數(shù)字通信外文翻譯

1、<p>　　附錄2：外文資料翻譯</p><p><b>　　譯文：</b></p><p><b>　　數(shù)字通信</b></p><p>　　作者：John Proakis</p><p>　　1.1數(shù)字通信系統(tǒng)的基本組成部分</p><p>　　圖1-1-1 顯

2、示了一個(gè)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(hào)，如音頻或視頻信號(hào);也可以是數(shù)字信號(hào)，如電傳機(jī)的輸出，該信號(hào)在時(shí)間上是離散的，并且只有有限個(gè)輸出字符。在數(shù)字通信系統(tǒng)中，由信源產(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(shuō).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)字信源的輸出有效地變換成二進(jìn)制數(shù)字序列的處理過(guò)程稱(chēng)為信源編碼或數(shù)據(jù)壓縮。

3、</p><p>　　由信源編碼器輸出的二進(jìn)制數(shù)字序列稱(chēng)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(lái)克服信號(hào)在信道中傳輸時(shí)所遭受的噪聲和干擾的影響。因此，所增加的冗余是用來(lái)提高接收數(shù)據(jù)的可靠性以及改善接收信號(hào)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(平凡的）形式的編碼就是將每個(gè)二進(jìn)制數(shù)字簡(jiǎn)

4、單重復(fù)m次.這里m為一個(gè)正整數(shù)。更復(fù)雜的（不平凡的）編碼涉及到一次取k個(gè)信息比特,并將每個(gè)k比特序列映射成惟一的n比特序列，該序列稱(chēng)為碼字。以這種方式對(duì)數(shù)據(jù)編碼所引人的冗余度的大小是由比率n/k作來(lái)度數(shù)的。該比率的倒數(shù)，即k/n，稱(chēng)為碼的速率或簡(jiǎn)稱(chēng)碼率。信道編碼器輸出的二進(jìn)制序列送至數(shù)字調(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(hào)（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(hào)波形。為了詳

5、細(xì)說(shuō)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(gè)比特傳輸,數(shù)字調(diào)制器可以簡(jiǎn)單地將二進(jìn)制數(shù)字“0”映射成波形s0(t)而二進(jìn)制數(shù)字“1”映射成波形s1(</p><p>　　圖1-1-1 數(shù)字通信系統(tǒng)的基本模型</p><p>　　通信信道是用來(lái)將發(fā)送機(jī)的信號(hào)發(fā)送給接收機(jī)的物理媒質(zhì)。在無(wú)線傳輸中，信道可以是大氣（自由空間）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

6、線路、光纜和無(wú)線(微波)等。無(wú)論用什么物理媒質(zhì)來(lái)傳輸信息，其基本特點(diǎn)是發(fā)送信號(hào)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(chē)點(diǎn)火噪聲）及大氣噪聲（如在雷雨時(shí)的閃電）。</p><p>　　在數(shù)字通信系統(tǒng)的接收端,數(shù)字解調(diào)器對(duì)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(gè)數(shù)的序列，該序列表示發(fā)送數(shù)據(jù)符號(hào)的估計(jì)值〔二進(jìn)制或M元〕。這個(gè)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

7、的關(guān)于碼的知識(shí)及接收數(shù)據(jù)所含的冗余度重構(gòu)初始的信息序列。</p><p>　　解調(diào)器和譯碼器工作性能好壞的—個(gè)度量是譯碼序列中發(fā)生差錯(cuò)的頻度。更準(zhǔn)確地說(shuō)，在譯碼器輸出端的平均比特錯(cuò)誤概率是解調(diào)器-譯碼器組合性能的一個(gè)度量。一般地，錯(cuò)誤概率是下列各種因素的函數(shù):碼特征、用來(lái)在信道上傳輸信息的波形的類(lèi)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(duì)性能的影

