Direct-sequence spread spectrum (DSSS)

In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation 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 that the carrier signals occur over the full bandwidth (spectrum) of a device's transmitting frequency.
Contents[hide]
1 Features
2 Transmission method
3 Benefits
4 Uses
5 References
6 See also
7 External links
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[edit] Features
It phase-modulates a sine wave pseudorandomly 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 faster chips. Therefore, the chip rate is much higher than the information signal bit rate.
It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori 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.

[edit] Transmission method
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 much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band.
The resulting signal resembles white noise, like an audio recording of "static". 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). 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.
For 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. 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 determine relative timing, which, in turn, can be used to calculate the receiver's position if the transmitters' positions are known. This is the basis for many satellite navigation systems.
The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing 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.
If an undesired transmitter transmits on 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) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the cross-correlation properties of their PN sequences.
As this description suggests, a plot of the transmitted 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 transmission.
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 whose width is determined by the output range of the pseudo-random number generator.

[edit] Benefits
Resistance to intended or unintended jamming
Sharing of a single channel among multiple users
Reduced signal/background-noise level hampers interception (stealth)
Determination of relative timing between transmitter and receiver

[edit] Uses
The United States GPS and European Galileo satellite navigation systems
DS-CDMA (Direct-Sequence Code Division Multiple 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.
Cordless phones operating in the 900 MHz, 2.4 GHZ and 5.8 GHz bands
IEEE 802.11b 2.4 GHz Wi-Fi, and its predecessor 802.11-1999. (Their successor 802.11g uses OFDM instead)
ZigBee / 802.15.4
Automatic meter reading
JR Spektrum RC DSM and DSM2 Radio Control Transmitters
IEEE 802.15.4

[edit] References
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management

[edit] See also
Frequency-hopping spread spectrum
Linear feedback shift register

[edit] External links
Civil Spread Spectrum History
Retrieved from "http://en.wikipedia.org/wiki/Direct-sequence_spread_spectrum"

頻技術(FHSS)及直接序列(DSSS)展頻技術
展頻技術主要分為「跳頻技術」及「直接序列」兩種方式。此兩種技術是在第二次世界大戰中軍隊所使用的技術，其目的是希望在惡劣的戰爭環境中，依然能保持通信信號的穩定性及保密性。展頻技術在無線區域網路的應用是依據FCC(Federal Communications Committee； 美國聯邦通訊委員會 )規定的ISM(Industrial Scientific, and Medical)，頻率範圍開放在 902M～ 928MHz及 2.4G～ 2.484GHz兩個頻段，所以並沒有所謂使用授權的限制。
一、跳頻技術 (FHSS)
跳頻技術 (Frequency-Hopping Spread Spectrum； FHSS)是在2.4GHz頻帶以1MHz的頻寬將其劃分為75-81個無線電頻率通道（Radio Frequency Channel；RFC），並且以使用接收和發送兩端一樣的頻率跳躍模式（Frequency Hopping）來接發訊號及防止資料擷取（跳躍頻率的最大時間間隔為250ms，也就是每秒跳頻至少4次）來。
二、直接序列展頻技術 (DSSS)
直接序列展頻技術 (Direct Sequence Spread Spectrum； DSSS)是將原本”0”與”1”的高功率、窄頻寬的位元訊號，透過虛擬隨機序列(Pseudo Random Sequence)和相位移轉技術（PSK；Phase Shift Keying），轉變成低功率、寬頻帶（在2.4Ghz頻帶，分以13個頻道，每個頻道頻寬為5MHz）的載波訊號，這些轉變後的載波訊號被稱為Spreading Chips，Chips數愈多可以增加資料安全性，愈低則增加使用者數目。一般普遍使用10-20 Chips。
三、FHSS vs DSSS調變差異
無線區域網路在性能和能力上的差異，主要是取決於所採用的是FHSS還是DSSS來實現、以及所採用的調變方式。然而，調變方式的選擇並不完全是隨意的，像FHSS並不強求某種特定的調變方式，而且，大部分既有的FHSS都是使用某些不同形式的高斯頻移鍵控(Gaussian Frequency Shift Keying, GFSK)的調變方式。至於DSSS則通過使用可變相位調變如PSK、QPSK(Quadrature Phase Shift Keying)、DQPSK(Differential Quadrature Phase Shift Keying)等， 可以得到最高的可靠性以及表現高數據速率性能 。
在抗噪聲能力方面，採用QPSK調變方式的DSSS與採用FSK調變方式的FHSS相比，可以發現這兩種不同技術的無線區域網路各自擁有的優勢。FHSS系統之所以選用FSK調變方式的原因是因為FHSS和FSK內在架構的簡單性，FSK無線訊號可使用非線性功率放大器，但這卻犧牲了傳輸距離和抗噪聲能力。而DSSS系統需要稍為貴一些的線性放大器，但卻可以獲得更多的回饋。
四、DSSS vs FHSS 孰優孰勝
　由於FHSS在抗干擾性上（FHSS需不斷切換頻道，而DSSS則需等待所有Chips都接收完成後才轉換訊號），和使用材料彈性度上較為優良（FHSS可以使用線性或非線性材料當功率放大器、DSSS則只能使用線性材料），所以大多數廠商選擇以FHSS為出發。但由於FHSS在1Mhz的窄頻寬上無法傳遞大量的資料（傳輸率約為1-2Mbps）和必須提供高功率輸出才能維持較長傳輸距離，並且WLAN早期在市場規劃上，趨向應用於企業用戶而可以忽略因材料限制所可能引發的價格過高、和避開在家庭市場中易與其他無線傳輸間（Bluetooth、HomeRF）發生干擾等諸多問題，所以在接下的IEEE 802.11b中，廠商都以DSSS為設計範疇（傳輸率為11Mbps）。但近來WLAN因技術規格成熟而吸引大量廠商進入競爭所導致的價格下降，卻無意間促成WLAN重新走回家庭市場。這樣的結果，對於DSSS所容易造成的干擾問題，又成為廠商們最傷腦筋的事。
　
表1

