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1、Spread Spectrum,Chapter 7,Spread Spectrum,Input is fed into a channel encoder Produces analog signal with narrow bandwidthSignal is further modulated using sequence of digits Spreading code or spreading sequence Generated by pseudonoise,or pseudo-random number generatorEffect of modulation is to inc
2、rease bandwidth of signal to be transmitted,Spread Spectrum,On receiving end,digit sequence is used to demodulate the spread spectrum signalSignal is fed into a channel decoder to recover data,Spread Spectrum,Spread Spectrum,What can be gained from apparent waste of spectrum?Immunity from various ki
3、nds of noise and multipath distortionCan be used for hiding and encrypting signalsSeveral users can independently use the same higher bandwidth with very little interference,Frequency Hoping Spread Spectrum(FHSS),Signal is broadcast over seemingly random series of radio frequenciesA number of channe
4、ls allocated for the FH signalWidth of each channel corresponds to bandwidth of input signalSignal hops from frequency to frequency at fixed intervalsTransmitter operates in one channel at a timeBits are transmitted using some encoding schemeAt each successive interval,a new carrier frequency is sel
5、ected,Frequency Hoping Spread Spectrum,Channel sequence dictated by spreading codeReceiver,hopping between frequencies in synchronization with transmitter,picks up messageAdvantagesEavesdroppers hear only unintelligible blipsAttempts to jam signal on one frequency succeed only at knocking out a few
6、bits,Frequency Hoping Spread Spectrum,FHSS Using MFSK,MFSK signal is translated to a new frequency every Tc seconds by modulating the MFSK signal with the FHSS carrier signalFor data rate of R:duration of a bit:T=1/R secondsduration of signal element:Ts=LT secondsTc Ts-slow-frequency-hop spread spec
7、trumTc Ts-fast-frequency-hop spread spectrum,FHSS Performance Considerations,Large number of frequencies usedResults in a system that is quite resistant to jammingJammer must jam all frequenciesWith fixed power,this reduces the jamming power in any one frequency band,Direct Sequence Spread Spectrum(
8、DSSS),Each bit in original signal is represented by multiple bits in the transmitted signalSpreading code spreads signal across a wider frequency band Spread is in direct proportion to number of bits usedOne technique combines digital information stream with the spreading code bit stream using exclu
9、sive-OR(Figure 7.6),Direct Sequence Spread Spectrum(DSSS),DSSS Using BPSK,Multiply BPSK signal,sd(t)=A d(t)cos(2 fct)by c(t)takes values+1,-1 to gets(t)=A d(t)c(t)cos(2 fct)A=amplitude of signalfc=carrier frequencyd(t)=discrete function+1,-1At receiver,incoming signal multiplied by c(t)Since,c(t)x c
10、(t)=1,incoming signal is recovered,DSSS Using BPSK,Code-Division Multiple Access(CDMA),Basic Principles of CDMAD=rate of data signalBreak each bit into k chipsChips are a user-specific fixed pattern Chip data rate of new channel=kD,CDMA Example,If k=6 and code is a sequence of 1s and-1sFor a 1 bit,A
11、 sends code as chip pattern For a 0 bit,A sends complement of codeReceiver knows senders code and performs electronic decode function=received chip pattern=senders code,CDMA Example,User A code=To send a 1 bit=To send a 0 bit=User B code=To send a 1 bit=Receiver receiving with As code(As code)x(rece
12、ived chip pattern)User A 1 bit:6-1User A 0 bit:-6-0User B 1 bit:0-unwanted signal ignored,CDMA for Direct Sequence Spread Spectrum,Categories of Spreading Sequences,Spreading Sequence Categories PN sequencesOrthogonal codesFor FHSS systemsPN sequences most commonFor DSSS systems not employing CDMAPN
13、 sequences most commonFor DSSS CDMA systemsPN sequencesOrthogonal codes,PN Sequences,PN generator produces periodic sequence that appears to be randomPN Sequences Generated by an algorithm using initial seedSequence isnt statistically random but will pass many test of randomnessSequences referred to
14、 as pseudorandom numbers or pseudonoise sequencesUnless algorithm and seed are known,the sequence is impractical to predict,Important PN Properties,RandomnessUniform distributionBalance propertyRun propertyIndependenceCorrelation propertyUnpredictability,Linear Feedback Shift Register Implementation
15、,Properties of M-Sequences,Property 1:Has 2n-1 ones and 2n-1-1 zerosProperty 2:For a window of length n slid along output for N(=2n-1)shifts,each n-tuple appears once,except for the all zeros sequenceProperty 3:Sequence contains one run of ones,length nOne run of zeros,length n-1One run of ones and
16、one run of zeros,length n-2Two runs of ones and two runs of zeros,length n-32n-3 runs of ones and 2n-3 runs of zeros,length 1,Properties of M-Sequences,Property 4:The periodic autocorrelation of a 1 m-sequence is,Definitions,Correlation The concept of determining how much similarity one set of data
17、has with anotherRange between 1 and 11 The second sequence matches the first sequence0 There is no relation at all between the two sequences-1 The two sequences are mirror imagesCross correlation The comparison between two sequences from different sources rather than a shifted copy of a sequence wit
18、h itself,Advantages of Cross Correlation,The cross correlation between an m-sequence and noise is lowThis property is useful to the receiver in filtering out noiseThe cross correlation between two different m-sequences is lowThis property is useful for CDMA applications Enables a receiver to discrim
19、inate among spread spectrum signals generated by different m-sequences,Gold Sequences,Gold sequences constructed by the XOR of two m-sequences with the same clockingCodes have well-defined cross correlation propertiesOnly simple circuitry needed to generate large number of unique codesIn following e
20、xample(Figure 7.16a)two shift registers generate the two m-sequences and these are then bitwise XORed,Gold Sequences,Orthogonal Codes,Orthogonal codes All pairwise cross correlations are zeroFixed-and variable-length codes used in CDMA systemsFor CDMA application,each mobile user uses one sequence i
21、n the set as a spreading codeProvides zero cross correlation among all usersTypesWelsh codesVariable-Length Orthogonal codes,Walsh Codes,Set of Walsh codes of length n consists of the n rows of an n n Walsh matrix:W1=(0)n=dimension of the matrixEvery row is orthogonal to every other row and to the l
22、ogical not of every other rowRequires tight synchronizationCross correlation between different shifts of Walsh sequences is not zero,Typical Multiple Spreading Approach,Spread data rate by an orthogonal code(channelization code)Provides mutual orthogonality among all users in the same cellFurther spread result by a PN sequence(scrambling code)Provides mutual randomness(low cross correlation)between users in different cells,
