擴(kuò)頻通信外文翻譯(中+英).doc

擴(kuò)頻通信系統(tǒng)的介紹摘要：本應(yīng)用筆記概述了擴(kuò)頻技術(shù)的原理，討論了涵蓋直接序列和快速跳頻的方法。相關(guān)理論方程的性能估算。以及討論直接序列擴(kuò)頻（DSSS）和跳頻（FHSS）這兩種擴(kuò)頻方式。 簡(jiǎn)介擴(kuò)頻技術(shù)越來越受歡迎，就連這一領(lǐng)域以外的電器工程師都渴望能夠深入理解這一技術(shù)。很多書和網(wǎng)站上都有關(guān)于這方面的書，但是，很多都很難理解或描述的不夠詳盡。（例如，直接序列擴(kuò)頻技術(shù)廣泛關(guān)注的是偽隨機(jī)碼的產(chǎn)生）。下面討論擴(kuò)頻技術(shù)（雙關(guān)語(yǔ)意）。簡(jiǎn)史一名女演員和一名音樂家首次以書面形式描述了擴(kuò)頻通信技術(shù)。1941年，好萊塢女星Hedy Lamarr和鋼琴家George Antheil描述一個(gè)安全的無線鏈路來控制魚雷。他們獲得了美國(guó)專利#2.292.387。但這一技術(shù)被遺忘了，沒有在當(dāng)時(shí)受到美軍的重視，直到20世紀(jì)80年代它才開始活躍起來。從那時(shí)起，這一技術(shù)在有關(guān)惡劣環(huán)境中的收音機(jī)鏈接方面越來越受歡迎。最典型的擴(kuò)頻技術(shù)應(yīng)用是數(shù)據(jù)收發(fā)器包括衛(wèi)星定位系統(tǒng)（GPS）、3G移動(dòng)通信、無限局域網(wǎng)(符合IEEE802.11a,IEEE 802.11b,IEEE 802.11g標(biāo)準(zhǔn))，還有藍(lán)牙技術(shù)也幫助了那些通訊落后和無線電通信條件有限的地方，因此，它是一種昂貴的資源。擴(kuò)頻通信的原理擴(kuò)頻是香農(nóng)定理的典型：C=Blog2(1+S/N) 公式（1）在公式中，C為信道容限，單位是比特/秒(bps),意指單位時(shí)間內(nèi)信道中無差錯(cuò)傳輸?shù)淖畲笮畔⒘?。B為信號(hào)頻帶寬度，單位是Hz，S/N為信噪比。也就是說，C為信道允許通過的信息量，也代表了擴(kuò)頻的性能。帶寬(B)是代價(jià)，因?yàn)轭l率是一個(gè)有限的資源。信噪比體現(xiàn)了環(huán)境條件或物理特性（如障礙、干擾器、干擾等）。上式說明，的情況下，在無差錯(cuò)傳輸?shù)男畔⑺俾蔆不變時(shí)，如果信噪比很低，則可以用足夠?qū)挼膸拋韨鬏斝盘?hào)，即使信號(hào)功率密度低于噪音水平。（公式可用！）改變公式（1）中對(duì)數(shù)的底數(shù)，2改為e，則為In=loge。因此，C/B=(1/ln2)ln(1+S/N)=1.443ln(1+S/N) 公式（2） 根據(jù)MacLaurin擴(kuò)展公式ln(1+x)=x-x2/2+x3/3-x4/4+(-1)k+1xk/k+:C/B=1.443(S/N-1/2(S/N)2+1/3(S/N)3-) 公式（3）在擴(kuò)頻應(yīng)用中，通常S/N很低。（正如剛才提到的，信號(hào)功率密度甚至低于噪音水平。）假定噪音水平即S/N1，香農(nóng)公式可簡(jiǎn)單表示為：C/B1.443S/N 公式（4）簡(jiǎn)化為：C/NS/N 公式（5）或者：N/SB/C 公式（6）向固定了信噪比的信道發(fā)送錯(cuò)誤的信息，只要執(zhí)行基本擴(kuò)頻信號(hào)的傳播操作：增加傳輸帶寬。盡管這一原則看起來很簡(jiǎn)單明確，但實(shí)現(xiàn)她卻很復(fù)雜，主要是因?yàn)檎箤捇鶐У碾娮釉O(shè)備必須同時(shí)存在展寬和解擴(kuò)的操作過程。定義不同的擴(kuò)頻技術(shù)都有一個(gè)共同之處：密鑰（也稱為代碼或序列）依附于傳輸信道。以插入代碼的形式準(zhǔn)確地定義擴(kuò)頻技術(shù)，術(shù)語(yǔ)“頻譜擴(kuò)展”是指擴(kuò)頻信號(hào)的幾個(gè)數(shù)量級(jí)的帶寬在有密鑰的傳輸信道中的擴(kuò)展。以傳統(tǒng)的方式定義擴(kuò)頻更為精確：在射頻通信系統(tǒng)中，將基帶信號(hào)擴(kuò)展為比原有信號(hào)的帶寬寬得多的高頻信號(hào)（如圖1）。在此過程中，傳輸寬帶信號(hào)產(chǎn)生的損耗，表現(xiàn)為噪聲。擴(kuò)頻信號(hào)帶寬與信息帶寬之比稱為處理增益。擴(kuò)頻過程的處理增益大都在10dB 到60dB之間。要應(yīng)用擴(kuò)頻技術(shù)，只需在天線（接收器）之前加入相應(yīng)的擴(kuò)頻碼。相反，你可以刪除一個(gè)點(diǎn)的擴(kuò)頻碼（稱為解擴(kuò)操作）接收發(fā)射鏈路數(shù)據(jù)恢復(fù)。解擴(kuò)過程是重新恢復(fù)原始帶寬的過程。很明顯，同樣的代碼必須在事先知道在傳輸通道兩端的信息。（在某些情況下，在調(diào)制和解調(diào)的過程中代碼應(yīng)該是知道的）。 輸電鏈擴(kuò)頻代碼接收鏈擴(kuò)頻代碼數(shù)據(jù)輸入射頻輸出射頻輸入RF IN射頻連接相同的配置序列數(shù)據(jù)輸出圖1.擴(kuò)頻通信系統(tǒng)傳播工作帶寬的影響圖2說明了信號(hào)帶寬的通信鏈路評(píng)估輸入的擴(kuò)頻碼頻率數(shù)據(jù)的處理增益數(shù)據(jù)輸入寬度擴(kuò)頻調(diào)制數(shù)據(jù)輸入能量能量PF載體圖2.擴(kuò)頻操作遍及一個(gè)更寬的頻率帶寬的信息能量擴(kuò)頻調(diào)制是一種適用于如BPSK或直接轉(zhuǎn)換。傳統(tǒng)的調(diào)制可以證明所有其他信號(hào)接收不到擴(kuò)頻代碼將保持它們?cè)械男畔ⅲ瑯O沒有被擴(kuò)展。解擴(kuò)過程中帶寬的影響同樣，解擴(kuò)過程如圖3。能量數(shù)據(jù)輸入寬度數(shù)據(jù)輸入解擴(kuò)調(diào)制能量輸入的擴(kuò)頻碼數(shù)據(jù)的處理增益PF載體頻率圖3,在解擴(kuò)過程中恢復(fù)的原有信號(hào)在這里，解擴(kuò)調(diào)制已經(jīng)取得了正常解調(diào)操作，也表明了干擾或干擾信號(hào)在解擴(kuò)傳輸過程中被擴(kuò)展！由于帶寬的浪費(fèi)抵消了傳播的多用戶擴(kuò)頻結(jié)果直接在一個(gè)更寬的頻帶使用，完全對(duì)應(yīng)之前的“處理增益”。 因此擴(kuò)頻并沒有節(jié)約有限的頻率資源。