基于Matlab的OFDM系統(tǒng)設(shè)計(jì)及分析

1、仁 人 教 育目錄設(shè)計(jì)總說(shuō)明IThe total designed to show thatV1緒論11.1課題背景11.2 無(wú)線通信11.2.1 無(wú)線通信概述11.2.2 無(wú)線信道特性21.3 OFDM系統(tǒng)介紹31.3.1 OFDM的概述31.3.2 OFDM的應(yīng)用41.3.3 OFDM的關(guān)鍵技術(shù)51.3.4 OFDM系統(tǒng)的優(yōu)點(diǎn)及缺點(diǎn)71.4 MATLAB特點(diǎn)與功能92 OFDM系統(tǒng)的基本原理112.1 OFDM技術(shù)原理112.2 基于IFFT/FFT 的OFDM 系統(tǒng)模型122.3 OFDM信號(hào)的頻譜特性142.4 串并轉(zhuǎn)換153 OFDM系統(tǒng)在MATLAB上的仿真分析163.1 0FDM

2、 系統(tǒng)調(diào)制與解調(diào)解析163.2 加窗183.3 AWGNA信道下的仿真203.3.1加入高斯白噪聲前后分析對(duì)比203.3.2 BER性能曲線214對(duì)系統(tǒng)誤碼率的改善分析234.1循環(huán)前綴234.2 OFDM系統(tǒng)的峰值平均功率比274.3信道估計(jì)284.3.1信道估計(jì)概述284.3.2基于導(dǎo)頻的信道估計(jì)方法294.3.3信道的插值方法294.3.4仿真結(jié)果及分析305總結(jié)325.1完成的主要工作325.2不足與展望32參考文獻(xiàn)34附錄35致謝501基于Matlab的OFDM系統(tǒng)設(shè)計(jì)及分析設(shè)計(jì)總說(shuō)明正交頻分復(fù)用是一種歷史悠久的多載波調(diào)制技術(shù)，最早在20世紀(jì)中期的時(shí)候，在通信領(lǐng)域，研究者們做了大量的

3、卓有成效的研究工作，對(duì)多載波調(diào)制理論進(jìn)行了研究，并論證了利用多載波調(diào)制技術(shù)能夠?qū)φw系統(tǒng)性能進(jìn)行優(yōu)化。當(dāng)時(shí)許多專家學(xué)者對(duì)這頻域非常密集的多載波調(diào)制的OFDM系統(tǒng)產(chǎn)生了巨大的關(guān)注。然而那個(gè)時(shí)候的數(shù)字信號(hào)處理技術(shù)還沒(méi)有得到足夠的成長(zhǎng)，OFDM也因此沒(méi)有得到普遍的關(guān)注和使用。在20世紀(jì)90年代，隨著數(shù)字信號(hào)處理技術(shù)的日漸成熟，OFDM技術(shù)在告訴數(shù)據(jù)傳輸方面得到了人們的重視，已經(jīng)普遍應(yīng)用于數(shù)字音頻廣播（如DAB和DBA）、無(wú)線局域網(wǎng)、電話上的數(shù)字傳輸（VDSL和ADSL）和電力線通信（homeplugav和BPL）等領(lǐng)域。2004年，已經(jīng)作為3G核心之一的WiMAX影響力的擴(kuò)大強(qiáng)烈迫使傳統(tǒng)電信領(lǐng)域的標(biāo)

4、準(zhǔn)化組織3GPP啟動(dòng)了它的長(zhǎng)期演進(jìn)計(jì)劃，以O(shè)FDM為基礎(chǔ)的多址接入技術(shù)在這次以空中接口技術(shù)的較量中，通過(guò)下行OFDMA和上行SC-FDMA奠定巨大優(yōu)勢(shì)獲得勝利，這也意味著未來(lái)的一段時(shí)間移動(dòng)通訊系統(tǒng)中將出現(xiàn)OFDM一統(tǒng)天下的局面。 正交頻分復(fù)用（OFDM）技術(shù)能夠有效的避免有符號(hào)間干擾信道，因?yàn)槠浜?jiǎn)易高效，現(xiàn)已成為現(xiàn)今無(wú)線高速通訊整體中不可或缺的主要技術(shù)之一。本文最開(kāi)始介紹了無(wú)線通信的發(fā)展史以及到現(xiàn)在的移動(dòng)通信系統(tǒng)中的OFDM采用多種新技術(shù)所以具備更高的帶寬利用率和克服符號(hào)間干擾與突發(fā)噪聲的能力，也因其子載波彼此之間存在正交性，能讓其頻譜互相疊加，對(duì)比一般的頻分復(fù)用，OFDM能夠盡可能地操控頻譜

