Digital Signal Processing Reference
In-Depth Information
Chapter 4
BENEFITS OF ISI
IN THE INDOOR ENVIRONMENT
In this section, the problems caused by a multipath channel are discussed in
more detail. First, a short descriptive overview will be given on the specific
categories under which a multipath environment can be classified. Along with
this, it is indicated how this is anticipated for in some of the wireless systems
(e.g. 802
.
11a/b/g) that have been discussed earlier in this topic. Towards the
end of this section, it should become clear to the reader under which particular
circumstances the extra costs of using a special architecture such as the multi-
finger rake receiver or a multi-antenna setup can be justified as a way to
increase diversity and as such also the reliability of a wireless link. First, in
the next few sections, some important concepts such as the delay spread of the
radio channel will be introduced. After this, the impact of the properties of the
wireless channel on the reliability of the link are discussed. It will be pointed
out that, in contrast to common belief, intersymbol interference is actually a
necessary precondition to improve the reliability of a wireless system that is
operated in a multipath indoor environment.
4.1
Power delay spread
The problems caused by multipath and fading - the latter one being essentially
the frequency domain representation of the equivalent multipath channel re-
sponse - have been mentioned several times before. However, a superficial use
of the word 'multipath' ignores the fact that this term covers a very broad range
of possible channel conditions. The actual properties of the wireless channel
as experienced by the user depend on several parameters such as the spectral
bandwidth being used and the stability of the channel over a certain period
of time. Probably the most important property is the time frame during which
delayed versions of the same transmitted symbol arrive at the antenna of the
