跳頻擴(kuò)頻技術(shù)中文資料與外文翻譯

1、<p><b>　　跳頻擴(kuò)頻技術(shù)</b></p><p>　　跳頻擴(kuò)頻（FHSS）的傳輸無線電信號(hào)，通過快速切換方法的載波頻率在許多渠道 ，使用偽隨機(jī)序列發(fā)射器和接收器 。 它是利用作為多址接入方法在跳頻碼分多址接入 （FH-CDMA）的計(jì)劃。 </p><p>　　擴(kuò)頻傳輸提供一個(gè)固定的頻率傳輸?shù)娜齻€(gè)主要優(yōu)點(diǎn)： </p><p>

2、　　1.擴(kuò)頻信號(hào)是高抗窄帶干擾 。重新收集傳播信號(hào)的過程中展開的干擾信號(hào)，使其回落到后臺(tái)。 </p><p>　　2.擴(kuò)頻信號(hào)是難以攔截。一個(gè)跳頻信號(hào)只出現(xiàn)在窄帶接收機(jī)的背景噪聲的增加。竊聽者只能夠攔截傳輸，如果被稱為偽隨機(jī)序列。 </p><p>　　3.擴(kuò)頻傳輸，可以與許多傳統(tǒng)的傳輸類型的頻帶，以最小的干擾。擴(kuò)頻信號(hào)加噪音極小狹窄的高頻通信，反之亦然。作為一個(gè)結(jié)果，可以更有效地利用帶寬

3、。</p><p><b>　　歷史</b></p><p>　　跳頻的概念跳頻首次提到在1903年美國專利723188 美國專利725605 特斯拉在1900年7月。特斯拉來到展示了世界上第一個(gè)無線電遙控潛水船在1898年，當(dāng)它成為明顯的控制無線信號(hào)的船需要的是從安全“受到干擾，攔截，或以任何方式干預(yù)后的想法。” 他的專利涉及兩個(gè)根本不同的技術(shù)實(shí)現(xiàn)的抗干擾能力，這兩

4、個(gè)的作用，通過改變載波的頻率或其他專屬特性。首先有一個(gè)發(fā)射器，同時(shí)在兩個(gè)或兩個(gè)以上的不同頻率和一個(gè)接收器，在每一個(gè)人的傳播頻率進(jìn)行調(diào)整，為了控制電路響應(yīng)，工作。第二種方法使用可變頻率的發(fā)射器，由一個(gè)編碼輪，在預(yù)定的方式改變發(fā)射頻率控制。這些專利描述跳頻頻率的基本原則和頻分復(fù)用，電子與門的邏輯電路。 </p><p>　　跳頻無線電先驅(qū)喬納森Zenneck的書無線電報(bào) （德國，1908年，英文翻譯McGraw Hi

5、ll出版社，1915）也提到，雖然Zenneck自己指出， 德律風(fēng)根已經(jīng)嘗試過了幾年前。zenneck的書是一個(gè)時(shí)間領(lǐng)先的文本，它是可能的，許多后來的工程師們意識(shí)到這一點(diǎn)。德國軍隊(duì)在第一次世界大戰(zhàn)中，英國的力量，沒有技術(shù)，按照順序，以防止竊聽有限使用固定指揮點(diǎn)之間的通信跳頻。一位波蘭工程師， 倫納德Danilewicz ，來到了在1929年的想法。在20世紀(jì)30年代被其他幾個(gè)專利，包括一個(gè)由威廉Broertjes（德國1929年， 美國

6、專利1869695 ，1932年）。第二次世界大戰(zhàn)期間，美國陸軍通信兵發(fā)明了一種稱為SIGSALY羅斯福和丘吉爾之間，其中納入擴(kuò)頻通信的通信系統(tǒng)，但由于其絕密性質(zhì)，SIGSALY的存在并沒有成為直到20世紀(jì)80年代聞名。 </p><p>　　跳頻頻率最有名的發(fā)明是七喜拉馬爾和作曲家喬治·安太爾 ，于1942年獲得美國專利2292387為他們的“秘密通訊系統(tǒng)”的女主角。拉馬爾學(xué)會(huì)了在國防她與她的前夫出席

