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What is Intersymbol Interference?

Mumbai University >IT>Sem3>Principles of Analog and Digital Communication

Marks: 5M

Year: May 2015

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• In a communication system when the data is being transmitted in the form of pulses (bits), the output produced at the receiver due to the other bits or symbol interferes with the output produced by the desired bit. This is called as intersymbol interference (ISI).

• The intersymbol interference will introduce errors in the detected signal.

• Consider the fig which shows the elements of a baseband binary PAM system. The input signal consists of a binary data sequence {${b_k}$} with a bit duration of $T_b$ .

• This sequence is applied to a pulse generator to produce a discrete PAM signal given by:

• Where v(t) denotes the basic pulse, normalized such that v (0) =1.The first block of the system i.e. pulse amplitude modulator converts this input sequence into polar form as follows:

If $b_k$ = 1 then $a_k$ =1

And

$b_k$ = 0 then $a_k$ =-1

• The PAM signal x (t) is then passed through transmitting filter. The output of the transmitting filter is then transmitted over transmission channel. The impulse response of the channel is h (t).

• A random noise is then added to the transmitted signal when it travels over the transmission channel. Thus the signal received at the receiving end is contaminated with noise.

• The channel output is passed through a receiving filter. This filter output is sampled synchronously with the transmitter. The sampling instants are determined by a clock or timing signal which is extracted from the receiving filter output.