5、資源去擴(kuò)大系統(tǒng)容量，最主要的是它的靈活開(kāi)放性能很好地符合多媒體通訊的特點(diǎn)，將包含數(shù)據(jù)、語(yǔ)音、影像等多種多樣的多媒體業(yè)務(wù)高效完善地傳達(dá)出去。并簡(jiǎn)略闡述了仿真必須用到的MATLAB的可靠的數(shù)值和符號(hào)運(yùn)算功能，簡(jiǎn)易的符號(hào)語(yǔ)言，可視化的特點(diǎn)。文章對(duì)OFDM 系統(tǒng)進(jìn)行了概述以及當(dāng)今社會(huì)普遍應(yīng)用的主要原因在于它能夠?qū)拵в蟹?hào)間干擾信道改造為一系列衰落信道，這也表明，帶寬利用率是非常高的，和高速離散傅里葉變換（DFT）技術(shù)來(lái)實(shí)現(xiàn)調(diào)制和解調(diào)，且選用聯(lián)合編碼技術(shù)，使其能夠有很強(qiáng)的抗衰減特點(diǎn)及可與多種接入方式結(jié)合使用的優(yōu)勢(shì)與需要補(bǔ)足的地方，如OFDM對(duì)系統(tǒng)定時(shí)和頻率偏移敏感和存在較高的峰值平均功率比，同時(shí)由于無(wú)

6、線信道的不穩(wěn)定性，且發(fā)射端載波和本地振蕩器的頻率偏差，所以產(chǎn)生的頻率存在一定偏差使OFDM整體出現(xiàn)問(wèn)題。同時(shí)，就OFDM系統(tǒng)的基本原理進(jìn)行了討論，事實(shí)上，它是用N個(gè)子載波信道切割成相應(yīng)的子信道，因?yàn)檫@些子信道調(diào)制，實(shí)現(xiàn)多個(gè)數(shù)據(jù)的頻域并行傳輸，這點(diǎn)和OFDM的原理差不多，唯一的區(qū)別就是OFDM技術(shù)很好的抓住了其控制特點(diǎn)，使其整個(gè)體系中每個(gè)子信道的載波相互正交，頻譜彼此疊加，所以頻譜利用率有所提升并減少了每一個(gè)子載波之間的影響。得到了OFDM為了保證不同子信道上承載的數(shù)據(jù)在接收端可以正常分離，必須保證自信道之間的“正交性”，也就是說(shuō)并行傳輸子信道并不是無(wú)限隨意劃分的。所謂的正交性在時(shí)域應(yīng)體現(xiàn)為：

7、在頻域體現(xiàn)為：OFDM所采用的信號(hào)設(shè)計(jì)形式是滿足正交性要求的最佳信號(hào)設(shè)計(jì)，滿足頻域奈奎斯特采樣定律的最密的采樣，它的正交性是根據(jù)在某一個(gè)積分時(shí)間內(nèi)所有子載波都是整周期而且不同載波之間周期個(gè)數(shù)不同來(lái)實(shí)現(xiàn)的。這樣，就利用了子載波在時(shí)域的正交性可以完全屏蔽其他子載波的干擾，恢復(fù)了本子載波承載的數(shù)據(jù)信息。從OFDM信號(hào)在頻域的圖像可以看出，每個(gè)子載波的頻譜實(shí)際上就是頻域的抽樣函數(shù)，在頻域的最佳采樣點(diǎn)無(wú)子載波間干擾。其實(shí)更本上OFDM調(diào)制和解調(diào)都是在數(shù)字基帶上進(jìn)行數(shù)字信號(hào)處理實(shí)現(xiàn)的，因此t =為起始時(shí)間可以得出OFDM的符號(hào)表達(dá)式：然后OFDM等效復(fù)基帶信號(hào)通過(guò)逆傅里葉變換（IDFT）的定義，能夠得到它

