擴(kuò)頻技術(shù)外文翻譯

1、<p><b>　　擴(kuò)頻技術(shù)</b></p><p><b>　　摘 要</b></p><p>　　擴(kuò)頻技術(shù)是信號（例如一個電氣、電磁，或聲信號）生成的特定帶寬頻率域中特意傳播，從而導(dǎo)致更大帶寬的信號的方法。這些技術(shù)用于各種原因包括增加抗自然干擾和干擾，以防止檢測，并限制功率流密度（如在衛(wèi)星下行鏈路）的安全通信設(shè)立的。</p&

2、gt;<p><b>　　頻率跳變的歷史：</b></p><p>　　跳頻的概念最早是歸檔在1903年美國專利723188和美國專利725605由尼古拉特斯拉在1900年7月提出的。特斯拉想出了這個想法后，在1898年時展示了世界上第一個無線電遙控潛水船，卻從“受到干擾，攔截，或者以任何方式干涉”發(fā)現(xiàn)無線信號控制船是安全的需要。他的專利涉及兩個實現(xiàn)抗干擾能力根本不同的技術(shù)，實

3、現(xiàn)這兩個功能通過改變載波頻率或其他專用特征的干擾免疫。第一次在為使控制電路發(fā)射機(jī)的工作，同時在兩個或多個獨(dú)立的頻率和一個接收器，其中的每一個人發(fā)送頻率調(diào)整，必須在作出回應(yīng)。第二個技術(shù)使用由預(yù)定的方式更改傳輸?shù)念l率的一個編碼輪控制的變頻發(fā)送器。這些專利描述頻率跳變和頻分多路復(fù)用，以及電子與門邏輯電路的基本原則。</p><p>　　跳頻在無線電報中也被無線電先驅(qū)約翰內(nèi)斯Zenneck提及(1908，德語，英語翻譯麥

4、克勞希爾，1915年)，雖然Zenneck自己指出德律風(fēng)根在早幾年已經(jīng)試過它。Zenneck的書是當(dāng)時領(lǐng)先的文本，很可能后來的許多工程師已經(jīng)注意到這個問題。一名波蘭的工程師(Leonard Danilewicz)，在1929年提出了這個想法。其他幾個專利被帶到了20世紀(jì)30年代包括威廉貝爾特耶斯（德國1929年，美國專利1869695，1932）。在第二次世界大戰(zhàn)中，美國陸軍通信兵發(fā)明一種稱為SIGSALY的通信系統(tǒng)，使得羅斯福和丘吉爾

5、之間能相互通信，這種系統(tǒng)稱為擴(kuò)頻，但由于其高的機(jī)密性，SIGSALY的存在直到20世紀(jì)80年代才知道。</p><p>　　最著名的跳頻發(fā)明是女演員海蒂拉瑪和作曲家喬治安太爾，他們的“秘密通信系統(tǒng)”1942年獲美國第2,292,387專利。拉瑪與前夫弗里德里希汀曼德這位奧地利武器制造商在國防會議上了解到這一問題。安太爾-拉馬爾版本的跳頻用鋼琴卷88個頻率發(fā)生變化，其旨在使無線電導(dǎo)向魚雷，讓敵人很難來檢測或干擾。該

6、專利來自五零年代ITT公司和其他私人公司開始時發(fā)展碼分多址(CDMA)，一個民間形式擴(kuò)頻，盡管拉馬爾專利有沒對后續(xù)技術(shù)有直接影響。它其實是在麻省理工學(xué)院林肯實驗室、樂華政府和電子工業(yè)公司、國際電話電報公司及萬年電子系統(tǒng)導(dǎo)致早期擴(kuò)頻技術(shù)在20世紀(jì)50年代的長期軍事研究。雷達(dá)系統(tǒng)的并行研究和一個稱為“相位編碼”的技術(shù)類似概念對擴(kuò)頻發(fā)展造成影響。</p><p><b>　　擴(kuò)頻通信</b><