8、響。</p><p>　　作為最后一步，當(dāng)需要模擬輸出時(shí)，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(shí)重構(gòu)由信源發(fā)出的原始信號(hào)。由于信道譯碼的差錯(cuò)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(hào)只是原始信源輸出的—個(gè)近似。在原始信號(hào)與重構(gòu)信號(hào)之間的信號(hào)差或信號(hào)差的函數(shù)是數(shù)字通信系統(tǒng)引入失真的一種度量。</p><p>　　1.2通信信道及其特征</p

9、><p>　　正如前面指出的，通信信道在發(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(hào)的一對(duì)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(hào)通過(guò)天線在空間輻射傳輸。可被表征為通信信道的其他媒質(zhì)是數(shù)據(jù)存儲(chǔ)媒質(zhì)如磁帶、磁盤(pán)和光盤(pán)。</p><p>　　在信號(hào)通過(guò)任何信道傳輸中的一個(gè)共同的問(wèn)題是加性噪聲。一般地，加性噪聲是由通信系統(tǒng)

10、內(nèi)部組成元器件所引起的，例如電阻和固態(tài)器件。有時(shí)將這種噪聲稱(chēng)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(lái)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(hào)占有同頻帶時(shí)，可通過(guò)對(duì)發(fā)送信號(hào)和接收機(jī)中解調(diào)器的適當(dāng)設(shè)計(jì)來(lái)使它們的影響最小。信號(hào)在信道上傳輸時(shí)可能會(huì)遇到的其他類(lèi)型損傷有信號(hào)衰減、幅度和相位失真、多徑失真等。</p><p>　　可以通過(guò)增加發(fā)送信號(hào)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

11、素限制了發(fā)送信號(hào)的功率電平，另一個(gè)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。</p><p><b>　　1.有線信道</b></p><p>　　電話網(wǎng)絡(luò)擴(kuò)大了有線線路的應(yīng)用，如話音信號(hào)

12、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(lái)連接用戶和中心機(jī)房的電話線的帶寬為幾百千赫(kHz)另一方面同軸電纜的可用寬帶是幾兆赫（MHz）。信號(hào)在這樣的信道上傳輸時(shí)，其幅度和相位都會(huì)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(lái)自物理鄰近信道的串音干擾。因?yàn)樵谌珖?guó)和全世界有線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(duì)傳輸特性的表征以及對(duì)信號(hào)傳輸時(shí)的幅度和相位失真的減緩方

13、法作了大量研究。在第9章中，我們將闡述最佳傳輸信號(hào)及其解調(diào)的設(shè)計(jì)方法。在第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(lái)補(bǔ)償信道的幅度和相位失真的。</p><p><b>　　2.光纖信道</b></p><p>　　光纖提供的信道帶寬比同軸電纜信道大幾個(gè)數(shù)量級(jí)。在過(guò)去的20年中，已經(jīng)研發(fā)出具有較低信號(hào)衰減的光纜，以及用于信號(hào)和信號(hào)檢測(cè)的可靠性光子器件。這

14、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(guó)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一系列電話業(yè)務(wù)，包括話音、數(shù)據(jù)、傳真和視頻等。</p><p>　　在光纖通信系統(tǒng)中，發(fā)送機(jī)或調(diào)制器是一個(gè)光源.或者是發(fā)光二極管（LED）或者是激光。通過(guò)消息信號(hào)改變（調(diào)制）光源的強(qiáng)度來(lái)發(fā)送信息。光像光波一樣通過(guò)光纖傳播，并沿著傳輸路徑被周期性地放大

15、以補(bǔ)償信號(hào)衰減（在數(shù)宇傳輸中，光由中繼器檢測(cè)和再生）。在接收機(jī)中，光的強(qiáng)度由光電二極管檢測(cè)，它的輸出電信號(hào)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。</p><p>　　3.無(wú)線電磁信道 </p><p>　　在無(wú)線通信系統(tǒng)中，電磁能是通過(guò)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

