无线通信与网络课件

,Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Spread Spectrum,Chapter 7,Spread SpectrumChapter 7,1,Spread Spectrum,Input is fed into a channel encoder,Produces analog signal with narrow bandwidth,Signal is further modulated using sequence of digits,Spreading code or spreading sequence,Generated by pseudonoise,or pseudo-random number generator,Effect of modulation is to increase bandwidth of signal to be transmitted,Spread SpectrumInput is fed in,Spread Spectrum,On receiving end,digit sequence is used to demodulate the spread spectrum signal,Signal is fed into a channel decoder to recover data,Spread SpectrumOn receiving en,Spread Spectrum,Spread Spectrum,Spread Spectrum,What can be gained from apparent waste of spectrum?,Immunity from various kinds of noise and multipath distortion,Can be used for hiding and encrypting signals,Several users can independently use the same higher bandwidth with very little interference,Spread SpectrumWhat can be gai,Frequency Hoping Spread Spectrum(FHSS),Signal is broadcast over seemingly random series of radio frequencies,A number of channels allocated for the FH signal,Width of each channel corresponds to bandwidth of input signal,Signal hops from frequency to frequency at fixed intervals,Transmitter operates in one channel at a time,Bits are transmitted using some encoding scheme,At each successive interval,a new carrier frequency is selected,Frequency Hoping Spread Spectr,Frequency Hoping Spread Spectrum,Channel sequence dictated by spreading code,Receiver,hopping between frequencies in synchronization with transmitter,picks up message,Advantages,Eavesdroppers hear only unintelligible blips,Attempts to jam signal on one frequency succeed only at knocking out a few bits,Frequency Hoping Spread Spectr,Frequency Hoping Spread Spectrum,Frequency Hoping Spread Spectr,FHSS Using MFSK,MFSK signal is translated to a new frequency every,T,c,seconds by modulating the MFSK signal with the FHSS carrier signal,For data rate of,R:,duration of a bit:,T,=1/,R,seconds,duration of signal element:,T,s,=,LT,seconds,T,c,T,s,-,slow-frequency-hop spread spectrum,T,c,T,s,-fast-frequency-hop spread spectrum,FHSS Using MFSKMFSK signal is,FHSS Performance Considerations,Large number of frequencies used,Results in a system that is quite resistant to jamming,Jammer must jam all frequencies,With fixed power,this reduces the jamming power in any one frequency band,FHSS Performance Consideration,Direct Sequence Spread Spectrum(DSSS),Each bit in original signal is represented by multiple bits in the transmitted signal,Spreading code spreads signal across a wider frequency band,Spread is in direct proportion to number of bits used,One technique combines digital information stream with the spreading code bit stream using exclusive-OR(Figure 7.6),Direct Sequence Spread Spectru,Direct Sequence Spread Spectrum(DSSS),Direct Sequence Spread Spectru,DSSS Using BPSK,Multiply BPSK signal,s,d,(,t,)=,A d,(,t,)cos(2,f,c,t,),by,c,(,t,),takes values+1,-1 to get,s,(,t,)=,A d,(,t,),c,(,t,)cos(2,f,c,t,),A,=amplitude of signal,f,c,=carrier frequency,d,(t)=discrete function+1,-1,At receiver,incoming signal multiplied by,c,(,t,),Since,c,(,t,)x,c,(,t,),=,1,incoming signal is recovered,DSSS Using BPSKMultiply BPSK s,DSSS Using BPSK,DSSS Using BPSK,Code-Division Multiple Access(CDMA),Basic Principles of CDMA,D,=rate of data signal,Break each bit into,k,chips,Chips are a user-specific fixed pattern,Chip data rate of new channel=,kD,Code-Division Multiple Access,CDMA Example,If,k,=6 and code is a sequence of 1s and-1s,For a 1 bit,A sends code as chip pattern,For a 0 bit,A sends complement of code,Receiver knows senders code and performs electronic decode function,=received chip pattern,=senders code,CDMA ExampleIf k=6 and code is,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(received chip pattern),User A 1 bit:6-1,User A 0 bit:-6-0,User B 1 bit:0-unwanted signal ignored,CDMA ExampleUser A code=1,CDMA for Direct Sequence Spread Spectrum,CDMA for Direct Sequence Sprea,Categories of Spreading Sequences,Spreading Sequence Categories,PN sequences,Orthogonal codes,For FHSS systems,PN sequences most common,For DSSS systems not employing CDMA,PN sequences most common,For DSSS CDMA systems,PN sequences,Orthogonal codes,Categories of Spreading Sequen,PN Sequences,PN generator produces periodic sequence that appears to be random,PN Sequences,Generated by an algorithm using initial seed,Sequence isnt statistically random but will pass many test of randomness,Sequences referred to as pseudorandom numbers or pseudonoise sequences,Unless algorithm and seed are known,the sequence is impractical to predict,PN SequencesPN generator produ,Important PN Properties,Randomness,Uniform distribution,Balance property,Run property,Independence,Correlation property,Unpredictability,Important PN Propert