通信工程專業(yè)畢業(yè)論文外文資料翻譯-- 正交頻分復用技術簡介

1、<p><b>　　中文3500字</b></p><p><b>　　譯文：</b></p><p>　　正交頻分復用技術簡介</p><p>　　正交頻分復用是一種多載波調制技術。其主要思想是：將信道分成若干正交子信道，將高速數(shù)據信號轉換成并行的低速子數(shù)據流，調制到在每個子信道上進行傳輸。正交信號可以通過在接

2、收端采用相關技術來分開，這樣可以減少子信道之間的相互干擾。每個子信道上的信號帶寬小于信道的相關帶寬，因此每個子信道上的可以看成平坦性衰落，從而可以消除碼間串擾。而且由于每個子信道的帶寬僅僅是原信道帶寬的一小部分，信道均衡變得相對容易。由于這種技術具有在雜波干擾下傳送信號的能力，因此常常會被利用在容易受外界干擾或者抵抗外界干擾能力較差的傳輸介質中。目前正交頻分復用技術已經被廣泛應用于廣播式的音頻、視頻領域和民用通信系統(tǒng)，主要的應用包括：非

3、對稱的數(shù)字用戶環(huán)路、歐洲電信標準協(xié)會的數(shù)字音頻廣播、數(shù)字視頻廣播、高清晰度電視、無線局域網等。</p><p>　　正交頻分復用并不是才發(fā)展起來的新技術，其應用已有40余年的歷史，在上個世紀60年代就已經有人提出了使用平行數(shù)據傳輸和頻分復用的概念。</p><p>　　70年代，韋斯坦和艾伯特等人應用離散傅里葉變換和離散傅里葉逆變換的方法研制了一個完整的多載波傳輸系統(tǒng)，叫做正交頻分復用系統(tǒng)

4、。正交頻分復用是一種特殊的多載波傳輸方案，它應用離散傅里葉變換和離散傅里葉逆變換的方法解決了產生多個互相正交的子載波以及從子載波中恢復原信號的問題。這就解決了多載波傳輸系統(tǒng)發(fā)送和傳送的難題。應用快速傅里葉變換和快速傅里葉逆變換更是使多載波傳輸系統(tǒng)的復雜度大大降低。從此正交頻分復用技術開始走向實用。但是應用正交頻分復用系統(tǒng)仍然需要大量繁雜的數(shù)字信號處理過程，而當時還缺乏數(shù)字處理功能強大的元器件，發(fā)射機和接收機振蕩器的穩(wěn)定性以及射頻功率放大

5、器的線性要求等因素也是正交頻分復用技術實現(xiàn)的制約條件。因此正交頻分復用技術遲遲沒有得到迅速發(fā)展。</p><p>　　80年代，集成電路獲得了突破性進展，大規(guī)模集成電路讓快速傅里葉變換和快速傅里葉逆變換的實現(xiàn)不再是難以逾越的障礙，一些其它難以實現(xiàn)的困難也都得到了解決，自此正交頻分復用走上了通信的舞臺，逐步邁向高速數(shù)字移動通信的領域。</p><p>　　進入90年代，由于技術的可實現(xiàn)性，正

6、交頻分復用的應用涉及到了利用移動調頻和單邊帶信道進行高速數(shù)據通信，陸地移動通信，高速數(shù)字用戶環(huán)路，非對稱數(shù)字用戶環(huán)路，高清晰度數(shù)字電視和陸地移動廣播等各種通信系統(tǒng)。1999年，國際電氣與電子工程師協(xié)會通過了一個的無線局域網標準IEEE802.lla，其中正交頻分復用調制技術被采用為物理層標準，使得傳輸速率可以達54Mbps。這樣，可提供25Mbps的無線ATM接口和10Mbps的以太網無線幀結構接口，并支持語音、數(shù)據、圖像業(yè)務。這樣的速

