擴頻通信技術(shù)外文翻譯

1、<p>　　A spread spectrum communication technology</p><p>　　1.1 spread spectrum communication system overview</p><p>　　Spread spectrum communication, namely the spread spectrum communication

2、( Spread Spectrum Communication ), and optical fiber communication, satellite communication, together known as entering the information age of three big high-tech communication transmission method. </p><p>　

3、　Spread spectrum communication is to transmit information data by the pseudo random code ( spread spectrum sequence: Spread Sequence ) modulation, realization of spread spectrum and transmission; receiving end uses the s

4、ame code demodulator and related treatment, recovery of the original information data. </p><p>　　This communication mode and the conventional narrow communication mode are different: one is the information s

5、pread spectrum after the formation of broadband transmission; two is related to recovery after treatment into narrowband information data. It is due to the two holding point, make the spread spectrum communication has th

6、e following advantages: high anti jamming noise anti multipath fading with the confidentiality of the power spectral density is low, has the nature of concealment and low</p><p>　　Because of the spread spect

7、rum communication technology has the advantages of the United States Army, from 50 time metaphase begins study convenient for military communications, has been the exclusive, widely used in military communication, electr

8、onic countermeasure and navigation, measurement and other fields. Until the early 80's before being used in civil communication field. In order to meet the growing needs of the civilian communication capacity and the

9、 efficient use of spectrum resources,</p><p>　　Two spread spectrum communication the basic concept</p><p>　　2.1spread spectrum communications</p><p>　　The spread spectrum communicat

10、ion, can be simply expressed as follows:" spread spectrum communication technique is a method for transmitting informationType, its signal the band width is far greater than the transmitted information required mini

11、mum bandwidth; bandwidth expansion is through an independent code sequence to complete, with the coding and modulation method, and the transmitted information data independence; at the receiving end with the same code sy

12、nchronization, correlation despread</p><p>　　2.2signal spectrum is broadened.</p><p>　　We know, transmission of any information requires a certain amount of bandwidth, called information bandwid

13、th. For example, in human speech information bandwidth of 300Hz -3400Hz, television image information bandwidth for the number MHz. In order to make full use of frequency resources, are usually used as roughly equivalent

14、 to that of the bandwidth of the signal to transmit information. In a radio communication in RF signal bandwidth and the transmitted information bandwidth is compared. As wi</p><p>　　The FM signal, or pulse

15、code modulated signals, their bandwidth and information bandwidth ratio is also only a few ten. Spread spectrum communication signal bandwidth and bandwidth ratio is as high as 100-1000, belonging to the broadband commun

16、ication.</p><p>　　Why use such wide band signal to transmit information? Wouldn't that is a waste of valuable frequency resources?</p><p>　　2.3 using the spread spectrum code sequence modula

17、tion way to stretch the signal spectrum.</p><p>　　We know, in terms of time limited signal, the spectrum is infinite. For example, a very narrow pulse signal, the spectrum is very wide. Signal frequency band

18、 width and its duration is approximately inversely proportional to. 1 microsecond pulse bandwidth is about 1MHz. Therefore, if with limited narrow pulse sequence is transmitted information modulation, can produce very wi

19、de band signal. As following the presentation of the direct sequence spread spectrum system is obtained with such a method </p><p>　　2.4on the receiving end of correlation demodulation despreading</p>

20、<p>　　As in the general communication, modulated signal at the receiving end are demodulated to recover the message. In the spread spectrum communication receiver and transmitter using the same spreading code seque

21、nce and the received spread spectrum signal correlation demodulation, restore message. In other words, this correlation demodulation despreading role play. That expansion signal back into the original message. At the beg

22、inning of the narrowband information that extended into the broadband si</p><p>　　2.5 spread spectrum communication theory</p><p>　　Long-term since, people always thought that the occupation of

23、signal spectrum as narrow as possible, in order to make full use of spectrum resources is very valuable. Why use such broadband signals to transmit information? The simple answer is mainly for communication security.<