8、的DFT轉(zhuǎn)換：從而可以得出由DFT和IDFT來(lái)分別代替OFDM系統(tǒng)的調(diào)制和解調(diào)。其中通過(guò)上式中OFDM 符號(hào)看出其功率譜的帶外衰減速率很慢，我們不妨對(duì)其每一個(gè)都進(jìn)行加窗調(diào)制，符號(hào)周期邊的幅值逐漸減小為零，信號(hào)的幅度也逐漸減小，即使對(duì)信號(hào)的誤碼率有一定的影響。通過(guò)以上OFDM 系統(tǒng)參數(shù)數(shù)學(xué)公式和加窗技術(shù)的原理及運(yùn)用IDFT 和DFT 完成了基于IFFT/FFT 的OFDM 系統(tǒng)設(shè)計(jì)，即在接收端把時(shí)域信號(hào)通過(guò)快速傅里葉變換(FFT)轉(zhuǎn)化為頻域信號(hào)，由此我們?cè)诔绦蚓幋a上配置調(diào)制解調(diào)相關(guān)參數(shù)和幀結(jié)構(gòu)等相關(guān)參數(shù)可以得到OFDM在AWGN信道下的BER性能曲線仿真圖，并粗略分析系統(tǒng)誤碼率與信噪比之間的關(guān)系

9、，而其代碼中也不會(huì)包含信道估計(jì)，循環(huán)前綴等模塊。接下來(lái)通過(guò)上述的AWGN信道編寫(xiě)的程序代碼的基礎(chǔ)上進(jìn)行添加修改下著重探討了了理想同步條件下,保護(hù)時(shí)隙(CP)、循環(huán)前綴加入前后和各種信道內(nèi)插方式在多徑瑞利衰落信道下對(duì)OFDM系統(tǒng)整體性能的影響。其中如果一個(gè)符號(hào)的長(zhǎng)度嚴(yán)格等于Ts，即N個(gè)采樣點(diǎn)，那么經(jīng)過(guò)符號(hào)間干擾信道之后連續(xù)的兩個(gè)符號(hào)之間的樣點(diǎn)之間必然發(fā)生重疊，而且符號(hào)之間的干擾還會(huì)影響每一個(gè)子載波間的正交性，帶來(lái)嚴(yán)峻的后果。因此，系統(tǒng)中往往需要增添保護(hù)時(shí)隙在兩個(gè)OFDM符號(hào)之間。 一種直接的思路是在兩個(gè)符號(hào)間加上一段時(shí)間為0的區(qū)域，這雖然消除了ISI，但是這種方法實(shí)際上無(wú)法克服符號(hào)間干擾所帶來(lái)的

10、子載波干擾。即在FFT積分時(shí)間的子載波的延遲路徑已經(jīng)不在整個(gè)周期中，子載波之間的正交性受其嚴(yán)重的損傷。我們?yōu)榱私鉀Q這一問(wèn)題，在目前實(shí)用化的OFDM系統(tǒng)中都采用了添加循環(huán)前綴（CP）的方法。所謂的循環(huán)前綴其實(shí)是將一個(gè)符號(hào)的后面若干個(gè)(Ncp）樣點(diǎn)按順序移到這個(gè)符號(hào)前面，代替保護(hù)間隔，其作用是在FFT積分時(shí)間內(nèi)，每一條延遲徑上的各個(gè)子載波都仍然是整周期的，保證了子載波間的正交性。循環(huán)前綴的引入雖然對(duì)抗了符號(hào)間干擾，但是帶來(lái)了傳輸效率的下降，因此在OFDM系統(tǒng)設(shè)計(jì)時(shí)循環(huán)前綴的長(zhǎng)度需要適當(dāng)?shù)倪x擇一個(gè)參數(shù)，其長(zhǎng)度一定要大于最大多徑時(shí)延時(shí)，才能擔(dān)保子載波間正交性。在添加循環(huán)前綴后，很明顯，整個(gè)系統(tǒng)都會(huì)產(chǎn)生

