Digital Signal Processing Reference
In-Depth Information
intersymbol interference will seriously cripple the performance of the link if
an unsuitable modulation method or a totally wrong coding strategy are thrown
into the game. However, if an efficient way can be found to disentangle the
different delayed signal streams at the receiving antenna, the problem which
causes isi can be turned into a surprising strength of the frequency-selective
multipath channel.
Definition - coherence bandwidth of a channel
The coherence bandwidth (B
c
) is a measurement of the average fre-
quency span over which the frequency response H
(f )
of a channel can
be considered as approximately 'flat' and it is not just a coincidence
that it is defined using the rms delay spread of the channel (4.4):
1
2
πτ
rms
B
c
=
[units: Hz]
(4.4)
Note: the interpretation of coherence bandwidth given here is only
very basic. For a more rigorous treatment, see [Jak94] and [Bar05].
An easily overlooked aspect of frequency-selective fading channels is that, in
order to effectively be able to make advantage of their
frequency diversity
,the
total bandwidth covered by all subbands together must be sufficiently larger
than the coherence bandwidth (see inset 4.4) of the channel. If this is not
the case, the wireless link can be easily corrupted since the entire frequency-
selective channel can suddenly disappear in a deep fade. The problem here is
not caused by isi, but by destructive interference caused by the multipath com-
ponents that arrive within the interval of a single symbol period (Figure 4.3).
This form of destructive interference within a single symbol interval, called
intra-symbol interference
, causes frequency-flat fading and comes on top of
the frequency-selectivity of the channel. In other words, the received signal
power will not only change over frequency, but the total combined power over
all subbands will also suffer from fading over time, which results in intermit-
tent link outages. A transmission with a sufficiently wide spectral footprint
with respect to the coherence bandwidth of the channel has less chance that all
frequency bands fade away at the same moment (Figure 4.4). It follows that isi
is indeed not only an annoying side-effect, but also a necessary precondition
for a frequency diverse - and thus reliable - wireless link.
5
5
The underlying problem described here is in fact related to the
multipath resolvability
of the system, and
will be discussed in more detail in Section 4.4.
