How Frequency Diversity works in CDMA?
Frequency diversity is inherent in a spread spectrum system. A fade of the entire signal is less likely than with narrow band systems.
- Combats Fading, Caused by Multipath
- Fading Acts like Notch Filter to a Wide Spectrum Signal
- May Notch only Part of Signal
Fading is caused by reflected images of an RF signal arriving at the receiver such that the phase of the delayed (reflected) signal is 180 degrees out of phase with the direct RF signal.
Since the direct signal and delayed signal are out of phase, they cancel each other causing the amplitude seen by the receiver to be greatly reduced. In the frequency domain, a fade appears as a notch filter that moves across a band. As the user moves, the frequency of the notch changes.
The width of the notch is on the order of one over the difference in arrival time of two signals. For a 1 usec delay, the notch will be approximately 1 MHz wide. The TIA CDMA system uses a 1.25 MHz bandwidth, so only those multipaths of time less than 1 usec actually cause the signal to experience a deep fade. In many environments, the multipath signals will arrive at the receiver after a much longer delay.
This means that only a narrow portion of the signal is lost. In the display shown, the fade is 200 to 300 kHz wide. This results in the complete loss of an analog or TDMA signal but only reduces the power in a portion of a CDMA signal. As the spreading width of a CDMA signal increases, so does its multipath fading resistance. Many spread spectrum systems use a 5 or 10 MHz wide channel to further improve fading resistance.