中英對(duì)照 擴(kuò)頻通信

1、淺談擴(kuò)頻通信的原理一、擴(kuò)頻通信的工作原理在發(fā)端輸人的信息先調(diào)制形成數(shù)字信號(hào),然后由擴(kuò)頻碼發(fā)生器產(chǎn)生的擴(kuò)頻碼序列去調(diào)制數(shù)字信號(hào)以展寬信號(hào) 的頻譜,展寬后的信號(hào)再調(diào)制到射頻發(fā)送出去。在接收端收到的寬帶射頻信號(hào),變頻至中頻,然后由本地產(chǎn)生的 與發(fā)端相同的擴(kuò)頻碼序列去相關(guān)解擴(kuò),再經(jīng)信息解調(diào),恢復(fù)成原始信息輸出?？梢?一般的擴(kuò)頻通信系統(tǒng)都要進(jìn) 行 3次調(diào)制和相應(yīng)的解調(diào)。一次調(diào)制為信息調(diào)制,二次調(diào)制為擴(kuò)頻調(diào)制,三次調(diào)制為射頻調(diào)制,以及相應(yīng)的信息 解調(diào)、解擴(kuò)和射頻解調(diào)。與一般通信系統(tǒng)比較,多了擴(kuò)頻調(diào)制和解擴(kuò)部分。擴(kuò)頻通信應(yīng)具備如下特征:(1數(shù)字傳 輸方式; (2傳輸信號(hào)的帶寬遠(yuǎn)大于被傳信息帶寬; (3帶寬的

2、展寬,是利用與被傳信息無(wú)關(guān)的函數(shù) (擴(kuò)頻函數(shù) 對(duì)被 傳信息的信元重新進(jìn)行調(diào)制實(shí)現(xiàn)的; (4接收端用相同的擴(kuò)頻函數(shù)進(jìn)行相關(guān)解調(diào) (解擴(kuò) ,求解出被傳信息的數(shù)據(jù)。 用擴(kuò)頻函數(shù) (也稱偽隨機(jī)碼 調(diào)制和對(duì)信號(hào)相關(guān)處理是擴(kuò)頻通信有別于其他通信的兩大特點(diǎn)。二、擴(kuò)頻通信技術(shù)的特點(diǎn)擴(kuò)頻信號(hào)是不可預(yù)測(cè)的、偽隨機(jī)的寬帶信號(hào),其帶寬遠(yuǎn)大于要傳輸?shù)臄?shù)據(jù) (信息 帶寬,同時(shí)接收機(jī)中必須有 與寬帶載波同步的副本。擴(kuò)頻系統(tǒng)具有以下特點(diǎn)。1.抗干擾性強(qiáng)擴(kuò)頻信號(hào)的不可預(yù)測(cè)性,使擴(kuò)頻系統(tǒng)具有很強(qiáng)的抗干擾能力。干擾者很難通過觀察進(jìn)行干擾,干擾起不了太 大作用。擴(kuò)頻通信系統(tǒng)在傳輸過程中擴(kuò)展了信號(hào)帶寬,所以即使信噪比很低,甚至在有用信

3、號(hào)功率低于干擾信號(hào) 功率的情況下,仍能不受干擾、高質(zhì)量地進(jìn)行通信,擴(kuò)展的頻譜越寬,其抗干擾性越強(qiáng)。2. 低截獲性擴(kuò)頻信號(hào)的功率均勻分布在很寬的頻帶上,傳輸信號(hào)的功率密度很低,偵察接收機(jī)很難監(jiān)測(cè)到,因此擴(kuò)頻通 信系統(tǒng)截獲概率很低。3. 抗多路徑干擾性能好多路徑干擾是電波傳播過程中因遇到各種非期望反射體 (如電離層、高山、建筑物等 引起的反射或散射,在 接收端的這些反射或散射信號(hào)與直達(dá)路徑信號(hào)相互干涉而造成的干擾。多路徑干擾會(huì)嚴(yán)重影響通信。擴(kuò)頻通信系 統(tǒng)中增加了擴(kuò)頻調(diào)制和解擴(kuò)過程,利用擴(kuò)頻碼序列間的相關(guān)特性,在接收端解擴(kuò)時(shí),從多徑信號(hào)中分離出最強(qiáng)的 有用信號(hào),或?qū)⒍鄰叫盘?hào)中的相同碼序列信號(hào)疊加,這