7、;/p><p>　　這是一種在其（電信）信號傳輸一個帶寬遠(yuǎn)遠(yuǎn)多于原始信息的頻率內(nèi)容的技術(shù)。</p><p>　　擴(kuò)頻通信是構(gòu)建技術(shù)，它采用直接序列、調(diào)頻，或多個訪問/多種功能可用這些的混合信號。這種技術(shù)減少了對其他接收機(jī)的潛在干擾，同時實現(xiàn)隱私。擴(kuò)頻通常會使用噪聲的連續(xù)的信號傳播結(jié)構(gòu)，通常使用窄帶上的信息信號分散一個相對寬帶（單選）的波段的頻率。 接收器接收信號的相關(guān)性檢索原始的信息信號。要么

8、努力抵御敵人的通信干擾（防堵塞，或簡稱AJ），或隱瞞事實，溝通，甚至發(fā)生，有時也稱為低截獲概率（LPI）的。</p><p>　　跳頻擴(kuò)頻(FHSS)，直接序列擴(kuò)頻（DSSS）、時間跳頻擴(kuò)頻(THSS)、線性擴(kuò)頻(CSS)，和這些技術(shù)的組合都是擴(kuò)頻的形式。每種方法采用了偽隨機(jī)數(shù)字序列使用的偽隨機(jī)數(shù)字生成器創(chuàng)建——以確定與控制信號通過分配帶寬的傳播模式。 超寬帶(UWB)是另一種調(diào)制技術(shù)，實現(xiàn)了基于傳輸短時間內(nèi)脈沖

9、相同的目的。無線以太網(wǎng)標(biāo)準(zhǔn)IEEE 802.11在其無線接口使用跳頻擴(kuò)頻或直接序列擴(kuò)頻。</p><p><b>　　備 注</b></p><p>　　自20世紀(jì)40年代以來已知和自20世紀(jì)50年代以來在軍事通信系統(tǒng)中使用的技術(shù)。</p><p>　　“傳播”的無線電信號較寬的頻率范圍內(nèi)若干程度高于最低要求。擴(kuò)頻的核心原則就是波載波噪聲樣，

10、使用和作為名稱意味著比相同的數(shù)據(jù)速率在簡單的點(diǎn)對點(diǎn)通信所需更多的帶寬。</p><p><b>　　兩種主要的方法： </b></p><p>　　1.直接序列(DS)</p><p><b>　　2.跳頻 (FH)</b></p><p>　　耐干擾。直接序列在抵御連續(xù)時間窄帶干擾更好，而跳頻抗脈

11、沖干擾是更好。在直接序列系統(tǒng)中，窄帶干擾會影響檢測性能如干擾功率量蔓延了整個信號的帶寬時，通常檢測性能不會比更強(qiáng)背景噪聲。相比之下，在那些低帶寬的窄帶信號系統(tǒng)，如果干擾功率恰巧集中在信號帶寬那么接收的信號質(zhì)量將會嚴(yán)重降低。</p><p>　　抗竊聽。擴(kuò)頻代碼（在直接序列系統(tǒng)）或跳頻模式（在跳頻系統(tǒng)）通常任何一方都不知道誰的信號是未定義的，在這種情況下“加密”信號，并降低對方的對其的判斷意識。更重要的是，有一個給

12、定的噪聲功率譜密度 (PSD)，擴(kuò)頻系統(tǒng)需要在每比特相同數(shù)量的能源之前傳播窄帶系統(tǒng)因此同樣的功率，如果比特率在擴(kuò)展前是相同的，但由于每比特能量信號功率擴(kuò)散超過一個大帶寬的擴(kuò)散，則信號PSD的要低得多，而往往大大低于噪聲PSD的，因此對手可能無法確定是否存在于所有的信號。不過，對于關(guān)鍵任務(wù)的應(yīng)用尤其是雇用商用無線電通訊設(shè)備，擴(kuò)頻無線電本質(zhì)上沒有提供足夠的安全“……只用擴(kuò)頻無線電通信本身是不足夠的安全?！?lt;/p><p&

