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Before you filter a signal using a channel object, you must ensure that the properties of the channel have suitable values for the situation you want to model. This section offers some guidelines to help you choose realistic values that are appropriate for your modeling needs. The topics are
The syntaxes for viewing and changing values of properties of channel objects are described in Specifying Fading Channels.
Here are some tips for choosing property values that describe realistic channels:
Path Delays
By convention, the first delay is typically set to zero. The first delay corresponds to the first arriving path.
For indoor environments, path delays after the first are typically between 1 ns and 100 ns (that is, between 1e-9 s and 1e-7 s).
For outdoor environments, path delays after the first are typically between 100 ns and 10 µs (that is, between 1e-7 s and 1e-5 s). Very large delays in this range might correspond, for example, to an area surrounded by mountains.
The ability of a signal to resolve discrete paths is related to its bandwidth. If the difference between the largest and smallest path delays is less than about 1% of the symbol period, then the signal experiences the channel as if it had only one discrete path.
Average Path Gains
The average path gains in the channel object indicate the average power gain of each fading path. In practice, an average path gain value is a large negative dB value. However, computer models typically use average path gains between -20 dB and 0 dB.
The dB values in a vector of average path gains often decay roughly linearly as a function of delay, but the specific delay profile depends on the propagation environment.
To ensure that the expected value of the path gains' total power is 1, you can normalize path gains via the channel object's NormalizePathGains property.
Maximum Doppler Shifts
Some wireless applications, such as standard GSM (Global System for Mobile Communication) systems, prefer to specify Doppler shifts in terms of the speed of the mobile. If the mobile moves at speed v (m/s), then the maximum Doppler shift is given below, where f is the transmission carrier frequency in Hz and c is the speed of light (3e8 m/s).
Based on the formula above in terms of the speed of the mobile, a signal from a moving car on a freeway might experience a maximum Doppler shift of about 80 Hz, while a signal from a moving pedestrian might experience a maximum Doppler shift of about 4 Hz. These figures assume a transmission carrier frequency of 900 MHz.
A maximum Doppler shift of 0 corresponds to a static channel that comes from a Rayleigh or Rician distribution.
K-Factor for Rician Fading Channels
The Rician K-factor specifies the ratio of specular-to-diffuse power for a direct line-of-sight path. The ratio is expressed linearly, not in dB.
For Rician fading, the K-factor is typically between 1 and 10.
A K-factor of 0 corresponds to Rayleigh fading.
Here are some tips for configuring a channel object to customize the filtering process:
If your data is partitioned into a series of vectors (that you process within a loop, for example), then you can invoke the filter function multiple times while automatically saving the channel's state information for use in a subsequent invocation. The state information is visible to you in the channel object's PathGains and NumSamplesProcessed properties, but also involves properties that are internal rather than visible.
Note To maintain continuity from one invocation to the next, you must set the ResetBeforeFiltering property of the channel object to 0. |
If you set the ResetBeforeFiltering property of the channel object to 0 and want the randomness to be repeatable, then use the reset function before filtering any signals, to reset both the channel and the state of the internal random number generator.
If you want to reset the channel before a filtering operation so that it does not use any previously stored state information, then either use the reset function or set the ResetBeforeFiltering property of the channel object to 1. The former method resets the channel object once, while the latter method causes the filter function to reset the channel object each time you invoke it.
If you want to normalize the fading process so that the expected value of the path gains' total power is 1, then set the NormalizePathGains property of the channel object to 1.
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