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 amplitudefading. If the multipath time delay spread equals Dseconds, then the coherence bandwidth in rad/s is given approximately by the equation:
Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. The period is the duration of time of one cycle in a repeating event, so the period is the reciprocal of the frequency. For example: if a newborn baby's heart beats at a frequency of 120 times a minute, its period—the time interval between beats—is half a second. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio signals (sound), radio waves, and light.
The amplitude of a periodic variable is a measure of its change over a single period. There are various definitions of amplitude, which are all functions of the magnitude of the difference between the variable's extreme values. In older texts the phase is sometimes called the amplitude.
In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a communication channel that experiences fading. In wireless systems, fading may either be due to multipath propagation, referred to as multipath-induced fading, weather, or shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading.
Also coherence bandwidth in Hz is given approximately by the equation:
It can be reasonably assumed that the channel is flat if the coherence bandwidth is greater than the data signal bandwidth. The coherence bandwidth varies over cellular or PCS communications paths because the multipath spread D varies from path to path.
A cellular network or mobile network is a communication network where the last link is wireless. The network is distributed over land areas called cells, each served by at least one fixed-location transceiver, but more normally three cell sites or base transceiver stations. These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell.
At the most basic level, Personal Communications Service (PCS) describes a set of communications capabilities which allows some combination of terminal mobility, personal mobility, and service profile management. More specifically, PCS refers to any of several types of wireless voice or wireless data communications systems, typically incorporating digital technology, providing services similar to advanced cellular mobile or paging services. In addition, PCS can also be used to provide other wireless communications services, including services which allow people to place and receive communications while away from their home or office, as well as wireless communications to homes, office buildings and other fixed havelocations. Described in more commercial terms, PCS is a generation of wireless-phone technology that combines a range of features and services surpassing those available in analog- and digital-cellular phone systems, providing a user with an all-in-one wireless phone, paging, messaging, and data service.
Application
Frequencies within a coherence bandwidth of one another tend to all fade in a similar or correlated fashion. One reason for designing the CDMAIS-95 waveform with a bandwidth of approximately 1.25 MHz is because in many urban signaling environments the coherence bandwidth Bc is significantly less than 1.25MHz. 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.
Example
If the delay spread D over a particular cellular communication path in an urban environment is 1.9 µs, then using equation above, the coherence bandwidth is approximately 0.53 MHz, which results in frequency selective fading over the IS-95 bandwidth.
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