過度的使用雖然得到了補(bǔ)償，但是可能有很多用戶共享這一擴(kuò)大頻率波段（如圖4）。用戶1+用戶2+用戶3+用戶N數(shù)據(jù)輸入獲得的擴(kuò)頻增益圖4.在相同的頻帶多個(gè)用戶共享擴(kuò)頻技術(shù)。擴(kuò)頻是寬帶技術(shù)相對(duì)于常規(guī)窄帶技術(shù)，擴(kuò)頻過程是一種寬帶技術(shù)。例如，W - CDMA和UMTS都是寬帶技術(shù)，與窄帶廣播相比，它需要一個(gè)比較大的頻率帶寬。擴(kuò)頻的優(yōu)點(diǎn)抗干擾性能和抗干擾的影響擴(kuò)頻技術(shù)有很多優(yōu)點(diǎn)。.抗干擾性是最重要的一個(gè)優(yōu)點(diǎn)。有意或無意的干擾和干擾信號(hào)都是不希望存在的因?yàn)樗鼈儾话瑪U(kuò)頻密鑰。只有期望信號(hào)才有密鑰，在解擴(kuò)過程中才會(huì)被接收器接收，如圖5。輸電鏈擴(kuò)頻代碼接收鏈擴(kuò)頻代碼數(shù)據(jù)輸入射頻輸出射頻輸入RF IN射頻連接數(shù)據(jù)輸出數(shù)據(jù) 干擾數(shù)據(jù)擴(kuò)展和干擾擴(kuò)展數(shù)據(jù)擴(kuò)展數(shù)據(jù)擴(kuò)展和干擾圖5.擴(kuò)頻通信系統(tǒng)。注意，解擴(kuò)鏈路中數(shù)據(jù)信號(hào)被傳輸?shù)耐瑫r(shí)干擾能源也被傳輸。無論在窄帶或?qū)拵е?，如果它不涉及解擴(kuò)過程，你幾乎可以忽略干擾。這種抑制反應(yīng)也適用于其他沒有正確密鑰的擴(kuò)頻信號(hào)。因此不同的擴(kuò)頻通信系統(tǒng)可以工作在同一頻段，例如CDMA。值得注意的是，擴(kuò)頻是寬帶技術(shù)，但反之則不然：寬帶技術(shù)不涉及擴(kuò)頻技術(shù)。抗截獲抗截獲是擴(kuò)頻通信技術(shù)的第二個(gè)優(yōu)勢(shì)。由于非法的聽眾沒有密鑰用于原始信號(hào)傳播，這些聽眾無法解碼。沒有合適的鑰匙，擴(kuò)頻信號(hào)會(huì)出現(xiàn)噪音或干擾。（掃描方法可以打破的這些密鑰，但是密鑰是短暫的。）甚至更好，信號(hào)電平可以低于噪聲水平，因?yàn)閿U(kuò)頻傳輸降低了頻譜密度，如圖6。（總能量是相同的，但它是廣泛存在于頻率的。）因此信息是無形的，這一影響在直接序列擴(kuò)頻（DSSS）技術(shù)上有充分的體現(xiàn)。（在下文的DSSS作更詳細(xì)說明。）其他接收機(jī)無法“看到”這種傳輸，它們只能出現(xiàn)在整體噪音水平略有增加的情況下！噪聲基準(zhǔn)擴(kuò)展后的數(shù)據(jù)噪聲基準(zhǔn)數(shù)據(jù)傳播之前圖6.在被噪音水平之下的擴(kuò)頻頻譜信號(hào)。在沒有正確的擴(kuò)頻傳輸密鑰的情況下，接收器不能“看到”傳輸過程?？顾ヂ洌ǘ鄰叫?yīng)）無線信道通常具有多徑傳播，即有一個(gè)以上的信號(hào)從發(fā)射機(jī)傳到接收器（如圖7）。這種多路徑可以通過空氣的反射或折射以及從地面反射或物體如這些路徑建筑物引起。RxRDTx圖7.信號(hào)是如何通過多個(gè)路徑到達(dá)接收器的。這種反射路徑（R）可干擾直接路徑（D）的現(xiàn)象稱為解擴(kuò)過程的同步衰落。因?yàn)榻鈹U(kuò)過程使信號(hào)D與信號(hào)R的同步被拒絕，即使它們包含了相同的密鑰。將反射路徑的信號(hào)應(yīng)用于解擴(kuò)是個(gè)有用的方法。擴(kuò)頻技術(shù)在CDMA的應(yīng)用請(qǐng)注意，擴(kuò)展頻譜不是一個(gè)擴(kuò)頻調(diào)制方案，不應(yīng)與其他調(diào)制方式相混淆。例如我們可以使用擴(kuò)頻技術(shù)發(fā)射一個(gè)由PSK或BPSK的已調(diào)信號(hào)。.感謝調(diào)制的信號(hào)的編碼基礎(chǔ)，使擴(kuò)頻頻譜也可用于其他類型的多址實(shí)現(xiàn)（即可以同時(shí)進(jìn)行多個(gè)通訊聯(lián)系和實(shí)際或表面上相同的物理介質(zhì)共存）。到目前為止，有三個(gè)主要的方法可用。FDMA-頻分多址FDMA分配一個(gè)特定的載波頻率給通信信道。不同用戶使用頻譜的切片數(shù)是受到限制的（如圖8）。在已有的三種多路存取方法中，F(xiàn)DMA在頻帶利用方面是效率最低的。FDMA的方法包括Methods包括無線電廣播，電視，高級(jí)移動(dòng)電話系統(tǒng)AMPS等。用戶1 用戶2 用戶3 用戶N頻率(kHz,MHz,GHz) Fc1 Fc2 Fc3 FcN圖8. FDMA系統(tǒng)中不同的用戶的載波頻率分配。TDMA-時(shí)分多址TDMA的不同用戶彼此間發(fā)言和聽取信息時(shí)，是根據(jù)定義的時(shí)隙分配來處理的（如圖9）。不同的通信信道可以建立一個(gè)唯一的載波頻率。TDMA的例子有全球移動(dòng)通信系統(tǒng)GSM，DECT，TETRA和IS - 136。用戶1 用戶2 用戶3 用戶N 用戶1 用戶2 用戶3 用戶N時(shí)間段時(shí)間段時(shí)間(ms,us)圖9.在TDMA系統(tǒng)中不同用戶的時(shí)隙分配。CDMA-碼分多址CDMA的傳播是由密鑰或代碼決定的（如圖10）。在這個(gè)意義上說，擴(kuò)頻就是一種CDMA 。在發(fā)射器和接收器密鑰必須提前被定義和確定。它的例子有IS - 95(DS)，IS- 98，藍(lán)牙和無線局域網(wǎng)。用戶1用戶5用戶4用戶3用戶2圖10.CDMA系統(tǒng)中相同頻帶有獨(dú)特的鑰匙或代碼。當(dāng)然，人們可以結(jié)合上述存取方法，例如，全球移動(dòng)通信系統(tǒng)GSM結(jié)合了TDMA和FDMA。GSM定義了不同的載波頻率（細(xì)胞）的拓?fù)漕I(lǐng)域，并設(shè)定時(shí)段內(nèi)每一個(gè)細(xì)胞。擴(kuò)頻和（的）編碼密鑰在這一點(diǎn)上，值得重申的是擴(kuò)頻的主要特點(diǎn)是一個(gè)代碼或密鑰必須在發(fā)射器和接收器之前就是已知的?，F(xiàn)代通訊的代碼是數(shù)字序列必須長(zhǎng)期存在和隨機(jī)出現(xiàn)的，盡可能地顯示為“噪音像”。在任何情況下，代碼必須確保是可再生的。或者接收器不能提取已發(fā)出去的消息。因此，該序列是幾乎是隨機(jī)的 。這樣的代碼被稱為偽隨機(jī)數(shù)（PRN）或序列。最常用的方法來產(chǎn)生偽隨機(jī)是基于反饋移位寄存器的。