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1、Unit 3 Mutiple Acess,Text A Multiple Access,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, multipl
2、e terminals or users share the bandwidth of the transmission medium. 译文：多址接入技术是多个通信终端共享通信媒介频段的技术。多址技术对于频段非常有限的无线通信系统而言是极其重要的。在多址接入技术中，多个通信终端或多个用户共享传输媒介频段,Unit 3,Unit 3,原文：Multiple access attains much importance in radio communications because the radio spectrum is a precious natural resource, and to
3、 make best use of the radio bandwidth is impotrant. We will discuss the various multiple access techniques with illustrative examples. 译文:在无线通信系统中，由于无线频谱是非常宝贵的自然资源，多址技术对于获得无线频谱的最好利用是至关重要的。我们将通过例子来讨论各种多址技术 。,Unit 3,原文：FDMA In radio communication system, a certain frequency band is allocated for speci
4、fic use. This band is divided into smaller bands, and this pool of bands is available for all the terminals to share. Depending on the need, the base station allocates a frequency for the terminal to transmit. 译文：频分多址（FDMA） 在无线通信系统中，不同的频谱分配给特定的用户使用。这些频段被划分为更小的频段，供所有的终端共享。,Unit 3,原文：This is known as
5、frequency division multiple access (FDMA). In FDMA systems, there is central station that allocates the band (or frequency) to different terminals, based on their needs. 译文：基站根据需要给不同终端支配不同的频段以传输信号，这就是频分多址 。在频分多址系统中有一个中心站，它根据终端的需求来分配频段。,Unit 3,原文：Consider the example of mobile communication system. T
6、he system consists of number of base stations. A base station has a pool of frequencies. 译文：考虑以移动通信系统为例.该系统由许多基站组成。一个基站拥有很多不同的频段。,Unit 3,原文：All frequencies are available for sharing by the mobile terminals located around the base station The frequency will be allocated based on need. Mobile terminal
7、 A will be allocated the frequency f1, and then mobile terminal B will be allocated the frequency f2. 译文：所有频段都可以被该基站周围的移动终端共享，这种频段指配方式是 按需的。移动终端A得到频率f1,移动终端B得到频率f2。,Unit 3,原文：To ensure that there is no overlap in transmission by different terminals using adjacent frequencies, a guard band is provide
8、d. A small separation between adjacent bands is called the guard band. 译文：为了确保使用相邻频率的不同终端不出现频率交迭，必须设置保护频段。将相邻频段彼此分开的小频段称为保护频段。,Unit 3,原文：SDMA Because radio 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 divi
9、sion multiple access (SDMA). 译文：空分多址（SDMA） 因为无线频谱是一种自然资源，我们要最好的利用。当一组频率在别的位置重复使用，就称为空分多址,Unit 3,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
10、between the two regions. 译文：再次考虑移动通信系统。一个基站会被分配一些频率。同样的这些频率可分配给其他地区，只要两个区域之间距离足够长。,Unit 3,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 separate
11、d by a distance called reuse distance. SDMA is used in mobile communication systems. 译文：在空分多址中，服务区别划分为几个小的地区称为小区并且每个小区都分配给一些固定的频率。两个小区可使用同一组频率，两个小区之间的距离称为复用距离。空分多址在移动通信系统中使用。,Unit 3,原文：In time division multiple access (TDMA), one frequency is shared by a number of terminals. Each station is given a
12、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. 译文：在时分多址中一个频率可以被多个终端共享。分配给每个站一个时隙用来发送，时隙的序号是确定的，终端在分配的时隙上发送信息。,Unit 3,原文：For example , in mobile communication systems, each frequency is divided into eight time slo
13、ts 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. 译文：例如，在移动通信系统中，每个频率划分为八个时隙 ，所以八个移动终端使用同个频率在不同的时隙上通信。终端所用的时隙是由基站决定的。,Unit 3,原文：In TDMA systems, the time slots can be fixed or dynamic
14、. In fixed TDMA, each station is given a fixed tine slot ( for example, time slot 1 for station A, time slot 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. 译文：在时分多址系统中，时隙可以是固定的或可变的。在静态的
15、时分多址中，分配给每个站一个固定时隙（例如，时隙1给站A，时隙2给站B，等等）.这导致系统的设计简单，但不利之处是如果站没有数据发送的时候时隙被浪费了。,Unit 3,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. 译文：在动态的时分多址中，一个站只在需要的时候才分配时隙。这使得系统更复杂，但信道能高效率的使用。,U
16、nit 3,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 when exactly tine slot 1 starts? 译文：在时分多址中的一个重要课题是同步：每