11、功率和信息速率的損耗，但它能夠克服符號(hào)間干擾和多徑產(chǎn)生的符號(hào)間干擾，本文也會(huì)通過(guò)仿真的實(shí)際結(jié)果重點(diǎn)說(shuō)明其兩面性的長(zhǎng)短。同時(shí)研究OFDM系統(tǒng)峰均功率比更高的原因，主要是該符號(hào)是由一個(gè)單一的調(diào)制的副載波信號(hào)。因此，它可能會(huì)導(dǎo)致一個(gè)非常高的信號(hào)峰值功率，具有很高的峰值均勻功率比。因此對(duì)比單載波系統(tǒng)，OFDM系統(tǒng)發(fā)射機(jī)的發(fā)送信號(hào)的瞬間時(shí)值極度不穩(wěn)定。這勢(shì)必要求系統(tǒng)內(nèi)的如A/D、D/A轉(zhuǎn)換器等一些器件具備非常大的線性動(dòng)態(tài)范圍；然而同時(shí)也會(huì)由于這些器件的非線性對(duì)動(dòng)態(tài)范圍較廣的信號(hào)引起非線性失真，以及彼此之間的干擾信道引起的諧波,從而降低整個(gè)OFDM體系的性能。在接收端的同時(shí)，由于無(wú)線信道的波動(dòng)范圍很廣和不

12、可預(yù)測(cè)性，且隨著多種原因的不穩(wěn)定性，從而致使收到的信號(hào)幅值、相位和頻率的失真，從而難以解析。這些問(wèn)題對(duì)接收機(jī)的設(shè)計(jì)造成了很大的阻礙，因此在接收機(jī)中，信道估計(jì)器是一個(gè)不可或缺的部分。信道估計(jì)器的設(shè)計(jì)在OFDM系統(tǒng)中首要直接就面臨著兩個(gè)困難：一是導(dǎo)頻信息的選擇，二是要有較低的復(fù)雜度。在最新建立于正交頻分復(fù)用的無(wú)線通信系統(tǒng)，因?yàn)槠鋫鬏斔俾矢?，它必然運(yùn)用相干檢測(cè)技術(shù)來(lái)得到更高的性能，因此一般選取非盲估計(jì)收獲很不錯(cuò)的估計(jì)結(jié)果，使其能夠很好的追尋無(wú)線信道的改變，提升接收端信號(hào)的質(zhì)量。此課題所研究的基于導(dǎo)頻信道估計(jì)的信道估計(jì)方法也是其中一種很實(shí)用的方法。由于其分為常值內(nèi)插、線性內(nèi)插和DFT插值，其中常值內(nèi)插

13、是相當(dāng)容易的方式，但是考慮到達(dá)成的困難度，信道估計(jì)準(zhǔn)則選取LS估計(jì)準(zhǔn)則，簡(jiǎn)單討論LS算法下不同插值方式下對(duì)信道估計(jì)的仿真。在基于IFFT/FFT的系統(tǒng)的OFDM系統(tǒng)內(nèi)加入保護(hù)間隔之前之后的模擬和循環(huán)前綴等模塊后，在完全不一樣的信道條件基礎(chǔ)上，,對(duì)比保護(hù)時(shí)隙、循環(huán)前綴、采用LS估計(jì)方法對(duì)OFDM系統(tǒng)誤碼率影響,得到了自己此次設(shè)計(jì)中想要的結(jié)論。最后對(duì)整個(gè)畢業(yè)設(shè)計(jì)歸納總結(jié)，并提出本文存在的不足的地方和對(duì)OFDM技術(shù)將來(lái)工作的研究展望。關(guān)鍵詞:正交頻分復(fù)用;系統(tǒng)仿真;誤碼率影響MATLAB Simulation and Performance Analysis of OFDM SystemThe to

14、tal designed to show that Orthogonal frequency division multiplexing is a multi-carrier modulation technology with a long history, as early as the middle of the 20th century, in the field of communication, the researchers have done a lot of fruitful work, studied the multicarrier modulation theory,

15、and demonstrates the use of multicarrier modulation technique to optimize the overall performance of the system. At that time, many experts and scholars on the frequency domain of very intensive multicarrier modulation OFDM system to produce a great deal of attention. At that time, however the growt

16、h of digital signal processing technology has not been enough, OFDM and therefore didnt get the attention and widely used.In the 1990 s, with the maturing of digital signal processing technology, OFDM technology in tell data transmission has got the attention of the people, has been widely applied i

