Image Processing Reference
In-Depth Information
A typical path-loss model for omnidirectional antennas is given by [, chap. .]:
n
d
d
)
E
[
PL
](
d
)=
C
⋅(
where
d
≥
d
is the distance between
T
and
R
E
isthemeanpathloss
d
is a reference distance which depends on the antenna technology
C
is a technology- and frequency-dependent scaling factor
n
is the so-called path-loss exponent
[
PL
](
d
)
Typical values for
n
are in the range between (free-space propagation) and (shadowed urban
cellular radio), see also [, chap. .].
Reflection occurs when a waveform impinges on a smooth surface with structures significantly
larger than the wavelength. Not all signal energy is reflected, some energy is absorbed by the material.
he mechanism of diffraction allows a wave to propagate into a shadowed region, provided that some
sharp edge exists. Scattering is produced when a wavefront hits a rough surface having structures
smaller than the wavelength; it leads to signal diffusion in many directions.
he most important types of interference are co-channel interference and adjacent-channel inter-
ference. In co-channel interference a signal transmitted from
T
to
R
on channel
c
is distorted by
a parallel transmission on the same channel. In case of adjacent channel interference the interferer
I
transmitsonanadjacentchannel
c
, but due to imperfect filters
R
captures frequency compo-
nents from
c
. Alternatively, an interferer
I
transmitting on channel
c
leaks some signal energy into
channel
c
due to nonperfect transmit circuitry (amplifier).
Noise can be thermal noise or man-made noise. Thermal noise is created in the channel or in
transceiver circuitry and can be found in almost any communications channel. Man-made noise
in industrial environments can have several sources, for example, remote controls, motors, or
microwave ovens.
24.4 Physical Layer: Transmission Problems and
Solution Approaches
The previously discussed wave propagation effects can lead to channel errors. In general, their impact
depends on a multitude of factors, including frequency, modulation scheme, and the current propa-
gation environment. he propagation environment is characterized by the distance between stations,
interferers, the number of different paths and their respective loss, and more. These factors can
change when a station or parts of the environment move. Consequently the transmission quality
is time-variable.
24.4.1 Effects on Transmission
The notion of slow fading refers to significant variations in the mean path loss, as they occur due to
signiicantchangesindistancebetweentransmitter
T
and receiver
R
or by moving beyond large
obstacles. Slow fading phenomena usually occur on longer timescales, they often coincide with
humanactivitylikemobility.Forshortdurationsintherangeofafewseconds,thechannelcanoten
be assumed to have constant path loss.
An immediate result of reflection, diffraction, and scattering is that multiple copies of a signal may
travel on different paths from
T
to
R
. Since these paths usually have different lengths, the copies arrive
