第3元TEXTAampB

1、unit 3 mutiple acesstext a multiple access v 原文：原文：multiple access is a technique with which multiple terminals share the bandwidth of the transmission medium. multiple access techniques are of paramount importance in radio systems where the channel bandwidth is very limited. in multiple access, mul

2、tiple terminals or users share the bandwidth of the transmission medium. v 譯文：多址接入技術(shù)是多個通信終端共享通信媒介頻段的譯文：多址接入技術(shù)是多個通信終端共享通信媒介頻段的技術(shù)。多址技術(shù)對于頻段非常有限的無線通信系統(tǒng)而言是技術(shù)。多址技術(shù)對于頻段非常有限的無線通信系統(tǒng)而言是極其重要的。在多址接入技術(shù)中，多個通信終端或多個用極其重要的。在多址接入技術(shù)中，多個通信終端或多個用戶共享傳輸媒介頻段戶共享傳輸媒介頻段unit 3unit 3v原文原文：multiple access attains much importance

3、 in radio communications because the radio spectrum is a precious natural resource, and to make best use of the radio bandwidth is impotrant. we will discuss the various multiple access techniques with illustrative examples. v譯文譯文:在無線通信系統(tǒng)中，由于無線頻譜是非常寶貴的自在無線通信系統(tǒng)中，由于無線頻譜是非常寶貴的自然資源，多址技術(shù)對于獲得無線頻譜的最好利用是至關(guān)重

4、然資源，多址技術(shù)對于獲得無線頻譜的最好利用是至關(guān)重要的。我們將通過例子來討論各種多址技術(shù)要的。我們將通過例子來討論各種多址技術(shù) 。unit 3v 原文：原文：fdmav in radio communication system, a certain frequency band is allocated for specific use. this band is divided into smaller bands, and this pool of bands is available for all the terminals to share. depending on the ne

5、ed, the base station allocates a frequency for the terminal to transmit.v 譯文：頻分多址（譯文：頻分多址（fdma）v 在無線通信系統(tǒng)中，不同的頻譜分配給特定的用戶使用。在無線通信系統(tǒng)中，不同的頻譜分配給特定的用戶使用。這些頻段被劃分為更小的頻段，供所有的終端共享。這些頻段被劃分為更小的頻段，供所有的終端共享。unit 3v 原文：原文：this is known as frequency division multiple access (fdma). in fdma systems, there is central

6、 station that allocates the band (or frequency) to different terminals, based on their needs.v譯文：基站根據(jù)需要給不同終端支配不同的頻段譯文：基站根據(jù)需要給不同終端支配不同的頻段以傳輸信號，這就是頻分多址以傳輸信號，這就是頻分多址 。在頻分多址系統(tǒng)。在頻分多址系統(tǒng)中有一個中心站，它根據(jù)終端的需求來分配頻段。中有一個中心站，它根據(jù)終端的需求來分配頻段。unit 3v原文：原文：consider the example of mobile communication system. the system

7、consists of number of base stations. a base station has a pool of frequencies.v譯文：考慮以移動通信系統(tǒng)為例譯文：考慮以移動通信系統(tǒng)為例.該系統(tǒng)由許多基該系統(tǒng)由許多基站組成。一個基站擁有很多不同的頻段。站組成。一個基站擁有很多不同的頻段。unit 3v 原文：原文：all frequencies are available for sharing by the mobile terminals located around the base station the frequency will be allocate

8、d based on need. mobile terminal a will be allocated the frequency f1, and then mobile terminal b will be allocated the frequency f2.v譯文：所有頻段都可以被該基站周圍的移動終端譯文：所有頻段都可以被該基站周圍的移動終端共享，這種頻段指配方式是共享，這種頻段指配方式是 按需的。移動終端按需的。移動終端a得到頻率得到頻率f1,移動終端移動終端b得到頻率得到頻率f2。unit 3v原文：原文：to ensure that there is no overlap in