17、n the digital audio broadcasting (DAB) and DBA), wireless local area network (LAN), the digital transmission (VDSL and ADSL) on the phone and power line communication (homeplugav and BPL), etc.In 2004, has been one of the 3 g core WiMAX influence to expand a strong force of traditional telecom organ

18、ization for standardization in the field of 3 GPP launched its long-term evolution plan, on the basis of OFDM multiple access technology in the air interface technology, through the downlink OFDMA uplink SC - FDMA and lay a huge advantage to win, it also means a period of time in the future mobile c

19、ommunication system will appear OFDM the situation of unify the whole country.Orthogonal frequency division multiplexing (OFDM) technology can effectively avoid the intersymbol interference channel, because of its simple and efficient, has now become the indispensable one of the main technology of h

20、igh-speed wireless communication overall. This paper at first introduced the wireless communication in the history of development, as well as to the present system for mobile communications OFDM USES the many kinds of new technology so with higher bandwidth utilization, and overcome the intersymbol

21、interference and burst noise strength, also because of its carrier orthogonality between each other, each other can make its spectrum superposition, compared with the general frequency division multiplexing, OFDM can be as much as possible manipulation of the spectrum resources to expand the system

22、capacity, the mainest is it flexible open performance conforms to the characteristics of multimedia communication, well will contain data, voice, image and so on a variety of multimedia business effective and perfect pass out. And briefly expounds the simulation must be used in the reliable numerica

23、l and symbolic operation function of MATLAB, a simple sign language, the characteristics of the visual.Articles on the OFDM system were summarized, and the main reason for general use in todays society is that it can be a broadband has transformed into a series of intersymbol interference channel fa

24、ding channel, this also shows that the bandwidth utilization rate is very high, and the discrete Fourier transform (DFT) technology to realize the modulation and demodulation, and selects the joint coding technology, enable it to have a strong resistance attenuation characteristics and can be used w

25、ith multiple access methods in combination with the advantages and the need to complement, such as OFDM is sensitive to timing and frequency offset and the existing high peak average power ratio, at the same time due to the instability of the wireless channel, the sender and the carrier and the loca

26、l oscillator frequency deviation, so there is a certain deviation of frequency OFDM the whole problem. At the same time, it discusses the basic principle of OFDM system, in fact, it is the carrier channel cut into corresponding person with N sub-channels, because these sub channel modulation, implem

27、ent multiple frequency domain parallel data transmission, which is similar to the principle of OFDM, the only difference is the OFDM technology good caught its control characteristic, make the whole system of orthogonal to each other, each channel in the carrier frequency spectrum superposition of e

28、ach other, so to improve spectrum efficiency and to reduce the influence between each child carrier. In different sub-channels of OFDM in order to ensure the bearing data can be separated at the receiving end, must ensure the orthogonality between self-confidence, that is to say, parallel transmissi

29、on is not infinite sub-channels randomly divided. The orthogonality of the so-called time domain should be embodied in:In the frequency domain embodied in:OFDM signal design form is adopted by the best signal that could satisfy the requirement of orthogonality design, satisfy the frequency domain Ny

30、quist sampling law is the most dense sampling, its orthogonality is according to all the subcarrier in a certain integral time is the whole cycle and cycle number is different between different carrier. So, it is using the orthogonality of the subcarrier in time domain can be completely shielded oth

31、er sub-carrier interference, restored the book data information carrier. Can be seen from the OFDM signal in the frequency domain image, each subcarrier frequency spectrum is actually the sample function of frequency domain, in the frequency domain of the best sampling points without interference be

32、tween subcarrier. More on the OFDM modulation and demodulation is carried out on the digital baseband digital signal processing, so t = as the starting time can be concluded that OFDM symbol expressions:Then OFDM complex equivalent baseband signal by inverse Fourier transform (IDFT) definition, to g

33、et its DFT transformation:Thus can draw by DFT and IDFT respectively instead of modulation and demodulation of OFDM system. Among them through the type on OFDM symbol to see its power spectrum out-of-band attenuation rate is slow, we might as well to each add window modulation, symbol of the periodi

34、c boundary amplitude decreases to zero, the signal amplitude decreases, even if the error rate of the signal has the certain influence. OFDM system parameters through the above mathematical formula and the principle of window technology and using IDFT based on IFFT/FFT and DFT of the OFDM system des

