【正文】 ment. The core principle of spread spectrum is the use of noiselike carrier waves, and, as the name implies, bandwidths much wider than that required for simple pointtopoint munication at the same data rate.s OmniTRACS system for munications to trucks. In the Qualm and Equatorial systems, spread spectrum enabled small antennas that viewed more than one satellite to be used since the processing gain of spread spectrum eliminated interference. The Del Norte system used the high bandwidth of spread spectrum to improve location accuracy.In 1981, the Federal Communications Commission started exploring ways to permit more 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 users and radio equipment manufacturers, although they were supported by the then HewlettPackard Corp. The laboratory group supporting the proposal would later bee part of Agilent.The May 1985 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 wele additional requests for spread spectrum in other resulting rules, now codified as 47 CFR permitted WiFi, Bluetooth, and many other products including cordless telephones. These rules were then copied in many other countries. Qualm was incorporated within 2 months after the decision to mercialize CDMA.Spreadspectrum telemunicationsThis is a technique in which a (telemunication) signal is transmitted on a bandwidth considerably larger than the frequency content of the original information.Spreadspectrum telemunications is a signal structuring technique that employs direct sequence, 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 spectrum generally makes use of a sequential noiselike signal structure to spread the normally narrowband information signal over a relatively wideband (radio) band of frequencies. The receiver correlates the received signals to retrieve the original information signal. Originally there were two motivations: either to resist enemy efforts to jam the munications (antijam, or AJ), or to hide the fact that munication was even taking place, sometimes called low probability of intercept (LPI).Frequencyhopping spread spectrum (FHSS), directsequence spread spectrum (DSSS), timehopping spread spectrum (THSS), chirp spread spectrum (CSS), and binations of these techniques are forms of spread spectrum. Each of these techniques employs pseudorandom number sequences — created using pseudorandom number generators — to determine and control the spreading pattern of the signal across the alloted bandwidth. Ultrawideband (UWB) is another modulation technique that acplishes the same purpose, based on transmitting short duration pulses. Wireless Ethernet standard IEEE uses either FHSS or DSSS in its radio interface.Notess very small aperture (VSAT) satellite terminal system for newspaper newswire services, Del Norte Technology39。 Industrial Electronics Corporation, ITT and Sylvania Electronic Systems that led to early spreadspectrum technology in the 1950s. Parallel research on radar systems and a technologically similar concept called phase coding also had an impact on spreadspectrum development.Commercial useThe 1976 publication of Spread Spectrum Systems by Robert Dixon, ISBN 0471216291, was a significant milestone in the mercialization of this technology. Previous publications were either classified military reports or academic papers on narrow subtopics. Dixon39。s book was a leading text of the time, and it is likely that many later engineers were aware of it. A Polish engineer, Leonard Danilewicz, came up with the idea in other patents were taken out in the 1930s, including one by Willem Broertjes (Germany 1929, . Patent 1,869,695, 1932). During World War II, the US Army Signal Corps was inventing a munication system called SIGSALY for munication between Roosevelt and Churchill, which incorporated spread spectrum, but due to its top secret nature, SIGSALY39。s first radiocontrolled submersible boat in 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 techniques for achieving immunity to interference, both of which functioned by altering the carrier frequency or other exclusive characteristic. The first had a transmitter that worked simultaneously at two or more separate frequencies and a receiver in which each of the individual transmitted frequencies had to be tuned in, in order for the control circuitry to respond. The second technique used a variablefrequency transmitter controlled by an encoding wheel that altered the transmitted frequency in a predetermined manner. These patents describe the basic principles of frequency hopping and frequencydivision multiplexing, and also the electronic ANDgate logic circuit.Frequency hopping is also mentioned in radio pioneer Johannes Zenneck39。這里就不再贅述。線性調(diào)頻擴(kuò)頻也可用于軍事應(yīng)用前景，因?yàn)樗欠浅＠щy的探測和攔截時(shí)，在低功耗工作。此外，Nanotron為該項(xiàng)目的執(zhí)行能夠工作在數(shù)據(jù)傳輸速率可高達(dá)2 Mbit/。但是，IEEE 。調(diào)頻擴(kuò)頻非常適合需要低功耗的應(yīng)用程序和需要的帶寬數(shù)量相對(duì)較少（1兆比特/秒或更少）的情況下。有些地方這種技術(shù)可能是有作用的是醫(yī)療應(yīng)用，物流（即容器需要被跟蹤），以及政府/安全應(yīng)用。目前，Nanotron科技，生產(chǎn)實(shí)時(shí)定位裝置，是唯一使用線性調(diào)頻擴(kuò)頻的無線設(shè)備賣方。使 用 GHz頻帶中的超寬帶競爭。然而，與直接序列擴(kuò)頻（DSSS）或頻率跳頻（FHSS），由于它不添加任何偽隨機(jī)的信號(hào)分量，以幫助區(qū)分的信道噪聲它擴(kuò)頻，而不是依靠線性調(diào)頻脈沖的線性性質(zhì)。概 述如同其他擴(kuò)頻方法，線性調(diào)頻頻譜使用其全部分配到的帶寬廣播信號(hào)，使信道噪聲強(qiáng)勁。一個(gè)線性調(diào)是在一定時(shí)間正弦信號(hào)頻率的增加或減少。線性調(diào)頻擴(kuò)頻可以被視為一種跳頻，只需通過可用頻率以連續(xù)順序掃描。這種情況往往發(fā)生在無節(jié)制的情況下使用頻譜。例如，如果有幾個(gè)同位跳頻網(wǎng)絡(luò)（如藍(lán)牙技術(shù)的微微網(wǎng)），那么他們是相互干擾且自適應(yīng)跳頻的策略未能避免這種干擾。因此，自適應(yīng)跳頻擴(kuò)頻應(yīng)從檢測好/壞信道的機(jī)制中得到補(bǔ)充。這種自適應(yīng)傳輸是調(diào)頻擴(kuò)頻比直接序列擴(kuò)頻更容易實(shí)現(xiàn)。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)備之間的隔離提高系統(tǒng)的性能。擴(kuò)頻系統(tǒng)過多的依賴信號(hào)信噪比的頻譜共享。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。美國聯(lián)邦通訊委員會(huì)（FCC）規(guī)定了渠道的最低數(shù)目和每個(gè)通