9、transmission by different terminals using adjacent frequencies, a guard band is provided. a small separation between adjacent bands is called the guard band.v譯文：為了確保使用相鄰頻率的不同終端不出現(xiàn)譯文：為了確保使用相鄰頻率的不同終端不出現(xiàn)頻率交迭，必須設(shè)置保護(hù)頻段。將相鄰頻段彼此頻率交迭，必須設(shè)置保護(hù)頻段。將相鄰頻段彼此分開的小頻段稱為保護(hù)頻段。分開的小頻段稱為保護(hù)頻段。unit 3v 原文：原文：sdmav because radi

10、o spectrum is a natural resource, we have to make best use of it. when a set of frequencies is reused in another location, it is called space division multiple access (sdma).v 譯文：空分多址（譯文：空分多址（sdma）v 因?yàn)闊o線頻譜是一種自然資源，我們要最好的利用。當(dāng)一因?yàn)闊o線頻譜是一種自然資源，我們要最好的利用。當(dāng)一組頻率在別的位置重復(fù)使用，就稱為空分多址組頻率在別的位置重復(fù)使用，就稱為空分多址unit 3v 原文：

11、原文：again, consider a mobile communication system. a base station will be allocated a few frequencies. the same set of frequencies can be allocated to another region, provided there is sufficient distance between the two regions.v譯文：再次考慮移動通信系統(tǒng)。一個基站會被分譯文：再次考慮移動通信系統(tǒng)。一個基站會被分配一些頻率。同樣的這些頻率可分配給其他地區(qū)，配一些頻率。同

12、樣的這些頻率可分配給其他地區(qū)，只要兩個區(qū)域之間距離足夠長。只要兩個區(qū)域之間距離足夠長。unit 3v 原文原文:in sdma, a service area is divided into small regions called cells and each cell is allocated certain frequencies. two cells can make use of the same set of frequencies, provided these two cells are separated by a distance called reuse distance

13、. sdma is used in mobile communication systems.v 譯文：在空分多址中，服務(wù)區(qū)別劃分為幾個小的地區(qū)稱為譯文：在空分多址中，服務(wù)區(qū)別劃分為幾個小的地區(qū)稱為小區(qū)并且每個小區(qū)都分配給一些固定的頻率。兩個小區(qū)可小區(qū)并且每個小區(qū)都分配給一些固定的頻率。兩個小區(qū)可使用同一組頻率，兩個小區(qū)之間的距離稱為復(fù)用距離?？帐褂猛唤M頻率，兩個小區(qū)之間的距離稱為復(fù)用距離?？辗侄嘀吩谝苿油ㄐ畔到y(tǒng)中使用。分多址在移動通信系統(tǒng)中使用。unit 3v 原文：原文：in time division multiple access (tdma), one frequency is s

14、hared by a number of terminals. each station is given a time slot in which it can transmit. the number of time slots is fixed ,and the terminals transmit in the time slots allocated to them. v 譯文：在時分多址中一個頻率可以被多個終端共享。分配譯文：在時分多址中一個頻率可以被多個終端共享。分配給每個站一個時隙用來發(fā)送，時隙的序號是確定的，終端給每個站一個時隙用來發(fā)送，時隙的序號是確定的，終端在分配的時隙上

15、發(fā)送信息。在分配的時隙上發(fā)送信息。unit 3v 原文：原文：for example , in mobile communication systems, each frequency is divided into eight time slots and hence eight mobile terminals use the same frequency, communicating in different time slots. which terminal gets which time slot is decided by the base station.v 譯文：例如，在移動

16、通信系統(tǒng)中，每個頻率劃分為八個時譯文：例如，在移動通信系統(tǒng)中，每個頻率劃分為八個時隙隙 ，所以八個移動終端使用同個頻率在不同的時隙上通信。，所以八個移動終端使用同個頻率在不同的時隙上通信。終端所用的時隙是由基站決定的。終端所用的時隙是由基站決定的。unit 3v 原文：原文：in tdma systems, the time slots can be fixed or dynamic. in fixed tdma, each station is given a fixed tine slot ( for example, time slot 1 for station a, time slo

