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Background: Overviews on Wireless Channel Modeling
We need to ask the following three important questions while designing a wireless
communication link:
1. Fading & Power Loss:
Is the signal to interference plus noise ratio (SINR) large enough for the receiver
to detect the transmitted signal?
2. Signal Distortion:
Can the signal distortion be ignored, predicted or removed so that we know how
to recover the transmitted information at the receiver?
3. Time Variation:
Can the receiver adapt faster enough to the variations of the above two features
(SINR & signal distortion)?
A complete wireless channel model should provide quantitative measures of SINR,
signal distortion and time variation.
For SINR (referred in question 1), we need only to consider the time invariant
transmission loss at a single frequency (that is the RF carrier frequency). The
frequency and time dependent properties of signal can be addressed in answering
questions 2 and 3.
Signal distortion (referred in question 2) is caused by the frequency dependent
variations of the received signal strength and phase. The primary source of
frequency dependent variations is multipath propagation. Here, we need only to
consider time invariant situations and leave the time varying features to question 3.
Motions of receivers, transmitters or wireless environments generate Doppler
effects. With Doppler effects, signal frequencies shift and spread. These Doppler
effects will cause time variations in the received signal strength and wave shape.
This kind of time varying features is usually random and can be modeled as
stochastic processes.
In order to addressing these three important issues, we divide wireless channel modeling
into three parts:
• Transmission loss
-single frequency (or narrowband signal)
-time invariant environment (or short observation time period)
• Frequency dependent channel impulse response or transfer function
-multiple frequencies (or broadband signal)
-time invariant environment (or short observation time period)
• Time varying channel impulse response or transfer function
-multiple frequencies (or broadband signal)
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2009-08-07
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