7、會(huì)議弗里德里希·曼德爾認(rèn)為，無線電制導(dǎo)導(dǎo)彈的信號(hào)很容易被卡住。安太爾拉馬爾版本頻率跳頻采用了鋼琴輥，其中88頻率改變，旨在使無線電制導(dǎo)魚雷的敵人更難檢測或堵塞。該專利在專利檢索來點(diǎn)亮?xí)r， ITT公司和其他私人公司開始開發(fā)拉馬爾專利雖然沒有直接影響到后續(xù)技術(shù)的碼分多址 （CDMA）的的擴(kuò)頻平民的形式，在20世紀(jì)50年代。這是在事實(shí)上正在進(jìn)行的軍事研究，在麻省理工學(xué)院林肯實(shí)驗(yàn)室 ， MAGNAVOX政府及工業(yè)電子公司，ITT公司和

8、Sylvania的電子系統(tǒng) ，導(dǎo)致在20世紀(jì)50年代早期的擴(kuò)頻技術(shù)。雷達(dá)系統(tǒng)和技術(shù)上稱為“相類似的概念，編碼”的平行研究也有一個(gè)擴(kuò)頻發(fā)展的影響。</p><p><b>　　軍事用途 </b></p><p>　　擴(kuò)頻信號(hào)具有很強(qiáng)的耐蓄意干擾 ，除非對(duì)手有知識(shí)的傳播特性。 軍事無線電使用加密技術(shù)，產(chǎn)生控制下的一個(gè)秘密通道序列傳輸?shù)陌踩越痂€ （TRANSEC），發(fā)送者

9、和接收者份額推進(jìn)。</p><p>　　跳頻本身，只提供有限的保護(hù)，防止竊聽和干擾。有一個(gè)簡單的算法，有效地發(fā)現(xiàn)序列的頻率。要解決這個(gè)弱點(diǎn)，最現(xiàn)代化的軍事跳頻電臺(tái)聘請(qǐng)獨(dú)立的加密設(shè)備如KY-57 。 美國軍用無線電設(shè)備使用跳頻包括JTIDS / MIDS的家庭， 有快速和SINCGARS 。</p><p><b>　　民用</b></p><p&g

10、t;　　由于美國聯(lián)邦通信委員會(huì) （FCC）的修訂規(guī)則，以允許在不受管制的2.4 GHz頻段的跳頻擴(kuò)頻系統(tǒng)，在這一帶的許多消費(fèi)電子設(shè)備采用不同的擴(kuò)頻模式。</p><p>　　已經(jīng)制定了無牌使用900兆赫頻段，采用跳頻擴(kuò)頻技術(shù)的一些步話機(jī) 。幾個(gè)這樣的收音機(jī)銷售的名義下的極端無線電服務(wù) （eXRS）。盡管名稱相似的FRS分配，該系統(tǒng)是一個(gè)專有的設(shè)計(jì)，而不是正式的美國聯(lián)邦通信委員會(huì)（FCC）分配服務(wù)。</p&g

11、t;<p><b>　　商業(yè)用途 </b></p><p>　　擴(kuò)頻系統(tǒng) 1976年由羅伯特·狄克遜，出版書號(hào)0-471-21629-1 ，是這項(xiàng)技術(shù)的商業(yè)化的一個(gè)重要的里程碑。以前的出版物，要么歸入軍事報(bào)告或?qū)W術(shù)論文在狹窄的副標(biāo)題。狄克遜的書是第一個(gè)全面的技術(shù)非保密審查，并為增加科研投入商業(yè)應(yīng)用的階段。 </p><p>　　最初的擴(kuò)頻商業(yè)用

12、途開始在20世紀(jì)80年代在美國三大體系：赤道幾內(nèi)亞通信系統(tǒng)是非常小的光圈（ VSAT ）衛(wèi)星終端系統(tǒng)為飛機(jī)導(dǎo)航報(bào)紙新聞專線服務(wù)，德爾北技術(shù)的無線電導(dǎo)航系統(tǒng)除塵作物和類似用途的，和高通公司的OmniTRACS系統(tǒng)通信車。在高通和赤道幾內(nèi)亞系統(tǒng)，擴(kuò)頻啟用以來，觀看多個(gè)衛(wèi)星小天線，擴(kuò)頻處理增益，消除干擾。城北系統(tǒng)采用擴(kuò)頻的高帶寬，提高定位精度。 </p><p>　　1981年，美國聯(lián)邦通信委員會(huì)開始探索在調(diào)查案卷的通