17、t 2 for station b, and so on). this results in a simple system design, but the disadvantage is that if a station has no data to transmit, that time slot is wasted. v 譯文：在時分多址系統(tǒng)中，時隙可以是固定的或可變的。譯文：在時分多址系統(tǒng)中，時隙可以是固定的或可變的。在靜態(tài)的時分多址中，分配給每個站一個固定時隙（例如，在靜態(tài)的時分多址中，分配給每個站一個固定時隙（例如，時隙時隙1給站給站a，時隙，時隙2給站給站b，等等），等等）.這

18、導(dǎo)致系統(tǒng)的設(shè)計(jì)簡這導(dǎo)致系統(tǒng)的設(shè)計(jì)簡單，但不利之處是如果站沒有數(shù)據(jù)發(fā)送的時候時隙被浪費(fèi)單，但不利之處是如果站沒有數(shù)據(jù)發(fā)送的時候時隙被浪費(fèi)了。了。unit 3v原文：原文：in dynamic tdma, a station is assigned a time slot only when it makes a request for it. this leads to a more complex system, but the channel is used effectively. v 譯文：在動態(tài)的時分多址中，一個站只在需要的譯文：在動態(tài)的時分多址中，一個站只在需要的時候才分配時隙。這使

19、得系統(tǒng)更復(fù)雜，但信道能時候才分配時隙。這使得系統(tǒng)更復(fù)雜，但信道能高效率的使用。高效率的使用。unit 3v 原文：原文：an important issue in tdma systems is synchronization: each station should know precisely when it can start transmission in its time slot. it is easy to till terminal a that it should transmit in time slot 1,but how does terminal a know whe

20、n exactly tine slot 1 starts? v 譯文：在時分多址中的一個重要課題是同步：每個站都要譯文：在時分多址中的一個重要課題是同步：每個站都要準(zhǔn)確的知道何時可在它的時隙中傳送信息。很容易知道終準(zhǔn)確的知道何時可在它的時隙中傳送信息。很容易知道終端端a在時隙在時隙1中發(fā)送，但中發(fā)送，但a如何知道時隙如何知道時隙1何時開始？何時開始？ unit 3v 原文：原文：if it starts transmission slightly early, the data will collide with the data in time slot 0. if it starts tr

21、ansmission slightly late, the data will collide with the data in time slot 2. the complexity of tdma lies in the synchronization. synchronization is achieved by the central station sending a bit pattern(101010101pattern), and all the stations use this bit pattern to synchronize their clocks. v 譯文：如果

22、開始傳送的早了，數(shù)據(jù)會和時隙譯文：如果開始傳送的早了，數(shù)據(jù)會和時隙0中的數(shù)據(jù)碰中的數(shù)據(jù)碰撞，如果開始的晚了數(shù)據(jù)會和時隙撞，如果開始的晚了數(shù)據(jù)會和時隙2中的數(shù)據(jù)碰撞中的數(shù)據(jù)碰撞.時分多時分多址的復(fù)雜位于同步上。通過中心站發(fā)送比特序列，所有的址的復(fù)雜位于同步上。通過中心站發(fā)送比特序列，所有的站使用這序列同步自己的時鐘來實(shí)現(xiàn)同步。站使用這序列同步自己的時鐘來實(shí)現(xiàn)同步。unit 3v 原文：原文：in tdma, a single frequency is shared by a number of terminals each terminal is assigned a small time sl

23、ot during which it can transmit. the time slot assignment can be fixed, or the assignment can be dynamic-a slot is assigned only when a terminal has data to transmit.v 譯文：在時分多址中，多終端共享一個頻率。每個終端被譯文：在時分多址中，多終端共享一個頻率。每個終端被分配一個時隙來傳送信息。分配的時隙可以是固定的也可分配一個時隙來傳送信息。分配的時隙可以是固定的也可是是 動態(tài)的動態(tài)的只在有數(shù)據(jù)傳動時分配時隙只在有數(shù)據(jù)傳動時分配時