24、/p><p>　　Spread spectrum communication the basic characteristics of signal transmission, is occupied by the band width ( W ) is much larger than the original information itself actual required minimum ( effecti

25、ve ) bandwidth (DF ), the ratio is called the processing gain Gp: Gp = W/DF ... (1) as everyone knows, any effective information transmission requires certain frequency width,1.7-3.1kHz such as voice, television image wi

26、dth to several mhz. In order to make full use of the limited frequency resource</p><p>　　Spread spectrum communication is discussed, from information theory and anti interference theory the basic formula of

27、extension to. Information theory of information capacity of the Shannon ( Shannon ) formula is: C = WLog2(1ten P/N ) ... (2) type : C - channel capacity ( with a transmission rate of W signal frequency band width measure

28、ment ) - P - N - white noise signal power power type (2), at a given transmission rate of C under the same conditions, the band width of W and the signal to noise</p><p>　　Spread spectrum communication feasi

29、bility of another theoretical basis, as Kejieer Metchnikoff on information transmission error probability formula: Powj »f ( E/N. ) (3): Powj - error probability in E - N signal energy. - noise power spectral densit

30、y because of the signal power, P = E / T ( T information for the duration of the noise power of N = WN ). ( W signal bandwidth ) information bandwidth D F = L / T type (3) can be reduced to: Powj, f ( TW.P/N ) = f ( P/N.

31、W/D F ) (4) type (4), for a ce</p><p>　　In short, we use the information bandwidth of100 times, or 1000 times more broadband signals to transmit information, is to improve communication anti-jamming ability,

32、 namely under strong interference to ensure the reliable and safe communication. This is the spread spectrum communication basic idea and theory basis.</p><p>　　2.6 spread spectrum communication main perform

33、ance index</p><p>　　Processing gain and the anti interference tolerance is spread spectrum communication system are two important performance indexes. Processing gain of G also known as spread spectrum gain

34、( Spreading Gain ) define it as a spread spectrum information before the DF bandwidth and bandwidth expansion after the signal bandwidth of W is G = W / DF in spread spectrum communication system receiver for spread spec

35、trum demodulation. After, only extraction of pseudo random code correlation processing after </p><p>　　Anti interference tolerance refers to the spread spectrum communication system can be in how old the int

36、erference environment under normal working ability, defined as: Mj = G [ ( S/N ) out + Ls]: Mj - anti interference capacity G - processing gain ( S / N ) out - information data to be correctly demodulated and minimum out

37、put signal noise ratio Ls - receiving system such as job loss, a spread spectrum processing gain of the system requirements for35dB. bit error rate less than l0 - 5information </p><p>　　(10 +3) = 22dB this d

38、escription, the system can input power level ratio in interference of spread spectrum signal power level high 22dB within the range of normal work, namely the system is able to receive the input SNR is greater than or eq

39、ual to22dB environment under normal operation.</p><p>　　Three major characteristics of spread spectrum communication</p><p>　　The spread spectrum communication can greatly expand the frequency s

40、pectrum of the signal, originating with spread spectrum code sequence spread spectrum modulation,</p><p>　　And at the receiving end by correlation demodulation technique, which has many narrow band communica

41、tion is difficult to replace the excellent performance,In" army turn civilian", quickly extended to various public and private communication network, basically have the followingCharacteristic:</p><p

42、>　　3.1 easy to reuse of frequencies, improves the spectrum utilization rate</p><p>　　Wireless spectrum is very valuable, although from the wave to the microwave have been developed using, still can not me

43、et the demand of society. In a narrow band communication, mainly rely on the channel to prevent interference between the division of channel. Therefore, all the countries in the world have established frequency managemen

44、t mechanism, the user can only use approved frequency.</p><p>　　Spread spectrum communication transmission power is very low (1-650mW ), the correlation receiving this high technology, and can work in the ch

45、annel noise and thermal noise in the background, is easy to be in the same area of the repeated use of the same frequency, and may also present a variety of narrow communication sharing the same frequency resource. There