13、gt;　　抗衰落。擴(kuò)頻信號所占用的高帶寬提供某些頻率的多樣性，也就是說，即是不可能的信號也會遇到整個帶寬的嚴(yán)重多徑衰落，而在其他情況下信號可以被檢測到使用，例如Rake接收機(jī)。</p><p>　　多種接入能力。多個用戶可以同時傳輸相同的頻率（范圍），只要他們使用不同的擴(kuò)頻碼。請參閱 CDMA。</p><p><b>　　主要技術(shù)：</b></p>&

14、lt;p><b>　　一、直接序列擴(kuò)頻</b></p><p>　　在電信中，直接序列擴(kuò)頻（DSSS）是一種調(diào)制技術(shù)。與其他擴(kuò)頻技術(shù)一樣傳輸?shù)男盘柋缺徽{(diào)制的信息信號的占用更多帶寬?！?dāng)U頻’名稱來自一個事實，即載波信號在整個帶寬（譜設(shè)備的發(fā)射頻率）發(fā)生。</p><p><b>　　功 能</b></p><p>　

15、　與它相調(diào)制正弦波偽隨機(jī)地與偽連續(xù)的字符串（PN）的代碼符號稱為“芯片”，各自有一個比信息比特更短的時間。也就是說每個信息位是由一個更快的芯片序列調(diào)制，因此芯片速率遠(yuǎn)高于信息信號的比特率。</p><p>　　它使用的信號接收器的眾所周知的先驗結(jié)構(gòu)，其中是由發(fā)射機(jī)生產(chǎn)的芯片序列。接收器就可以使用相同的偽隨機(jī)碼序列，以抵消對接收信號的偽隨機(jī)碼序列的影響，以重建信息信號。</p><p>&l

16、t;b>　　傳輸方法</b></p><p>　　直接序列擴(kuò)頻傳輸數(shù)據(jù)乘以由一個“噪音”信號傳送。這種噪聲信號是1和-1偽隨機(jī)序列值，其頻率比原始信號為高，從而帶能量延伸到更廣泛的原信號。</p><p>　　產(chǎn)生的信號類似于白噪聲，像“靜態(tài)”的音頻錄音。不過，這個類似噪聲的信號可用于乘以相同的偽隨機(jī)序列完全重建接收端的原始數(shù)據(jù)（因為 1 × 1 = 1，?1

17、×?1 = 1)。這個過程稱為“解擴(kuò)”的過程在數(shù)學(xué)上構(gòu)成傳播的 PN 序列，接收方認(rèn)為使用發(fā)射器PN序列的相關(guān)性。</p><p>　　對于解擴(kuò)的正常運(yùn)行，發(fā)送和接收序列必須同步。這需要通過某種形式的時間搜索過程使發(fā)射器的序列與接收器序列同步。但是，這種明顯的缺點(diǎn)可以是一個重要好處：如果多個發(fā)射器的序列是相互同步的，那么相對的同步接收器必須使它們之間可以用來確定相對時間，而反過來，如果已知發(fā)射器的位置，

18、可用于計算接收器的位置。這是許多衛(wèi)星導(dǎo)航系統(tǒng)的基礎(chǔ)。</p><p>　　調(diào)用過程中加強(qiáng)對通道信噪比造成的影響被稱為處理增益。這種影響可通過采用較大較長PN序列和每比特更多的芯片，但用來生成PN序列的物理設(shè)備的多個芯片上可達(dá)到的處理增益實際限制。</p><p>　　如果在同一信道發(fā)送器發(fā)送同一頻道，但使用不同的PN序列（或根本沒有序列） 解擴(kuò)過程導(dǎo)致該信號沒有獲得處理。這種效果是碼分多址