24、隙 unit 3v 原文：原文：fdma/tdmav in many radio systems, a combination of fdma and tdma is used. for example, in cellular mobile communication systems, each base station is given a set of frequencies, and each frequency is divided into time slots to be shared by different terminals.v 譯文：在許多無線系統(tǒng)中共同使用頻分多址和時分

25、多址。譯文：在許多無線系統(tǒng)中共同使用頻分多址和時分多址。例如，蜂窩移動通信系統(tǒng)中，每個基站分配一組頻率，每例如，蜂窩移動通信系統(tǒng)中，每個基站分配一組頻率，每個頻率劃分為多個時隙來被不同終端共享。個頻率劃分為多個時隙來被不同終端共享。unit 3v 原文：原文：in fdma/tdma, a pool of frequencies is shared by a number of terminals. in addition, each frequency is shared in different time slots by the terminals. fdma/tdma is used

26、in mobile communication systems an satellite communication systems. the combination of fdma an tdma increases the capacity of the system.v 譯文：在譯文：在fdma/dma中，一組頻率由多終端共享。此中，一組頻率由多終端共享。此外，每個頻率由不同時隙的終端共享。外，每個頻率由不同時隙的終端共享。fdma/tdma使使用在移動通信系統(tǒng)和衛(wèi)星通信系統(tǒng)中。二者的綜合增加了用在移動通信系統(tǒng)和衛(wèi)星通信系統(tǒng)中。二者的綜合增加了系統(tǒng)的容量。系統(tǒng)的容量。 unit 3v 原

27、文：原文：cdmav code division multiple access (cdma) technology has been developed for defense application where secure communication is very important. in cdma systems, a channel with very large bandwidth is required, many times more than the bandwidth occupied by the information to be transmitted. for

28、instance, if the actual bandwidth required is 1mhz, in cdma systems, perhaps 80 mhz is allocated. v 譯文：發(fā)展碼分多址技術(shù)在安全通信很重要的防護(hù)應(yīng)用。譯文：發(fā)展碼分多址技術(shù)在安全通信很重要的防護(hù)應(yīng)用。在碼分多址系統(tǒng)中，信道需要很高的帶寬，是信息傳輸所在碼分多址系統(tǒng)中，信道需要很高的帶寬，是信息傳輸所需帶寬的很多倍。比如說，如果實(shí)際所需的帶寬是一兆赫需帶寬的很多倍。比如說，如果實(shí)際所需的帶寬是一兆赫茲，在碼分多址中可能就需要八十兆赫茲的帶寬。茲，在碼分多址中可能就需要八十兆赫茲的帶寬。unit 3

29、 原文：原文： such large bandwidths were available only with defense organizations, and hence cdma was used initially only for defense applications. because the spectrum is spread, these systems are also known as spread spectrum multiple access (ssma) systems. in this category, there are two types of tech

30、niques: frequency hopping and hopping and direct sequence. 譯文譯文 ：只有安全組織才可以利用這么多的帶寬。所以：只有安全組織才可以利用這么多的帶寬。所以碼分多址最初用在安全領(lǐng)域。因?yàn)轭l譜擴(kuò)寬了，這個系統(tǒng)碼分多址最初用在安全領(lǐng)域。因?yàn)轭l譜擴(kuò)寬了，這個系統(tǒng)也被稱為擴(kuò)頻多址系統(tǒng)。在這個范疇內(nèi)有兩種技術(shù)：跳頻也被稱為擴(kuò)頻多址系統(tǒng)。在這個范疇內(nèi)有兩種技術(shù)：跳頻和直接序列。和直接序列。unit 3v 原文：原文：frequency hopping (fh)v consider a system in which 1 mhz bandwidth i

