16-10-2012, 12:36 PM
Channel Models A Tutorial
Channel Models.pdf (Size: 219.03 KB / Downloads: 186)
Channel Models: A Tutorial
Many readers may be experts in modeling, programming, or higher layers of networking but may not
be familiar with many PHY layer concepts. This tutorial on Channel Models has been designed for
such readers. This information has been gathered from various IEEE and ITU standards and
contributions and published books.
Basic Concepts
Channel
The term channel refers to the medium between the transmitting antenna and the receiving antenna as
shown in Figure A.1.1
The characteristics of wireless signal changes as it travels from the transmitter antenna to the receiver
antenna. These characteristics depend upon the distance between the two antennas, the path(s) taken
by the signal, and the environment (buildings and other objects) around the path. The profile of
received signal can be obtained from that of the transmitted signal if we have a model of the medium
between the two. This model of the medium is called channel model.
In general, the power profile of the received signal can be obtained by convolving the power profile of
the transmitted signal with the impulse response of the channel. Convolution in time domain is
equivalent to multiplication in the frequency domain.
Shadowing
If there are any objects (such buildings or trees) along the path of the signal, some part of the
transmitted signal is lost through absorption, reflection, scattering, and diffraction. This effect is
called shadowing. As shown in Figure A.1.3, if the base antenna were a light source, the middle
building would cast a shadow on the subscriber antenna. Hence, the name shadowing.
Doppler Spread
The power delay profile gives the statistical power distribution of the channel over time for a signal
transmitted for just an instant. Similarly, Doppler power spectrum gives the statistical power
distribution of the channel for a signal transmitted at just one frequency f. While the power delay
profile is caused by multipath, the Doppler spectrum is caused by motion of the intermediate objects in
ITU Path Loss Models
Another commonly used set of empirical channel models is that specified in ITU-R recommendation
M.1225. The recommendation specifies three different test environments: Indoor office, outdoor-toindoor
pedestrian, and vehicular high antenna. Since the delay spread can vary significantly, the
recommendation specifies two different delay spreads for each test environment: low delay spread (A),
and medium delay spread (B). In all there are 6 cases. For each of these cases, a multipath tap delay
profile is specified. The number of multipath components in each model is different. The following
three tables list the specified parameters [M.1225].