4、樣就可有效消除無(wú)線通信中因多徑干擾造成的信號(hào)衰落現(xiàn) 象,使擴(kuò)頻通信系統(tǒng)具有良好的抗多徑衰落特性。4. 保密性好在一定的發(fā)射功率下,擴(kuò)頻信號(hào)分布在很寬的頻帶內(nèi),無(wú)線信道中有用信號(hào)功率譜密度極低,這樣信號(hào)可以 在強(qiáng)噪聲背景下,甚至在有用信號(hào)被噪聲淹沒的情況下進(jìn)行可靠通信,使外界很難截獲傳送的信息,要想進(jìn)一步 檢測(cè)出信號(hào)的特征參數(shù)就更難了.所以擴(kuò)頻系統(tǒng)可實(shí)現(xiàn)隱蔽通信。同時(shí),對(duì)不同用戶使用不同碼,旁人無(wú)法竊聽 通信,因而擴(kuò)頻系統(tǒng)具有高保密性。5. 易于實(shí)現(xiàn)碼分多址在通信系統(tǒng)中,可充分利用在擴(kuò)頻調(diào)制中使用的擴(kuò)頻碼序列之間良好的自相關(guān)特性和互相關(guān)特性,接收端利 用相關(guān)檢測(cè)技術(shù)進(jìn)行解擴(kuò),在分配給不同用戶不同

5、碼型的情況下,系統(tǒng)可以區(qū)分不同用戶的信號(hào),這樣同一頻帶 上許多用戶可以同時(shí)通話而互不干擾。在擴(kuò)頻系統(tǒng)中, 信號(hào)頻譜的擴(kuò)展是通過信息數(shù)據(jù)與擴(kuò)頻碼異或?qū)崿F(xiàn)的 , 因此擴(kuò)頻系統(tǒng)性能的好壞直接取 決于擴(kuò)頻碼的特性。通常來(lái)說(shuō) , 擴(kuò)頻碼應(yīng)滿足以下要求:1. 易于產(chǎn)生; 2. 具有隨機(jī)性; 3. 具有盡可能長(zhǎng)的周期; 4. 具有尖銳的自相關(guān)特性和良好的互相關(guān)特 性;從理論上說(shuō),用純隨機(jī)序列去擴(kuò)展信號(hào)頻譜是最理想的。但是接收機(jī)中為了解擴(kuò)應(yīng)當(dāng)有一個(gè)與發(fā)送端 擴(kuò)頻碼同步的副本, 而真正的隨機(jī)序列是不可能重復(fù)產(chǎn)生的 , 因此實(shí)際上, 我們只能用偽隨機(jī)或偽噪聲 (PN 序列作為擴(kuò)頻碼。偽隨機(jī)序列具有貌似噪聲性質(zhì),但