46、fore, in the United States and most of the countries in the world, spread spectrum communication need not apply for any frequency, indivi</p><p>　　3.2strong anti-interference, low bit error rate</p>&

47、lt;p>　　Spread spectrum communication in the space occupied by the transmission bandwidth is relatively wide, and the receiving end and the correlation detection approach to despread, make useful wideband information s

48、ignal into the narrow band signal, and the desired signal spread into wideband signal, and then through the narrow band filtering technique to extract useful signal. This auspicious, for various kinds of interference sig

49、nal at the receiving end, because of its non correlation, de-spread na</p><p>　　For broadband interference and pulse interference in spread spectrum apparatus to inhibit physical process, for various forms o

50、f human ( such as electronic warfare ) interference or other narrowband or broadband ( DSSS) system disturbance, as long as the waveform, time and symbol is slightly different, de-spread still maintains its broad band, a

51、nd the useful signal will be compressed, the pulse interference, bandwidth will be expanded to B, and the useful signal recovery ( compression ), ensure </p><p>　　As a result of spread spectrum signal in a r

52、elatively wide frequency band was expanded, the unit within the frequency band of the power is very small, signal submerged in noise, are generally not easily found, and would like to further detection signal parameters

53、( such as pseudorandom coded sequence ) is more difficult, so that its concealment. Furthermore, due to the spread spectrum signal has very low power spectral density, the current use of the narrowband communication inte

54、rference is very</p><p>　　3.3 realize the code division multiple access</p><p>　　Spread spectrum communication improves the anti-interference performance, but paid the occupied bandwidth costs,

55、if many users share the bandwidth, can greatly improve the utilization rate of frequency band. Due to the spread spectrum communication in presence of the spreading code sequence spread spectrum modulation, make full use

56、 of all sorts of different type of code spreading code sequences between excellent auto-correlation and cross-correlation properties, at the receiving end using correl</p><p>　　3.4anti multipath interference

57、</p><p>　　In a wireless communication in various frequency bands, long-term since, multipath interference is always a difficult problem to solve. In the past the narrow band communication, using two methods

58、to improve the ability of anti multipath interference : one is to put the most useful signal is separated, the exclusion of other paths of the interference signal, which uses diversity / receiving technology; two is to t

59、ry to put different paths to different delay times, different phase signal at the r</p><p>　　The two techniques in spread spectrum communication is easy to realize. Using a spreading code auto-correlation pr

60、operties, at the receiving end from the multipath signal extracting and separating out the strongest of the useful signal, or the multiple paths to the same code sequence waveform additive synthesis, which is equivalent

61、to the comb filter effect. In addition, using frequency hopping spread spectrum modulation spread spectrum system, by using multiple frequency signal is transmitted w</p><p>　　3.5precise timing and ranging&l

62、t;/p><p>　　We know that the electromagnetic wave propagation in the space velocity is constant speed of light. It is natural to wonder if capable of accurately measuring the electromagnetic wave in the two obje

63、cts are transmitted between the time, also be equal to measure the distance between two objects.</p><p>　　In the spread spectrum communication if spread spectrum is very wide, it means the code rate is very

64、high, each chip time is very short. When the transmitted spread spectrum signal reflected back upon the object to be measured, in the receiver demodulates the spread spectrum code sequence, and then compare the transceiv

65、er two code sequence phase difference, can accurately detect the spread spectrum signal and time difference, so as to calculate the distance between two people. Measurement accura</p><p><b>　　一 擴頻通信技術(shù)&

66、lt;/b></p><p>　　1.1擴頻通信系統(tǒng)概述 </p><p>　　擴頻通信，即擴展頻譜通信(Spread Spectrum Communication)，它與光纖通信、衛(wèi)星通信，一同被譽為進(jìn)入信息時代的三大高技術(shù)通信傳輸方式?！?lt;/p><p>　　擴頻通信是將待傳送的信息數(shù)據(jù)被偽隨機編碼(擴頻序列：Spread Sequence)調(diào)

