2. • The basic idea in spread-spectrum systems is, as the
name implies, to spread the signal over a much wider
portion of the spectrum than usual.
• A simple audio signal that would normally occupy only
a few kilohertz of spectrum can be expanded to cover
many megahertz.
• Thus only a small portion of the signal is likely to be
masked by any interfering signal.
• Of course, the average power density, expressed in
watts per hertz of bandwidth, is also reduced, and this
often results in a signal-to-noise ratio of less than one
(that is, the signal power in any given frequency range is
less than the noise power in the same bandwidth).
Spread Spectrum
3. 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
4. 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
