Coherence bandwidth
Encyclopedia
Coherence bandwidth is a statistical measurement of the range of frequencies over which the channel can be considered "flat", or in other words 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 second
s, then the coherence bandwidth
in rad/s is given approximately by the equation:
The coherence bandwidth varies over cellular
or PCS
communications paths because the multipath spread D varies from path to path.
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.This is the bandwidth over which the channel transfer function remains virtually constant.
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
interval over which two frequencies of a signal are likely to experience comparable or correlated amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
. If the multipath time delay spread
Delay spread
In telecommunications, the delay spread is a measure of the multipath richness of a communications channel.In general, it can be interpreted as the difference between the time of arrival of the earliest significant multipath component and the time of arrival of the latest multipath component.The...
equals D second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s, then the coherence bandwidth
in rad/s is given approximately by the equation:The coherence bandwidth varies over cellular
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...
or PCS
Personal Communications Service
At the most basic level Personal Communications Service or PCS describes a set of wireless communications capabilities that allows some combination of terminal mobility, personal mobility, and service profile management...
communications paths because the multipath spread D varies from path to path.
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-95IS-95
Interim Standard 95 is the first CDMA-based digital cellular standard by Qualcomm. The brand name for IS-95 is cdmaOne. IS-95 is also known as TIA-EIA-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.This is the bandwidth over which the channel transfer function remains virtually constant.