劉中建外文翻譯

1、23 黃河科技學(xué)院畢業(yè)設(shè)計(jì)（文獻(xiàn)翻譯）數(shù)字通信1 引 言在本書中,我們將介紹作為數(shù)字通信系統(tǒng)分析和設(shè)計(jì)基礎(chǔ)的基本原理。數(shù)字通信的研究主題包括數(shù)字形式的信息從產(chǎn)生該信息的信源到一個或多個目的地的傳輸問題。在通信系統(tǒng)的分析和設(shè)計(jì)中，特別重要的是信息傳輸所通過的物理信道的特征。信道的特征-般會影響通信系統(tǒng)基本組成部分的設(shè)計(jì)。下面闡述一個通信系統(tǒng)的基本組成部分及其功能。1.1數(shù)字通信系統(tǒng)的基本組成部分圖1.1 顯示了一個數(shù)字通信系統(tǒng)的功能性框圖和基本組成部分。輸出的可以是模擬信號，如音頻或視頻信號;也可以是數(shù)字信號，如電傳機(jī)的輸出，該信號在時間上是離散的，并且只有有限個輸出字符。在數(shù)字通信系統(tǒng)中，由信

2、源產(chǎn)生的消息變換成二進(jìn)制數(shù)字序列。理論上，應(yīng)當(dāng)用盡可能少的二進(jìn)制數(shù)字表示信源輸出（消息）。換句話說.我們要尋求一種信源輸出的有效的表示方法,使其很少產(chǎn)生或不產(chǎn)生冗余。將模擬或數(shù)宇信源的輸出有效地變換成二進(jìn)制數(shù)字序列的處理過程稱為信源編碼或數(shù)據(jù)壓縮。由信源編碼器輸出的二進(jìn)制數(shù)字序列稱為信息序列，它被傳送到信道編碼器。信道編碼器的目的是在二進(jìn)制信息序列中以受控的方式引人一些冗余，以便于在接收機(jī)中用來克服信號在信道中傳輸時所遭受的噪聲和干擾的影響。因此，所增加的冗余是用來提高接收數(shù)據(jù)的可靠性以及改善接收信號的逼真度的。實(shí)際上，信息序列中的冗余有助于接收機(jī)譯出期望的信息序列。例如,二進(jìn)制信息序列的一種

3、(平凡的）形式的編碼就是將每個二進(jìn)制數(shù)字簡單重復(fù)m次.這里m為一個正整數(shù)。更復(fù)雜的（不平凡的）編碼涉及到一次取k個信息比特,并將毎個k比特序列映射成惟一的n比特序列，該序列稱為碼字。以這種方式對數(shù)據(jù)編碼所引人的冗余度的大小是由比率n/k作來度擻的。該比率的倒數(shù)，即k/n，稱為碼的速率或簡稱碼率。信道編碼器輸出的二進(jìn)制序列送至數(shù)宇調(diào)制器，它是通信信道的接口。因?yàn)樵趯?shí)際中遇到的幾乎所有的通信信道都能夠傳輸電信號（波形）,所以數(shù)字調(diào)制的主要目的是將二進(jìn)制信息序列映射成信號波形。為了詳細(xì)說明這一點(diǎn)，假定已編碼的信息序列以均勻速率R（b/s）次一個比特傳輸,數(shù)字調(diào)制器可以簡單地將二進(jìn)制數(shù)字“0”映射成波

4、形s0(t)而二進(jìn)制數(shù)字“1”映射成波形s1(t)。在這種方式中，信道編碼器輸出的毎一比特是分別傳輸?shù)摹Ｎ覀儼阉Q為二進(jìn)制調(diào)制。另一種方式，調(diào)制器目一次傳輸b個已編碼的信息比特，其方法是采用M = 2s個不同的波形ST(t)i=1,2，M,每一個波形用來傳輸2s個可能的b比特序列中的一個序列。我們稱這種方式為M元調(diào)制（M2）。注意，每b/R秒就有一個新的b比特序列進(jìn)入調(diào)制器。因此，當(dāng)信道比特率R固定,與一個b比特序列相應(yīng)的似個波形之一的傳輸時間量是二進(jìn)制調(diào)制系統(tǒng)時間周期的b倍。圖1.1 數(shù)字通信系統(tǒng)的基本模型通信信道是用來將發(fā)送機(jī)的信號發(fā)送給接收機(jī)的物理媒質(zhì)。在無線傳輸中，信道可以是大氣（自由

