B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

Figure 7.21
Regenerative
repeater.
7.5 Digital Receivers and Regenerative Repeaters 359
Sampler
and
decision
no □
..------ LI
Regenerator
Transmission
medium
Preamplifier
and
equalizer
Noise
Timing
extraction
is normally accomplished by transformer-coupling the signals and bypassing the de around
the transformers to the power supply circuitry.*
7.5.1 Equalizers
A pulse train is attenuated and distorted by the transmission medium. The attenuation can
be compensated by the preamplifier, whereas the distortion is compensated by the equalizer.
Channel distortion is in the form of dispersion, which is caused by an attenuation of certain
critical frequency components of the data pulse train. Theoretically, an equalizer should have a
frequency characteristic that is the inverse of that of the transmission medium. This will restore
the critical frequency components and eliminate pulse dispersion. Unfortunately, this also
enhances the received channel noise by boosting its components at these critical frequencies.
This undesirable phenomenon is known as noise amplification.
For digital signals, however, complete equalization is really not necessary, because a
detector only needs to make relatively simple decisions-such as whether the pulse is positive
or negative ( or whether the pulse is present or absent). Therefore, considerable pulse dispersion
can be tolerated. Pulse dispersion results in ISi and the consequent increase in error detection.
Noise increase resulting from the equalizer (which boosts the high frequencies) also increases
the detection error probability. For this reason, design of an optimum equalizer involves an
inevitable compromise between reducing ISi and reducing the channel noise. A judicious choice
of the equalization characteristics is a central feature in all well-designed digital communication
systems. 6
Zero-Forcing Equalizer
It is really not necessary to eliminate or minimize ISi (interference) with neighboring pulses
for all t. All that is needed is to eliminate or minimize interference with neighboring pulses
at their respective sampling instants only. This is because the receiver decision is based on
sample values only. This kind of (relaxed) equalization can be accomplished by equalizers
using the transversal filter structure encountered earlier. Unlike traditional filters, transversal
* The repeater usually includes circuitry to protect the 'electronics of the regenerator from high-voltage transients
induced by power surges and lightning. Special transformer windings may be provided to couple fault-locate signals
into a cable pair dedicated to the purpose.