1、安徽建筑大學(xué)畢業(yè)設(shè)計(jì)英文翻譯專 業(yè) 電子信息工程 班 級(jí) 2010級(jí) 學(xué)生姓名 方芳 學(xué) 號(hào) 課 題 碼分多址通信系統(tǒng)的建模、仿真和設(shè)計(jì) 初始化模塊、基站接收模塊 指導(dǎo)教師 馮俊 2014 年 06 月 10 日譯文原文1.1 The basic concept of spread-spectrum communicationSpread spectrum communications basic characteristics, is used to transmit information to the signal bandwidth(W) is far greater than pra

2、ctical required minimum(effective) bandwidth (),as the radio of processing gain .As we well know,the ordinary AM,FM,or pulse code modulation,GP value in the area more than 10 times,collectively,the “narrow-band communication”,and spread-spectrum communication GP values as hundred or even thousands o

3、f times, can be called “broadband communication”.Due to the spread-spectrum signal,it is very low power transmitters,transmission space mostly drowned in the noise,it is difficult to intercepted by the other receiver ,only spreading codes with the same (or random PN code) receiver, Gain can be dealt

4、 with ,and despreading resume the original signal.1.2 The technology superiority of spread-spectrum communication.Strong anti-interference, bit error rate is low. As noted above, the spread spectrum communication system due to the expansion of the transmitter signal spectrum, the receiver despreadin

5、g reduction signal produced spreading gain, thereby greatly enhancing its interference tolerance. Under the spreading gain, or even negative in the signal-to-noise ratio conditions, can also signal from the noise drowned out Extraction, in the current business communications systems, spread spectrum

6、 communications systems, spread spectrum communication is only able to work in a negative signal-to-noise ratio under the conditions of communication .Anti-multi-path interference capability, increase the reliability of system. Spread-spectrum systems as used in the PN has a good correlation, correl

7、ation is very weak. Different paths to the transmission signal can easily be separated and may in time and re-alignment phase, formation of several superimposed signal power, thereby improving the systems performance to receive increased reliability of the system.Easy to use the same frequency, impr

8、oving the wireless spectrum utilization. Wireless spectrum is very valuable,although long-wave microwave have to be exploited, and still can not meet the needs of community. To this end, countries around the world are designed spectrum management, users can only use the frequency applications,rely o

9、n the channel to prevent the division between the channel interference.Due to the use of spread-spectrum communication related receive this high-tech,low signal output power(“a W,as a general-100mW),and will work in the channel noise and thermal noise in the background,easy to duplicate in the same

10、area using the same frequency,can now all share the same narrow-band frequency communication resources.Spread-spectrum communication is digital communication,particularly for digital voice and data transmission with their own encryption, only in the same PN code communication between users, is good

11、for hiding and confidential in nature, facilitating communication business. Easy to use spread-spectrum CDMA communications, voice compression and many other new technologies, more applicable to computer networks and digitization of voice,image information transmission.Communication in the most digi

12、tal circuits, equipment, highly integrated, easy installation, easy maintenance, but also very compact and reliable. The average failure rate no time was very long.1.3 Spread spectrum communication systemSpread spectrum communication,namely, spread spectrum communications (Spread spectrum communicat

13、ion), with fiber-optic communications,satellite communications,with access to the information age as the three major high-tech communications transmission. Spread spectrum communication is to send the information to be pseudo-random data is coded(Spread spectrum sequence: spread sequence) modulation

14、, spread spectrum and then the realization of transmission; the receiving end is using the same modem code and related processing, the restoration of the original data. Spread spectrum communication system has three main characteristics.(1) Carrier is an unpredictable, or so-called pseudo-random bro

15、adband signal.(2) Carrier data bandwidth than the modulation bandwidth is much wilder.(3) Receiving process is generated by local broadband carrier signal and receiving a copy of the signal to the broadband signal to achieve.The main way of spread spectrum are as follows: Direct Sequence Spread Spec

16、trum(DSSS) using high-speed pseudo-random code on to the low-speed data transmission spread spectrum modulation; Frequency-hopping system using pseudo-random code to control the carrier frequency in a wider band of the change; TH is the data transmission time slot is a pseudo-random; chirp frequency

17、 system is a linear extension of the process of change. Combination of a number of ways of hybrid systems are often applied.The most important measure pf spread-spectrum system is an indicator of spreading gain, also known as processing gain. It is precisely because of the spread spectrum system its