17、个站都要准确的知道何时可在它的时隙中传送信息。很容易知道终端A在时隙1中发送，但A如何知道时隙1何时开始？,Unit 3,原文：If it starts transmission slightly early, the data will collide with the data in time slot 0. If it starts transmission slightly late, the data will collide with the data in time slot 2. The complexity of TDMA lies in the synchronizatio
18、n. Synchronization is achieved by the central station sending a bit pattern(101010101pattern), and all the stations use this bit pattern to synchronize their clocks. 译文：如果开始传送的早了，数据会和时隙0中的数据碰撞，如果开始的晚了数据会和时隙2中的数据碰撞.时分多址的复杂位于同步上。通过中心站发送比特序列，所有的站使用这序列同步自己的时钟来实现同步。,Unit 3,原文：In TDMA, a single frequency
19、is shared by a number of terminals Each terminal is assigned a small time slot 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. 译文：在时分多址中，多终端共享一个频率。每个终端被分配一个时隙来传送信息。分配的时隙可以是固定的也可是 动态的只在
20、有数据传动时分配时隙,Unit 3,原文：FDMA/TDMA 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. 译文：在许多无线系统中共同使用频分多址
21、和时分多址。例如，蜂窝移动通信系统中，每个基站分配一组频率，每个频率划分为多个时隙来被不同终端共享。,Unit 3,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 in mobile communication systems an satellite communication systems. T
22、he combination of FDMA an TDMA increases the capacity of the system. 译文：在FDMA/DMA中，一组频率由多终端共享。此外，每个频率由不同时隙的终端共享。FDMA/TDMA使用在移动通信系统和卫星通信系统中。二者的综合增加了系统的容量。,Unit 3,原文：CDMA Code division multiple access (CDMA) technology has been developed for defense application where secure communication is very impor
23、tant. 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 instance, if the actual bandwidth required is 1MHZ, in CDMA systems, perhaps 80 MHZ is allocated. 译文：发展码分多址技术在安全通信很重要的防护应用。在码分多址系统中，信道需要很高的带宽，
24、是信息传输所需带宽的很多倍。比如说，如果实际所需的带宽是一兆赫兹，在码分多址中可能就需要八十兆赫兹的带宽。,Unit 3,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 (SSM
25、A) systems. In this category, there are two types of techniques: frequency hopping and hopping and direct sequence. 译文 ：只有安全组织才可以利用这么多的带宽。所以码分多址最初用在安全领域。因为频谱扩宽了，这个系统也被称为扩频多址系统。在这个范畴内有两种技术：跳频和直接序列。,Unit 3,原文：Frequency Hopping (FH) Consider a system in which 1 MHz bandwidth is required to transmit the
26、 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. 译文：考虑需要一兆赫兹带宽来传输数据的系统。不只是给信道分配一兆赫兹的带宽，还需要分配一组无线信道（七十九），每个信道的带宽为一兆赫兹。我们需要很大的带宽，是
27、实际需要的七十七倍。,Unit 3,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 again switch over to another channel and so on. This is known as frequency hopping (FH). 译文：当信道发送数据时，在一个信道中发送一段时间，转移到另一个信道
28、上发送更多的数据，然后在转移到一个信道上，这样不断的继续。这就是跳频。,Unit 3,原文：When the transmitting station hops its frequency of transmission, 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 th
29、e receiving stations. 译文：当发送站改变它的传输频率是，只有知道跳频序列的那些基站可以接受到数据。如果发送站和接收站之间的跳频序列是保密的那么传输系统是安全的。,Unit 3,原文：Frequency hopping is used in Global System for Mobile Communications (GSM) and Bluetooth radio systems. 译文：跳频使用在全球移动通信系统和蓝牙无线系统中。,Unit 3,原文：Direct Sequence CDMA In direct sequence CDMA(DS-CDMA), eac
30、h bit 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 requiremen
31、t by16 times. 译文:在直扩码分多址中，每个传送的比特被多个比特代替。简单的说，用十六个1和0组成的序列代替一个1来传输，用另外的十六个1和0组成的序列来代替0来传输。我们提高了数据的速率，所以带宽需要成了原来的 十六倍。,Unit 3,原文：The number of bits to be transmitted in place of 1 or 0 is known as chipping rate. If the chipping code is kept a secret, only those stations that have the chipping code ca
32、n decode the information. When multiple stations have to transmit, the chipping codes will be different for each station. 译文：代替1和0的比特序列称为码片率。如果码片编码是保密的，只有那些有码片编码的站可以解码信息。当多各站发送时，码片的编码是不同的。,原文：If they are chosen in such a way they are orthogonal to each other, then the data from different stations ca
33、n be pushed on to the channel simultaneously without interference. 译文：如果码片编码是一这种方式选择的，那么它们都是相互正交的，来自不同站的数据可以没有干扰的同时装到信道中。,Unit 3,Unit 3,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 a