7、率完全能滿足室內、室外的各種應用場合。歐洲電信組織的寬帶射頻接入網的局域網標準HiperiLAN2也把正交頻分復用定為它的物理層標準調制技術。</p><p>　　正交頻分復用有許多關鍵技術。</p><p>　　（1）時域和頻域同步。正交頻分復用系統(tǒng)對定時和頻率偏移敏感，特別是實際應用中可能與頻分多址、時分多址和碼分多址等多址方式結合使用時，時域和頻率同步顯得尤為重要。與其它數(shù)字通信系統(tǒng)

8、一樣，同步分為捕獲和跟蹤兩個階段。在下行鏈路中，基站向各個移動終端廣播式發(fā)同步信號，所以，下行鏈路同步相對簡單，較易實現(xiàn)。在上行鏈路中，來自不同移動終端的信號必須同步到達基站，才能保證子載波間的正交性?；靖鶕饕苿咏K端發(fā)來的子載波攜帶信息進行時域和頻域同步信息的提取，再由基站發(fā)回移動終端，以便讓移動終端進行同步。具體實現(xiàn)時，同步將分為時域同步和頻域同步，也可以時頻域同時進行同步。</p><p>　?。?）信道

9、估計。在正交頻分復用系統(tǒng)中，信道估計器的設計主要有兩個問題：一是導頻信息的選擇。由于無線信道常常是衰落信道，需要不斷對信道進行跟蹤，因此導頻信息也必須不斷的傳送。二是既有較低的復雜度又有良好的導頻跟蹤能力的信道估計器的設計。在實際設計中，導頻信息選擇和最佳估計器的設計通常又是相互關聯(lián)的，因為估計器的性能與導頻信息的傳輸方式有關。</p><p>　　（3）信道編碼和交織。為了提高數(shù)字通信系統(tǒng)性能，信道編碼和交織是

10、通常采用的方法。對于衰落信道中的隨機錯誤，可以采用信道編碼；對于衰落信道中的突發(fā)錯誤，可以采用交織。實際應用中，通常同時采用信道編碼和交織，進一步改善整個系統(tǒng)的性能。在正交頻分復用系統(tǒng)中，如果信道衰落不是太深，均衡是無法再利用信道的分集特性來改善系統(tǒng)性能的，因為正交頻分復用系統(tǒng)自身具有利用信道分集特性的能力，一般的信道特性信息已經被正交頻分復用這種調制方式本身所利用了。但是正交頻分復用系統(tǒng)的結構卻為在子載波間進行編碼提供了機會，形成編碼

11、正交頻分復用。編碼可以采用各種碼，如分組碼、卷積碼等，卷積碼的效果要比分組碼好。</p><p>　?。?）降低峰均功率比。由于正交頻分復用信號時域上表現(xiàn)為N個正交子載波信號的疊加，當這N個信號恰好均以峰值占相加時，正交頻分復用信號也將產生最大峰值，該峰值功率是平均功率的N倍。盡管峰值功率出現(xiàn)的概率較低，但為了不失真地傳輸這些高峰均功率比的信號，發(fā)送端對高功率放大器的線性度要求很高且發(fā)送效率極低，接收端對前端放大

12、器以及模數(shù)轉換器的線性度要求也很高。因此，高的峰均功率比使得正交頻分復用系統(tǒng)的性能大大下降甚至直接影響實際應用。為了解決這一問題，人們提出了基于信號畸變技術、信號擾碼技術和基于信號空間擴展等降低正交頻分復用系統(tǒng)峰均功率比的方法。</p><p>　　作為通信方面的應用，正交頻分復用存在很多技術優(yōu)點。</p><p>　　（1）在窄帶帶寬下也能夠發(fā)出大量的數(shù)據。正交頻分復用技術能同時分開至少