18、elf with its performance characteristics with a series of advantages.1.4 Code division multiple accessCode division multiple access (CDMA) is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000(t

19、he 3G evolution of cdmaOne) and WCDMA (the 3Gstandard used by GSM carrier), which are often referred to as simply CDMA, and use CDMA as an underlying channel access method.One of the concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a

20、 signal communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called multiple access. CDMA employs spread-spectrum technology and a special coding scheme( where each transmitter is assigned a code) to allow multiple user to be multiplexed ov

21、er the same physical channel. By contrast, time division multiple access (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated. 1.5 Spread-spectrum characteristic of CDMAMost mod

22、ulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military app

23、lications including guidance and communication systems. These system were designed using spread spectrum because if its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread spectru

24、m signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. CDMA also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer ca

25、n either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal.CDMA can also effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filter

26、ing without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signal are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to mult

27、ipath at any give time. Like the narrow band interference this will result in only a small loss of data and can be overcome.Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudo-random code, and will thus appear as another user, which is

28、 ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, 天the multipath channel induces at least one chip of delay,the multipath signals will arrive at the receiver.in other words, as long as the multipath channel induces at least one chip of del

29、ay, the multipath signals will arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it i

30、s thus ignored.Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a sim

31、ple receiver with a signal correlation tuned to the path delay of the strongest signal.Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is and important consideration. The frequencie

32、s used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Reusing the same frequency in every cell eliminat

33、es the need for frequency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.Since adjacent cell use the same frequencies, CDMA systems have the abil

34、ity to perform soft handoffs. Soft handoffs allow the mobile telephone to communication simultaneously with two or more cells. The best signal quality in selected until the handoff is complete. This is different from hard handoffs utilized in other cellular systems. In a hard handoff situation, as t

35、he mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal.Concluding remarksspread-spectrum technology in the initial stages of development, it has become a

36、 theory and a major technological breakthrough. Later in the development process is the improvement and hardware performance improved. Development to the present,spread-spectrum technology and the theory has been almost perfect,mainly from the point of view of overall performance, and the other new

37、technology applications. Therefore, the application has been driven by the development of spread-spectrum technology is a power driving force, the future wireless communication systems, such as mobile communication. Wireless LAN, global personal communications, spread-spectrum technology will certai

38、nly play an important role.譯文正文1.擴(kuò)頻通信系統(tǒng)概述擴(kuò)頻通信，即擴(kuò)展頻譜通信（Spread spectrum communication)，它與光纖通信、衛(wèi)星通信，一同譽(yù)為進(jìn)入信息時(shí)代的三大高技術(shù)通信傳輸方式，擴(kuò)頻通信是將待傳送的信息數(shù)據(jù)被偽隨機(jī)碼調(diào)制，實(shí)現(xiàn)頻譜擴(kuò)展后再傳輸；接收端則采用相同的編碼進(jìn)行解調(diào)及相關(guān)處理，恢復(fù)原始信息數(shù)據(jù)。擴(kuò)頻通信系統(tǒng)具有3個(gè)主要特征：（1）載波是不可預(yù)測的，或者稱為偽隨機(jī)的寬帶信號(hào)（2）載波的帶寬比調(diào)制數(shù)據(jù)的帶寬要寬得多（3）接收過程是通過本地將本地產(chǎn)生的寬帶載波信號(hào)的復(fù)制信號(hào)與接收到的寬帶信號(hào)相關(guān)來實(shí)現(xiàn)的。擴(kuò)展頻譜的方式主要有一下幾

39、種：直接序列擴(kuò)頻（DSSS）使用高速偽隨機(jī)碼對(duì)要傳輸?shù)牡退俾蕯?shù)據(jù)進(jìn)行擴(kuò)頻調(diào)制；跳頻系統(tǒng)則采用偽隨機(jī)碼控制載波頻率在一個(gè)更寬的頻帶內(nèi)變化；跳時(shí)則是數(shù)據(jù)傳輸時(shí)隙是偽隨機(jī)的；線性調(diào)頻系統(tǒng)中的頻率擴(kuò)展則是一個(gè)線性變化的過程。幾種方式組合的混合系統(tǒng)也經(jīng)常得到應(yīng)用。衡量擴(kuò)頻系統(tǒng)最重要的一個(gè)指標(biāo)是擴(kuò)頻增益，又稱為處理增益。正是因?yàn)閿U(kuò)頻系統(tǒng)本身具有的特征使其性能具有一系列的優(yōu)勢。1.2擴(kuò)頻通信技術(shù)的基本概念擴(kuò)頻通信的基本特點(diǎn)：其傳輸信息所用信號(hào)的帶寬（W）遠(yuǎn)大于信息本身實(shí)際所需的最小（有效）帶寬（），其比值稱為處理增益 眾所周知，在普通的調(diào)幅、調(diào)頻或者脈沖編碼調(diào)制通信中，值一般都在十多倍范圍內(nèi)，稱為“窄帶通信