67、制，實現(xiàn)頻譜擴展后再傳輸；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)。　</p><p>　　這種通信方式與常規(guī)的窄道通信方式是有區(qū)別的： 一是信息的頻譜擴展后形成寬帶傳輸；二是相關(guān)處理后恢復(fù)成窄帶信息數(shù)據(jù)。正是由于這兩大持點，使擴頻通信有如下的優(yōu)點：  抗干擾 抗噪音 抗多徑衰落 具有保密性 功率譜密度低，具有隱蔽性和低的截獲概率 可多址復(fù)用和任意選址 高精度測量等　</p>

68、;<p>　　正是由于擴頻通信技術(shù)具有上述優(yōu)點，自50年代中期美國軍方便開始研究，一直為軍事通信所獨占，廣泛應(yīng)用于軍事通信、電子對抗以及導(dǎo)航、測量等各個領(lǐng)域。直到80年代初才被應(yīng)用于民用通信領(lǐng)域。為了滿足日益增長的民用通信容量的需求和有效地利用頻譜資源，各國都紛紛提出在數(shù)字峰窩移動通信、衛(wèi)星移動通信和未來的個人通信中采用擴頻技術(shù)，擴頻技術(shù)已廣泛應(yīng)用于蜂窩電話、無繩電話、微波通信、無線數(shù)據(jù)通信、遙測、監(jiān)控、報警等系統(tǒng)中<

69、;/p><p>　　二 擴展頻譜通信的基本概念</p><p>　　2.1 擴展頻譜通信的定義 </p><p>　　所謂擴展頻譜通信，可簡單表述如下：“擴頻通信技術(shù)是一種信息傳輸方式，其信號所占有的頻帶寬度遠(yuǎn)大于所傳信息必需的最小帶寬；頻帶的擴展是通過一個獨立的碼序列來完成，用編碼及調(diào)制的方法來實現(xiàn)的，與所傳信息數(shù)據(jù)無關(guān)；在接收端則用同樣的碼進(jìn)行相關(guān)同步接收、解擴及

70、恢復(fù)所傳信息數(shù)據(jù)”。</p><p>　　2.2信號的頻譜被展寬了。 </p><p>　　我們知道，傳輸任何信息都需要一定的帶寬，稱為信息帶寬。例如人類的語音的信息帶寬為300Hz --- 3400Hz，電視圖像信息帶寬為數(shù)MHz。為了充分利用頻率資源，通常都是盡量采用大體相當(dāng)?shù)膸挼男盘杹韨鬏斝畔ⅰＴ跓o線電通信中射頻信號的帶寬與所傳信息的帶寬是相比擬的。如用調(diào)幅信號來傳送語音信息，其帶

71、寬為語音信息帶寬的兩倍；電視廣播射頻信號帶寬也只是其視頻信號帶寬的一倍多。這些都屬于窄帶通信。</p><p>　　一般的調(diào)頻信號，或脈沖編碼調(diào)制信號，它們的帶寬與信息帶寬之比也只有幾到十幾。擴展頻譜通信信號帶寬與信息帶寬之比則高達(dá)100 --- 1000，屬于寬帶通信。 為什么要用這樣寬的頻帶的信號來傳輸信息呢? 這樣豈不太浪費寶貴的頻率資源了嗎?</p><p>　　2.3采用擴頻碼序

72、列調(diào)制的方式來展寬信號頻譜。</p><p>　　我們知道，在時間上有限的信號，其頻譜是無限的。例如很窄的脈沖信號，其頻譜則很寬。信號的頻帶寬度與其持續(xù)時間近似成反比。1微秒的脈沖的帶寬約為1MHz。因此，如果用限窄的脈沖序列被所傳信息調(diào)制，則可產(chǎn)生很寬頻帶的信號。如下面介紹的直接序列擴頻系統(tǒng)就是采用這種方法獲得擴頻信號。這種很窄的脈沖碼序列，其碼速率是很高的，稱為擴頻碼序列。這里需要說明的一點是所采用的擴頻碼序