31、s required to transmit the data, instead of allocating a radio channel of 1mhz only, a number of radio channels( say 79) will be allocated, each channel with 1 mhz bandwidth. we need a very large spectrum, 79 times that of the actual requirement. v 譯文：考慮需要一兆赫茲帶寬來傳輸數(shù)據(jù)的系統(tǒng)。不只是譯文：考慮需要一兆赫茲帶寬來傳輸數(shù)據(jù)的系統(tǒng)。不只是給

32、信道分配一兆赫茲的帶寬，還需要分配一組無線信道給信道分配一兆赫茲的帶寬，還需要分配一組無線信道（七十九），每個信道的帶寬為一兆赫茲。我們需要很大（七十九），每個信道的帶寬為一兆赫茲。我們需要很大的帶寬，是實(shí)際需要的七十七倍。的帶寬，是實(shí)際需要的七十七倍。unit 3v 原文：原文：when a station has to transmit its data, it will send the data in one channel for some time , switch over to another channel and transmit some more data,and ag

33、ain switch over to another channel and so on. this is known as frequency hopping (fh). v 譯文：當(dāng)信道發(fā)送數(shù)據(jù)時，在一個信道中發(fā)送一段時間，譯文：當(dāng)信道發(fā)送數(shù)據(jù)時，在一個信道中發(fā)送一段時間，轉(zhuǎn)移到另一個信道上發(fā)送更多的數(shù)據(jù)，然后在轉(zhuǎn)移到一個轉(zhuǎn)移到另一個信道上發(fā)送更多的數(shù)據(jù)，然后在轉(zhuǎn)移到一個信道上，這樣不斷的繼續(xù)。這就是跳頻。信道上，這樣不斷的繼續(xù)。這就是跳頻。unit 3v 原文：原文：when the transmitting station hops its frequency of transmiss

34、ion, only those stations that know the hopping sequence can receive the data. this will be a secure communication system if the hopping sequence is kept a secret between the transmitting and the receiving stations.v 譯文：當(dāng)發(fā)送站改變它的傳輸頻率是，只有知道跳頻序列譯文：當(dāng)發(fā)送站改變它的傳輸頻率是，只有知道跳頻序列的那些基站可以接受到數(shù)據(jù)。如果發(fā)送站和接收站之間的的那些基站可以接受

35、到數(shù)據(jù)。如果發(fā)送站和接收站之間的跳頻序列是保密的那么傳輸系統(tǒng)是安全的。跳頻序列是保密的那么傳輸系統(tǒng)是安全的。unit 3v原文：原文：frequency hopping is used in global system for mobile communications (gsm) and bluetooth radio systems.v譯文：跳頻使用在全球移動通信系統(tǒng)和藍(lán)牙無線譯文：跳頻使用在全球移動通信系統(tǒng)和藍(lán)牙無線系統(tǒng)中。系統(tǒng)中。unit 3v 原文：原文：direct sequence cdmav in direct sequence cdma(ds-cdma), each bit

36、to be transmitted is represented by multiple bits. for instance instead of transmitting a 1,a pattern of 16 1s and 0s is transmitted, and instead of transmitting a 0, another pattern of 16 1s and 0s is transmitted. effectively, we are increasing the data rate and hence the bandwidth requirement by16

37、 times. v 譯文譯文:在直擴(kuò)碼分多址中，每個傳送的比特被多個比特代在直擴(kuò)碼分多址中，每個傳送的比特被多個比特代替。簡單的說，用十六個替。簡單的說，用十六個1和和0組成的序列代替一個組成的序列代替一個1來傳來傳輸，用另外的十六個輸，用另外的十六個1和和0組成的序列來代替組成的序列來代替0來傳輸。我來傳輸。我們提高了數(shù)據(jù)的速率，所以帶寬需要成了原來的們提高了數(shù)據(jù)的速率，所以帶寬需要成了原來的 十六倍。十六倍。unit 3v 原文：原文：the number of bits to be transmitted in place of 1 or 0 is known as chipping r