40、”，而擴(kuò)頻通信的值高達(dá)幾百甚至幾千倍，可以稱為“寬帶通信”。由于擴(kuò)頻后的信號(hào)，它的發(fā)射功率很低，傳輸時(shí)大多數(shù)淹沒在空間的噪聲中，因此很難被其他接收機(jī)截獲，只有具有相同的擴(kuò)頻碼（或稱PN碼）的接收機(jī)，才能獲得處理增益，解擴(kuò)并恢復(fù)出原發(fā)的信號(hào)。1.3擴(kuò)頻技術(shù)的優(yōu)越性抗干擾能力強(qiáng)，誤碼率低，如上所述，擴(kuò)頻通信系統(tǒng)由于在發(fā)送端擴(kuò)展信號(hào)頻譜，在接收端解擴(kuò)還原信號(hào)，產(chǎn)生擴(kuò)頻增益，從而大大提高了抗干擾容限。根據(jù)擴(kuò)頻增益的不同，甚至在負(fù)的信噪比條件下，也可以將信號(hào)從噪聲的淹沒中提取出來，在目前商用的通信系統(tǒng)中，擴(kuò)頻通信是唯一能夠工作于負(fù)信噪比條件下的通信方式。抗多徑干擾能力強(qiáng)，增加了系統(tǒng)的可靠性。由于擴(kuò)頻系統(tǒng)

41、中采用的PN碼具有很好的自相關(guān)性，互相關(guān)性很弱，不同路徑傳來的信號(hào)很容易被分開，并可在時(shí)間相位上重新對(duì)齊，形成幾路信號(hào)功率的疊加，從而改善了接收系統(tǒng)的性能增加了系統(tǒng)的可靠性，易于同頻使用，提高了無線頻譜利用率。無線頻譜十分寶貴，雖然從長波、微波都已得到開發(fā)利用，仍然滿足不了社會(huì)的需求。為此，世界各地都設(shè)立了頻譜管理機(jī)構(gòu)，用戶只能使用申請(qǐng)獲得的頻率，依靠頻率劃分來防止信道之間的干擾。由于擴(kuò)頻通信又采取了相關(guān)接收這一技術(shù)，信號(hào)發(fā)送功率極低（1W，一般為1100mW），且可工作在信道噪聲和熱噪聲背景中，易于在同一地區(qū)重復(fù)使用同一頻率，也可以在現(xiàn)今各種窄帶通信共享同一頻率資源。擴(kuò)頻通信是數(shù)字通信，特別

42、適合數(shù)字語音和數(shù)據(jù)同時(shí)傳輸，擴(kuò)頻通信自身具有加密功能，只能在PN碼相同的用戶之間進(jìn)行通信，隱蔽性好，保密性強(qiáng)，便于開展各種通信業(yè)務(wù)。擴(kuò)頻通信容易采用碼分多址、語音壓縮等多項(xiàng)新技術(shù)，更加適用于計(jì)算機(jī)網(wǎng)絡(luò)以及數(shù)字化的語音、圖像信息傳輸。擴(kuò)頻通信絕大多數(shù)是數(shù)字電路，設(shè)備高度集成，安裝便捷，易于維護(hù)，也十分小巧可靠，平均無故障時(shí)間也很長。1.4碼分多址碼分多址（CDMA）是一種用于各種無線通信技術(shù)的信道接入方法。它不應(yīng)該與被用在移動(dòng)電話標(biāo)準(zhǔn)的要求的cdmaOne，CDMA2000（cdmaOne的的3G演進(jìn)）和WCDMA相混淆（用于GSM的載波3G標(biāo)準(zhǔn)），它往往被簡稱為CDMA，并且作為一個(gè)潛在的信道接入方法。在數(shù)據(jù)通信中有一種想法是允許多個(gè)發(fā)射機(jī)同時(shí)發(fā)送的信息通過信號(hào)通信信道。這允許多個(gè)用戶共享一個(gè)頻帶（見帶寬）。這個(gè)概念