19、（CDMA）屬性的直接序列擴(kuò)頻，它允許多個發(fā)射機(jī)內(nèi)共享他們的偽碼序列的互相關(guān)特性來限制相同的頻道。</p><p>　　由于這說明表明，一個傳輸?shù)牟ㄐ螆D有一個大致的鐘形信封的載波頻率為中心，就像AM傳播, 除了增加的傳輸噪音導(dǎo)致的分配要大大高于一個AM信號的更廣泛的傳播。</p><p>　　相比之下，跳頻擴(kuò)頻偽隨機(jī)重新調(diào)整載波信號，而不是添加偽隨機(jī)噪聲數(shù)據(jù)，結(jié)果導(dǎo)致在一個統(tǒng)一的頻率分布，

20、其寬度是由偽隨機(jī)數(shù)發(fā)生器的輸出范圍決定。</p><p><b>　　優(yōu) 點(diǎn)</b></p><p>　　對預(yù)期的或非預(yù)期抗干擾 </p><p>　　共享多個用戶間的單信道</p><p>　　減少信號/背景噪聲級別包裝截?。[身）</p><p>　　發(fā)射器與接收器之間的相對時間的測定<

21、;/p><p><b>　　使 用</b></p><p>　　美國全球定位系統(tǒng)和歐洲伽利略衛(wèi)星導(dǎo)航系統(tǒng)</p><p>　　基于直接序列擴(kuò)頻系統(tǒng)（直接序列碼分多址）是一種在擴(kuò)頻多址接入方案的基礎(chǔ)上，從信號的傳播，到不同的用戶有不同的代碼。這是CDMA的最廣泛使用的類型。</p><p>　　無繩電話在900兆赫，2.4吉

22、赫和5.8吉赫頻帶操作</p><p>　　電氣和電子工程師協(xié)會802.11b 2.4 GHz無線網(wǎng)絡(luò)和其前身802.11-1999。(正交頻分復(fù)用技術(shù)繼任802.11g技術(shù))</p><p><b>　　自動抄表</b></p><p>　　電氣和電子工程師協(xié)會802.15.4標(biāo)準(zhǔn)（例如用作物理層和鏈路層的紫蜂)</p><

23、;p><b>　　二、跳頻擴(kuò)頻</b></p><p>　　跳頻擴(kuò)頻(FHSS)通過很多渠道快速切換頻率，其中一個運(yùn)載體發(fā)射無線電信號的一種方法是，使用一個發(fā)射機(jī)和接收機(jī)已知的偽隨機(jī)序列。它被利用作為多個訪問方法中跳頻碼分多址(FH-CDMA)計劃。</p><p>　　擴(kuò)頻傳輸通過三個主要優(yōu)點(diǎn)提供了固定頻率傳輸：</p><p>　　擴(kuò)

24、頻信號高度抗窄帶干擾。再收集傳播信號傳播出了干擾信號的過程，導(dǎo)致其退到背景的干擾信號。</p><p>　　擴(kuò)頻信號難以進(jìn)行攔截。一個跳頻擴(kuò)頻信號顯示為一個簡單的背景噪聲增加至窄帶接收機(jī)。如果竊聽者知道了偽隨機(jī)序列，他們只能夠攔截傳輸。</p><p>　　擴(kuò)頻傳輸可以與許多類型的最小干擾的常規(guī)傳輸共享一個頻帶。擴(kuò)頻信號添加最小噪聲窄頻的通信，反之亦然。這樣一來可以更有效地利用帶寬。<

25、;/p><p><b>　　基本的算法</b></p><p>　　通常，一個調(diào)頻通信的啟動是如下所示</p><p>　　發(fā)起方發(fā)送請求通過預(yù)定義的頻率或控制通道。</p><p>　　接收方發(fā)送一個數(shù)字，像已知的種子。</p><p>　　發(fā)起方作為變量的計算順序，必須使用的頻率的一個預(yù)定義算法中

26、使用該號碼。最經(jīng)常的頻率變化的時期是預(yù)定義的，以允許一個基站，服務(wù)多個連接。</p><p>　　發(fā)起方通過第一次發(fā)送同步信號的頻率計算，從而為接受確認(rèn)它有正確的計算順序。</p><p>　　在通信開始，發(fā)送方和接收沿該計算的順序在同一點(diǎn)開始的時間更改其頻率。</p><p><b>　　技術(shù)的幾點(diǎn)思考</b></p><