16、輻射，天線必須比波長(zhǎng)的1/10更長(zhǎng)。因此,在調(diào)幅（AM）頻段發(fā)射的無(wú)線電臺(tái)，譬如說(shuō)在f=1MHz時(shí)（相當(dāng)于波長(zhǎng)= C/f=300m）要求天線至少為30m。無(wú)線傳輸天線的其他重要特征和屬性將在第5章闡述。</p><p>　　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(lèi)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(zhǎng)超過(guò)10km，地球和電離層對(duì)電磁波傳播的作用如同波導(dǎo)。在這些頻段,通信

17、信號(hào)實(shí)際上環(huán)繞地球傳播，由于這個(gè)原因，這些頻段主要用來(lái)在世界范圍內(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(guò)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(dòng)產(chǎn)生的，特別是在熱帶地區(qū)。干擾來(lái)自這些頻段上的用戶。</p><p>　　在高頻（HF）頻段范圍內(nèi)，電磁波經(jīng)由天波傳播時(shí)經(jīng)常發(fā)生的問(wèn)題是信號(hào)多徑。信號(hào)多徑發(fā)生

18、在發(fā)送信號(hào)經(jīng)由多條傳播路徑以不同的延遲到達(dá)接收機(jī)的時(shí)候，一般會(huì)引起數(shù)字通信系統(tǒng)中的符號(hào)間干擾。而且經(jīng)由不同傳播路徑到達(dá)的各信號(hào)分量會(huì)相互削弱，導(dǎo)致信號(hào)衰落的現(xiàn)象.許多人在夜晚收聽(tīng)遠(yuǎn)地?zé)o線電臺(tái)廣播時(shí)會(huì)對(duì)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熱噪聲的組合。</p><p>　　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30~60MHZ頻段有可能進(jìn)行電離層

19、散射傳播，這是由較低電離層的信號(hào)散射引起的。也可利用在40~300MHZ頻率范圍內(nèi)的對(duì)流層散射在幾百英里的距離通信。對(duì)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(hào)散射造成的，一般地，電離層散射和對(duì)流層散射具有大的信號(hào)傳播損耗，要求發(fā)射機(jī)功率大和天線比較長(zhǎng)。</p><p>　　在30MHZ以上頻率通過(guò)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

20、，電磁傳播的最主要模式是LOS傳播。對(duì)于陸地通信系統(tǒng)這意味著發(fā)送機(jī)和接收機(jī)的天線必須是直達(dá)LOS,沒(méi)有什么障礙。由于這個(gè)原因VHF和特高頻(UHF)頻段發(fā)射的電視臺(tái)的天線安裝在髙塔上,以達(dá)到更寬的覆蓋區(qū)域。</p><p>　　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(méi)有物理障礙(如山)那么到無(wú)線地平線的距離近似為d=15H KM，例如電視天線安裝在300m

21、高的塔上.它的覆蓋范圍大約67km另一個(gè)例子，工作在1GHZ以上頻率，用來(lái)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。</p><p>　　對(duì)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，大氣層環(huán)境在信號(hào)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(shí)的0.003 dB/KM左

22、右到大雨時(shí)的0.3dB/KM；在100GHZ,衰減范圍從小雨時(shí)的0.1dB左右到大雨時(shí)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(huì)導(dǎo)致業(yè)務(wù)中斷(通信系統(tǒng)完全中斷)。</p><p>　　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(jiàn)光區(qū)，它們可用來(lái)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。</p><p>

23、;<b>　　4.水聲信道 </b></p><p>　　在過(guò)去的幾十年中.海洋探險(xiǎn)活動(dòng)不斷增多。與這種增多相關(guān)的是對(duì)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。</p><p>　　除極低頻率外，電磁波在水下不能長(zhǎng)距離傳播。在低頻率的信號(hào)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