35、ign, namely the time-domain signal at the receiving end through the fast Fourier transform (FFT) is converted into frequency domain signal, thus we configure modem related parameters on the program code and frame structure parameters can be related to OFDM BER performance under AWGN channel simulati

36、on curve graph, and a rough analysis of the relationship between the system bit error rate and signal to noise ratio, and the code will not be included in channel estimation, cyclic prefix and other modules.Then through the program code written in AWGN channel on the basis of adding modified item is

37、 emphatically discussed, under the condition of ideal synchronization protection time slot (CP), and a variety of before and after cyclic prefix to join channel interpolation method in multipath Rayleigh fading channel in OFDM system under the influence of the overall performance. One if the symbol

38、is equal to the length of the strict Ts, namely N sampling points, then after the intersymbol interference channel between two consecutive symbol of sample must be overlap, and the interference among the symbols can also affect the orthogonality between each child carrier, serious consequences. Ther

39、efore, systems often need to add protection time slot between two OFDM symbol. A direct approach is combined with a period of time between two symbols of 0, it eliminates ISI, but this method is actually unable to overcome sub-carrier interference brought by the intersymbol interference. In FFT subc

40、arrier delay path integral time isnt the whole cycle, the subcarrier orthogonality between its serious damage. We have to solve this problem, in the current practical OFDM systems are adopted the approach of adding cyclic prefix (CP). So-called cyclic prefix is actually a symbol behind a number of s

41、ample point (Ncp) moved to the front of the symbol according to the order instead of protection interval, and its role is in the FFT integral time, each delay diameter on each subcarrier is still a whole cycle, ensure the orthogonality between the subcarrier. The introduction of the cyclic prefix al

42、though against the intersymbol interference, but lead to a drop in efficiency, so when the length of the cyclic prefix in OFDM system design need to select a proper parameter, when most of its length must be greater than the diameter delay, to guarantee the subcarrier orthogonality between. After ad

43、ding cyclic prefix, obviously, the whole system will produce loss of power and information rate but it can overcome the intersymbol interference and multipath intersymbol interference, this paper will through the simulation of the actual results based on the length of the two sides. Research OFDM sy

44、stem peak power is higher than at the same time, is the symbol is dominated by a single modulated subcarrier signal. Therefore, it may lead to a very high signal peak power, has the very high peak power ratio. So comparing single carrier system, OFDM system transmitter signal instantaneous value ext

45、remely unstable. This will inevitably request in the system, such as A/D, D/A converter and some other devices with very large linear dynamic range; But also because of these devices with wider range of nonlinear dynamic nonlinear distortion caused by the signal, and the interference between each ch

46、annel caused by harmonics, thus reducing the whole performance of the OFDM system. On the receiving end at the same time, because of the wireless channel fluctuation range is very wide and unpredictability, and as the instability of a variety of reasons, thus received signal amplitude, phase and fre

47、quency distortion, which is difficult to parse. These problems to the design of receiver caused great obstacles, so in the receiver, channel estimation is an indispensable part. The design of the channel estimator in OFDM systems directly to face two difficulties: first one is the choice of pilot in

48、formation, 2 it is to have a lower complexity. In the newly established in orthogonal frequency division multiplexing wireless communication system, because of its high transmission rate, it must use coherent detection technology to get a higher performance, therefore generally choose the blind esti

49、mation harvest good estimation results, enable it to very good after the change of the wireless channel, improve the quality of the receiver signal. This topic research of channel estimation method based on pilot channel estimation is one of the very practical method. Because its can be divided into

50、 constant interpolation and linear interpolation and DFT interpolation, the constant interpolation is fairly easy way, but given the difficulty, channel estimation criteria to select the LS estimate criterion, under the simple discussion of the LS algorithm for channel estimation under different int

51、erpolation methods in the simulation. In the OFDM system based on IFFT/FFT system incorporating protection interval before and after simulation and cyclic prefix module, on the basis of completely different channel conditions, and compare the protection time slot, cyclic prefix, the LS estimation me

52、thods for OFDM system ber, got myself want to in the design of the conclusion.Finally, the graduation design, the conclusion and put forward in this paper, the deficiencies of place and the research of OFDM technique in the future job prospects.Key words:Orthogonal frequency division multiplexing; S

