Digital Signal Processing Reference
In-Depth Information
signal magnitude [dBr]
noise boosting
due to equalization
frequency-selective
channel response
average
signal power
after equalization
noise floor
lost information
frequency
= dead frequency band
Figure 2.8.
Channel equalization prevents isi, but causes excessive noise when
the channel response has spectral nulls (deep fades). Information in
deep fades is submerged by the background noise of the channel.
domain equalization filter,
13
equalization may result in excessive noise boost-
ing (Figure 2.8). This problem is inherent to equalization filters and is caused
by deep fades in the frequency domain. After all, information which is lost
during transmission cannot be recovered and is gone forever. From the point
of error probability it can more advantageous to allow a limited amount of isi
and optimize the equalization filter for the smallest overall mean square error
(mse) ([Cou97] p. 184).
This seemingly innocent observation leads us to another, more problematic
weakness of wideband single-carrier modulation: equalization offers no pro-
tection whatsoever against in-band interference. Even a single narrowband in-
terferer occupying a small fraction of the total spectral footprint can (and will)
bring down the performance of a whole wideband single-carrier system. Re-
moving the interferer with a notch filter causes isi and trying to remove the
resulting isi with a channel equalization filter introduces, again, a lot of noise.
Direct sequence spread spectrum
Despite all the drawbacks, there are wideband communication systems out
there that stubbornly use a single-carrier modulation method. It is instructive to
see how these systems cope with the difficulties of multipath and narrowband
interference. Maybe the best known example from this category is the Global
Positioning System (gps). The encoding method used by the gps system is
direct sequence spread spectrum
(dsss).
13
Equalization requires a cyclic prefix in order to deal with the causality of realizable filters.
