Digital Signal Processing Reference
In-Depth Information
Communication over a wideband radio channel
From the considerations of the system in Chapter 2, it has become clear that an
appropriate channel coding and modulation technique is indispensable for the
optimal functioning of a communication system. Of course, since the original
topic of this text includes the design of a broadband
wireless
radio system, it
seems quite logical to shift the center of attention a little, from a wireline to a
wideband wireless channel. While the broad outlines and the ideas made for
narrowband wireline communication systems still hold, the characteristics of
the wideband wireless channel make it everything but fit for use in a communi-
cation application. First of all, there is the problem of multipath (Chapter 4):
the transmitted radio signal has not exactly the intention to travel in a straight
line between the transmitter and the receiver. Instead, several reflections of the
transmitted symbol stream arrive at the receiver, each with a different time
delay. Depending on the ratio between the
delay spread
of the channel and
the period of the transmitted symbols, two different things may happen: If
the multipath delay spread is shorter than the duration of a symbol, the re-
ceived streams will start to interfere with each other at the symbol level. This
means that several delayed versions of the same symbol arrive at the receive
antenna at the same moment, each with a different phase rotation. The result,
frequency-flat fading
, causes intermittent channel outages during which com-
munication becomes impossible. The only feasible way to deal with such a
channel is to use an error correction scheme that spreads information over a
longer duration in time, which hopefully can bridge the entire period of a link
failure. However, this method does not bring any relief under static channel
conditions: when both transmitter and receiver are stationary devices in a sta-
ble environment, the duration of periods of destructive interference may grow
beyond the time-diversity capabilities of the error correction mechanism, with
the obvious tragic consequences.
The situation becomes completely different for a broadband communication
system. The large spectral footprint of such a radio transmission makes that
the channel has a frequency-selective instead of a frequency-flat profile. From
a system-level point-of-view this is a good thing, because the diversity of a
frequency-selective channel makes it very unlikely that all frequency bands
suffer from destructive fading at exactly the same moment. Using the appro-
priate subchannel loading techniques, it should be possible to pull off a reliable
communication link over a frequency-selective channel. Finding out the cor-
rect way to inject the information in the wireless channel is not evident, though:
from a time domain point-of-view, the frequency dependent channel response
is again caused by the delay spread in a multipath environment. This time, due
to the wideband character of high-speed communication, the delay spread of
the channel may become larger than the symbol period of the transmitter. This