表2 各主要無線網路廠商產品比較

目前全球無線區域網路的前4大公司為Lucent、Aironet(已被Cisco購併)、Symbol及Proxim。Symbol及Proxim的晶片組支援FHSS，而Lucent及Intersil則採用DSSS，其中使用Intersil晶片的公司除了友訊外，更包括了Aironet、Compaq、Nortel及Dell等大廠。而未來幾年無線區域網路的成長便是來自使用DSSS調變的802.11b產品，因此2001年4月初Proxim主要對無線區域網路晶片組供應商Intersil提出「Direct Sequence」專利的侵權告訴，但是目前Intersil的無線區域網路 晶片組在市場佔有率最高，影響所及，使用Intersil晶片組的廠商友訊及其他Intersil的客戶如3Com、思科等都被告，就個別產品來看，無線網路卡2000年銷售量為344.5萬片，較前一年成長了186%，主要的成長仍來自DSSS的產品，DSSS產品在2000年的銷售量達289.4萬片，較前一年大幅成長了368%，而在FHSS方面，則為551萬片，較前一年衰退6%。在Access Point(接入點，係指於媒體存取控制層中扮演無線工作站及有線區域網路的橋樑)方面，去年整體銷售量為516萬台，年成長率為189%，主要成長亦為DSSS，成長率為448%，而FHSS則持平。因此可看出FHSS的日漸式微，也可理解Proxim急欲打壓DSSS心態。
單就台灣廠商來看，根據工研院的資料顯示，2000年我國無線區域網路(包括無線網卡、Access Point 及橋接器)的產量為124萬台，較前一年成長了101.6%，產值則為39.78億台幣，較前一年成長了122%，呈現高度的成長。預估2001年我國無線區域網路的產量可達209萬台，成長68%，產值可達63.57億台幣，成長60%，2000年無線區域網路出貨量主要集中在中華電訊、正文、禾翔、智捷及環隆電氣，其中以FHSS為主的只有中華電訊，而環隆電氣替Lucent代工，因此使用Lucent的晶片組，其餘國內公司大都使用Intersil的晶片組。這次台灣廠商遭Proxim控告的公司除了友訊外尚包括了立碁公司，其他使用Intersil的公司雖未被控告但仍需小心防範Proxim的後續動作。