13、知，允許一般民用擴(kuò)頻。這案卷，提出了FCC和FCC的工作人員由邁克爾·馬庫斯然后定向。立案的提案頻譜使用者和無線電設(shè)備制造商的普遍反對(duì)，雖然他們支持的惠普公司，由當(dāng)時(shí)的實(shí)驗(yàn)室組，支持該建議，以后將成為部分安捷倫 。 </p><p>　　1985年5月決定允許在此案卷3階權(quán)力擴(kuò)頻未經(jīng)許可使用1瓦。FCC當(dāng)時(shí)表示，它歡迎在擴(kuò)頻的額外請(qǐng)求其他bands.The導(dǎo)致成文的規(guī)則，現(xiàn)在為47 CFR 15.247

14、 允許的Wi-Fi ， 藍(lán)牙 ，和許多其他產(chǎn)品，包括無繩電話。這些規(guī)則，然后將其復(fù)制在其他許多國家。.高通公司CDMA的商業(yè)化的決定后2個(gè)月內(nèi)成立。</p><p><b>　　技術(shù)的考慮</b></p><p>　　跳頻所需的總帶寬是遠(yuǎn)遠(yuǎn)超過傳輸所需的更廣泛的信息只使用一個(gè)載波頻率 。 然而，由于傳輸只發(fā)生在任何特定時(shí)間的一小部分在此帶寬，有效的干擾帶寬是真的一樣。

15、雖然熱噪聲對(duì)寬帶沒有提供額外的保護(hù)，跳頻方式減少窄帶干擾所造成的退化。 </p><p>　　跳頻系統(tǒng)的挑戰(zhàn)之一是同步發(fā)射器和接收器。 一種方法是有保證的發(fā)射機(jī)將使用在一個(gè)固定的時(shí)間內(nèi)的所有通道。接收器可以找到發(fā)射機(jī)挑選一個(gè)隨機(jī)的信道和監(jiān)聽該通道上的有效數(shù)據(jù)。變送器的數(shù)據(jù)是由一個(gè)特殊的數(shù)據(jù)序列，這是不可能發(fā)生在此通道和段可以有一個(gè)數(shù)據(jù)段的校驗(yàn)，完整性和進(jìn)一步鑒定確認(rèn)。 發(fā)射器和接收器，可以使用固定的信道序列表，因

16、此，一旦同步，他們可以按照表保持溝通。每個(gè)通道上段，變送器可以在表發(fā)送其當(dāng)前位置。 </p><p>　　在美國， FCC第15部分無牌制度，在900兆赫和2.4千兆赫頻段，允許更多比非擴(kuò)頻系統(tǒng)的電源。跳頻和直接序列系統(tǒng)可以在1瓦傳輸。限額增加1毫瓦至1瓦或增加千倍。 美國聯(lián)邦通信委員會(huì) （FCC）的規(guī)定的渠道和最大的停留時(shí)間，為每個(gè)通道的最低數(shù)量。 </p><p>　　在一個(gè)真正的多點(diǎn)

17、廣播系統(tǒng)，空間允許多個(gè)相同的頻率上傳輸可能使用多個(gè)無線電在一個(gè)地理區(qū)域。 創(chuàng)建系統(tǒng)的數(shù)據(jù)傳輸速率高于渠道單一的香農(nóng)極限的可能性。擴(kuò)頻系統(tǒng)不違反Shannon極限。擴(kuò)頻系統(tǒng)依靠過剩的信號(hào)信噪比的頻譜共享。這個(gè)屬性也可以看到MIMO和DSSS系統(tǒng)中波束定向天線，也有利于提高系統(tǒng)性能，提供遠(yuǎn)程無線電通訊設(shè)備之間的隔離。</p><p>　　技術(shù)稱為20世紀(jì)40年代以來，自20世紀(jì)50年代，在軍事通信系統(tǒng)中使用的“擴(kuò)散”