73、列與所傳信息數(shù)據(jù)是無關(guān)的，也就是說它與一般的正弦載波信號一樣，絲毫不影響信息傳輸?shù)耐该餍?。擴頻碼序列僅僅起擴展信號頻譜的作用。</p><p>　　2.4在接收端用相關(guān)解調(diào)來解擴</p><p>　　正如在一般的窄帶通信中，已調(diào)信號在接收端都要進(jìn)行解調(diào)來恢復(fù)所傳的信息。在擴頻通信中接收端則用與發(fā)送端相同的擴頻碼序列與收到的擴頻信號進(jìn)行相關(guān)解調(diào)，恢復(fù)所傳的信息。換句話說，這種相關(guān)解調(diào)起到解擴

74、的作用。即把擴展以后的信號又恢復(fù)成原來所傳的信息。這種在發(fā)端把窄帶信息擴展成寬帶信號，而在收端又將其解擴成窄帶信息的處理過程，會帶來一系列好處。弄清楚擴頻和解擴處理過程的機制，是理解擴頻通信本質(zhì)的關(guān)鍵所在。</p><p>　　2.5擴頻通信的理論基礎(chǔ)</p><p>　　長期以來，人們總是想法使信號所占領(lǐng)譜盡量的窄，以充分利用十分寶貴的頻譜資源。為什么要用這樣寬頻帶的信號來傳送信息呢?

75、簡單的回答就是主要為了通信的安全可靠。 </p><p>　　擴頻通信的基本特點，是傳輸信號所占用的頻帶寬度(W)遠(yuǎn)大于原始信息本身實際所需的最小(有效)帶寬(F)，其比值稱為處理增益Gp: Gp = W/F ...... (1)眾所周知，任何信息的有效傳輸都需要一定的頻率寬度，如話音為1.7 --- 3.1kHz，電視圖像則寬到數(shù)兆赫。為了充分利用有限的頻率資源，增加通路數(shù)目，人們廣泛選擇不同調(diào)制方式，

76、采用寬頻信道(同軸電纜、微波和光纖等)，和壓縮頻帶等措施，同時力求使傳輸?shù)拿浇橹袀鬏數(shù)男盘栒加帽M量窄的帶寬。因現(xiàn)今使用的電話、廣播系統(tǒng)中，無論是采用調(diào)幅、調(diào)頻或脈沖編碼調(diào)制制式，Gp值一般都在十多倍范圍內(nèi)，統(tǒng)稱為“窄帶通信”。而擴頻通信的Gp值，高達(dá)數(shù)百、上千，稱為 “寬帶通信”。</p><p>　　擴頻通信的可行性， 是從信息論和抗干擾理論的基本公式中引伸而來的。 信息論中關(guān)于信息容量的仙農(nóng)(Shanno

77、n)公式為： C ＝ WLog2(1十P/N) ...... (2)式中：C --- 信道容量(用傳輸速率度量) W --- 信號頻帶寬度 P --- 信號功率 N --- 白噪聲功率 式(2)說明，在給定的傳輸速率C不變的條件下，頻帶寬度W和信噪比P／N是可以互換的。即可通過增加頻帶寬度的方法，在較低的信噪比P／N(S／N)情況下，傳輸信息。擴展頻譜換取信噪比要求的降低，正是擴頻通信的重要特點，并由此為擴頻通信的應(yīng)用奠定了基礎(chǔ)

78、。</p><p>　　擴頻通信可行性的另一理論基礎(chǔ)，為柯捷爾尼可夫關(guān)于信息傳輸差錯概率的公式：Powj f(E/N。)(3)式中：Powj --- 差錯概率 E --- 信號能量 N。--- 噪聲功率譜密度 因為，信號功率 P＝E／T (T為信息持續(xù)時間) 噪聲功率 N＝WN。 (W為信號頻帶寬度) 信息帶寬 F＝l／T 則式(3)可化為： Powj f(TW.P/N) = f(P/