5、空間）另一方面，電話信道通常使用各種各樣的物理媒質(zhì)，包括有線線路、光纜和無線(微波)等。無論用什么物理媒質(zhì)來傳輸信息，其基本特點(diǎn)是發(fā)送信號隨機(jī)地受到各種可能機(jī)理的惡化，例如由電子器件產(chǎn)生的加性熱噪聲、人為噪聲（如汽車點(diǎn)火噪聲）及大氣噪聲（如在雷賽雨時的閃電）。在數(shù)字逋信系統(tǒng)的接收端,數(shù)字解調(diào)器對受到信道惡化的發(fā)送波形進(jìn)行處理，并將該波形還原成一個數(shù)的序列，該序列表示發(fā)送數(shù)據(jù)符號的估計(jì)值二進(jìn)制或M元。這個數(shù)的序列披送至信道譯碼器，它根據(jù)信進(jìn)編碼器所用的關(guān)于碼的知識及接收數(shù)據(jù)所含的冗余度重構(gòu)初始的信息序列。解調(diào)器和譯碼器工作性能好壞的個度量是譯碼序列中發(fā)生差錯的頻度。更準(zhǔn)確地說，在譯碼器輸出端的平

6、均比特錯誤概率是解調(diào)器-譯碼器組合性能的一個度量。一般地，錯誤概率是下列各種因素的函數(shù):碼特征、用來在信道上傳輸信息的波形的類型、發(fā)送功率信道的特征（即噪聲的大小、干擾的性質(zhì)等)以及解調(diào)和譯碼的方法。在后續(xù)各章中將詳細(xì)討論這些因素及其對性能的影晌。作為最后一步，當(dāng)需要模擬輸出時，信源譯碼器從信道譯碼器接收其輸出序列并根據(jù)所采用的信源編碼方法的有關(guān)知識重構(gòu)由信源發(fā)出的原始信號。由于信道譯碼的差錯以及信源編碼器可能引入的失真,在信源譯碼器輸出端的信號只是原始信源輸出的個近似。在原始信號與重構(gòu)信號之間的信號差或信號差的函數(shù)是數(shù)字通信系統(tǒng)引入失真的一種度量。 2 通信信道及其特征正如前面指出的，通信信

7、道在發(fā)送機(jī)與接收機(jī)之間提供了連接。物理信道也許是攜帶電信號的一對明線；或是在已調(diào)光波束上攜帶信息的光纖；或是水下海洋信道其中信息以聲波形式傳輸；或是自由空間，攜帶信息的信號通過天線在空間輻射傳輸?？杀槐碚鳛橥ㄐ判诺赖钠渌劫|(zhì)是數(shù)據(jù)存儲媒質(zhì)如磁帶、磁盤和光盤。在信號通過任何信道傳輸中的一個共同的問題是加性噪聲。一般地，加性噪聲是由通信系統(tǒng)內(nèi)部組成元器件所引起的，例如電阻和固態(tài)器件。有時將這種噪聲稱為熱噪聲。其他噪聲和干擾源也許是系統(tǒng)外面引起的，例如來自信道上其他用戶的干擾。當(dāng)這樣的噪聲和干擾與期望信號占有同頻帶時，可通過對發(fā)送信號和接收機(jī)中解調(diào)器的適當(dāng)設(shè)計(jì)來使它們的影響最小。信號在信道上傳輸時可