18、，在較寬的頻率范圍比最低要求高出幾個(gè)數(shù)量級(jí)的無線電信號(hào)。使用類似于噪聲的載波擴(kuò)頻的核心原則是，顧名思義，帶寬遠(yuǎn)遠(yuǎn)超過簡單的點(diǎn)至點(diǎn)的通訊需要，在相同的數(shù)據(jù)速率廣泛。 </p><p>　　DS（直接序列）是更好的抗窄帶干擾的連續(xù)時(shí)間，而跳頻（跳頻）是在抗脈沖干擾。 在DS系統(tǒng)，窄帶干擾影響檢測性能一??樣多，如果干擾功率量在整個(gè)信號(hào)帶寬，傳播時(shí)，往往會(huì)不會(huì)有太大的比背景噪聲強(qiáng)。 相比之下，在信號(hào)帶寬是低的窄帶系統(tǒng)，

19、接收信號(hào)的質(zhì)量將受到嚴(yán)重的干擾功率降低，如果發(fā)生信號(hào)帶寬集中。 </p><p>　　抗竊聽的傳播代碼（DS系統(tǒng)）或的頻率，跳頻模式（FH系統(tǒng)）是經(jīng)常未知任何對(duì)他們的信號(hào)是意想不到的，在這種情況下，“加密”信號(hào)，并降低對(duì)手的決策意識(shí)的機(jī)會(huì)它。此外，對(duì)于一個(gè)給定的噪聲功率譜密度 （PSD）擴(kuò)頻系統(tǒng)需要窄帶系統(tǒng)，因此如果之前傳播的比特率是相同的功率相同金額的蔓延之前，每比特的相同數(shù)量的能量，但由于信號(hào)傳播的力量是在一

20、個(gè)大的帶寬，信號(hào)的PSD要低得多 - 往往比噪聲PSD明顯降低 - 可能使對(duì)手無法確定是否存在于所有的信號(hào)。然而，對(duì)于關(guān)鍵任務(wù)應(yīng)用，特別是那些采用市售無線電，擴(kuò)頻無線電沒有本質(zhì)提供足夠的安全性?！?.....只用擴(kuò)頻無線電本身是沒有足夠的通信安全” </p><p>　　耐褪色 。擴(kuò)頻信號(hào)占用高帶寬提供某些頻率的多樣性，即它是不太可能的信號(hào)會(huì)遇到嚴(yán)重的多徑超過其整個(gè)帶寬的衰落，而在其他情況下，可以檢測到的信號(hào)，如

21、使用Rake接收機(jī) 。 </p><p>　　多址接入能力。多個(gè)用戶可以同時(shí)傳送相同的頻率（范圍），只要他們使用不同的擴(kuò)頻碼?？吹紺DMA的 。 </p><p><b>　　跳頻變化 </b></p><p>　　自適應(yīng)頻率跳頻擴(kuò)頻（AHF）（使用藍(lán)牙 ），提高抗射頻干擾，避免擁擠的頻率使用的跳頻序列。這種自適應(yīng)傳輸比實(shí)施更容易與跳頻

22、擴(kuò)頻 。 </p><p>　　AFH的背后的關(guān)鍵思想是使用只有“好”的頻率，以避免“壞”的頻率，頻道，也許那些“壞”頻道正在經(jīng)歷頻率選擇性衰落 ，或者一些第三方試圖對(duì)這些頻段通信，或者那些樂隊(duì)正在積極卡住。因此，人力資源管理司應(yīng)輔之以一個(gè)好/壞的渠道檢測機(jī)制。 </p><p>　　然而，如果無線電頻率的干擾本身是動(dòng)態(tài)的，那么“壞通道清除”應(yīng)用在人力資源管理司，戰(zhàn)略可能無法正常工作。例如