53、ystem simulation; Ber influence VII1緒論1.1課題背景 縱觀移動(dòng)通信的發(fā)展史，第一代模擬系統(tǒng)僅提供語(yǔ)音服務(wù)，不能傳輸數(shù)據(jù)。第二代數(shù)字移動(dòng)通信系統(tǒng)的數(shù)據(jù)傳輸速率也只有9.6bit/s，最高可達(dá)32kbit/s；第三代移動(dòng)通信系統(tǒng)數(shù)據(jù)傳輸速率可達(dá)到2Mbit/s；而我們目前所致力研究的第四代移動(dòng)通信系統(tǒng)可以達(dá)10Mbit/s至20Mbit/s。雖然第三代移動(dòng)通信可以比現(xiàn)有傳輸速率快上千倍，但是仍無(wú)法滿足未來(lái)多媒體通信的要求，第四代移動(dòng)通信系統(tǒng)的提出便是希望能滿足提供更大的頻寬需求。第四代移動(dòng)通信系統(tǒng)計(jì)劃以O(shè)FDM(正交頻分復(fù)用)為核心技術(shù)提供增值服務(wù)，它在寬帶領(lǐng)

54、域的應(yīng)用具有很大的潛力。較之第三代移動(dòng)通信系統(tǒng)，采用多種新技術(shù)的OFDM具有更高的頻譜利用率和良好的抗多徑干擾能力，它不僅僅可以增加系統(tǒng)容量，更重要的是它能更好地滿足多媒體通信要求，將包括語(yǔ)音、數(shù)據(jù)、影像等大量信息的多媒體業(yè)務(wù)通過(guò)寬頻信道高品質(zhì)地傳送出去。OFDM（Orthogonal Frequency Division Multiplexing）是目前已知的頻譜利用率最高的一種通信系統(tǒng)，它將數(shù)字調(diào)制、數(shù)字信號(hào)處理、多載波傳輸?shù)燃夹g(shù)有機(jī)結(jié)合在一起，使得它在系統(tǒng)的頻譜利用率、功率利用率、系統(tǒng)復(fù)雜性方面綜合起來(lái)有很強(qiáng)的競(jìng)爭(zhēng)力，是支持未來(lái)移動(dòng)通信特別是移動(dòng)多媒體通信的主要技術(shù)之一1。1.2 無(wú)線通

55、信1.2.1 無(wú)線通信概述 到目前為止，無(wú)線通信大約經(jīng)歷了四個(gè)階段。 (1)1G階段 主要采用的是模擬技術(shù)和頻分多址(FDMA)技術(shù)。由于受到傳輸帶寬的限制，不能進(jìn)行移動(dòng)通信的長(zhǎng)途溫游，只能是一種區(qū)域性的移動(dòng)通信系統(tǒng)2。第一代移動(dòng)通信有很多不足之處，比如容量有限、制式太多、互不兼容、保密性差、通話質(zhì)量不高、不能提供數(shù)據(jù)業(yè)務(wù)、不能提供自動(dòng)漫游等。(2)2G階段 主要采用的是數(shù)字的時(shí)分多址(TDMA)技術(shù)和碼分多址(CDMA)技術(shù)。全球主要有GSM和CDMA兩種體制。GSM技術(shù)標(biāo)準(zhǔn)是歐洲提出的，目前全球絕大多數(shù)國(guó)家使用這一標(biāo)準(zhǔn)。我國(guó)移動(dòng)通信也主要是GSM體制。目前使用GSM的用戶占國(guó)內(nèi)市場(chǎng)的97。

56、CDMA是美國(guó)高通公司提出的標(biāo)準(zhǔn)。由于第二代采用不同的制式，移動(dòng)通信標(biāo)準(zhǔn)不統(tǒng)一，用戶只能在同一制式覆蓋的范圍內(nèi)進(jìn)行漫游，因而無(wú)法進(jìn)行全球漫游，由于第二代數(shù)字移動(dòng)通信系統(tǒng)帶寬有限，限制了數(shù)據(jù)業(yè)務(wù)的應(yīng)用，也無(wú)法實(shí)現(xiàn)高速率的業(yè)務(wù)如移動(dòng)的多媒體業(yè)務(wù)3。(3)3G階段與從前以模擬技術(shù)為代表的第一代和目前正在使用的第二代移動(dòng)通信技術(shù)相比，3G將有更寬的帶寬，其傳輸速度最低為384K，最高為2M，帶寬可達(dá)5MHz以上。目前全球有三大標(biāo)準(zhǔn)，分別是歐洲提出的WCDMA、美國(guó)提出的CDMA2000和我國(guó)提出的TDSCDMA。第三代移動(dòng)通信網(wǎng)絡(luò)能將高速移動(dòng)接入和基于互聯(lián)網(wǎng)協(xié)議的服務(wù)結(jié)合起來(lái)，提高無(wú)線頻率利用效率6。