6、它是周期性有規(guī)律,既容易產(chǎn)生,又可以加工和復(fù)制 的序列。常用的擴(kuò)頻技術(shù)是跳頻擴(kuò)頻, 其實(shí)現(xiàn)方法是載頻信號(hào)以一定的速度和順序, 在多個(gè)頻率點(diǎn)上跳變傳遞, 接收端以相應(yīng)的速度和順序接收并解調(diào)。這個(gè)預(yù)先設(shè)定的頻率跳變的序列就是 PN 碼。在 PN 碼的控制下,收發(fā)雙方按照設(shè)定的序列在不同的頻點(diǎn)上進(jìn)行通信。由于系統(tǒng)的工作頻率在不停的跳變,在每個(gè)頻率點(diǎn)上 停留的時(shí)間僅為毫秒或微秒級(jí),因此在一個(gè)相對(duì)的時(shí)間段內(nèi),就可以看作在一個(gè)寬的頻段內(nèi)分布了傳輸信 號(hào),也就是寬帶傳輸。跳頻通信系統(tǒng)的頻率跳頻速度反映了系統(tǒng)的性能,好的跳頻系統(tǒng)每秒的跳頻次數(shù)可 以達(dá)到上萬(wàn)跳 . 跳頻通信系統(tǒng)在每個(gè)跳頻點(diǎn)上的瞬時(shí)通信實(shí)際上還是

7、窄帶通信。三、擴(kuò)頻技術(shù)的 發(fā)展 與應(yīng)用在過去由于技術(shù)的限制,人們一直在走增加信號(hào)功率,減少噪聲,提高信噪比的道路。即使到了 70年代, 偽碼技術(shù)已經(jīng)出現(xiàn),但作為相關(guān)器的 “ 碼環(huán) ” 的鐘頻只能做到幾千赫茲也無(wú)助于事.近幾年,由于大規(guī)模集成電路 的發(fā)展,幾十兆赫茲,甚至幾百兆赫茲的偽碼發(fā)生器及其相關(guān)部件都已成為現(xiàn)實(shí),擴(kuò)頻通信獲得極其迅速的發(fā) 展.通信的發(fā)展史又到了一個(gè)轉(zhuǎn)折點(diǎn),由用信噪比換帶寬的年代進(jìn)入了用寬帶換信噪比的年代.從最佳通信系統(tǒng) 的角度看擴(kuò)頻通信.最佳通信系統(tǒng)一最佳發(fā)射機(jī) +最佳接收機(jī).幾十年來(lái),最佳接收理論已經(jīng)很成熟,但最佳發(fā) 射問題一直沒有很好解決,偽碼擴(kuò)頻是一種最佳的信號(hào)形式和

8、調(diào)制制度,構(gòu)成了最佳發(fā)射機(jī).因此,有了最佳通 信系統(tǒng)一偽碼擴(kuò)頻 +相關(guān)接收這種認(rèn)識(shí), 人們就不難預(yù)測(cè)擴(kuò)頻通信的未來(lái)前景. 從 9O 年代無(wú)線通信開始步人擴(kuò)頻 通信和自適應(yīng)通信的年代.擴(kuò)頻通信的熱浪已經(jīng)波及短波、超微波、微波通信和衛(wèi)星通信,碼分多址 (CDMA已 開始廣泛用于未來(lái)的峰窩通信、無(wú)繩通信和個(gè)人通信以及各種無(wú)線本地環(huán)路,發(fā)揮越來(lái)越大的作用.接入網(wǎng)是由 傳統(tǒng)的用戶線、 用戶環(huán)路和用戶接入系統(tǒng), 逐步發(fā)展、 演變和升級(jí)而形成的. 現(xiàn)代 電信 網(wǎng)絡(luò) 分為 3部分:傳輸網(wǎng)、 交換網(wǎng)和接入網(wǎng).由于接入網(wǎng)發(fā)展較晚,往往成為電信發(fā)展的 “ 瓶頸 ” ,各國(guó)都很重視接入網(wǎng)的發(fā)展,因此各類接 人技術(shù)和系