24、以用表面深度來(lái)表示,它是信號(hào)衰減l/e的距離。對(duì)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(hào)能在幾十甚至幾百千米距離上傳播。</p><p>　　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(duì)信號(hào)反射的緣故。因?yàn)椴ǖ倪\(yùn)動(dòng),信號(hào)多徑分量的傳播延遲是時(shí)變的，這就導(dǎo)致了信號(hào)的衰落。此外,還存在與頻率相關(guān)的衰減，它與信號(hào)頻率的平方近似成正比。聲音速度通常大約為

25、1 500m/s，實(shí)際值將在正常值上下變化,這取決于信號(hào)傳播的深度。</p><p>　　海洋背景噪聲是由蝦、魚(yú)和各種哺乳動(dòng)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(zhǎng)距離地傳輸數(shù)字信號(hào)。</p><p><b>　　5.存儲(chǔ)信道</b></p><p>　

26、　信息存儲(chǔ)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(lái)存儲(chǔ)大量計(jì)算機(jī)數(shù)據(jù)的磁盤(pán)、用作計(jì)算機(jī)數(shù)據(jù)存儲(chǔ)器的光盤(pán)以及只讀光盤(pán)都是數(shù)據(jù)存儲(chǔ)系統(tǒng)的例子,它們可以表征為通信信道。在磁帶或磁盤(pán)或光盤(pán)上存儲(chǔ)數(shù)據(jù)的過(guò)程，等效于在電話或在無(wú)線信道上發(fā)送數(shù)據(jù)。回讀過(guò)程以及在存儲(chǔ)系統(tǒng)中恢復(fù)所存儲(chǔ)的數(shù)據(jù)的信號(hào)處理等效于在電話和無(wú)線通信系統(tǒng)中恢復(fù)發(fā)送信號(hào)。</p><p>　　由電子元器件產(chǎn)生的加性

27、噪聲和來(lái)自鄰近軌道的干擾一般會(huì)呈現(xiàn)在存儲(chǔ)系統(tǒng)的回讀信號(hào)中，這正如電話或無(wú)線通信系統(tǒng)中的情況。</p><p>　　所能存儲(chǔ)的數(shù)據(jù)量一般受到磁盤(pán)或磁帶尺寸及密度（每平方英寸存儲(chǔ)的比特?cái)?shù)）的限制，該密度是由寫(xiě)/讀電系統(tǒng)和讀寫(xiě)頭確定的。例如在磁盤(pán)存儲(chǔ)系統(tǒng)中，封裝密度可達(dá)每平方英寸比特(1 in=2.54cm)。磁盤(pán)或磁帶上的數(shù)據(jù)的讀寫(xiě)速度也受到組成信息存儲(chǔ)系統(tǒng)的機(jī)械和電子子系統(tǒng)的限制。</p><p

28、>　　信道編碼和調(diào)制是良好設(shè)計(jì)的數(shù)字磁或存儲(chǔ)系統(tǒng)的最重要的組成部分。在回讀過(guò)程中，信號(hào)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(hào)中的差錯(cuò)。</p><p>　　1.3 通信信道的數(shù)學(xué)模型</p><p>　　在通過(guò)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)，建立一個(gè)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

29、的解調(diào)器和信道譯碼器的設(shè)計(jì)。下面，我們將簡(jiǎn)要的描述信道的模型，它們常用來(lái)表征實(shí)際的物理信道。</p><p><b>　　加性噪聲信道</b></p><p>　　通信信道最簡(jiǎn)單的數(shù)學(xué)模型是加性噪聲信道，如圖1-3-1所示。在這個(gè)模型中，發(fā)送信號(hào)s（t）被加性隨機(jī)噪聲過(guò)程n（t）惡化。在物理上，加性噪聲過(guò)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

30、引起（正如在無(wú)線電信號(hào)傳輸中那樣）。</p><p>　　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(guò)程。因此，該信道的數(shù)學(xué)模型通常稱(chēng)為加性高斯噪聲信道。因?yàn)檫@個(gè)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(hào)通過(guò)信道傳輸而受到衰減時(shí)，接收信號(hào)是<