34、re used in wireless local loops. 译文:码分多址系统现在被广泛的使用在蜂窝移动通信系统中，像3G系统一样通过无线网络接入因特网。码分多址系统使用在无线本地回路。,Unit 3,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 sequen
35、ce of 1s and 0s . This sequence is unique for each terminal. 译文：在直扩码分多址中多个终端同时在同一信道上通过不同的码片编码发送。如果码片编码的长度为十一比特，1和0都被十一比特的1，0序列取代。每个终端的序列是唯一的。,Unit 3,原文：Summary Multiple access techniques provide the mechanism to share the bandwidth efficiently by multiple terminals. Particularly in radio systems whe
36、re the spectrum bandwidth is very limited, multiple access techniques are very important. 译文：多址接入技术为多终端有效的共享带宽提供了机制，尤其是在频谱带宽受限制的无线系统中多址接入技术是非常重要的。,Unit 3,原文：In frequency division multiple access (FDMA), a pool of frequencies is assigned to a base station, and the terminals are allocated a specific f
37、requency for communication. In time division multiple access (TDMA), one frequency will be shared by different terminals in different time slots . 译文：在频分多址中，一组频率被分给一个基站，给终端分配一个固定的频率来传输。在时分多址中，一个频率被不同时隙的不同终端共享。,Unit 3,原文：Each terminal is assigned a specific time slot during which the terminal has to
38、send the data. In TDMA frequency division duplex (FDD), a pair of frequencies is used for communication uplink and downlink. 译文：每个终端分配一个可以在上面传输的时隙。在频分双工中，一对频率在通信的上行和下行链路上使用。,Unit 3,原文：In TDMA time division duplex (TDD), a single frequency is used for communication in both directions the time slots w
39、ill be divided into two portions- downlink slots and uplink slots. The code division multiple access technique requires large bandwidth, but the advantage is that it provides secure communication. 译文：时分双工时，通信的两个方向上使用一个频率，时隙被划分成上行时隙和下行时隙两部分。码分多址接入技术需要大的带宽，但好处是提供了安全的通信。,Unit 3,原文：In CDMA, there are tw
40、o 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. 译文：在码分多址中，有两种机制：跳频和直扩。在跳频中，传输的频率按随机的可能快速的改变。只有知
41、道跳变序列的站可以解调数据，在直扩码分多址中 ，码片编码用来传送1和0，而不是直接发送比特流。,Unit 3 Mutiple Acess,Text B Multiplexing,原文： In a communication system, the costliest element may be the transmission medium. To make the best use of the medium, we have to ensure that the bandwidth of the channel is utilized to its fullest capacity .
42、译文：通信媒体也许是通许系统中成本最昂贵的一部分。为了充分利用通信媒体，必须确保信道带宽的利用达到最大容量。,Unit 3,Unit 3,原文：Multiplexing is the technique used to combine a number of 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. 译文：复用技术先将许多信
43、道合并，再将它们一起发送到通信媒体上，以实现通信媒体的 最大利用。本节我们将讨论各种复用技术。,Unit 3,原文：The use of 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 t
44、hese three sources can be combined together (multiplexed) and sent through a single transmission channel. 译文：复用技术在信道的容量大于信源的数据速率是可使用。假设通信系统中有三个信源的例子。来自三个信源的信号合并在一起在一个传输信道上发送。,Unit 3,原文:At the receiving end, the signals are separated (demultiplexed) . At the transmitting end. Equipment known as a mul
45、tiplexer (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. 译文：在接受端，信号被分解。在发送端需要复用设备。在接收端需要解复用设备。从概念上说，复用是一种简单
46、的操作可以促进信道带宽的利用。,Unit 3,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 band carries the signal corresponding to one source. 译文：在频分复用中，信号以不同的频率发
47、送然后在媒介上传送。通信信道划分成不同的频段，每个频段承载一个信号响应。,Unit 3,原文：FDM is used in cable TV transmission, where signals corresponding to different TV channels are multiplexed and sent through the cable . At the TV receiver, by applying the filter, the signals of aparticular channel can be viewed. 译文：频分复用使用在有线电视传输，不同电视信道
48、的信号响应被复用然后在电缆上发送，在电视接受端 ，采用滤波器收看特定信道的信号。,Unit 3,原文：Radio an TV transmission, are also done using FDM, 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. 译文：无线和电视传输都使用频分复用，每个广播站分配给频谱中的一个小的带宽。频段的中心频率称为载频
49、,Unit 3,原文：Each voice channel occupies a bandwidth 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. 译文：每路话音信道占用3.4kHZ的带宽。然而，每个信道分配的带宽是4KHZ。第二路话音信号传输的频率的频带范围从4KHZ到8KHZ。同样的，第三路话音信号在8KHZ到12KHZ范围传送。,Unit 3,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 channels.