23、，如果有幾個(gè)同位網(wǎng)絡(luò)跳頻（藍(lán)牙微微網(wǎng) ），然后他們相互干擾和AFH的戰(zhàn)略失敗，以避免這種干擾。 </p><p>　　在這種情況下，有必要使用跳頻模式的動(dòng)態(tài)適應(yīng)的戰(zhàn)略。這種情況可以使用授權(quán)頻譜中的場景，經(jīng)常發(fā)生。 </p><p>　　此外，動(dòng)態(tài)無線電頻率干擾，預(yù)計(jì)將出現(xiàn)在有關(guān)情況下認(rèn)知無線電網(wǎng)絡(luò)和設(shè)備應(yīng)表現(xiàn)出敏捷的頻率操作。 </p><p>　　調(diào)頻調(diào)制可以被看

24、作是一種跳頻，僅僅通過連續(xù)順序的可用頻率掃描。 </p><p>　　GSM蜂窩系統(tǒng)采用跳頻，頻率交織的形式，在視圖，以避免這兩個(gè)相鄰的單元格的電話不斷互相干擾。然而，這是不考慮擴(kuò)頻，因?yàn)樵谕粏卧竦膬蓚€(gè)電話中從來不使用相同的頻率。</p><p><b>　　擴(kuò)頻時(shí)鐘信號(hào)的產(chǎn)生</b></p><p>　　擴(kuò)頻時(shí)鐘發(fā)生器（SSCG）是用來在

25、一些同步數(shù)字系統(tǒng) ，尤其是那些含有微處理器，以減少這些系統(tǒng)產(chǎn)生的電磁干擾 （EMI）的譜密度。同步數(shù)字系統(tǒng)是由一個(gè)時(shí)鐘信號(hào)，因?yàn)槠渲芷谛再|(zhì)，有一個(gè)不可避免的窄頻譜。事實(shí)上，一個(gè)完美的時(shí)鐘信號(hào)，將其所有的能量集中在一個(gè)單一的頻率及其諧波。實(shí)際同步數(shù)字系統(tǒng)窄波段上的時(shí)鐘頻率及其諧波傳播，導(dǎo)致在頻譜，在某些頻率可以超過法規(guī)限制電磁干擾（例如那些輻射的電磁能量在美國FCC美國在日本和IEC ， JEITA ）在歐洲。 </p>&

26、lt;p>　　擴(kuò)頻時(shí)鐘避免這個(gè)問題，通過使用前面描述的方法之一，以降低峰值輻射能量，因此，它的電磁輻射等符合電磁兼容性 （EMC）規(guī)定。 </p><p>　　它已經(jīng)成為一種流行的技術(shù)，因?yàn)樗鼉H需要簡單的設(shè)備改造，以獲得監(jiān)管部門批準(zhǔn)。這是在便攜式電子設(shè)備的更受歡迎，因?yàn)楦斓臅r(shí)鐘速度和更小的設(shè)備越來越多的高清晰度液晶顯示器集成。由于這些設(shè)備的設(shè)計(jì)是重量輕，價(jià)格低廉，傳統(tǒng)的被動(dòng)式，電子措施，以降低EMI，如電

27、容器或金屬屏蔽，是不是可行。 有源EMI降低技術(shù)，如擴(kuò)頻時(shí)鐘在這些情況下需要。 </p><p>　　然而，擴(kuò)頻時(shí)鐘，也可以為設(shè)計(jì)師創(chuàng)造的挑戰(zhàn)。其中主要是時(shí)鐘/數(shù)據(jù)不對(duì)，或時(shí)鐘歪斜 【擴(kuò)大如何發(fā)生這種情況，或參考另一篇文章。】 </p><p>　　請(qǐng)注意，這種方法不會(huì)減少總輻射能量，因此，系統(tǒng)不一定是不太可能會(huì)造成干擾。遍布更大的帶寬能有效地降低窄帶內(nèi)的電和磁的讀數(shù)。典型的測量接收機(jī) E

28、MC測試實(shí)驗(yàn)室所使用的電磁頻譜分成約120千赫寬的頻帶。如果被測系統(tǒng)的輻射都在窄帶寬的能源，它會(huì)注冊一個(gè)大的高峰。分發(fā)到一個(gè)更大的帶寬相同的能量，防止把任何一個(gè)窄帶超過法定限度的足夠的能量系統(tǒng)。這種方法作為一種手段，以減少干擾的有效性經(jīng)常辯論，因?yàn)樗徽J(rèn)為擴(kuò)頻時(shí)鐘隱藏，而不是解決高輻射的能源問題，在EMC立法或認(rèn)證程序簡單的漏洞剝削。這種情況的結(jié)果，在窄帶寬（S），經(jīng)歷了干擾少，敏感的電子設(shè)備，而寬帶的敏感性，甚至在其他頻率（如無線電接