9、統(tǒng)應(yīng)運(yùn)而生.由于 ISM(Industry Scientific Medica1頻段的開放性,經(jīng)營(yíng)者和用戶不需申請(qǐng)授權(quán)就可以 自由地使用這些頻段, 而無(wú)線擴(kuò)頻技術(shù)所使用的頻段 (2. 4002. 483 正是全世界通用的 ISM 頻段, 包括 IEEE802.11協(xié)議架構(gòu)的無(wú)線局域網(wǎng)也大部分選用此頻段. 在無(wú)線接人系統(tǒng)中, 擴(kuò)頻微波與常規(guī)微波相比有著 3個(gè)顯著的優(yōu)點(diǎn):抗干擾性強(qiáng)、頻點(diǎn)問題容易處理、價(jià)格比較便宜.而且,擴(kuò)頻微波接入技術(shù)相對(duì)有線接入技術(shù)來(lái)說(shuō),有成本低、 使用靈活、建設(shè)快捷的優(yōu)勢(shì),在接入網(wǎng)中起著不可替代的作用 .擴(kuò)頻微波主要應(yīng)用在以下幾個(gè)方面.語(yǔ)音接入 (點(diǎn)對(duì)點(diǎn) ;數(shù)據(jù)接入;視頻接入

10、;多媒體接入; 因特網(wǎng) (Internet接入。四、結(jié)語(yǔ)擴(kuò)頻通信是通信的一個(gè)重要分支和發(fā)展方向,是擴(kuò)頻技術(shù)與通信相結(jié)合的產(chǎn)物。本文主要論述了擴(kuò)頻通信的 特點(diǎn)、理論可行性及典型的工作方式。擴(kuò)頻通信的強(qiáng)抗干擾性、低截獲性、良好的抗多路徑干擾性和安全性等特 點(diǎn),使它的應(yīng)用迅速?gòu)能娪脭U(kuò)展到民用通信中,它的易于實(shí)現(xiàn)碼分多址的特點(diǎn),使它能與第三代移動(dòng)通信系統(tǒng)完 美結(jié)合,發(fā)展前景極為廣闊。Discussion on Spectrum Communication PrincipleOne , Spread Spectrum Communication work Modulation of the inform

11、ation in the beginning people to lose the formation of digital signal and the Spread Spectrum code generator to produce Spread Spectrum code - sequence modulation signal of digital signal in broadening the spectrum broadening of the signal modulation to send out radio. Broadband RF signal received a

12、t the receiving end, the frequency conversion to intermediate frequency, and then by local Associated with the Spread Spectrum code sequence of beginning the same solution of expansion and demodulation and restore the original message output. We can see that most Spread Spectrum Communication System

13、 will be 3 modulation and demodulation accordingly. Modulation of information modulation, two modulation for Spread Spectrum modulation, three modulation of RF modulation and demodulation, and radio frequency demodulation solution of expansion. General communication systems, part of Spread Spectrum

14、modulation settlement expansion. Spread spectrum function is called pseudo - random code modulation and its signal processing is Spread Spectrum Communication is different from the other two major characteristics of communications.Two. characteristics of Spread Spectrum Communication Technology Spre

15、ad Spectrumsignal is unpredictable, pseudo - random wide - band signal and its bandwidth than to data information transmission bandwidth and receiver must have and copies of broadband carrier synchronization. Spread Spectrum system has the following characteristics. 1. strong anti - interference Spr

16、ead Spectrum signal unpredictability, Spread Spectrum system with strong anti - interference ability. Interference difficult by observing interference and interference not be too big. Spread Spectrum Communication System in the process of transmission expansion of signal bandwidth, so even if the si

17、gnal - to - noise ratio is very low, even lower than interfering signal power available signal power circumstances, can still without interference, high - quality communication, expand the spectrum width and its anti - jamming the stronger. 2. low intercepted Spread Spectrum signal power distributio

18、n in wide frequency band and transmit power density is low, reconnaissance receiver difficult to monitor, it Spread Spectrum Communication System for low probability of intercept. 3. anti - multipath interference performance better Multi - path interference is due to meet with various non - expectat

19、ions reflected in the process of radio wave propagation ionospheric, mountains, buildings caused by reflecting and scattering, at the receiving end of these reflecting and scattering signal interference caused by direct path signal interference. Multi - path interference would seriously affect traff

