08-08-2012, 01:50 PM
Spread Spectrum
SpreadSpectrum.ppt (Size: 389 KB / Downloads: 406)
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
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
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
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
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
PN Sequences
PN generator produces periodic sequence that appears to be random
PN Sequences
Generated by an algorithm using initial seed
Sequence isn’t 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
SpreadSpectrum.ppt (Size: 389 KB / Downloads: 406)
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
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
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
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
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
PN Sequences
PN generator produces periodic sequence that appears to be random
PN Sequences
Generated by an algorithm using initial seed
Sequence isn’t 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