【正文】 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 Ether sta ndard IEEE uses either FHSS or DSSS in its radio interface. Notes ? Techniques known since 1940s and used in military munication system since 1950s ? Spread radio signal over a wide frequency range several magnitudes higher than minimum requirement. 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. ? Two main techniques: sequence (DS) hopping (FH) ? Resistance to jamming (interference). DS is better at resisting continuoustime narrowband jamming, while FH is better at resisting pulse jamming. In DS systems, narrowband jamming affects detection performance about as much as if the amount of 13 jamming power is spread over the whole signal 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 quality will be severely lowered if the jamming power happens to be concentrated on the signal bandwidth. ? Resistance to eavesdropping. The spreading code (in DS systems) or the frequencyhopping 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 chance of an adversary39。s 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 use The 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(cè)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/s 比 任務(wù)指定的速率要高。但是， 盡管 標(biāo)準(zhǔn)指定 個(gè)人區(qū)域網(wǎng)絡(luò) 包含 10米的或較少的區(qū)域 ， IEEE 指定線性調(diào)頻擴(kuò)頻在物理層使用時(shí)延長(zhǎng)范圍和設(shè)備在高速移動(dòng)作為是您的網(wǎng)絡(luò)運(yùn)行的一部分。 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，并獲得線性調(diào)頻擴(kuò)頻主要力量后加入到電機(jī)及電子學(xué)工程師聯(lián)合會(huì) 標(biāo)準(zhǔn)，是唯一使用線性調(diào)頻擴(kuò)頻的無(wú)線設(shè)備賣(mài)方。 使 用 線性調(diào)頻 擴(kuò)頻最初設(shè)計(jì) 是 為與測(cè)距精度及低速率無(wú)線網(wǎng)絡(luò)在 GHz 頻帶中的 超寬帶 競(jìng)爭(zhēng)。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(guò)可用頻率以連續(xù)順序掃描 。 這種情況往往發(fā)生在無(wú)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) ）， 8 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(cè)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(guò)提供遠(yuǎn)程無(wú)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(guò)多的依賴(lài)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(guó)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)通道的最大駐留時(shí)間。 調(diào)頻 和直接序列系統(tǒng)可以在 1 瓦傳輸。 每個(gè)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。發(fā)送器的數(shù)據(jù)都是通過(guò)一個(gè)特殊的數(shù)據(jù)序列不像發(fā)生在這個(gè)渠道為數(shù)據(jù)段和段可以有一個(gè)完整的校驗(yàn)和進(jìn)一步鑒定。 一種方法是有將保證的發(fā)射機(jī)使用在固定時(shí)間內(nèi)的所有渠道。 雖然沒(méi)有提供額外的熱噪聲對(duì)寬帶的保護(hù) ， 跳頻方法確實(shí)降低窄帶干擾造成的退化。 技術(shù)的幾點(diǎn)思考 所需頻率跳變的整體帶寬是比需要來(lái)傳輸僅一個(gè)相同 信息 使用 載波頻率 更 大 。 若要繞過(guò)此弱點(diǎn)最現(xiàn)代軍事頻率跳躍收音機(jī)經(jīng)常采用單獨(dú)的加密設(shè)備如 KY57。 軍用 無(wú)線電通訊設(shè)備使用 加密 技術(shù)來(lái)生成所控制的 傳輸安全密鑰 (TRANSEC)，發(fā)送方和接收方共享一個(gè)秘密通道序列。 5. 在通信開(kāi)始，發(fā)送方和接收沿該計(jì)算的順序在同一點(diǎn)開(kāi)始的時(shí)間更改其頻率。 最經(jīng)常的頻率變化的時(shí)期是預(yù)定義的，以允許一個(gè)基站，服務(wù)多個(gè)連接 。 2. 接收方發(fā)送一 個(gè)數(shù)字，像已知的種子 。這樣一來(lái)可以更有效地利用帶寬。 3. 擴(kuò)頻傳輸可以與許多類(lèi)型的最小干擾的常規(guī)傳輸共享一個(gè)頻帶。 一個(gè)跳頻擴(kuò)頻信號(hào)顯示為一個(gè)簡(jiǎn)單的背景噪聲增加至窄帶接收機(jī)。 再收集 傳播信號(hào)傳播出