13、1000個數(shù)字信號，而且在干擾的信號周圍可以安全運行，這種能力將直接威脅到目前已經開始流行的碼分多址技術的進一步發(fā)展和壯大，正是由于具有了這種特殊的信號穿透能力使得正交頻分復用技術深受歐洲通信營運商以及手機生產商的喜愛和歡迎。</p><p>　　(2) 正交頻分復用技術能夠持續(xù)不斷地監(jiān)控傳輸介質上通信特性的突然變化。由于通信路徑傳送數(shù)據的能力會隨時間發(fā)生變化，所以正交頻分復用能動態(tài)地與之相適應，并且接通和切斷相

14、應的載波以保證持續(xù)地進行成功的通信。</p><p>　　(3) 正交頻分復用可以自動地檢測到傳輸介質下哪一個特定的載波存在高的信號衰減或干擾脈沖，然后采取合適的調制措施來使指定頻率下的載波進行成功通信。</p><p>　　(4) 正交頻分復用技術特別適合使用在高層建筑物、居民密集和地理上突出的地方以及將信號散播的地區(qū)。高速的數(shù)據傳播及數(shù)字語音廣播都希望降低多徑效應對信號的影響。<

15、/p><p>　　(5) 正交頻分復用技術的最大優(yōu)點是對抗頻率選擇性衰落或窄帶干擾。在單載波系統(tǒng)中，單個衰落或干擾能夠導致整個通信鏈路失敗，但是在多載波系統(tǒng)中，僅僅有很小一部分載波會受到干擾。對這些子信道還可以采用糾錯碼來進行糾錯。</p><p>　　(6) 可以有效地對抗信號波形間的干擾，適用于多徑環(huán)境和衰落信道中的高速數(shù)據傳輸。當信道中因為多徑傳輸而出現(xiàn)頻率選擇性衰落時，只有落在頻帶凹陷

16、處的子載波以及其攜帶的信息受影響，其他的子載波未受損害，因此系統(tǒng)總的誤碼率性能要好得多。</p><p>　　(7) 通過各個子載波的聯(lián)合編碼，具有很強的抗衰落能力。正交頻分復用技術本身已經利用了信道的頻率分集，如果衰落不是特別嚴重，就沒有必要再加時域均衡器。通過將各個信道聯(lián)合編碼，則可以使系統(tǒng)性能得到提高。</p><p>　　(8) 正交頻分復用技術抗窄帶干擾性很強，因為這些干擾僅僅影

17、響到很小一部分的子信道。</p><p>　　(9) 信道利用率很高，這一點在頻譜資源有限的無線環(huán)境中尤為重要；當子載波個數(shù)很大時，系統(tǒng)的頻譜利用率趨于2Baud/Hz。 </p><p>　　雖然正交頻分復用有上述優(yōu)點，但是同樣其信號調制機制也使得其信號在傳輸過程中存在著一些劣勢。</p><p>　　(1)對相位噪聲和載波頻偏十分敏感。這是正交頻分復用技術一個非

18、常致命的缺點，整個正交頻分復用系統(tǒng)對各個子載波之間的正交性要求格外嚴格，任何一點小的載波頻偏都會破壞子載波之間的正交性，引起符號間干擾，同樣，相位噪聲也會導致碼元星座點的旋轉、擴散，從而形成信道間干擾。而單載波系統(tǒng)就沒有這個問題，相位噪聲和載波頻偏僅僅是降低了接收到的信噪比，而不會引起互相之間的干擾。</p><p>　　(2)峰均比過大。正交頻分復用信號由多個子載波信號組成，這些子載波信號由不同的調制符號獨立調

19、制。同傳統(tǒng)的恒包絡的調制方法相比，正交頻分復用調制存在一個很高的峰值因子。因為其信號是很多個小信號的總和，這些小信號的相位是由要傳輸?shù)臄?shù)據序列決定的。對某些數(shù)據，這些小信號可能同相，而在幅度上疊加在一起從而產生很大的瞬時峰值幅度。而峰均比過大，將會增加模數(shù)轉換器和數(shù)模轉換器的復雜性，而且會降低射頻功率放大器的效率。同時，在發(fā)射端，放大器的最大輸出功率就限制了信號的峰值，這會在正交頻分復用頻段內和相鄰頻段之間產生干擾。</p>