57、提供包括衛(wèi)星在內(nèi)的全球覆蓋并實(shí)現(xiàn)有線和無(wú)線以及不同無(wú)線網(wǎng)絡(luò)之間業(yè)務(wù)的無(wú)縫連接。 (4)4G階段由于第三代移動(dòng)通信系統(tǒng)仍是基于地面標(biāo)準(zhǔn)不一的區(qū)域性通信系統(tǒng)，盡管其傳輸速率可高達(dá)2 Mb/s，但仍無(wú)法滿足多媒體通信的要求，因此，第四代移動(dòng)通信系統(tǒng)(4G)的研究隨之應(yīng)運(yùn)而生。第四代移動(dòng)通信技術(shù)的概念可稱為廣帶(Broadband)接入和分布網(wǎng)絡(luò)，具有非對(duì)稱超過(guò)2 Mb/s的數(shù)據(jù)傳輸能力，對(duì)全速移動(dòng)用戶能提供150 Mb/s的高質(zhì)量影像服務(wù)，將首次實(shí)現(xiàn)三維圖像的高質(zhì)量傳輸。他包括廣帶無(wú)線固定接入、廣帶無(wú)線局域網(wǎng)、移動(dòng)廣帶系統(tǒng)和互操作的廣播網(wǎng)絡(luò)(基于地面和衛(wèi)星系統(tǒng))。其廣帶無(wú)線局域網(wǎng)(WLAN)能與B-

58、ISDN和ATM兼容，實(shí)現(xiàn)廣帶多媒體通信，形成綜合廣帶通信網(wǎng)(IBCN)，他還能提供信息之外的定位定時(shí)、數(shù)據(jù)采集、遠(yuǎn)程控制等綜合功能7。1.2.2 無(wú)線信道特性 無(wú)線信道包括了電波的多徑衰落，時(shí)延擴(kuò)展，以及多普勒效應(yīng)，在移動(dòng)通信中，必須要充分考慮這些特性，并提出相關(guān)的解決方案。(1)時(shí)延擴(kuò)展無(wú)線信道中電波的傳播不是單一路徑，而是許多路徑來(lái)的眾多反射波的合成。由于電波通過(guò)各個(gè)路徑的距離不同，因而各個(gè)路徑來(lái)的反射波到達(dá)時(shí)間不同，也就是各信號(hào)的時(shí)延不同。當(dāng)發(fā)送端發(fā)送一個(gè)極窄的脈沖信號(hào)時(shí)，移動(dòng)臺(tái)接收的信號(hào)由許多不同時(shí)延的脈沖組成，這被稱之為時(shí)延擴(kuò)展。(2)多徑衰落由于各個(gè)路徑來(lái)的反射波到達(dá)時(shí)間不同，相位也就不同。不同相位的多個(gè)信號(hào)在接收端迭加，有時(shí)迭加而加強(qiáng)（方向相同），有時(shí)迭加而減弱（方向相反）。這樣，接收信號(hào)的幅度將急劇變化，即產(chǎn)生了快衰落。此外，接收信號(hào)除瞬時(shí)值出現(xiàn)快衰落之外，場(chǎng)強(qiáng)中值（平均值）也會(huì)出現(xiàn)緩慢變化。主要是由地區(qū)位置的改變以及氣象條件變化造成的，以致電波的折射傳播隨時(shí)間變化而變化，多徑傳播到達(dá)固定接收點(diǎn)的信號(hào)的時(shí)延隨之變化。這種由陰影效應(yīng)和氣象原因引起的信號(hào)變化，稱為慢衰落。這種衰落是由多種路徑引起的，所