得到高容量（大量的用戶）的一種措施是，通過減小蜂窩系統(tǒng)簇的大小N，來縮短信道復(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候都是隨機的，必須通過對蜂窩之間的射頻傳播環(huán)境和移動用戶的位置進行建模才能仿真。另外，每個用戶話務(wù)量的統(tǒng)計特性以及基站中信道分配方案的類型決定了瞬時干擾電平和系統(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）來估計，這里信干比定義為有用信號的功率S與總干擾信號的功率I之比。由于無線傳播影響，用戶移動性以及話務(wù)量的變化，功率級S和I都是隨機變量，SIR也是一

52、個隨機變量。因此，同頻干擾對系統(tǒng)性能產(chǎn)生影響的嚴重程度，通常用系統(tǒng)的中斷概率來進行分析。在這個特定場合下，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典型用戶可接受的移動性能所需的SIR閾值。</p><p>　　如第11章所述，用來估計蜂窩系統(tǒng)中斷概率的解析方法，需要已知射頻傳播影響、用戶移動性和話務(wù)量變化等隨機量的易于處理的模型，以求得 的解析表達式。然而，由于這些影響和接受信號電平間的復(fù)雜關(guān)系，很難對這些影響采用解析模型。因此，主要靠仿真來估計蜂窩系統(tǒng)的中斷概率，仿真還為分析提供了靈活性。本章我們