20、<p>　　(3)所需線性范圍寬。由于正交頻分復用系統(tǒng)峰值平均功率比大，對非線性放大更為敏感，故正交頻分復用調制系統(tǒng)比單載波系統(tǒng)對放大器的線性范圍要求更高。</p><p><b>　　原文：</b></p><p>　　Orthogonal frequency division multiplexing</p><p>　　te

21、chnology introduction</p><p>　　OFDM is a multi carrier modulation technique. The main idea is: divides the channel into several orthogonal sub channels, the high-speed data signal into a low flow sub data pa

22、rallelism, modulation to transmit in each sub channel. Orthogonal signal can be separated by the use of related technologies at the receiving end, thus reducing the mutual interference between the channel. Relative bandw

23、idth signal bandwidth of each sub channel is shorter than the channel, so each sub channel can be seen f</p><p>　　The new technology of orthogonal frequency division multiplexing is not development, its appl

24、ication has a history of 40 years, 60 years in the last century, it has been put forward the concept of using parallel data transmission and frequency division multiplexing.</p><p>　　In 70, Weinsein and Albe

25、rt et al. Application of discrete Fu Liye transform and the discrete Fu Liye transform is developed for multi carrier transmission system of a complete, called orthogonal frequency division multiplexing system. Orthogona

26、l frequency division multiplexing is a special multi carrier transmission scheme, which using the discrete Fu Liye transform and discrete Fu Liye transform solution to restore the original signal to generate a plurality

27、of mutually orthogonal subcarriers a</p><p>　　In 80, the integrated circuit to be a breakthrough, realize large-scale integrated circuit to make the fast Fu Liye transform and fast Fu Liye transform is not i

28、nsurmountable obstacles, some other difficulties of implementation have been solved, since orthogonal frequency division multiplexing on the communication stage, gradually moving towards high speed digital mobile communi

29、cation field.</p><p>　　Enter 90 age, the realization of the technology, application of orthogonal frequency division multiplexing involves the use of mobile FM and single sideband channel high speed data com

30、munication, mobile communication, high speed digital subscriber loop, asymmetric digital subscriber loop, high-definition digital TV and land mobile radio communication system. In 1999, the International Association of e

31、lectrical and electronic engineers through the IEEE802.lla wireless LAN standard one, the orthog</p><p>　　Orthogonal frequency division multiplexing many key technology.</p><p>　　the time domain

32、 and frequency domain synchronization. Orthogonal frequency division multiplexing system is sensitive to the timing and frequency offset, in particular may be used in actual application combined with FDMA, TDMA and CDMA,

33、 FDMA, time and frequency synchronization is very important. As with other digital communication systems, synchronization is divided into two stages to capture and track. In the downlink, base station to each mobile term

34、inal to broadcast type synchronous signal, th</p><p>　　Channel estimation. In orthogonal frequency division multiplexing system, the design of channel estimator has two main problems: one is the selection of

35、 pilot information. Since the wireless channel is often a fading channel, need to continue to track the channel, the pilot information must also be continuous conveyor. The two is to design both the channel estimator wit

36、h low complexity and good pilot tracking capability. In practical design, the design of pilot information selection and optimal </p><p>　　Channel coding and interleaving. In order to improve the performance

37、of digital communication system, channel coding and interleaving is a commonly used method. For fading random errors in the channel, the channel coding for fading; burst error channel, can the interleaving. In practical

38、application, usually at the same time, channel coding and interleaving, to further improve the performance of the whole system. In orthogonal frequency division multiplexing system, if the channel fading is not </p>

39、;<p>　　Reducing the peak to average power ratio. The orthogonal frequency division multiplexing signal in time domain on the performance of the superposition of N orthogonal sub carrier signal, when the N signal i