38、ate. if the chipping code is kept a secret, only those stations that have the chipping code can decode the information. when multiple stations have to transmit, the chipping codes will be different for each station. v 譯文：代替譯文：代替1和和0的比特序列稱為碼片率。如果碼片編碼的比特序列稱為碼片率。如果碼片編碼是保密的，只有那些有碼片編碼的站可以解碼信息。當(dāng)多是保密的，只有那些

39、有碼片編碼的站可以解碼信息。當(dāng)多各站發(fā)送時，碼片的編碼是不同的。各站發(fā)送時，碼片的編碼是不同的。v原文：原文：if they are chosen in such a way they are orthogonal to each other, then the data from different stations can be pushed on to the channel simultaneously without interference.v譯文：如果碼片編碼是一這種方式選擇的，那么譯文：如果碼片編碼是一這種方式選擇的，那么它們都是相互正交的，來自不同站的數(shù)據(jù)可以沒它們都是相互正

40、交的，來自不同站的數(shù)據(jù)可以沒有干擾的同時裝到信道中。有干擾的同時裝到信道中。unit 3unit 3v原文：原文：cdma systems are now being widely deployed for cellular communications as well as 3g systems for accessing the internet through wireless networks. cdma systems are used in wireless local loops. v譯文譯文:碼分多址系統(tǒng)現(xiàn)在被廣泛的使用在蜂窩移動碼分多址系統(tǒng)現(xiàn)在被廣泛的使用在蜂窩移動通信系統(tǒng)中

41、，像通信系統(tǒng)中，像3g系統(tǒng)一樣通過無線網(wǎng)絡(luò)接入因系統(tǒng)一樣通過無線網(wǎng)絡(luò)接入因特網(wǎng)。碼分多址系統(tǒng)使用在無線本地回路。特網(wǎng)。碼分多址系統(tǒng)使用在無線本地回路。unit 3v 原文：原文： ds- cdma, multiple terminals transmit on the same channel simultaneously, with different chipping codes .if the chipping code length is 11 bits , both 1 and 0 are replaced by the 11 bit sequence of 1s and 0s .

42、this sequence is unique for each terminal.v 譯文：在直擴(kuò)碼分多址中多個終端同時在同一信道上通過譯文：在直擴(kuò)碼分多址中多個終端同時在同一信道上通過不同的碼片編碼發(fā)送。如果碼片編碼的長度為十一比特，不同的碼片編碼發(fā)送。如果碼片編碼的長度為十一比特，1和和0都被十一比特的都被十一比特的1，0序列取代。每個終端的序列是序列取代。每個終端的序列是唯一的。唯一的。unit 3v 原文：原文：summary v multiple access techniques provide the mechanism to share the bandwidth effic

43、iently by multiple terminals. particularly in radio systems where the spectrum bandwidth is very limited, multiple access techniques are very important. v 譯文：多址接入技術(shù)為多終端有效的共享帶寬提供了機(jī)制，譯文：多址接入技術(shù)為多終端有效的共享帶寬提供了機(jī)制，尤其是在頻譜帶寬受限制的無線系統(tǒng)中多址接入技術(shù)是非尤其是在頻譜帶寬受限制的無線系統(tǒng)中多址接入技術(shù)是非常重要的。常重要的。unit 3v 原文：原文：in frequency divisi

44、on multiple access (fdma), a pool of frequencies is assigned to a base station, and the terminals are allocated a specific frequency for communication. in time division multiple access (tdma), one frequency will be shared by different terminals in different time slots .v 譯文：在頻分多址中，一組頻率被分給一個基站，給終端譯文：

45、在頻分多址中，一組頻率被分給一個基站，給終端分配一個固定的頻率來傳輸。在時分多址中，一個頻率被分配一個固定的頻率來傳輸。在時分多址中，一個頻率被不同時隙的不同終端共享。不同時隙的不同終端共享。unit 3v 原文：原文：each terminal is assigned a specific time slot during which the terminal has to send the data. in tdma frequency division duplex (fdd), a pair of frequencies is used for communication uplink

