Spread Spectrum Communications & Code Division Multiple Access (CDMA)

,National Chiao Tung University,Institute of Electronics,Page,56,按一下以編輯母片標題樣式,按一下以編輯母片本文樣式,第二層,第三層,第四層,第五層,Institute of Electronics,Page,1,National Chiao Tung University,2024/9/22,W,LECTRONICS,Institute of Electronics,IRELESS,DESIGN,E,按一下以編輯母片標題樣式,按一下以編輯母片本文樣式,第二層,第三層,第四層,第五層,WS Wuen,按一下以編輯母片標題樣式,按一下以編輯母片,第二層,第三層,第四層,第五層,*,*,*,WS Wuen,按一下以編輯母片標題樣式,按一下以編輯母片,第二層,第三層,第四層,第五層,*,*,*,WS Wuen,按一下以編輯母片標題樣式,按一下以編輯母片,第二層,第三層,第四層,第五層,*,Overview of Wireless Communications,*,*,WS Wuen,WS Wuen,按一下以編輯母片標題樣式,按一下以編輯母片,第二層,第三層,第四層,第五層,*,*,*,Multiple Access,Contents,Multiplexing & Multiple Access,Spread Spectrum Communications,Properties of CDMA,Pseudo Noise (PN) Sequence,Multiplexing Techniques,Frequency Division Duplexing (FDD),FDD provides two simplex channels at the same time,Time Division Duplexing (TDD),TDD provides two simplex time slots on the same frequency,Multiple Access Techniques,Frequency Division Multiple Access (FDMA),Time Division Multiple Access (TDMA),Code Division Multiple Access (CDMA),Space Division Multiple Access (SDMA),Polarization Division Multiple Access (PDMA),Multiple Access,FDMA,Frequency Division Multiple Access,TDMA,Time Division Multiple Access,CDMA,Code Division Multiple Access,TDMA,CDMA,time,FDMA,freq,time,freq,TDMA,time,freq,code,TDMA Frame Structure,Other Multiple Access Techniques,Space Division Multiple Access (SDMA),SDMA systems control the radiated energy for each user in space,Polarization Division Multiple Access (PDMA),Dual Polarization Frequency Reuse,Orthogonal polarizations are used to separate signals and allow for reuse of the same frequency band,Multiple Access Techniques Used inDifferent Wireless Communication Systems,Contents,Multiplexing & Multiple Access,Spread Spectrum Communications,Properties of CDMA & IMT-2000,Pseudo Noise (PN) Sequence,Outline,Introduction,Direct Sequence Spread Spectrum (DSSS),Frequency Hopping Spread Spectrum (FHSS),Time Hopping Spread Spectrum (THSS),Hybrid Systems Basic Principle of Spread Spectrum Systems,Code Division Multiple Access,Why Spread Spectrum?,Hide a signal below the noise floor,Resistance to narrowband jamming and interference,Mitigate performance degradation due to intersymbol and narrowband interference,In conjunction with RAKE receiver, SS can provide coherent combining of different multipath components,Allow multiple users to share the same signal bandwidth,Wide bandwidth of SS signals is useful for location and timing acquisition,Spread Spectrum Properties,Signal occupies a bandwidth much larger than is needed for the information signal,Spread spectrum modulation is done using a,spreading code,independent of the data in the signal,Despreading at the receiver is done by correlating the received signal with a,synchronized,copy of the spreading code,Spread Spectrum System: DSSS & FHSS,Basic Spread Spectrum Techniques,Direct Sequence Spread Spectrum (DSSS),Input,Binary data d,t,with symbol rate R,s,=1/T,s,(=Rb for BPSK),Pseudo-noise code: pn,t,with chip rate R,c,=1/T,c,(an integer of R,s,),Spreading,Transmitted baseband signal: tx,b,=d,t,*,pn,t,Spreading,Processing gain,Despreading,Direct Sequence Spread Spectrum Example,C=AxB,A=CxB,Direct Sequence Spread Spectrum Using BPSK Example,Narrowband Interference,Wideband Interference,Gaussian Noise,Processing Gain,Frequency Hopping Spread Spectrum (FHSS),Signal broadcast over seemingly random series of frequencies,Receiver hops between frequencies in sync with transmitter,Eavesdroppers hear unintelligible blips,Jamming on one frequency affects only a few bits,Basic Operation,Typically 2,k,carriers frequencies forming 2,k,channels,Channel spacing corresponds with bandwidth of input,Each channel used for fixed interval,Frequency Hopping Spread Spectrum System,Transmitter,Receiver,Slow and Fast Frequency Hopping,Frequency shifted every T,c,seconds,Duration of signal element is T,s,seconds,Slow FHSS has T,c, T,s,Fast FHSS has T,c, T,s,Generally fast FHSS gives improved performance in noise (or jamming),Fast Frequency Hop Spread Spectrum Using MFSK (M=4, k=2),Slow Frequency Hop Spread Spectrum Using MFSK (M=4, k=2),Fast Hopping versus Slow Hopping,Fast Hopping,Fast Hopping versus Slow Hopping,Fast Hopping,Fast Hopping versus Slow Hopping,Slow Hopping,Fast Hopping versus Slow Hopping,Slow Hopping,Hybrid DS/FH Spread Spectrum System,Hybrid DS/FH Spread Spectrum System,Contents,Multiplexing & Multiple Access,Spread Spectrum Communications,CDMA,Pseudo Noise (PN) Sequence,CDMA: Past, Present and Future,Properties of CDMA,Multiple Access Capability,Protection Against Multi-path Interference,Privacy,Interference Rejection,Anti-Jamming Capability,Low Probablity of Interception (LPI),Classification of CDMA,Code Division Multiple Access (CDMA),Code Division Multiple Access (CDMA),Multiplexing Technique used with spread spectrum,Start with data signal rate,D,Called bit data rate,Break each bit into,k,chips according to fixed pattern specific to each user,Users code,New channel has chip data rate,kD,chips per second,E.g.,k,=6, three users (A,B,C) communicating with base receiver R,Code for A = ,Code for B = ,Code for C = ,CDMA Example,CDMA Explanation,Consider A communicating with base,Base knows As code,Assume communication already synchronized,A wants to send a 1,Send chip pattern ,As code,A wants to send 0,Send chip pattern ,Complement of As code,Decoder ignores other sources when using As code to decode,Orthogonal codes,CDMA for DSSS,n users each using different orthogonal PN sequence,Modulate each users data stream,Using BPSK,Multiply by spreading code of user,Seven Channel CDMA Encoding and Decoding,CDMA Power Control,Contents,Multiplexing & Multiple Access,Spread Spectrum Communications,Properties of CDMA,Pseudo Noise (PN) Sequence,Pseudo Noise (PN) Sequence,Three Properties of PN Sequence,Balance Property,N(+1) = N(-1), 1,Run-Length Property,1,2(runs): length 1,1,4(runs): length 2,1,8(runs): length 3 etc.,Auto-correlation Property,PN Auto-correlation Function,Barker Sequence,Maximal Length Sequence (m-sequence),IS-95: Long PN Sequence,PN Sequence Length: 2,42,1,Characteristic Polynomial:,IS-95: Short PN Sequence,PN Sequence Length: 2,15,1,Characteristic Polynomial:,Gold Sequence,IMT-2000: Gold Sequence,Mutually Orthogonal,Gold,Codes (256),Generator Polynomials:,Gold Sequence Generator,