8、能會遇到的其他類型損傷有信號衰減、幅度和相位失真、多徑失真等。可以通過增加發(fā)送信號功率的方法使噪聲的影響最小。然而，設(shè)備和其他實(shí)際因素限制了發(fā)送信號的功率電平，另一個基本的限制是可用的信道帶寬。帶寬的限制通常是由于媒質(zhì)以及發(fā)送機(jī)和接牧機(jī)中組成器件和部件的物理限制產(chǎn)生的。這兩種限制因素限制了在任何通信信道上能可靠傳輸?shù)臄?shù)據(jù)量,我們將在以后各章中討論這種情況。下面描述幾種通信信道的重要特征。2.1 有線信道電話網(wǎng)絡(luò)擴(kuò)大了有線線路的應(yīng)用，如話音信號傳輸以及數(shù)據(jù)和視頻傳輸。雙絞線和同軸電纜是基本的導(dǎo)向電磁信道，它能提供比較適度的帶寬。通常用來連接用戶和中心機(jī)房的電話線的帶寬為幾百千赫(khz)另一方面

9、同軸電纜的可用寬帶是幾兆赫（Mhz）。信號在這樣的信道上傳輸時，其幅度和相位都會發(fā)生失真，還受到加性噪聲的惡化。雙絞線信道還易受到來自物理鄰近信道的串音干擾。因?yàn)樵谌珖腿澜缬芯€信道上通信在日常通信中占有相當(dāng)大的比例，因此,人們對傳輸特性的表征以及對信號傳輸時的幅度和相位失真的減緩方法作了大量研究。在第9章中，我們將闡述最佳傳輸信號及其解調(diào)的設(shè)什方法。在笫10章和第11章中，我們將研究信道均衡器的設(shè)計(jì),它是用來補(bǔ)償信道的幅度和相位失真的。2.2 光纖信道光纖提供的信道帶寬比同軸電纜信道大幾個數(shù)量級。在過去的20年中，已經(jīng)研發(fā)出具有較低倌號衰減的光纜，以及用于信號和信號檢測的可靠性光子器件。這

10、些技術(shù)上的進(jìn)展導(dǎo)致了光纖信道應(yīng)用的快速發(fā)展，不僅應(yīng)用在國內(nèi)通信系統(tǒng)中，也應(yīng)用于跨大西洋和跨太平洋的通信中。由于光纖信道具有大的可用帶寬，因此有可能使電話公司為用戶提供寬系列電店業(yè)務(wù)，包括話音、數(shù)據(jù)、傳真和視頻等。在光纖通信系統(tǒng)中，發(fā)送機(jī)或調(diào)制器是一個光源.或者是發(fā)光二極管（LED）或者是激光。通過消息信號改變（調(diào)制）光源的強(qiáng)度來發(fā)送信息。光像光波一樣通過光纖傳播，并沿著傳輸路徑被周期性地放大以補(bǔ)償信號衰減（在數(shù)宇傳輸中，光由中繼器檢測和再生）。在接收機(jī)中，光的強(qiáng)度由光電二極管檢測，它的輸出電信號的變化直接與照射到光電二極管上的光的功率成正比。光纖信道中的噪聲源是光電二極管和電子放大器。2.3

11、無線電磁信道 在無線通信系統(tǒng)中，電磁能是通過作為輻射器的天線耦合到傳播媒質(zhì)的。天線的物理尺寸和配置主要決定于運(yùn)行的頻率。為了獲得有效的電磁能量的輻射，天線必須比波長的1/10更長。因此,在調(diào)幅（AM）頻段發(fā)射的無線電臺，譬如說在f=1MHz時（相當(dāng)于波長= C/f=300m）要求天線至少為30m。無線傳輸天線的其他重要特征和屬性將在第5章闡述。在大氣和自由空間中，電磁波傳播的模式可以劃分為3種類型，即地波傳播、天波傳播和視線傳播。在甚低頻（VLF）和音頻段，其波長超過10km，地球和電離層對電磁波傳播的作用如同波導(dǎo)。在這些頻段,通信信號實(shí)際上環(huán)繞地球傳播，由于這個原因，這些頻段主要用來在世界范

