Inter Symbol Interference (ISI)
Intersymbol interference (ISI) is a form of distortion of a signal in
which one symbol interferes with subsequent symbol.
1. Multipath propogation.
2. Bandlimited channels
Baseband Binary Data Transmission
Nyquist’s Criterion for zero ISI
The pulse shaping function p(t) with a fourier transform P(f) is given by
p(iTb-kTb)=1 for i=k
P(iTb-kTb)=0 for i≠k this condition is knows as Nyquist criterion for zero ISI.
The frequency response of an ideal low pass filter decreases towards zero
abruptly which is practically unrealizable.
To overcome this problem raised cosine-roll off Nyquist filter is used.
The amplitude response is as shown:
Specifically, α governs the bandwidth occupied by the pulse and the rate at
which the tails of the pulse decay.
A value of α = 0 offers the narrowest bandwidth, but the slowest rate of
decay in the time domain.
When α = 1, the transmission bandwidth is 2B0, which is twice that of ideal
Conversely, inverse when α = 0, the bandwidth is reduced to B0, implying a
factor-of-two increase in data rate for the same bandwidth occupied by a
rectangular pulse. But there is a slow rate of decay in the tails of the pulse.
Thus, the parameter α gives the system designer a trade-off between
increased data rate and the tail suppression.
Transmission Bandwidth Requirement
From the earlier figure we know that the transmission bandwidth is:
B = 2B0 - f1 ………………(1)
where B0 =1 / 2Tb
a = 1- (f1/B0)
» f1= B0(1-a)
Therefore (1) becomes,
B= 2B0- B0(1-a)
» B= B0(1+a)
B = 2B0
Information given by Eye Diagram
The width of the eye opening defines the time interval over which
the received wave can be sampled without error from ISI. It is
apparent that the preferred time for sampling is the instant of time
at which the eye is open widest.
The sensitivity of the system to timing error is determined by the rate
of closure of the eye as the sampling time is varied.
The height of the eye opening, at a specified sampling time, defines
the margin over noise.