29、收機(jī)調(diào)諧到一個(gè)不同的站）經(jīng)營者，會(huì)遇到更多的干擾。 </p><p>　　FCC認(rèn)證測試往往是為了啟用，以減少測量排放量在可接受的法律限制的擴(kuò)頻功能完成。然而，擴(kuò)頻功能可以由用戶在某些情況下被禁用。作為一個(gè)例子，在個(gè)人電腦領(lǐng)域，某些BIOS作家包括能夠?yàn)橛脩粼O(shè)置禁用擴(kuò)頻時(shí)鐘發(fā)生器，從而擊敗了EMI規(guī)范的對(duì)象。這可能被視為一個(gè)漏洞 ，但一般只要忽視擴(kuò)頻是默認(rèn)啟用的。 </p><p>　　禁

30、用計(jì)算機(jī)系統(tǒng)中的擴(kuò)頻時(shí)鐘的能力被認(rèn)為是有用的， 超頻 ，可以降低擴(kuò)頻最大時(shí)鐘速度實(shí)現(xiàn)時(shí)鐘歪斜 。</p><p>　　The technology of frequency-hopping spread spectrum</p><p>　　Frequency-hopping spread spectrum ( FHSS ) is a method of transmitting radi

31、o 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 code d

32、ivision multiple access (FH-CDMA) scheme. </p><p>　　A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission: </p><p>　　1.Spread-spectrum signals are highl

33、y 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>　　2. Spread-spectrum signals are d

34、ifficult 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 the pseudorandom sequence was kn

35、own. </p><p>　　3. 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 narro

36、w-frequency communications, and vice versa. As a result, bandwidth can be utilized more efficiently. </p><p><b>　　History </b></p><p>　　Frequency hopping The concept of frequency ho

37、pping was first alluded to in the 1903 US Patent 723,188 and US Patent 725,605 filed by Nikola Tesla in July 1900. Tesla came up with the idea after demonstrating the world's first radio-controlled submersible boat i

38、n 1898, when it became apparent the wireless signals controlling the boat needed to be secure from "being disturbed, intercepted, or interfered with in any way." His patents covered two fundamentally different

39、techniques for achieving immu</p><p>　　Frequency hopping is also mentioned in radio pioneer Jonathan Zenneck 's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Ze

40、nneck himself states that Telefunken had already tried it several years earlier. Zenneck's book was a leading text of the time, and it is likely that many later engineers were aware of it.The German military made lim

41、ited use of frequency hopping for communication between fixed command points in World War I to prevent eavesdropping by </p><p>　　The most celebrated invention of frequency hopping was that of actress Hedy L

42、amarr and composer George Antheil , who in 1942 received US Patent 2,292,387 for their "Secret Communications System". Lamarr had learned at defense meetings she had attended with her former husband Friedrich M

43、andl that radio-guided missiles' signals could easily be jammed. The Antheil–Lamarr version of frequency hopping used a piano-roll to change among 88 frequencies, and was intended to make radio-guided torpedoes h<

44、;/p><p>　　Military use</p><p>　　Spread-spectrum signals are highly resistant to deliberate jamming , unless the adversary has knowledge of the spreading characteristics. Military radios use cryptog

45、raphic techniques to generate the channel sequence under the control of a secret Transmission Security Key ( TRANSEC ) that the sender and receiver share in advance.</p><p>　　By itself, frequency hopping pro

46、vides only limited protection against eavesdropping and jamming. There is a simple algorithm that effectively discovers the sequence of frequencies. To get around this weakness most modern military frequency hopping radi

47、os employ separate encryption devices such as the KY-57 . US military radios that use frequency hopping include the JTIDS/MIDS family, HAVE QUICK and SINCGARS .</p><p>　　Civilian use </p><p>　　S

48、ince the Federal Communications Commission (FCC) amended rules to allow frequency hopping spread spectrum systems in the unregulated 2.4 GHz band, many consumer devices in that band have employed various spread-spectrum