12、圍內(nèi)提供從海洋到船舶的導(dǎo)航幫助。在此頻段中可用的帶寬較小（通常是中心頻率的1% 10%）因此通過這些信道傳輸?shù)男畔⑺俾瘦^低，且一般限于數(shù)字傳輸。在這些頻率上，最主要的一種噪聲是由地球上的雷暴活動產(chǎn)生的，特別是在熱帶地區(qū)。干擾來自這些頻段上的用戶。在高頻（HF）頻段范圍內(nèi)，電磁波經(jīng)由天波傳播時經(jīng)常發(fā)生的問題是信號多徑。信號多徑發(fā)生在發(fā)送信號經(jīng)由多條傳播路徑以不同的延遲到達(dá)接收機(jī)的時侯，一般會引起數(shù)字通信系統(tǒng)中的符號間干擾。而且經(jīng)由不同傳播路徑到達(dá)的各信號分量會相互削弱，導(dǎo)致信號衰落的現(xiàn)象.許多人在夜晚收聽遠(yuǎn)地?zé)o線電臺廣播時會對此有體驗(yàn)。在夜晚，天波是主要的傳播模式。HF頻段的加性噪聲是大氣噪聲和

13、熱噪聲的組合。在大約30MHZ之上的頻率,即頻段的邊緣，就不存在天波電離層傳播。然而，在3060MHZ頻段有可能進(jìn)行電離層散射傳播，這是由較低電離層的信號散射引起的。也可利用在40300MHZ頻率范圍內(nèi)的對流層散射在幾百英里的距離通信。對流層散射是由在10mile或更低高度大氣層中的粒子引起的信號散射造成的，一般地，電離層散射和對流層散射具有大的信號傳播損耗，要求發(fā)射機(jī)功率大和天線比較長。在30MHZ以上頻率通過電離層傳播具有較小的損耗,這使得衛(wèi)星和超陸地通信成為可能。因此,在甚高頻(VHF)頻段和更高的頻率，電磁傳播的最主要模式是LOS傳播。對于陸地通信系統(tǒng)這意味著發(fā)送機(jī)和接收機(jī)的天線必須是

14、直達(dá)LOS,沒有什么障礙。由于這個原因VHF和特高頻(UHF)頻段發(fā)射的電視臺的天線安裝在髙塔上,以達(dá)到更寬的覆蓋區(qū)域。一般地LOS傳播所能覆蓋的區(qū)域受到地球曲度的限制。如果發(fā)射天線安裝在地表面之上H米的高度,并假定沒有物理障礙(如山)那么到無線地平線的距離近似為d=15H KM，例如電視天線安裝在300m高的塔上.它的覆蓋范圍大約67km另一個例子，工作在1GHZ以上頻率，用來延伸電話和視頻傳輸?shù)奈⒉ㄖ欣^系統(tǒng)將天線安裝在離塔上或高的建筑物頂部。對工作在VHF和UHF頻率范圍的通信系統(tǒng)限制性能的最主要噪聲是接收機(jī)前端所產(chǎn)生的熱噪聲和天線接收到的宇宙噪聲。在10GHZ以上的超髙頻（SHF)頻段，

15、大氣層環(huán)境在信號傳播中擔(dān)負(fù)主要角色。例如,在10GHZ頻率，衰減范圍從小雨時的0.003 dB/KM左右到大雨時的0.3dB/KM；在100GHZ,衰減范圍從小雨時的0.1dB左右到大雨時的6dB左右。因此，在此頻率范圍,大雨引起了很大的傳播損耗，這會導(dǎo)致業(yè)務(wù)中斷(通信系統(tǒng)完全中斷)。在極高頻(EHF)頻段以上的頻率是電磁頻譜的紅外區(qū)和可見光區(qū)，它們可用來提供自由空間的LOS光通信。到目前為止,這些頻段已經(jīng)用于實(shí)驗(yàn)通信系統(tǒng),例如，衛(wèi)星到衛(wèi)星的通信鏈路。2.4 水聲信道 在過去的幾十年中.海洋探險活動不斷增多。與這種增多相關(guān)的是對傳輸數(shù)據(jù)的需求。數(shù)據(jù)是由位于水下的傳感器傳送到海洋表面的,從那里可