46、 and downlink. v譯文：每個終端分配一個可以在上面?zhèn)鬏數(shù)臅r隙。譯文：每個終端分配一個可以在上面?zhèn)鬏數(shù)臅r隙。在頻分雙工中，一對頻率在通信的上行和下行鏈在頻分雙工中，一對頻率在通信的上行和下行鏈路上使用。路上使用。unit 3v 原文：原文：in tdma time division duplex (tdd), a single frequency is used for communication in both directions the time slots will be divided into two portions- downlink slots and uplin

47、k slots. the code division multiple access technique requires large bandwidth, but the advantage is that it provides secure communication. v 譯文：時分雙工時，通信的兩個方向上使用一個頻率，時譯文：時分雙工時，通信的兩個方向上使用一個頻率，時隙被劃分成上行時隙和下行時隙兩部分。碼分多址接入技隙被劃分成上行時隙和下行時隙兩部分。碼分多址接入技術(shù)需要大的帶寬，但好處是提供了安全的通信。術(shù)需要大的帶寬，但好處是提供了安全的通信。unit 3v 原文：原文：in

48、cdma, there are two mechanisms: frequency hopping (fh) and direct sequence. in fh, the frequency of transmission changes will be able to decode the data. in direct sequence cdma ,instead of sending the bit stream directly , chipping codes are used to transmit 1s and 0s.v 譯文：在碼分多址中，有兩種機(jī)制：跳頻和直擴(kuò)。在跳頻譯文：

49、在碼分多址中，有兩種機(jī)制：跳頻和直擴(kuò)。在跳頻中，傳輸?shù)念l率按隨機(jī)的可能快速的改變。只有知道跳變中，傳輸?shù)念l率按隨機(jī)的可能快速的改變。只有知道跳變序列的站可以解調(diào)數(shù)據(jù)，在直擴(kuò)碼分多址中序列的站可以解調(diào)數(shù)據(jù)，在直擴(kuò)碼分多址中 ，碼片編碼用，碼片編碼用來傳送來傳送1和和0，而不是直接發(fā)送比特流。，而不是直接發(fā)送比特流。unit 3 mutiple acesstext b multiplexingv 原文：原文：v in a communication system, the costliest element may be the transmission medium. to make the b

50、est use of the medium, we have to ensure that the bandwidth of the channel is utilized to its fullest capacity . v譯文：通信媒體也許是通許系統(tǒng)中成本最昂貴的譯文：通信媒體也許是通許系統(tǒng)中成本最昂貴的一部分。為了充分利用通信媒體，必須確保信道一部分。為了充分利用通信媒體，必須確保信道帶寬的利用達(dá)到最大容量。帶寬的利用達(dá)到最大容量。unit 3unit 3v 原文：原文：multiplexing is the technique used to combine a number of

51、channels and send them over the medium to make the best use of the transmission medium. we will discuss the various multiiplexing techniques in this chapter.v 譯文：復(fù)用技術(shù)先將許多信道合并，再將它們一起發(fā)送到譯文：復(fù)用技術(shù)先將許多信道合并，再將它們一起發(fā)送到通信媒體上，以實(shí)現(xiàn)通信媒體的通信媒體上，以實(shí)現(xiàn)通信媒體的 最大利用。本節(jié)我們將討最大利用。本節(jié)我們將討論各種復(fù)用技術(shù)。論各種復(fù)用技術(shù)。 unit 3v 原文：原文：the use o

52、f multiplxing technique is possible if the capacity of the channel is higher than the data rates of the individual data source . consider the example of a communication system in which there are three data sources. the signals from these three sources can be combined together (multiplexed) and sent