79、N.W/ F ) (4) 式(4)說明，對于一定帶寬 F的信息而言，用Gp值較大的寬帶信號來傳輸，可以提高通信抗干擾能力，保證強干擾條件下， 通信的安全可靠。 亦即式(4)與式(2)一樣，說明信噪比和帶寬是可以互換的。 </p><p>　　總之，我們用信息帶寬的100倍，甚至1000倍以上的寬帶信號來傳輸信息，就是為了提高通信的抗干擾能力，即在強干擾條件下保證可靠安全地通信。這就是擴展頻譜通信的基本思想和理

80、論依據(jù)。</p><p>　　2.6擴頻通信的主要性能指標(biāo) </p><p>　　處理增益和抗干擾容限是擴頻通信系統(tǒng)的兩個重要性能指標(biāo)。 處理增益G也稱擴頻增益(Spreading Gain) 它定義為頻譜擴展前的信息帶寬F與頻帶擴展后的信號帶寬W之比G＝W／F 在擴頻通信系統(tǒng)中．接收機作擴頻解調(diào)后，只提取偽隨機編碼相關(guān)處理后的帶寬為F 的信息，而排除掉寬頻帶W中的外部干擾、噪音和其

81、地用戶的通信影響。因此，處理增益G反映了擴頻通信系統(tǒng)信噪比改善的程度。</p><p>　　抗干擾容限是指擴頻通信系統(tǒng)能在多大干擾環(huán)境下正常工作的能力，定義為：Mj = G － [(S/N)out + Ls] 其中：Mj --- 抗干擾容G --- 處理增益 (S／N)out --- 信息數(shù)據(jù)被正確解調(diào)而要求的最小輸出信噪比 Ls --- 接收系統(tǒng)的工作損耗 例如, 一個擴頻系統(tǒng)的處理增益為35dB．要求誤碼率小

82、于l0－5的信息數(shù)據(jù)解調(diào)的最小的輸出信噪比(S/N)out ＜10 dB，系統(tǒng)損耗Ls＝3dB，則干擾容限Mj ＝35 － </p><p>　　(10 +3) ＝ 22dB 這說明，該系統(tǒng)能在干擾輸入功率電平比擴頻信號功率電平高22dB的范圍內(nèi)正常工作，也就是該系統(tǒng)能夠在接收輸入信噪比大于或等于－22dB的環(huán)境下正常工作。</p><p>　　三  擴展頻譜通信的主要特點<

83、;/p><p>　　由于擴頻通信能大大擴展信號的頻譜，發(fā)端用擴頻碼序列進(jìn)行擴頻調(diào)制，以及在收端用相關(guān)解調(diào)技術(shù)，使其具有許多窄帶通信難于替代的優(yōu)良性能，能在“軍轉(zhuǎn)民”后，迅速推廣到各種公用和專用通信網(wǎng)絡(luò)之中，主要有以下幾項特點： 　</p><p>　　3.1易于重復(fù)使用頻率，提高了無線頻譜利用率</p><p>　　無線頻譜十分寶貴，雖然從長波到微波都得到了開發(fā)

84、利用，仍然滿足不了社會的需求。在窄帶通信中，主要依靠波道劃分來防止信道之間發(fā)生干擾。 為此，世界各國都設(shè)立了頻率管理機構(gòu)，用戶只能使用申請獲準(zhǔn)的頻率。 </p><p>　　擴頻通信發(fā)送功率極低(1 --- 650mW)，采用了相關(guān)接收這一高技術(shù)，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, 也可與現(xiàn)今各種窄道通信共享同一頻率資源。所以，在美國及世界絕大多數(shù)國家，擴頻通信不需申請頻率，任何

85、個人與單位可以無執(zhí)照使用。 </p><p>　　3.2抗干擾性強，誤碼率低</p><p>　　擴頻通信在空間傳輸時所占有的帶寬相對較寬，而收端又采用相關(guān)檢測的辦法來解擴，使有用寬帶信息信號恢復(fù)成窄帶信號，而把非所需信號擴展成寬帶信號，然后通過窄帶濾波技術(shù)提取有用的信號。這祥，對于各種干擾信號，因其在收端的非相關(guān)性，解擴后窄帶信號中只有很微弱的成份，信噪比很高，因此抗干擾性強。