31、;/p><p><b>　　式中，是衰減因子。</b></p><p>　　圖1-3-1 加性噪聲信道</p><p><b>　　線性濾波器信道</b></p><p>　　在某些物理信道中，例如有線電話信道，采用濾波器來(lái)保證傳輸信號(hào)不超過(guò)規(guī)定的帶寬限制，從而不會(huì)引起相互干擾。這樣的信道通常在數(shù)學(xué)上表

32、征為帶有加性噪聲的線性濾波器，如圖1-3-2所示。因此，如果信道輸入信號(hào)為s（t），那么信道輸出信號(hào)是</p><p>　　式中，是信道的沖激響應(yīng)，表示卷積。</p><p>　　圖1-3-2 帶有加性噪聲的線性濾波器信道</p><p><b>　　線性時(shí)變?yōu)V波器信道</b></p><p>　　像水聲信道和電離層無(wú)

33、線電信道這樣的物理信道，它們會(huì)導(dǎo)致發(fā)送信號(hào)的時(shí)變多徑傳播，這類(lèi)物理信道在教學(xué)上可以表征為時(shí)變線性濾波器。該線性濾波器可以表征為時(shí)變信道沖激響應(yīng)c（τ；t），這里c（τ；t）是信道在t-τ時(shí)刻加入沖激而在τ時(shí)刻的響應(yīng)。因此，τ表示“歷時(shí)（經(jīng)歷時(shí)間）”變量。</p><p>　　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(shū)將這3</p><p>　　種模型用于通信系統(tǒng)的分析

34、和設(shè)計(jì)。</p><p><b>　　原文：</b></p><p>　　Digital Communications</p><p>　　Autor：John Proakis</p><p>　　1-1 ELEMENTS OF A DIGITAL COMMUNICATION SYSTEM</p><

35、p>　　Figure 1-1-1 illustrates the functional diagram and the basic 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

36、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 messages produced by the source are converted into a sequence of binary d

37、igits. Ideally, we should like to represent the source output (mess</p><p>　　The sequence of binary digits from the source encoder, which we call the information sequence, is passed lo the channel encoder. T

38、he purpose 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

39、 in the transmission of the signal through the channel. Thus, the added redundancy serves to increase the reliability of the received data and improve</p><p>　　The binary sequence at the output of the channe

40、l encoder is passed to the digital modulator, which serves as the interface to the communications channel. Since nearly all of the communication channels encountered in practice are capable of transmitting electrical sig

41、nals (waveforms), the primary purpose of the digital modulator is to map the binary information sequence into signal waveforms. To elaborate on this point, let us suppose that the coded information sequence is to be tran

42、smitted one b</p><p>　　bits 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 Mary modulation (M>2). Note that

43、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 t

44、ime period in a system that uses binary modulation.</p><p>　　The communication channel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, t

45、he 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 wireless (microwave radio). Whatever the ph

46、ysical medium used for transmission of the information, the essential feature is that the transmitted signal is corrupted in a random </p><p>　　At the receiving end of a digital communications system, the di

47、gital demodulator processes the channel-corrupted transmitted waveform and reduces the waveforms to a sequence of numbers that represent estimates of the transmitted data symbols (binary or Mary). This sequence of number

48、s 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.</p><

49、;p>　　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

50、 measure of the performance of the demodulator-decoder combination. In general, the probability of error is a function of the code characteristics, the types of waveforms used to transmit the information over the channel

51、, the transmitter power, the characteristics of the channel, i.e., t</p><p>　　As a final step, when an analog output is desired, the source decoder accepts the output sequence from the channel decoder and, f

52、rom knowledge of the source encoding method used, attempts to reconstruct the original signal from the source. Due to channel decoding errors and possible distortion introduced by the source encoder and, perhaps, the sou