許多書籍都在介紹偽隨機(jī)碼的發(fā)展與特征，但是，實(shí)際的發(fā)展已超出了這些教材所敘述的。注意的是，建立或選擇適當(dāng)?shù)男蛄谢蛐蛄屑⒉皇俏⒉蛔愕赖?。為了保證有效的擴(kuò)頻通信，偽隨機(jī)序列必須尊重一定的規(guī)律如長(zhǎng)度、自相關(guān)、互相關(guān)、正交。比較受歡迎偽隨機(jī)序列有Barker碼，M序列碼，Gold碼，Walsh碼等。考慮到存在更復(fù)雜的序列集，給它提供了一個(gè)更強(qiáng)大的擴(kuò)展頻譜鏈路。但是這產(chǎn)生了成本問題：擴(kuò)頻和解擴(kuò)都需要在速度和性能都更復(fù)雜的電子產(chǎn)品，數(shù)字?jǐn)U頻解擴(kuò)芯片包含幾百萬(wàn)個(gè)等效的2輸入與非門在幾十兆赫間切換。An Introduction to Spread-Spectrum CommunicationsAbstract:This application note is a tutorial overview of spread-spectrum principles.The discussion covers both direct-sequence and fast-hopping methods.Theoretical equations are given to allow performance estimates.Relation direct-sequence spread-spectrum(DSSS) and frequency-hopping spread-spectrum(FHSS) methods.Introduction As spread-spectrum techmiques become increasingly popular,electrical engineers outside the field are eager for understandable explanations of the technology.There are books and websites on the subject,but many are hard to understand or describe some aspects while ignoring others(e.g.,the DSSS technique with extensive focus on PRN-code generation).The following discussion covers the full spectrum(pun intended).A Short HistorySpread-spectrum communications technology was first described on paper by an actress and a musician!In 1941 Hollywood actress Hedy Lamarr and pianist George Antheil described a secure radio link to control torpedos.They received U.S.Patent #2.292.387.The technology was not taken seriously at that time by the U.S.Army and was forgotten until the 1980s,when it became active.Since then the technology has become increasingly popular for application that involve radio links in hostile environments.Typical applications for the resulting short-range data transceivers include satellite-positioning systemsGPS，3G mobile telecommunications,W-LAN(IEEE802.11a,IEEE 802.11b,IEEE 802.11g),and Bluetooth.Spread-spectrum techniques also aid in the endless race between communication needs and radio-frequency availability-situations where the radio spectrum is limited and is,therefore,an expensive resource.Theoretical Justification for Spread Spectrum Spread-spectrum is apparent in the Shannon and Hartley channel-capacity theorem: C=Blog2(1+S/N) (Eq.1)In this equation,C is the channel capacity in bits per second(bps),which is the maximum data rate for a theoretical bit-error rate(BER).B is the required channel bandwidth in Hz,and S/N is the signal-to-nosie power ratio.To be more explicit,one assumes that C,which represents the amount of information allowed by the communication channel,also represents the desired performance.Bandwidth (B) is the price to be paid,bacause frequency is a limited resource.The S/N ratio expresses the environmental conditions or the physical characteristics (i.e., obstacles ,presence of jammers ,interferences,etc.).There is an elegant interpretation of this equation,applicable for difficult environments,for example,when a low S/N ratio is caused by noise and