30-05-2012, 02:44 PM
Spread Spectrum
Spread Spectrum.ppt (Size: 470 KB / Downloads: 23)
Spread Spectrum
Analog or digital data
Analog signal
Spread data over wide bandwidth
Makes jamming and interception harder
Frequency hoping
Signal broadcast over seemingly random series of frequencies
Direct Sequence
Each bit is represented by multiple bits in transmitted signal
Chipping code.
Spread Spectrum Concept
Input fed into channel encoder
Produces narrow bandwidth analog signal around central frequency
Signal modulated using sequence of digits
Spreading code/sequence
Typically generated by pseudonoise/pseudorandom number generator
Increases bandwidth significantly
Spreads spectrum
Receiver uses same sequence to demodulate signal
Demodulated signal fed into channel decoder.
Direct Sequence Spread Spectrum
Each bit represented by multiple bits using spreading code
Spreading code spreads signal across wider frequency band
In proportion to number of bits used
10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code
One method:
Combine input with spreading code using XOR
Input bit 1 inverts spreading code bit
Input zero bit doesn’t alter spreading code bit
Data rate equal to original spreading code
Performance similar to FHSS.
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
User’s 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.
Spread Spectrum.ppt (Size: 470 KB / Downloads: 23)
Spread Spectrum
Analog or digital data
Analog signal
Spread data over wide bandwidth
Makes jamming and interception harder
Frequency hoping
Signal broadcast over seemingly random series of frequencies
Direct Sequence
Each bit is represented by multiple bits in transmitted signal
Chipping code.
Spread Spectrum Concept
Input fed into channel encoder
Produces narrow bandwidth analog signal around central frequency
Signal modulated using sequence of digits
Spreading code/sequence
Typically generated by pseudonoise/pseudorandom number generator
Increases bandwidth significantly
Spreads spectrum
Receiver uses same sequence to demodulate signal
Demodulated signal fed into channel decoder.
Direct Sequence Spread Spectrum
Each bit represented by multiple bits using spreading code
Spreading code spreads signal across wider frequency band
In proportion to number of bits used
10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code
One method:
Combine input with spreading code using XOR
Input bit 1 inverts spreading code bit
Input zero bit doesn’t alter spreading code bit
Data rate equal to original spreading code
Performance similar to FHSS.
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
User’s 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.