20、ic. meanwhile, in different user using different code, others cannot interception of communications, which spread spectrum system with high secrecy. 5. easy to implement code division multiple access。In Spread Spectrum systems, signal spectrum expansion is through information and spread differences

21、or achieve, so Spread Spectrum system performance will have a direct depends on the characteristics of Spread Spectrum code . Generally speaking, Spread Spectrum code should meet the following requirements : 1. easy to produce ; 2with randomness. 3. a long period as far as possible ;. with acute sel

22、f - correlation characteristic and good cross - correlation ;theoretically, pure random sequence to expand signal frequency spectrum is the best. however, there should be a receiver to understand the expansion and send copies of the end of Spread Spectrum code synchronous, and the true random sequen

23、ce cannot be repeated.And so, in fact, we can only use pseudo - random or pseudo noise of PN sequences as spreading codes. Pseudo - random sequence with properties, but it is periodic law is easy, and can process and replication sequence.Spread spectrum technology is the frequency hopping spread spe

24、ctrum, its implementation is a carrier signal to a certain speed and order in multiple frequency jump pass, the receiving end and order of receiving and demodulation at the appropriate speed. This predetermined frequency hopping sequences are PN code. Under the Control of PN code in, sending the two

25、 sides in accordance with the set of sequences at different frequency communication. Due to the jump of system frequency in all, at each frequency length of stay only as MS or microsecond, therefore, in a period of time, they can be thought on distribution of a wide - band signal transmission, which

26、 is the Broadband transmission. Frequency Hopping speed of frequency hopping communication system reflects the performance of the system and frequency - hopping system of frequency hopping frequency can reach tens of thousands per second jump. frequency hopping communication system at each jump freq

27、uency instantaneous communication in fact, or narrowband communication.Three, the development and application of spread spectrum technologyIn the past, due to technical restrictions, people have been going to increase signal power,reduce noise and improve the road of signal - to - noise.Even in the

28、70, the pseudo - code technology has occurred, but as a code Rings clock frequency only thousands of Hertz but also help in the matter.In recent years, due to the development of large - scale integrated circuits, dozens of MHZ or even hundreds of Mhz pseudo - code generator and its related component

29、s have already become a reality. Spread Spectrum Communication is extremely rapid development. development history of communication and a turning point, from entering the broadband signal - to - noise ratio for bandwidth in the change of signal to noise ratio. from the angle of the best communicatio

30、n system spread spectrum communication.A best of the best communication system best receiver transmitter. over the past dozens of years, the best reception theory is already very mature, but the best emission problem has not been solved, PN spread is one of the best form of signals and modulation sy

31、stem, constitutes the best transmitter. therefore, It is the best one of pseudo - random code Spread Spectrum Communication system related to receive this understanding, it is not difficult to predict the future prospects of Spread Spectrum communication.Starting 9O of wireless communication in huma

32、n Spread Spectrum Communication and adaptive communication age. heat wave has affected the short - wave Spread Spectrum Communications and super microwave and microwave communication and satellite communications, Code Division Multiple Access CDMA has begun to be widely used in future peak socket co

33、mmunications, cordless communications and personal communication and wireless local loop, playing an increasingly big role.Access Network is from the traditional subscriber line, subscriber and user access system, the development of evolution and upgrade formed. Modern telecommunications network is

34、divided into 3 : transmission networks, switching network and access networks. as the access network development rather late, often telecommunications development bottleneck All countries have attached great importance to the development of the access network, and therefore throw in technology and s

35、ystem came into being.Due to the opening of ISMIndustry Scientific Medica1band, operators and users do not need to apply for authorization are free to use these bands.Wireless Spread spectrum technology used in band 2.4 2.483 are general ISM frequency band in the world, most of the IEEE802.11 protocol architecture of wireless LAN selection of the band.Wireless throw in system, Spread Spectrum microwave and conventional microwave compared with more than 3 significant advantages :strong anti - interference, frequency