49、modes. </p><p>　　Some walkie-talkies that employ frequency-hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Several such radios are marketed under the name eXtre

50、me Radio Service (eXRS). Despite the name's similarity to the FRS allocation, the system is a proprietary design, rather than an official US Federal Communications Commission (FCC) allocated service.</p><p

51、>　　Commercial use </p><p>　　The 1976 publication of Spread Spectrum Systems by Robert Dixon, ISBN 0-471-21629-1 , was a significant milestone in the commercialization of this technology.Previous publicati

52、ons were either classified military reports or academic papers on narrow subtopics. Dixon's book was the first comprehensive unclassified review of the technology and set the stage for increasing research into commer

53、cial applications. </p><p>　　Initial commercial use of spread spectrum began in the 1980s in the US with three systems: Equatorial Communications System's very small aperture ( VSAT ) satellite terminal

54、system for newspaper newswire services, Del Norte Technology's radio navigation system for navigation of aircraft for crop dusting and similar applications, and Qualcomm 's OmniTRACS system for communications to

55、trucks. In the Qualcomm and Equatorial systems, spread spectrum enabled small antennas that viewed more than one s</p><p>　　In 1981, the Federal Communications Commission started exploring ways to permit mor

56、e general civil uses of spread spectrum in a Notice of Inquiry docket. This docket was proposed to FCC and then directed by Michael Marcus of the FCC staff. The proposals in the docket were generally opposed by spectrum

57、users and radio equipment manufacturers, although they were supported by the then Hewlett-Packard Corp. The laboratory group supporting the proposal would later become part of Agilent . </p><p>　　The May 198

58、5 decision in this docket permitted unlicensed use of spread spectrum in 3 bands at powers up to 1 Watt. FCC said at the time that it would welcome additional requests for spread spectrum in other bands.The resulting ru

59、les, now codified as 47 CFR 15.247 permitted Wi-Fi , Bluetooth , and many other products including cordless telephones. These rules were then copied in many other countries.Qualcomm was incorporated within 2 months after

60、 the decision to commercialize CDMA.</p><p>　　Technical considerations</p><p>　　The overall bandwidth required for frequency hopping is much wider than that required to transmit the same informa

61、tion using only one carrier frequency . However, because transmission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. Whilst providing no extr

62、a protection against wideband thermal noise , the frequency-hopping approach does reduce the degradation caused by narrowband interferers. </p><p>　　One of the challenges of frequency-hopping systems is to s

63、ynchronize the transmitter and receiver. One approach is to have 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 chan

64、nel and listening for valid data on that channel. 。 The transmitter'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 ch

65、ecksum</p><p>　　In the US, FCC part 15 on unlicensed system in the 900 MHz and 2.4 GHz bands permits more power than non-spread spectrum systems. Both frequency hopping and direct sequence systems can trans

66、mit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. The Federal Communications Commission (FCC) prescribes a minimum number of channels and a maximum dwell time for each channe

67、l. </p><p>　　In a real multipoint radio system, space allows multiple transmissions on the same frequency to be possible using multiple radios in a geographic area. This creates the possibility of system dat

68、a rates that are higher than the Shannon limit for a single channel.Spread spectrum systems do not violate the Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing of spectrum. This

69、property is also seen in MIMO and DSSS systems. Beam steering and directional antennas</p><p>　　Techniques Techniques known since the 1940s and used in military communication systems since the 1950s "sp

70、read" a 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, bandwidths

71、much wider than that required for simple point-to-point communication at the same data rate. </p><p>　　Resistance to jamming (interference). DS (direct sequence) is better at resisting continuous-time narrow

72、band jamming, while FH (frequency hopping) is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of jamming power is spread over the whole si

73、gnal bandwidth, when it will often not be much stronger than background noise. By contrast, in narrowband systems where the signal bandwidth is low, the received signal qualit</p><p>　　Resistance to eavesdr

74、opping . The spreading code (in DS systems) or the frequency-hopping pattern (in FH systems) is often unknown by anyone for whom the signal is unintended, in which case it "encrypts" the signal and reduces the