27、p>　　所需頻率跳變的整體帶寬是比需要來傳輸僅一個相同信息使用載波頻率更大。不過，由于在任何給定時間只能在此帶寬的一小部分上發(fā)生傳播，實在是一樣有效的干擾帶寬是。雖然沒有提供額外的熱噪聲對寬帶的保護(hù)，跳頻方法確實降低窄帶干擾造成的退化。</p><p>　　對跳頻系統(tǒng)的挑戰(zhàn)之一是如何同步發(fā)射器和接收器。一種方法是有將保證的發(fā)射機(jī)使用在固定時間內(nèi)的所有渠道。接收器隨機(jī)選擇一個頻道就可以找到發(fā)送器，該頻道提供有

28、效的數(shù)據(jù)傾聽變送器。發(fā)送器的數(shù)據(jù)都是通過一個特殊的數(shù)據(jù)序列不像發(fā)生在這個渠道為數(shù)據(jù)段和段可以有一個完整的校驗和進(jìn)一步鑒定。發(fā)射器和接收器可以使用固定的渠道序列表，以便他們按照表中的能保持同步。每個通道段上發(fā)射器表中，可以將其當(dāng)前位置的進(jìn)行發(fā)送。</p><p>　　在美國的通信委員會第15部分無牌系統(tǒng)900兆赫茲和2.4兆赫茲頻帶上允許更多非擴(kuò)頻系統(tǒng)功率。調(diào)頻和直接序列系統(tǒng)可以在1瓦傳輸。該限制從1毫瓦增加到1瓦

29、或增加一千倍。美國聯(lián)邦通訊委員會（FCC）規(guī)定了渠道的最低數(shù)目和每個通道的最大駐留時間。</p><p>　　在實際的多點(diǎn)式無線電系統(tǒng)，空間允許的多個相同頻率的傳輸，在一個地理區(qū)域內(nèi)可能使用多個無線電設(shè)備。這將創(chuàng)建系統(tǒng)數(shù)據(jù)速率高于香農(nóng)極限的單通道的可能性。擴(kuò)頻系統(tǒng)沒有違反香農(nóng)極限。擴(kuò)頻系統(tǒng)過多的依賴信號信噪比的頻譜共享。多輸入多輸出和直接序列擴(kuò)頻系統(tǒng)中也看到此屬性。電波傳導(dǎo)和定向天線也通過提供遠(yuǎn)程無線電通訊設(shè)備之

30、間的隔離提高系統(tǒng)的性能。</p><p>　　Spread Spectrum Techniques</p><p><b>　　Abstract:</b></p><p>　　Spread-spectrum techniques are methods by which a signal (e.g. an electrical, electro

31、magnetic, or acoustic signal ) generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, includi

32、ng the establishment of secure communications, increasing resistance to natural interference and jamming, to prevent detection, and to limit power flux density (e.g. in satellite downlinks).</p><p>　　History

33、 Frequency hopping:</p><p>　　The concept of frequency hopping was first alluded to in the 1903 U.S. Patent 723,188 and U.S. Patent 725,605 filed by Nikola Tesla in July 1900. Tesla came up with the idea afte

34、r demonstrating the world's first radio-controlled submersible boat in 1898, when it became apparent the wireless signals controlling the boat needed to be secure from "being disturbed, intercepted, or interfere

35、d with in any way." His patents covered two fundamentally different techniques for achieving immunity to interf</p><p>　　Frequency hopping is also mentioned in radio pioneer Johannes Zenneck's book

36、Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken had already tried it several years earlier. Zenneck's book was a leading text of the time, an

37、d it is likely that many later engineers were aware of it. A Polish engineer, Leonard Danilewicz, came up with the idea in 1929.Several other patents were taken out in the 1930s, including one by Willem Broe</p>&

38、lt;p>　　The most celebrated invention of frequency hopping was that of actress Hedy Lamarr and composer George Antheil, who in 1942 received U.S. Patent 2,292,387 for their "Secret Communications System". Lam