16、能將數(shù)據(jù)經(jīng)由衛(wèi)星轉(zhuǎn)發(fā)給數(shù)據(jù)采集中心。除極低頻率外，電磁波在水下不能長距離傳播。在低頻率的信號傳輸?shù)难由焓艿较拗?，因?yàn)樗枰蟮那夜β蕪?qiáng)的發(fā)送機(jī)。電磁波在水下的衰減可以用表面深度來表示,它是信號衰減l/e的距離。對于海水,表面深度 250/f，其中f以HZ為單位。例如，在10 khz上,表面深度是2.5m。聲信號能在幾十甚至幾百千米距離上傳播。水聲信道可以表征為多徑信道,這是由于海洋表面和底部對信號反射的緣故。因?yàn)椴ǖ倪\(yùn)動,信號多徑分量的傳播延遲是時變的，這就導(dǎo)致了信號的衰落。此外,還存在與頻率相關(guān)的衰減，它與信號頻率的平方近似成正比。聲音速度通常大約為1 500m/s，實(shí)際值將在正常值上下變化

17、,這取決于信號傳播的深度。海洋背景噪聲是由蝦、魚和各種哺乳動物引起的。在靠近港口處，除了海洋背景噪聲外也有人為噪聲。盡管有這些不利的環(huán)境，還是可能設(shè)計(jì)并實(shí)現(xiàn)有效的且高可靠性的水聲通信系統(tǒng),以長距離地傳輸數(shù)字信號。2.5 存儲信道信息存儲和恢復(fù)系統(tǒng)構(gòu)成了日常數(shù)據(jù)處理工作的非常重要的部分。磁帶（包括數(shù)字的聲帶和錄像帶）、用來存儲大量計(jì)箅機(jī)數(shù)據(jù)的磁盤、用作計(jì)箅機(jī)數(shù)據(jù)存儲器的光盤以及只讀光盤都是數(shù)據(jù)存儲系統(tǒng)的例子,它們可以表征為通信信道。在磁帶或磁盤或光盤上存儲數(shù)據(jù)的過程，等效于在電話或在無線信道上發(fā)送數(shù)據(jù)?；刈x過程以及在存儲系統(tǒng)中恢復(fù)所存儲的數(shù)據(jù)的信號處理等效于在電話和無線通信系統(tǒng)中恢復(fù)發(fā)送信號。由

18、電子元器件產(chǎn)生的加性噪聲和來自鄰近軌道的干擾一般會呈現(xiàn)在存儲系統(tǒng)的回讀信號中，這正如電話或無線通信系統(tǒng)中的情況。所能存儲的數(shù)據(jù)量一般受到磁盤或磁帶尺寸及密度（每平方英寸存儲的比特數(shù)）的限制，該密度是由寫/讀電系統(tǒng)和讀寫頭確定的。例如在磁盤存儲系統(tǒng)中，封裝密度可達(dá)每平方英寸比特(1 in=2.54cm)。磁盤或磁帶上的數(shù)據(jù)的讀寫速度也受到組成信息存儲系統(tǒng)的機(jī)械和電子子系統(tǒng)的限制。信道編碼和調(diào)制是良好設(shè)計(jì)的數(shù)字磁或存儲系統(tǒng)的最重要的組成部分。在回讀過程中，信號被解調(diào)。由信道編碼器引入的附加冗余度用于糾正回讀信號中的差錯。3 通信信道的數(shù)學(xué)模型在通過物理信道傳輸信息的通信系統(tǒng)設(shè)計(jì)中，我們發(fā)現(xiàn)，建立一

19、個能反映傳輸媒質(zhì)最重要特征的數(shù)學(xué)模型是很方便的。信道的數(shù)學(xué)模型可以用于發(fā)送機(jī)中的信道編碼器和調(diào)制器，以及接收機(jī)中的解調(diào)器和信道譯碼器的設(shè)計(jì)。下面，我們將簡要的描述信道的模型，它們常用來表征實(shí)際的物理信道。3.1 加性噪聲信道通信信道最簡單的數(shù)學(xué)模型是加性噪聲信道，如圖1.3所示。在這個模型中，發(fā)送信號s（t）被加性隨機(jī)噪聲過程n（t）惡化。在物理上，加性噪聲過程由通信系統(tǒng)接收機(jī)中的電子元部件和放大器引起，或者由傳輸中的干擾引起（正如在無線電信號傳輸中那樣）。如果噪聲主要是由接收機(jī)中的元部件和放大器引起，那么，它可以表征為熱噪聲。這種模型的噪聲統(tǒng)計(jì)地表征為高斯噪聲過程。因此，該信道的數(shù)學(xué)模型通常

