光纤通信课程设计课件Class12-03.pptx

Class 12Class 12Topic:OCDMAOCDMA historyFiber OCDMACode&DecodeImplementation by FBGExerciseCDMA Introduction The acronymCDMA =Code Division Multiple Access Code Division Multiple Access分碼多重存取式分碼多重存取式 CDMA：展頻系統，展頻系統將傳輸資訊(通常是語音)所需的頻寬展開極寬頻，可讓同頻或是鄰頻使用者所造成的干擾減至最低CDMA由指定不同的手機使用不同代碼的方式，允許多個使用者在同一時間使用同一個頻道CDMA Introduction The History1985 ITU-FPLMTS(Future Public Land Mobile Telecommunications Systems)成立1991 Qualcomm 發表一套CDMA系統測試結果，是當時AMPS的十倍容量1993 IS-95 標準正式製定1994 FPLMTS 改名 IMT-20001997 IS-95B 標準完成CDMA Introduction The development groupCDG:CDMA Development GroupTIA :Telecommunication Industry Association ETSI:European Telecommunication Standards InstituteTl.PG:Telecommunication Planning Group CDMA Principles 一個好的比喻 1/3在室內一群參加宴會的人們-TDMA CDMA Principles 一個好的比喻 2/3在室內一群參加宴會的人們 FDMACDMA Principles 一個好的比喻 3/3在室內一群參加宴會的人們-CDMAHistory Overview(CDMA)1938,in the field of radar systems,examples of frequency hopping signal patterns were patentedDuring and after WWII the fact of BW expansion without pulse narrowing could also provide finer time resolution.1950s Sol Golomb first Books on CDMAEarly system are ARC-50 by Magnavox and satellite radios as well as few GPS systemsHistory Overview(Optical CDMA)Relatively new technique in Optical Fiber communications and seems to outperform traditional TDM and WDMFirst applied in mid 1980s by A Salehi Prucnal and some others.Aim for Terabit performance on Broadband Networks by reducing the cost of every aspect in optical network.IntroductionCDMA used extensively in radio frequency communication systems,especially in 2G and 3G cellular telephone networks.Basic Advantage is the way it handles a finite BW among a large number of users(more users can transmit the same data over the same Bandwidth)introduction(comparison)TDMA and WDMA schemes present significant drawbacks in Local Area Systems when large number of users must be considered.TDMA:one user tx at a time System capacity=users*tx rateWDMA:Four wave mixing as discussed(next slide)Drawbacks DWDM(revisited)DispersionAttenuationFour wave mixingNon-linear nature of refractive index of optical fiberLimits channel capacity of the DWDM System Difficult to construct for dynamic set of multiple users because of the significant amount of coordination among the nodes required for successful parisonOptical CDMA does not need time and frequency management because all the users transmit using the whole BW at the same time!It can also operate asynchronously(as in wireless applications)without packet collisions.Slot allocation requirements are not needed here in contradiction to TDMA and WDMAcomparison(Advantages)Simple implementation,using existing fiber networksReduce the cost in every aspect:Equipment,outside plantFacilities,Operational Support systemsSECURITYEliminate many of intermediate time-division multiplexing steps required by SONETHow does o-CDMA workThe principle is the same as in wireless application.Each user is assigned a unique code(spreading length-L-)which is multiplied by each bit.This code is only known to the receiver in order to demodulate the data.The most important part for correct detection is the code.This code must be uncorrelated from other users codes and be orthogonal.Multiplication by either+1 or-1Two optical orthogonal Code with length L=32 and =c=1.(a)First code is represented by placing a pulse at the 1st,10th 13th and 28th chip positions.(b)Second code is represented by placing a pulse at the 1st,5th 12th and 31st chip positions.Orthogonal Code examplehow does o-CDMA worksO-CDMA divides the fiber spectrum into individual codes,all derived from a single broadband optical source(WDM divides the spectrum into narrow optical wavelengths)It is a simple 3 step process:Source Filter ModulatorFilter:Spatial Filter can be thought an optical Bar code(fixed or programmable)Spectral CodingSpectral DecodingHow to choose wavelength/channels=xDecodingDecoding:X1 user:calculate rX1-rX1=12-8=4 units of power-1X2 user:calculate rX2-rX2=10-10=0 units of power-0Orthogonal Correlation FunctionHadamard CodeBER v.s.number of gratings(chip positions)and usersNumber of chipsNumber of UsersOne way to save the number of gratings:Modified Double Weight Code(MDW)-shared wavelengthOnly correlated chip!EncoderChannel 5,6Channel 8,9Channel 2,3Channel 7,8Decoder:AND subtraction techniqueCross-Correlation ConfirmationBERAny better coding other than this“WDM”example?-Not again!Any better coding other than this“WDM”example?