53、rce 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 d</p><p>　　1-2 COMMUNICATION CHANNELS AND THEIR CHARACTER

54、ISTICS</p><p>　　As indicated in the preceding discussion, the communication channel provides the connection between the transmitter and the receiver. The physical channel may be a pair of wires that carry th

55、e electrical signal, or an optical fiber that 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 informati

56、on-bearing signal is radiated by use of an antenna. Other media that can be characterized</p><p>　　One common problem in signal transmission through any channel is additive noise. In general, additive noise

57、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 extern

58、ally to the system, such as interference from other users of the channel. When such noise and interference occupy the same frequency band as the desired signal, its effect </p><p>　　The effects of noise may

59、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 channel bandwi

60、dth. 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 amou

61、nt of data that can be transmitted </p><p>　　Wire-line Channels The telephone network makes extensive use of wire lines for voice signal transmission, as well as data and video transmission. Twisted-pair wir

62、e 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 hundre

63、d kilohertz (kHz). On the other hand, coaxial cable has a usable bandwidth of several megahertz (MHz). Figure 1-2-1 il</p><p>　　Signals transmitted through such channels are distorted in both amplitude and p

64、hase and further corrupted by additive noise. Twisted-pair wire line channels arc also prone to crosstalk interference from physically adjacent channels. Because wire line channels carry a large percentage of our daily c

65、ommunications around the country and the world, much research has been performed on the characterization of their transmission properties and on methods for mitigating the amplitude and phase distortio</p><p&g

66、t;　　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 ha

67、ve been developed that have a relatively low signal attenuation, and highly reliable photonic devices have been developed for signal generation and signal detection. These technological advances have resulted in a rapid

68、deployment of optical fiber channels, both in domestic telecommu</p><p>　　The transmitter or modulator in a fiber optic communication system is a light source, cither a light-emitting diode (LED) or a laser.

69、 Information is transmitted by varying (modulating) the intensity of the light source with the message signal. The light propagates through the fiber as a light wave and is amplified periodically (in the case of digital

70、transmission, it is detected and regenerated by repeaters) along the transmission path to compensate for signal attenuation. At the receiver, the l</p><p>　　It is envisioned that optical fiber channels will

71、replace nearly all wire line channels in the telephone network by the turn of the century.</p><p>　　Wireless Electromagnetic Channels In wireless communication systems, electromagnetic energy is coupled to t

72、he propagation medium by an antenna which serves as the radiator. The physical size and the configuration of the antenna depend primarily on the frequency of operation. To obtain efficient radiation of electromagnetic en

73、ergy, the antenna must be longer than of the wavelength. Consequently, a radio station transmitting in the AM frequency band, say at f - 1 MHz (corresponding to a wavelength</p><p>　　Figure 1-2-2 illustrates

74、 the various frequency bands of the electromagnetic spectrum. The mode of propagation of electromagnetic waves in the at mo-sphere and in free space may be subdivided into three categories, namely, ground-wave propagatio

75、n, sky-wave propagation, and line-of-sight (LOS) propagation. In the VLF and audio frequency bands, where the wavelengths exceed 10 km, the earth and the ionosphere act as a waveguide for electromagnetic wave propagation

76、. In these frequency ranges, communi</p><p>　　Ground-wave propagation, as illustrated in Fig. 1-2-3, is the dominant mode of propagation for frequencies in the MF band (0.3-3 MHz). This is the frequency band

77、 used for AM broadcasting and maritime radio broadcasting. In AM broadcasting, the range with ground-wave propagation of even the more powerful radio stations is limited to about 150 km. Atmospheric noise, man-made noise

78、, and thermal noise from electronic components at the receiver are dominant disturbances for signal transmission in the</p><p>　　Sky-wave propagation, as illustrated in Fig. 1-2-4 results from transmitted si