interference.This approach says that one can maintain or even increase communication performance (high C) by allowing or injecting more bandwidth (high B),even when signal power is below the noise floor. (The equation does not forbid that condition!)Modify Equation 1 by changing the log base from 2 to e (the Napierian number) and by noting that In=loge.Therefore:C/B=(1/ln2)ln(1+S/N)=1.443ln(1+S/N) (Eq.2)Applying the MacLaurin series development forln(1+x)=x-x2/2+x3/3-x4/4+(-1)k+1xk/k+:C/B=1.443(S/N-1/2(S/N)2+1/3(S/N)3-) (Eq.3)S/N is usually low for spread-spectrum applications. (As just mentioned, the signal power density can even be below the noise level.) Assuming a noise level such that S/N 1,Shannons expression becomes simply:C/B1.443S/N (Eq.4)Very roughly:C/NS/N (Eq.5)Or:N/SB/C (Eq.6)To send error-free information for a given noise-to-signal ratio in the channel,therefore,one need only perform the fundamental spread-spectrum signal-spreading operation:increase the transmitted bandwidth.That principle seems simple and evident.Nonetheless,implementation is complex,mainly because spreading the baseband (by a factor that can be several orders of magnitude) forces the electronics to act and react accordingly,which,in turn,makes the spreading and despreading operations necessary.DefinitionsDifferent spread-spectrum techniques are available,but all have one idea in common:the key (also called the code or sequence) attached to the communication channel.The manner of inserting this code defines precisely the spread-spectrum technique.The term spread spectrum refers to the expansion of signal bandwidth,by several orders of magnitude in some cases,which occurs when a key is attached to the communication channel.The formal definition of spread spectrum is more precise:an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency signal (Figure 1).As a direct consequence,energy used in transmitting the signal is spread over a wider bandwidth,and appears as noise.The ratio (in dB) between the spread baseband and the original signal is called processing gain.Typical spread-spectrum processing gains run from 10dB to 60dB.To apply a spread-spectrum technique,simply inject the corresponding spread-spectrum code somewhere in the transmitting chain before the antenna (receiver).Conversely,you can remove the spread-spectrum code (called a despreading operation) at a point in the receive chain before data retrieval.A despreading operation reconstitutes the information into its original bandwidth.Obviously,the same code must be known in advance at both ends of the transmission channel. (In some circumstances,the code should be known only by those two parties.)Figure 1.Spread-spectrum communication systemBandwidth Effects of the Spreading OperationFigure 2 illustrates the evaluation of signal bandwidths in a communication link.Figure 2.Spreading operation spreads the signal energy over a wider frequency bandwidth.Spread-spectrum