Test: Pulse Shaping and Intersymbol Interference - Electronics and Communication Engineering (ECE) MCQ

Pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to its purpose or the communication channel, typically by limiting the effective bandwidth of the transmission.

The transmission bandwidth for Nyquist criteria (raised cosine pulse) with roll-off factor α is given by:

Inter-symbol Interference:

• This is the effect of dispersion on optical fibre.
• Intersymbol interference occurs when the pulse spreading caused by dispersion causes the output pulses of a system to overlap.

Scattering:

• Scattering is the loss of signal caused by the diffusion of a light beam, where the diffusion itself is caused by microscopic variations in the transmission medium.
• Scattering typically happens when a light signal hits an impurity in the fibre.
  

Dispersion:

• Dispersion is defined as pulse spreading in an optical fibre
• As a pulse of light propagates through a fibre, elements such as numerical aperture, core diameter, refractive index profile, wavelength, and laser linewidth cause the pulse to broaden
  

Attenuation:

• The attenuation of an optical fibre measures the amount of light lost between input and output
• Total attenuation is the sum of all losses
• For a given fibre, these losses are wavelength-dependent

Concept: By using raised cosine pulse bandwidth required for M-ary modulation is   where α is called roll-off factor.
Application: Bandwidth = 2000 - 500 = 1500 Hz 
Bandwidth required for 16 QAM

Rb(1+α ) = 1500 x 4 = 6000
(1+α) = 6000/4800
α = 0.25

Baseband communication is given.
Minimum Bandwidth will be required if transmission of data takes place using sinc pulses and the bandwidth is given by:

Where R_b = n  fs
n = number of bits used to represent samples
f_s = sampling rate.
Calculation:
In the Question, it is asked about minimum Bandwidth. ∴ we will consider that the transmission of data is taking place through sinc pulses.
Given f_s = 8 kHz
n = 8 bits/sample
M = 4
R_b = nf_s

For ISI free transmission according to Nyquist criterion for Baseband signal is:

But for Bandpass signal, the amplitude spectrum get shifted to some high frequency and B.W ≥ R_b
For M-ary signaling B.W 
Calculation:
Bit Rate R_b = R k bits/sec
32-ary signaling is used:

The bit rate of base baseband binary pulse amplitude modulation (PAM) with a raised-cosine spectrum is given as:

Where,
BW = Transmission Bandwidth of PAM signal.
α = Roll-off factor. (α < 1)
Calculation: 
Given: Bit rate = 56 kbps, α =.25
we know that,

B.W. = 28 × 1.25
B.W. = 35 kHz

During sampling:
We sample at 2W. With this rate signal of bandwidth, W gets sampled properly.
The signal of bandwidth 2W is sampled properly by being fed twice in the commutator switch.
And signal with bandwidth 3W is fed thrice into the commutator switch in order to get sampled at an effective rate of 6W. So, the total bit rate is R_b = 14W
Now, the minimum bandwidth required is: R_b/2 = 7W.