20、稱為加性高斯噪聲信道。因?yàn)檫@個信道模型適用于很廣的物理通信信道，并且因?yàn)樗跀?shù)學(xué)上易于處理，所以是在通信系統(tǒng)分析和設(shè)計(jì)中所用的最主要的信道模型。信道的衰減很容易加入到該模型。信號通過信道傳輸而受到衰減時，接收信號是 式中，是衰減因子。圖1.2 加性噪聲信道3.2 線性濾波器信道在某些物理信道中，例如有線電話信道，采用濾波器來保證傳輸信號不超過規(guī)定的帶寬限制，從而不會引起相互干擾。這樣的信道通常在數(shù)學(xué)上表征為帶有加性噪聲的線性濾波器，如圖1.3所示。因此，如果信道輸入信號為s（t），那么信道輸出信號是 式中，是信道的沖激響應(yīng)，表示卷積。 圖1.3 帶有加性噪聲的線性濾波器信道3.3 線性時變?yōu)V波

21、器信道像水聲信道和電離層無線電信道這樣的物理信道，它們會導(dǎo)致發(fā)送信號的時變多徑傳播，這類物理信道在教學(xué)上可以表征為時變線性濾波器。該線性濾波器可以表征為時變信道沖激響應(yīng)c（；t），這里c（；t）是信道在t-時刻加入沖激而在時刻的響應(yīng)。因此，表示“歷時（經(jīng)歷時間）”變量。上面描述的三種數(shù)學(xué)模型適當(dāng)?shù)谋碚髁藢?shí)際中的絕大多數(shù)物理信道。本書將這3種模型用于通信系統(tǒng)的分析和設(shè)計(jì)。4 數(shù)字通信發(fā)展的回顧與展望值得注意的是，最早的電通信形式，即電報，是一個數(shù)字通信系統(tǒng)。電報由S莫爾斯研制，并在1837年進(jìn)行了演示試驗(yàn)。莫爾斯設(shè)計(jì)出一種可變長度的二進(jìn)制碼，其中英文字母用點(diǎn)劃線的序列（碼字）表示。在這種碼中，較

22、頻繁發(fā)生的字母用短碼字表示，不常發(fā)生的字母用較長的碼字表示。因此，莫爾斯碼是第三章所述可變長度信源編碼方法的先驅(qū)。差不多在40年之后，1875年，E博多設(shè)計(jì)出一種電報碼，其中每一個字母編成一個固定長度為5的二進(jìn)制碼字。在博多碼中，二進(jìn)制碼的元素是等長度的，且指定為傳號和空號。雖然莫爾斯在研制第一個點(diǎn)的數(shù)字通信系統(tǒng)（電報）中起了重要的作用，但是現(xiàn)在我們所指的現(xiàn)代數(shù)字通信系統(tǒng)起源于奈奎斯特的研究。奈奎斯特研究了再給定帶寬的電報信道上，無符號間干擾的最大信號傳輸速率。他用公式表達(dá)了一個電報系統(tǒng)的模型，其中發(fā)送信號的一般形式為 式中，g（t）表示基本的脈沖形狀，是以速率1/T bit/s發(fā)送的二進(jìn)制數(shù)

23、據(jù)序列。奈奎斯特提出了帶寬限于W Hz的最佳脈沖形狀，并且在脈沖抽樣時刻Kt(k=0，1，2，。)無符號間干擾的條件下的最大比特率。他得出結(jié)論：最大脈沖速率是2W脈沖/s，該速率稱為奈奎斯特速率。附：英文原文Digital Communications1INTRODUCTIONIn this book, we present the basic principles that underlie the analysis and design of digital communication systems.The subject of digital communications involv