modulation is applies on top of a conventional modulation such as BPSK or direct conversion.One can demonstrate that all other signals not receiving the spread-spectrum code will remain ad they are,that is,unspread.Bandwidth Effects of the Despreading Operation Similarly,despreading can be seen in Figure 3.Figure 3. The despreading operation recovers the original signal.Here a spread-spectrum demodulation has been made on top of the normal demodulation operations.One can also demonstrate that signals such as an interferer or jammer added during the transmission will be spread during the despreading operation!Waste of Bandwidth Due to Spreading Is Offset by Multiple UsersSpreading results directly in the use of a wider frequency band by a factor that corresponds exactly to the processing gain mentioned earlier.Therefore spreading does not spare the limited frequency resource.That overuse is well compensated,however,by the possibility that many users will share the enlarged frequency band (Figure 4).Figure 4. The same frequency band can be shared by multiple users with spread-spectrum techniques.Spread Spectrum Is a Wideband Technology In contrast to regular narrowband technology,the spread-spectrum process is a wideband technology.W-CDMA and UMTS, for example,are wideband technologies that require a relatively large frequency bandwidth, compared to narrowband radio.Benefits of Spread SpectrumResistance to Interference and Antijamming EffectsThere are many benefits to spread-spectrum technology.Resistance to interference is the most important advantage.Intentional or unintentional interference and jamming signals are rejected because they do not contain the spread-spectrum key.Only the desired signal,which has the key, will be seen at the receiver when the despreading operation is exercised.See Figure 5.Figure 5. A spread-spectrum communication system.Note that the interferers energy is spread while the data signal is despread in the receive chain.You can practically ignore the interference,narrowband or wideband,if it does not include the key used in the dispreading operation.That rejection also applies to other spread-spectrum signals that do not have the right key.Thus different spread-spectrum communications can be active simultaneously in the same band,such as CDMA.Note that spread-spectrum is a wideband technology,but the reverse is not true:wideband techniques need not involve spread-spectrum technology.Resistance to Interception Resistance to interception is the second advantage provided by spread-spectrum techniques.Because nonauthorized listeners do not have the key used to spread the original signal,those listeners cannot decode it.Without the right key,the spread-spectrum signal appears as noise or as an interferer.(Scanning methods can break the code,however,if the key is short.) Even better,signal levels can be below the noise floor,because the spreading operation reduces the spectral density.See Figure 6.