40、s in the peak of overtime, orthogonal frequency division multiplexing signal will produce the maximum peak, the peak power is N times the average power. Although the probability of peak power of the lower, but in order n

41、ot to transmit the signal distortion of the peak to average power ratio, t</p><p>　　As an application of communication, OFDM has many technical advantages.</p><p>　　(1) In the narrow bandwidth c

42、an be a lot of data sent. Orthogonal frequency division multiplexing technology can also separated at least 1000 digital signal, and can safely run around in interference signal, this ability will be a direct threat to t

43、he further development of the current has begun to CDMA technology popular and expands, it is because of the special signal penetration capability of the orthogonal frequency division multiplexing technology by European

44、telecommunications operators an</p><p>　　(2) A sudden change in orthogonal frequency division multiplexing technology can constantly monitor the communication characteristics of the transmission medium. Beca

45、use of the ability to transmit data communication path will change over time, so the orthogonal frequency division multiplexing can dynamically adapt, and connect and disconnect the corresponding carrier to ensure ongoin

46、g successful communication.</p><p>　　(3) Orthogonal frequency division multiplexing can automatically detect the transmission medium under which a specific carrier has high signal attenuation and interferenc

47、e pulse, and then take the appropriate measures to make the specified carrier frequency modulation of the successful communication.</p><p>　　(4) Orthogonal frequency division multiplexing technique is especi

48、ally suitable for use in high-rise buildings, highlighting the densely populated and geographical place and signals of spreading area. Data communication and digital audio broadcasting high hope to reduce multipath effec

49、ts on signal.</p><p>　　(5) The biggest advantages of orthogonal frequency division multiplexing technique is to combat frequency selective fading or narrowband interference. In single carrier systems, a sing

50、le fading or interference can cause the entire communication link failure, but in a multi carrier system, only a very small part of carrier interference. The sub channel can also use error correcting codes for error corr

51、ection.</p><p>　　(6) Can effectively resist the interference between the signal waveform, high speed data transmission in multipath environment and fading channels. When the channel because of multipath tran

52、smission frequency selective fading, only fell on the band depression sub carrier and carry information affected, subcarrier other unimpaired, therefore better BER performance than the general.</p><p>　　(7)

53、Through the joint coding of each sub carrier, has a strong ability of anti fading. Orthogonal frequency division multiplexing technology itself has taken advantage of frequency diversity channel fading, if not serious, t

54、here is no need to add the time domain equalizer. Through the various channel coding, it can improve the performance of system.</p><p>　　(8) Orthogonal frequency division multiplexing technology of narrowban

55、d interference is very strong, because these disturbances affect only a small fraction of the sub channel.</p><p>　　(9) The channel utilization rate is high, this is an important point in the spectrum is a l

56、imited resource in wireless environment; when the subcarrier number is large, the system spectrum utilization tends to 2Baud/Hz.</p><p>　　Although orthogonal frequency division multiplexing has these advanta

57、ges, but also its modulation mechanism also makes the signal in the transmission process exist some disadvantages.</p><p>　　Is very sensitive to the phase noise and carrier frequency. This is orthogonal freq

58、uency division multiplexing technology is a very fatal flaw, the orthogonal frequency division multiplexing system is strictly on the orthogonality between sub carrier requirements, any small carrier frequency offset wil

59、l destroy the orthogonality between subcarriers, causes inter symbol interference, similarly, phase noise can also cause rotation diffusion, symbol constellation points, thus forming the inter chan</p><p>　　

60、Peak to average ratio is too large. Orthogonal frequency division multiplexing signal is composed of a plurality of sub carrier signal, the carrier signal modulated by different independent modulation symbols. Compared w

61、ith the traditional method of constant envelope modulation, OFDM has a very high peak factor. Because the signal is the sum of many small signal, the phase of these small signal is determined by the data sequence to be t

62、ransmitted. Some of the data, these small signal may be same</p><p>　　The wide linear range. The orthogonal frequency division multiplexing system peak to average power ratio, the nonlinear amplification is