75、 chance of an adversary's making sense of it. Moreover, for a given noise power spectral density (PSD), spread-spectrum systems require the same amount of energy per bit before spreading as narrowband systems and the

76、refore the same amount of power if the bitrate before</p><p>　　Resistance to fading . The high bandwidth occupied by spread-spectrum signals offer some frequency diversity, ie it is unlikely that the signal

77、will encounter severe multipath fading over its whole bandwidth, and in other cases the signal can be detected using eg a Rake receiver . </p><p>　　Multiple access capability. Multiple users can transmit sim

78、ultaneously on the same frequency (range) as long as they use different spreading codes. See CDMA .</p><p>　　Variations of FHSS </p><p>　　Adaptive Frequency-hopping spread spectrum (AFH) (as us

79、ed in Bluetooth ) improves resistance to radio frequency interference by avoiding using crowded frequencies in the hopping sequence.This sort of adaptive transmission is easier to implement with FHSS than with DSSS . <

80、;/p><p>　　AThe key idea behind AFH is to use only the “good” frequencies, by avoiding the "bad" frequency channels—perhaps those "bad" frequency channels are experiencing frequency selective

81、 fading , or perhaps some third party is trying to communicate on those bands, or perhaps those bands are being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good/bad channels. </

82、p><p>　　However, if the radio frequency interference is itself dynamic, then the strategy of “bad channel removal”, applied in AFH might not work well. For example, if there are several colocated frequency-hopp

83、ing networks (as Bluetooth Piconet ), then they are mutually interfering and the strategy of AFH fails to avoid this interference. </p><p>　　In this case, there is a need to use strategies for dynamic adapta

84、tion of the frequency hopping pattern. Such a situation can often happen in the scenarios that use unlicensed spectrum . </p><p>　　In addition, dynamic radio frequency interference is expected to occur in th

85、e scenarios related to cognitive radio , where the networks and the devices should exhibit frequency-agile operation. </p><p>　　Chirp modulation can be seen as a form of frequency-hopping that simply scans t

86、hrough the available frequencies in consecutive order. </p><p>　　Spread-spectrum clock signal generation</p><p>　　Spread-spectrum clock generation (SSCG) is used in some synchronous digital syst

87、ems , especially those containing microprocessors, to reduce the spectral density of the electromagnetic interference (EMI) that these systems generate. A synchronous digital system is one that is driven by a clock signa

88、l and, because of its periodic nature, has an unavoidably narrow frequency spectrum. In fact, a perfect clock signal would have all its energy concentrated at a single frequency and its harmonics. Pra</p><p>

89、;　　Spread-spectrum clocking avoids this problem by using one of the methods previously described to reduce the peak radiated energy and, therefore, its electromagnetic emissions and so comply with electromagnetic compati

90、bility (EMC) regulations. </p><p>　　It has become a popular technique to gain regulatory approval because it requires only simple equipment modification. It is even more popular in portable electronics devic

91、es because of faster clock speeds and increasing integration of high-resolution LCD displays into ever smaller devices. Since these devices are designed to be lightweight and inexpensive, traditional passive, electronic

92、measures to reduce EMI, such as capacitors or metal shielding, are not viable. Active EMI reduction technique</p><p>　　However, spread-spectrum clocking can also create challenges for designers.Principal amo

93、ng these is clock/data misalignment, or clock skew [expand how that happens, or refer to another article]. </p><p>　　Note that this method does not reduce total radiated energy, and therefore systems are not

94、 necessarily less likely to cause interference. Spreading energy over a larger bandwidth effectively reduces electrical and magnetic readings within narrow bandwidths.Typical measuring receivers used by EMC testing labor

95、atories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to radiate all its energy in a narrow bandwidth, it would register </p><p>　　FCC cer

96、tification testing is often completed with the spread-spectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, the spread-spectrum functionality may be disabled by t

97、he user in some cases.As an example, in the area of personal computers, some BIOS writers include the ability to disable spread-spectrum clock generation as a user setting, thereby defeating the object of the EMI regulat

98、ions.This might be considered a loophole , but is gen</p><p>　　An ability to disable spread-spectrum clocking in computer systems is considered useful for overclocking , as spread spectrum can lower maximum