86、如上述例子（第二講），當(dāng)Gp = 35dB時，抗干擾容限Mj = 22dB，即在負(fù)信噪聲比(－22dB)條件下，可以將信號從噪聲的湮滅中提取出來。在目前商用的通信系統(tǒng)中， 擴頻通信是唯一能夠工作于負(fù)信噪比條件下的通信方式。 </p><p>　　對于寬帶干擾和脈沖干擾在擴頻設(shè)備中如何被抑制的物理過程， 對于各種形式人為的(如電子對抗中)干擾或其他窄帶或?qū)拵?擴頻)系統(tǒng)的干擾， 只要波形、時間和碼元稍有差異，解擴

87、后仍然保持其寬帶性，而有用信號將被壓縮，對于脈沖干擾，帶寬將被展寬到B，而有用信號恢復(fù)(壓縮)后，保證高于干擾，由于擴頻系統(tǒng)這一優(yōu)良性能，誤碼率很低，正常條件下可低到l 0-10, 最差條件下約10-6，完全能滿足國內(nèi)相關(guān)系統(tǒng)對通道傳輸質(zhì)量的要求。 </p><p>　　由于擴頻信號在相對較寬的頻帶上被擴展了，單位頻帶內(nèi)的功率很小，信號湮沒在噪聲里，一般不容易被發(fā)現(xiàn)，而想進(jìn)一步檢測信號的參數(shù)(如偽隨機編碼序列)

88、就更加困難，因此說其隱蔽性好。再者，由于擴頻信號具有很低的功率譜密度，它對目前使用的各種窄帶通信系統(tǒng)的干擾很小。 </p><p>　　3.3可以實現(xiàn)碼分多址 </p><p>　　擴頻通信提高了抗干擾性能，但付出了占用頻帶寬的代價，如果讓許多用戶共用這一寬頻帶，則可大為提高頻帶的利用率。由于在擴頻通信中存在擴頻碼序列的擴頻調(diào)制，充分利用各種不同碼型的擴頻碼序列之間優(yōu)良的自相關(guān)特性和互相

89、關(guān)特性，在接收端利用相關(guān)檢測技術(shù)進(jìn)行解擴，則在分配給不同用戶碼型的情況下可以區(qū)分不同用戶的信號，提取出有用信號。這樣一來，在一寬頻帶上許多對用戶可以同時通話而互不干擾。</p><p><b>　　3.4抗多徑干擾</b></p><p>　　在無線通信的各個頻段，長期以來，多徑干擾始終是一個難以解決的問題之一。在以往的窄帶通信中，采用兩種方法來提高抗多徑干擾的能力：

90、 一是把最強的有用信號分離出來，排除其他路徑的干擾信號，即采用分集／接收技術(shù)； 二是設(shè)法把不同路徑來的不同延遲、不同相位的倍號在接收端從時域上對齊相加，合并成較強的有用信號，即采用梳狀濾波器的方法。 </p><p>　　這兩種技術(shù)在擴頻通信中都易于實現(xiàn)。利用擴頻碼的自相關(guān)特性，在接收端 從多徑信號中提取和分離出最強的有用信號，或把多個路徑來的同一碼序列的波形相加合成，這相當(dāng)于梳狀濾波器的作用。另外，采用頻率跳變

91、擴頻調(diào)制方式的擴頻系統(tǒng)中，由于用多個頻率的信號傳送同一個信息，實際上起到了頻率分集的作用。</p><p>　　3.5能精確地定時和測距</p><p>　　我們知道電磁波在空間的傳播速度是固定不變的光速。人們自然會想到如果能夠精確測量電磁波在兩個物體之間傳播的時間，也就等于測量兩個物體之間的距離。</p><p>　　在擴頻通信中如果擴展頻譜很寬，則意味著所采用的