53、through a single transmission channel. v 譯文：復(fù)用技術(shù)在信道的容量大于信源的數(shù)據(jù)速率是可使譯文：復(fù)用技術(shù)在信道的容量大于信源的數(shù)據(jù)速率是可使用。假設(shè)通信系統(tǒng)中有三個信源的例子。來自三個信源的用。假設(shè)通信系統(tǒng)中有三個信源的例子。來自三個信源的信號合并在一起在一個傳輸信道上發(fā)送。信號合并在一起在一個傳輸信道上發(fā)送。unit 3v 原文原文:at the receiving end, the signals are separated (demultiplexed) . at the transmitting end. equipment known as

54、a multiplexer (abbreviated to mux ) is required. at the receiving end, equipment known as a multiplexer (abbreviated to demux ) is required, conceptually, multiplexing is a very simple operation that facilitates good utilization of the channel bandwidth.v 譯文：在接受端，信號被分解。在發(fā)送端需要復(fù)用設(shè)備。譯文：在接受端，信號被分解。在發(fā)送端需

55、要復(fù)用設(shè)備。在接收端需要解復(fù)用設(shè)備。從概念上說，復(fù)用是一種簡單在接收端需要解復(fù)用設(shè)備。從概念上說，復(fù)用是一種簡單的操作可以促進(jìn)信道帶寬的利用。的操作可以促進(jìn)信道帶寬的利用。unit 3v原文：原文：in frequency division multiplexing( fdm), the signals at translated into different frequency bands and sent over the medium. the communication channel is divided into different frequency bands and each

56、 band carries the signal corresponding to one source.v譯文：在頻分復(fù)用中，信號以不同的頻率發(fā)送然譯文：在頻分復(fù)用中，信號以不同的頻率發(fā)送然后在媒介上傳送。通信信道劃分成不同的頻段，后在媒介上傳送。通信信道劃分成不同的頻段，每個頻段承載一個信號響應(yīng)。每個頻段承載一個信號響應(yīng)。unit 3v 原文：原文：fdm is used in cable tv transmission, where signals corresponding to different tv channels are multiplexed and sent throug

57、h the cable . at the tv receiver, by applying the filter, the signals of aparticular channel can be viewed. v 譯文：頻分復(fù)用使用在有線電視傳輸，不同電視信道的信譯文：頻分復(fù)用使用在有線電視傳輸，不同電視信道的信號響應(yīng)被復(fù)用然后在電纜上發(fā)送，在電視接受端號響應(yīng)被復(fù)用然后在電纜上發(fā)送，在電視接受端 ，采用濾，采用濾波器收看特定信道的信號。波器收看特定信道的信號。unit 3v 原文：原文：radio an tv transmission, are also done using fdm,

58、where each broadcasting station is given a small band in the frequency spectrum. the center frequency of this band is known as the carrier frequency.v 譯文：無線和電視傳輸都使用頻分復(fù)用，每個廣播站分配譯文：無線和電視傳輸都使用頻分復(fù)用，每個廣播站分配給頻譜中的一個小的帶寬。頻段的中心頻率稱為載頻給頻譜中的一個小的帶寬。頻段的中心頻率稱為載頻 unit 3v 原文：原文：each voice channel occupies a bandwidt

59、h of 3.4khz. however, each channel is assigned a bandwidth of 4 khz . the second voice channel is frequency translated to the band 48 khz. similarly, the third voice channel is translated to 812 khz and so on. v 譯文：每路話音信道占用譯文：每路話音信道占用3.4khz的帶寬。然而，每個信的帶寬。然而，每個信道分配的帶寬是道分配的帶寬是4khz。第二路話音信號傳輸?shù)念l率的頻。第二路話音信

60、號傳輸?shù)念l率的頻帶范圍從帶范圍從4khz到到8khz。同樣的，第三路話音信號在。同樣的，第三路話音信號在8khz到到12khz范圍傳送。范圍傳送。unit 3v 原文：原文：slightly higher bandwidth is assigned (4khz instead of 3.4khz) mainly because it is very difficult to design filters of high accuracy .hence an additional bandwidth, known as guard band, separates two successive c