79、gnals being reflected (bent or refracted) from the ionosphere, which consists of several layers of charged particles ranging in altitude from 50 to 400 km above the surface of the earth. During the daytime hours, the hea

80、ting of the lower atmosphere by the sun causes the formation of the lower layers at altitudes below 120 km. These lower layers, especially the D-layer, serve to absorb frequencies below 2 MHz, thus seve</p><p&

81、gt;　　A frequently occurring problem with electromagnetic wave propagation via sky wave in the HF frequency range is signal multipath. Signal multipath occurs when the transmitted signal arrives at the receiver via multip

82、le propagation paths at different delays, generally results in inter-symbol interference in a digital communication system. Moreover, the signal components arriving via different propagation paths may add destructively,

83、resulting in a phenomenon called signal fading, which most people</p><p>　　Sky-wave ionosphere propagation ceases to exist at frequencies above approximately 30 MHz, which is the end of the HF band. However,

84、 it is possible to have ionosphere scatter propagation at frequencies in the range 30-60 MHz, resulting from signal scattering from the lower ionosphere. It is also possible to communicate over distances of several hundr

85、ed miles by use of troposphere scattering at frequencies in the range 40-300 Mhz. Troposphere results from signal scattering due to particles in the</p><p>　　Frequencies above 30 MHz propagate through the io

86、nosphere with relatively little loss and make satellite and extraterrestrial communications possible. Hence, at frequencies in the VHF band and higher, the dominant mode of electromagnetic propagation is line-of-sight (L

87、OS) propagation. For terrestrial communication systems, this means that the transmitter and receiver antennas must be in direct LOS with relatively little or no obstruction. For this reason, television stations transmitt

88、ing in the </p><p>　　In general, the coverage area for LOS propagation is limited by the curvature of the earth. If the transmitting antenna is mounted at a height h m above the surface of the earth, the dis

89、tance to the radio horizon, assuming no physical obstructions such as mountains, is approximately d - km. For example, a TV antenna mounted on a tower of 300 m in height provides a coverage of approximately 67 km. As ano

90、ther example, microwave radio relay systems used extensively for telephone and video transmiss</p><p>　　The dominant noise limiting the performance of a communication system in VHF and UHF frequency ranges i

91、s thermal noise generated in the receiver front end and cosmic noise picked up by the antenna. At frequencies in the SHF band above 10 GHz, atmospheric conditions play a major role in signal propagation. For example, at

92、10 GHz, the attenuation ranges from about 0.003 dB/km in light rain to about 0.3 dB/km in heavy rain. At 100 GHz, the attenuation ranges from about 0.1 dB/km in light rain to ab</p><p>　　At frequencies above

93、 the EHF (extremely high frequency) band, we have the infrared and visible light regions of the electromagnetic spectrum, which can be used to provide LOS optical communication in free space. To date, these frequency ban

94、ds have been used in experimental communication systems, such as satellite-to-satellite links.</p><p>　　Underwater Acoustic Channels Over the past few decades, ocean exploration activity has been steadily in

95、creasing. Coupled with this increase is the need to transmit data, collected by sensors placed under water, to the surface of the ocean. From there, it is possible to relay the data via a satellite to a data collection c

96、enter.</p><p>　　Electromagnetic waves do not propagate over long distances under water except at extremely low frequencies. However, the transmission of signals at such low frequencies is prohibitively expen

97、sive because of the large and powerful transmitters required. The attenuation of electromagnetic waves in water can be expressed in terms of the skin depth, which is the distance a signal is attenuated by 1/r. For sea wa

98、ter, the skin depth 250/v7, where f is expressed in Hz and 8 is in m. For example, at 10 </p><p>　　An underwater acoustic channel is characterized as a multipath channel due to signal reflections from the su

99、rface and the bottom of the sea. Because of wave motion, the signal multipath components undergo time-varying propagation delays that result in signal fading. In addition, there is frequency dependent attenuation, which