fading

fading

Perhaps the most challenging technical problem facing communications systems
engineers is fading in a mobile environment. The term fading refers to the time variation
of received signal power caused by changes in the transmission medium or
path(s). In a fixed environment, fading is affected by changes in atmospheric conditions,
such as rainfall. But in a mobile environment, where one of the two antennas
is moving relative to the other, the relative location of various obstacles changes
over time, creating complex transmission effects.
Multipath Propagation
Three propagation mechanisms, illustrated in Figure 5.11, playa role. Reflection
occurs when an electromagnetic signal encounters a surface that is large relative to
the wavelength of the signal. For example, suppose a ground-reflected wave near
the mobile unit is received. Because the ground-reflected wave has a 1800 phase
shift after reflection, the ground wave and the line-of-sight (LOS) wave may tend to
cancel, resulting in high signalloss.3 Further, because the mobile antenna is lower
than most human-made structures in the area, multipath interference occurs. These
reflected waves may interfere constructively or destructively at the receiver.


Diffraction occurs at the edge of an impenetrable body that is large compared
to the wavelength of the radio wave. When a radio wave encounters such an edge,
waves propagate in different directions with the edge as the source. Thus, signals can
be received even when there is no unobstructed LOS from the transmitter.
.If the size of an obstacle is on the order of the wavelength of the signal or less,
scattering occurs. An incoming signal is scattered into several weaker outgoing signals.
At typical cellular microwave frequencies, there are numerous objects, such as
lamp posts and traffic signs, that can cause scattering. Thus, scattering effects are difficult
to predict.
These three propagation effects influence system performance in various ways
depending on local conditions and as the mobile unit moves within a celL If a mobile
unit has a clear LOS to the transmitter, then diffraction and scattering are generally
minor effects, although reflection may have a significant impact. If there is no clear
LOS, such as in an urban area at street level, then diffraction and scattering are the
primary means of signal reception.
The Effects of Multipath Propagation As just noted, one unwanted effect
of multipath propagation is that multiple copies of a signal may arrive at different
phases. If these phases add destructively, the signal level relative to noise declines,
making signal detection at the receiver more difficult.
A second phenomenon, of particular importance for digital transmission, is
intersymbol interference (lSI). Consider that we are sending a narrow pulse at a
given frequency across a link between a fixed antenna and a mobile unit. Figure 5.12
shows what the channel may deliver to the receiver if the impulse is sent at two different
times. The upper line shows two pulses at the time of transmission. The lower
line shows the resulting pulses at the receiver. In each case the first received pulse is
the desired LOS signaL The magnitude of that pulse may change because of changes
in atmospheric attenuation. Further, as the mobile unit moves farther away from the



fixed antenna, the amount of LOS attenuation increases. But in addition to this primary
pulse, there may be multiple secondary pulses due to reflection, diffraction,
and scattering. Now suppose that this pulse encodes one or more bits of data. In that
case, one or more delayed copies of a pulse may arrive at the same time as the primary
pulse for a subsequent bit. These delayed pulses act as a form of noise to the
subsequent primary pulse, making recovery of the bit information more difficult.
As the mobile antenna moves, the location of various obstacles changes; hence
the number, magnitude, and timing of the secondary pulses change. This makes it
difficult to design signal processing techniques that will filter out multipath effects
so that the intended signal is recovered with fidelity.
Types of Fading Fading effects in a mobile environment can be classified as
either fast or slow. Referring to Figure 5.11, as the mobile unit moves down a
street in an urban environment, rapid variations in signal strength occur over
distances of about one-half a wavelength. At a frequency of 900 MHz, which is
typical for mobile cellular applications, a wavelength is 0.33 m. The rapidly
changing waveform in Figure 5.13 shows an example of the spatial variation of
received signal amplitude at 900 MHz in an urban setting. Note that changes of
amplitude can be as much as 20 or 30 dB over a short distance. This type of
rapidly changing fading phenomenon, known as fast fading, affects not only
mobile phones in automobiles, but even a mobile phone user walking down an
urban street.
As the mobile user covers distances well in excess of a wavelength, the urban
environment changes, as the user passes buildings of different heights, vacant lots,
intersections, and so forth. Over these longer distances, there is a change in the average
received power level about which the rapid fluctuations occur. This is indicated
by the slowly changing waveform in Figure 5.13 and is referred to as slow fading.
Fading effects can also be classified as flat or selective. Flat fading, or nonselective
fading, is that type of fading in which all frequency components of the
received signal fluctuate in the same proportions simultaneously. Selective fading
affects unequally the different spectral components of a radio signal. The term

selective fading is usually significant only relative to the bandwidth of the overall
communications channel. If attenuation occurs over a portion of the bandwidth of
the signal, the fading is considered to be selective; nonselective fading implies that
the signal bandwidth of interest is narrower than, and completely covered by, the
spectrum affected by the fading.