(Total energy is the same,but it is widely spread in frequency.) The message is thus made invisible,an effect that is particularly strong with the direct-sequence spread-spectrum (DSSS) technique.(DSSS is discussed in greater detail below.) Other receivers cannot “see” the transmission;they only register a slight increase in the overall noise level!Figure 6.Spread-spectrum signal is buried under noise level.The receiver cannot “see” the transmission without the right spread-spectrum keys.Resistance to Fading (Multipath Effects)Wireless channels often include multiple-path propagation in which the signal has more that one path from the transmitter to the receiver (Figure 7).Such multipaths can be caused by atmospheric reflection or refraction, and by reflection from the ground or from objects such as buildings.Figure 7.Illustration of how the signal can reach the receiver over multiple paths.The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading.Because the dispreading process synchronizes to signal D,signal R is rejected even though it contains the same key. Methods are available to use the reflected-path signals by dispreading them and adding the extracted results to the main one.Spread Spectrum Allows CDMANote that spread spectrum is not a modulation scheme,and should not be confused with other types of modulation.One can,for example,use spread-spectrum techniques to transmit a signal modulated by PSK or BPSK.Thanks to the coding basis,spread spectrum can also be used as another method for implementing multiple access (i.e.,the real or apparent coexistence of multiple and simultaneous communication links on the same physical media).So far,three main methods are available. FDMA-Frequency Division Multiple AccessFDMA allocates a specific carrier frequency to a communication channel.The number of different users is limited to the number of “slices” in the frequency spectrum (Figure 8).Of the three methods for enabling multiple access,FDMA is the least efficient in term of frequency-band usage.Methods of FDMA access include radio broadcasting,TV,AMPS,and TETRAPOLE.Figure 8.Carrier-frequency allocations among different users in a FDMA system.TDMA-Time Division Multiple Access With TDMA the different users speak and listen to each other according to a defined allocation of time slots (Figure 9).Different communication channels can then be established for a unique carrier frequency.Examples of TDMA are GSM,DECT,TETRA,and IS-136.Figure 9. Time-slot allocations among different users in a TDMA system.CDMA-Code Division Multiple AccessCDMA access to the air is determined by a key or code (Figure 10).In that sence,spread spectrum is a CDMA access.The key must be defined and known in advance at the transmitter and receiver ends.Growing examples are IS-95 (DS),IS-98,Bluetooth,and WLAN.Figure 10.CDMA systems access the same frequency band with unique keys or codes.One can,of course,combine the above