39、arr had learned about the problem at defense meetings she had attended with her former husband Friedrich Mandl, who was an Austrian arms manufacturer. The Antheil-Lamarr version of frequency hopping used a piano-roll to

40、change among 88 frequencies, and was intended to make radio-guided torpedoes har</p><p>　　Spread-spectrum telecommunications</p><p>　　This is a technique in which a (telecommunication) signal is

41、 transmitted on a bandwidth considerably larger than the frequency content of the original information.</p><p>　　Spread-spectrum telecommunications is a signal structuring technique that employs direct seque

42、nce, frequency hopping, or a hybrid of these, which can be used for multiple access and/or multiple functions. This technique decreases the potential interference to other receivers while achieving privacy. Spread spectr

43、um generally makes use of a sequential noise-like signal structure to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies. The receiv</p><p>　　Frequency-h

44、opping spread spectrum (FHSS), direct-sequence spread spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and combinations of these techniques are forms of spread spectrum. Each of these te

45、chniques employs pseudorandom number sequences — created using pseudorandom number generators — to determine and control the spreading pattern of the signal across the alloted bandwidth. Ultra-wideband (UWB) is another m

46、odulation technique that accomplishes the same pu</p><p><b>　　Notes</b></p><p>　　Techniques known since 1940s and used in military communication system since 1950s</p><p&g

47、t;　　"Spread" radio signal over a wide frequency range several magnitudes higher than minimum requirement. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies,

48、bandwidths much wider than that required for simple point-to-point communication at the same data rate.</p><p>　　Two main techniques: </p><p>　　1.Direct sequence (DS) </p><p>　　2.Fr

49、equency hopping (FH)</p><p>　　Resistance to jamming (interference). DS is better at resisting continuous-time narrowband jamming, while FH is better at resisting pulse jamming. In DS systems, narrowband jamm

50、ing affects detection performance about as much as if the amount of jamming power is spread over the whole signal bandwidth, when it will often not be much stronger than background noise. By contrast, in narrowband syste

51、ms where the signal bandwidth is low, the received signal quality will be severely lowered if the jammi</p><p>　　Resistance to eavesdropping. The spreading code (in DS systems) or the frequency-hopping patte

52、rn (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it "encrypts" the signal and reduces the chance of an adversary's making sense of it. What's more, for a gi

53、ven noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and therefore the same amount of power if the bitrate befor</p><

54、p>　　Resistance to fading. The high bandwidth occupied by spread-spectrum signals offer some frequency diversity, i.e. it is unlikely that the signal would encounter severe multipath fading over its whole bandwidth, an

55、d in other cases the signal can be detected using e.g. a Rake receiver.</p><p>　　Multiple access capability. Multiple users can transmit simultaneously on the same frequency (range) as long as they use diffe

56、rent spreading codes. See CDMA.</p><p>　　Main techniques:</p><p>　　1、Direct-sequence spread spectrum</p><p>　　In telecommunications, direct-sequence spread spectrum (DSSS) is a modu

57、lation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 'spread spectrum' comes from the fact tha

58、t the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.</p><p><b>　　Features</b></p><p>　　1.It phase-modulates a sine wave pseudorand

59、omly with a continuous string of pseudonoise (PN) code symbols called "chips", each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much

60、 faster chips. Therefore, the chip rate is much higher than the information signal bit rate.</p><p>　　2. It uses a signal structure in which the sequence of chips produced by the transmitter is known a prior

61、i by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal.</p><p>　　Transmission meth

62、od</p><p>　　Direct-sequence spread-spectrum transmissions multiply the data being transmitted by a "noise" signal. This noise signal is a pseudorandom sequence of 1 and ?1 values, at a frequency mu

63、ch higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.</p><p>　　The resulting signal resembles white noise, like an audio recording of "s

64、tatic". However, this noise-like signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 × 1 = 1, and ?1 × ?1 = 1).

65、This process, known as "de-spreading", mathematically constitutes a correlation of the transmitted PN sequence with the PN sequence that the receiver believes the transmitter is using.</p><p>　　For

66、 de-spreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter's sequence via some sort of timing search process.