24、es the transmission of information in digital form from a source that generates the information to one or more destinations. Of particular importance in the analysis and design of communication systems are the characteristics of the physical channels through which the information is transmitted. The

25、 characteristics of the channel generally affect the design of the basic building blocks of the communication system. Below, we describe the elements of a communication system and their functions.1.1 ELEMENTS OF A DIGITAL COMMUNICATION SYSTEMFigure 1.1 illustrates the functional diagram and the basi

26、c elements of a digital communication system. The source output may be either an analog signal, such as audio or video signal, or a digital signal, such as the output of a teletype machine, that is discrete in time and has a finite number of output characters. In a digital communication system, the

27、messages produced by the source are converted into a sequence of binary digits. Ideally, we should like to represent the source output (message) by as few binary digits as possible. In other words, we seek an efficient representation of the source output that results in little or no redundancy. The

28、process of efficiently converting the output of either an analog or digital source into a sequence of binary digits is called source encoding or data compression.The sequence of binary digits from the source encoder, which we call the information sequence, is passed lo the channel encoder. The purpo

29、se of the channel encoder is to introduce, in a controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission of the signal through the channel. Thus, the added redundancy serve

30、s to increase the reliability of the received data and improves the fidelity of the received signal.In effect, redundancy in the information sequence aids the receiver in decoding the desired information sequence. For example, a (trivial) form of encoding of the binary information sequence is simply

31、 to repeat each binary digit m times,where m is some positive integer. More sophisticated (nontrivial) encoding involves talcing k information bits at a time and mapping each k-bit sequence into a unique n-bit sequence, called a code word. The amount of redundancy introduced by encoding the data in

32、this manner is measured by the ratio n/k.The reciprocal of this ratio, namely k/n, is called the rate of the code or,simply, the code rate.FIGURE 1.1 Basic elements of a digital communication systemThe binary sequence at the output of the channel encoder is passed to the digital modulator, which ser

33、ves as the interface to the communications channel.Since nearly all of the communication channels encountered in practice are capable of transmitting electrical signals (waveforms), the primary purpose of the digital modulator is to map the binary information sequence into signal waveforms. To elabo

34、rate on this point, let us suppose that the coded information sequence is to be transmitted one bit at a time at some uniform rate R bits/s. The digital modulator may simply map the binary digit 0 into a waveform s0(t) and the binary digit 1 into a waveform j,(i). In this manner,each bit from the ch

35、annel encoder is lransmitted separately. We call this binary modulation. Alternatively, the modulator may transmit b coded informationbits at a time by using M = 2s distinct waveforms j.(r), i = 0,1M - 1, one waveform for each of the 2 possible 6-bit sequences. We call this M-ary modulation (M2). No

36、te that a new 6-bit sequence enters the modulator every b/R seconds. Hence, when the channel bit rate R is fixed, the amount of time available to transmit one of the M waveforms corresponding to a 6-bit sequence is b times the time period in a system that uses binary modulation.The communication cha

37、nnel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, the channel may be the atmosphere (free space). On the other hand, telephone channels usually employ a variety of physical media, including wire lines,optical fiber cables, and

38、 wireless (microwave radio). Whatever the physical medium used for transmission of the information, the essential feature is that the transmitted signal is corrupted in a random manner by a variety of possible mechanisms, such as additive thermal noise generated by electronic devices,man-made noise,

39、 e.g., automobile ignition noise,and atmospheric noise,e.g.electrical lightning discharges during thunderstorms.At the receiving end of a digital communications system, the digital demodulator processes the channel-corrupted transmitted waveform and reduces the waveforms to a sequence of numbers tha

40、t represent estimates of the transmitted data symbols (binary or M-ary). This sequence of numbers is passed to the channel decoder, which attempts to reconstruct the original information sequence from knowledge of the code used by the channel encoder and the redundancy contained in the received data

41、.A measure of how well the demodulator and decoder perform is thefrequency with which errors occur in the decoded sequence. More precisely,the average probability of a bit-error at the output of the decoder is a measure of the performance of the demodulator-decoder combination. In general, the proba

