Digital Signal Processing Reference
In-Depth Information
broadcast fm nearly always survives serious drops in the signal quality without
noticeable interruptions. Somehow, it may not surprise the attentive reader that
boosting the transmission power a few orders of magnitude may not entirely be
the most energy efficient solution for a battery-powered communication sys-
tem. It is the goal of this section to find better ways to deal with the unstable
character of the multipath channel, without losing out too much in terms of
performance and power efficiency.
4.2
Frequency-selective versus flat fading
The power delay spread is a very important statistical parameter during the de-
sign of a wireless communication system: it provides a measure for the time
dispersion of the signal as seen at the antenna terminal of the receiver. For ev-
ery symbol that the transmitter injects into a multipath channel, the receiver
will pick up several delayed versions of that symbol, each with a randomly
distributed attenuation and phase rotation. If the duration of a symbol (T
s
)be-
comes shorter than 10
τ
rms
, then delayed versions of one transmitted symbol
start to interfere with adjacent (future) symbols. In the time domain this phe-
nomenon is referred to as the well-known intersymbol interference (isi) prob-
lem. From the equivalent frequency domain point of view, this type of wireless
channel is characterized as
frequency-selective
.
On the other hand, when the duration of a single symbol becomes longer than
a few times the delay spread, the consecutive symbols will not corrupt each
other, but the interference now takes place within the duration of a single sym-
bol (Figure 4.2). From a frequency-domain point of view, the difference in
the length of the propagation paths causes a random phase rotation. As a con-
sequence, individual signal components that arrive at the receiving antenna
within one symbol period can interfere in a constructive or a destructive way
with each other. Within the frequency band of a narrowband receiver, this ef-
fect is experienced as
flat fading
: all frequencies in the whole frequency range
of the wireless link experience the same path loss. Destructive interference is
a major issue under flat fading circumstances, because the received signal can
completely disappear at any moment, no matter how much rf signal power is
being supplied from transmission side.
Apart from the example of broadcast fm, another nice illustration of a
frequency-flat fading channel is provided by the periodic fading of short-
wave radio signals. Due to the partial reflection of radio signals at different
heights in the ionospheric layers of the earth, several reflected versions of the
same signal arrive at the receiving antenna with a different delay. The very
typical periodic fading of shortwave radio stations is caused by the vertical
movements of the ionospheric layers, which results in an alternating sequence
of periods with constructive and destructive interference.
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