67、 However, this apparent drawback can be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to

68、determine relative timing, which, in turn, can be used to cal</p><p>　　The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employi

69、ng a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain.</p><p>　　If an undesired transmitter transmits on

70、the same channel but with a different PN sequence (or no sequence at all), the de-spreading process results in no processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) pro

71、perty of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.</p><p>　　As this description suggests, a plot of the t

72、ransmitted waveform has a roughly bell-shaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transm

73、ission.</p><p>　　In contrast, frequency-hopping spread spectrum pseudo-randomly re-tunes the carrier, instead of adding pseudo-random noise to the data, which results in a uniform frequency distribution whos

74、e width is determined by the output range of the pseudo-random number generator.</p><p><b>　　Benefits</b></p><p>　　Resistance to intended or unintended jamming </p><p>　

75、　Sharing of a single channel among multiple users </p><p>　　Reduced signal/background-noise level hampers interception (stealth) </p><p>　　Determination of relative timing between transmitter an

76、d receiver </p><p><b>　　Uses</b></p><p>　　The United States GPS and European Galileo satellite navigation systems</p><p>　　DS-CDMA (Direct-Sequence Code Division Multipl

77、e Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with different codes. It is the most widely used type of CDMA.</p><p>　　Cordless phones operating in the

78、900 MHz, 2.4 GHz and 5.8 GHz bands</p><p>　　IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead)</p><p>　　Automatic meter reading </p&

79、gt;<p>　　IEEE 802.15.4 (used e.g. as PHY and MAC layer for ZigBee)</p><p>　　2、Frequency-hopping spread spectrum</p><p>　　Frequency-hopping spread spectrum (FHSS) is a method of transmitti

80、ng radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is utilized as a multiple access method in the frequency-hopping cod

81、e division multiple access (FH-CDMA) scheme.</p><p>　　A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:</p><p>　　Spread-spectrum signals are highly

82、 resistant to narrowband interference. The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background.</p><p>　　Spread-spectrum signals are difficul

83、t to intercept. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper would only be able to intercept the transmission if they knew the pseudorandom sequence.</

84、p><p>　　Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency co

85、mmunications, and vice versa. As a result, bandwidth can be utilized more efficiently.</p><p>　　Basic algorithm</p><p>　　Typically, the initiation of an FHSS communication is as follows</p>

86、;<p>　　The initiating party sends a request via a predefined frequency or control channel. </p><p>　　The receiving party sends a number, known as a seed. </p><p>　　The initiating party us

87、es the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. Most often the period of the frequency change is predefined, as to allow a single base station to ser

88、ve multiple connections.</p><p>　　The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated

89、 the sequence.</p><p>　　The communication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time.</p><p>　　T

90、echnical considerations</p><p>　　The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because tran

91、smission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. Whilst providing no extra protection against wideband thermal noise, the frequency-hop

92、ping approach does reduce the degradation caused by narrowband interferers.</p><p>　　One of the challenges of frequency-hopping systems is to synchronize the transmitter and receiver. One approach is to have

93、 a guarantee that the transmitter will use all the channels in a fixed period of time. The receiver can then find the transmitter by picking a random channel and listening for valid data on that channel. The transmitter&

94、#39;s data is identified by a special sequence of data that is unlikely to occur over the segment of data for this channel and the segment can have a checksum for</p><p>　　In the US, FCC part 15 on unlicense

95、d system in the 900MHz and 2.4GHz bands permits more power than non-spread spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or

96、 a thousand times increase. The Federal Communications Commission (FCC) prescribes a minimum number of channels and a maximum dwell time for each channel.</p><p>　　In a real multipoint radio system, space al

97、lows multiple transmissions on the same frequency to be possible using multiple radios in a geographic area. This creates the possibility of system data rates that are higher than the Shannon limit for a single channel.

98、Spread spectrum systems do not violate the Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing of spectrum. This property is also seen in MIMO and DSSS systems. Beam steering and dire