Coherence Bandwidth
Definition: The approximate maximum bandwidth or frequency interval over which two frequencies of a signal are likely to experience comparable or correlated amplitude fading. If the multipath time delay spread equals D seconds, then the coherence bandwidth Wc in Hertz is given approximately by the equation:
Wc ˜ 1/{2pD} (1)
The coherence bandwidth varies over cellular or PCS communications paths because the multipath spread D varies from path to path.
USE YOUR BROWSER "BACK" BUTTON TO RETURN
© 1999 DR. ERNEST SIMO - SPACE2000
Coherence Bandwidth
Application: Frequencies within a coherence bandwidth of one another tend to all fade in a similar or correlated fashion. One reason for designing the CDMA IS-95 waveform with a bandwidth of approximately 1.25 MHz is because in many urban signaling environments the coherence bandwidth Wc is significantly less than 1.25 MHz. Therefore, when fading occurs it occurs only over a relatively small fraction of the total CDMA signal bandwidth. The portion of the signal bandwidth over which fading does not occur typically contains enough signal power to sustain reliable communications.
USE YOUR BROWSER "BACK" BUTTON TO RETURN
© 1999 DR. ERNEST SIMO - SPACE2000
Coherence Bandwidth
Example: If the delay spread D over a particular cellular communication path in an urban environment is 3 µsec, then using Eq. (1), the coherence bandwidth is approximately 53 kHz, which is only about 4.2% of the IS-95 bandwidth.
USE YOUR BROWSER "BACK" BUTTON TO RETURN
© 1999 DR. ERNEST SIMO - SPACE2000
|
