21-07-2014, 09:44 AM
Spread Spectrum
Spread Spectrum.ppt (Size: 347 KB / Downloads: 103)
Introduction to Spread Spectrum
Problems such as capacity limits, propagation effects, synchronization occur with wireless systems
Spread spectrum modulation spreads out the modulated signal bandwidth so it is much greater than the message bandwidth
Independent code spreads signal at transmitter and despreads signal at receiver
Frequency multiplex
Separation of spectrum into smaller frequency bands
Channel gets band of the spectrum for the whole time
Advantages:
no dynamic coordination needed
works also for analog signals
Disadvantages:
waste of bandwidth if traffic distributed unevenly
inflexible
guard spaces
Spread Spectrum Technology
Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference
Solution: spread the narrow band signal into a broad band signal using a special code
DS/SS Comments IV
Secure and Jamming Resistant
Both receiver and transmitter must know c(t)
Since PSD is low, hard to tell if signal present
Since wide response, tough to jam everything
Multiple access
If ci(t) is orthogonal to cj(t), then users do not interfere
Near/Far problem
Users must be received with the same power
RAKE Receiver
RAKE Receiver has to estimate:
Multipath delays
Phase of multipath components
Amplitude of multipath components
Number of multipath components
Main challenge is receiver synchronization in fading channels