42、bility of error is a function of the codc characteristics, the types of waveforms used to transmit the information over the channci, the transmitter power, the characteristics of the channel, i.e., the amount of noise, the nature of the interference, etc., and the method of demodulation and decoding

43、. These items and their effect on performance will be discussed in detail in subsequent chapters.As a final step, when an analog output is desired, the source decoder accepts the output sequence from the channel decoder and, from knovtledge of the source encoding method used, attempts to reconstruct

44、 the original signal from the source. Due to channel decoding errors and possible distortion introduced by the source encoder and, perhaps, the source decoder, the signal at the output of the source decoder is an approximation to the original source output.The difference or some function of the diff

45、erence between the original signal and the reconstructed signal is a measure of the distortion introduced by the digital communication system.2 COMMUNICATION CHANNELS AND THEIR CHARACTERISTICSAs indicated in the preceding discussion, the communication channel provides the connection between the tran

46、smitter and the receiver. The physical channel may be a pair of wires that carry the electrical signal, or an optical fiber thai carries the information on a modulated light beam, or an underwater ocean channel in which the information is transmitted acoustically, or free space over which the inform

47、ation-bearing signal is radiated by use of an antenna. Other media that can be characterized as communication channels are data storage media, such as magnetic tape, magnetic disks, and optical disks.One common problem in signal transmission through any channel is additive noise. In general, additiv

48、e noise is generated internally by components such as resistors and solid-state devices used to implement the communication system.This is sometimes called thermal noise. Other sources of noise and interference may arise externally to the system, such as interference from other users of the channel.

49、 When such noise and interference occupy the same frequency band as the desired signal, its effect can be minimized by proper design of the transmitted signal and its demodulator at the receiver. Other types of signal degradations (hat may be encountered in transmission over the channel are signal a

50、ttenuation, amplitude and phase distortion, and multipath distortion.The effects of noise may be minimized by increasing the power in the transmitted signal. However, equipment and other practical constraints limit the power level in the transmitted signal. Another basic limitation is the available

51、channel bandwidth. A bandwidth constraint is usually due to the physical limitations of the medium and the electronic components used to implement the transmitter and the receiver. These two limitations result in constraining the amount of data that can be transmitted reliably over any communication

52、s channel as we shall observe in later chapters. Below, we describe some of the important characteristics of several communication channels.2.1 Wireline Channels The telephone network makes extensive use of wire lines for voice signal transmission, as well as data and video transmission.Twisted-pair

53、 wire lines and coaxial cable are basically guided electromagnetic channels that provide relatively modest bandwidths. Telephone wire generally used to connect a customer to a central office has a bandwidth of several hundred kilohertz (kHz). On the other hand, coaxial cable has a usable bandwidth o

54、f several megahertz (MHz). Figure 1.2 illustrates the frequency range of guided electromagnetic channels, which include waveguides and optical fibers.Signals transmitted through such channels are distored in both amplitude and phase and further corrupted by additive noise. Twisted-pair wireline chan

55、nels arc also prone to crosstalk interference from physically adjacent channels. Becausc wireline channels carry a large percentage of our daily communications around the country and the world, much research has been performed on the characterization of their transmission properties and on methods f

56、or mitigating the amplitude and phase distortion encountered in signal transmission. In Chapter 9, we describe methods for designing optimum transmitted signals and their demodulation: in Chapters 10 and 11, we consider the design of channel equalizers that compensate for amplitude and phase distort

57、ion on these channels.2.2 Fiber Optic Channels Optical fibers offer the communications system designer a channel bandwidth that is several orders of magnitude larger than coaxial cable channels. During the past decade, optical fiber cables have been developed that have a relatively low signal attenu

58、ation, and highly reliable photonic devices have been developed for signal generation and signal detection. These technological advances have resulted in a rapid deployment of optical fiber channels, both in domestic telecommunication systems as well as for trans-Atlantic and trans-Pacific communications. With the large bandwidth available on fiber optic channels, it is possible for telephone companies to offer subscribers a wide array of telecommunication services, including voice, data,facsimile, and video.The transmitter or modulator in a fiber optic communication sy