EC 6501 – DIGITAL COMMUNICATION

UNIT - III

PART – A

1. Define correlative level coding. [N/D-16]

Correlative level coding is used to transmit a baseband signal with the signaling rate of

2Bo over the channel of bandwidth Bo. This is made physically possible by allowing

ISI in the transmitted in controlled manner. This ISI is known to receiver. The

correlative coding is implemented by duobinary signaling and modified duobinary

signaling.

2. For the binary data 101001110 draw the unipolar and RZ signal. [N/D-16]

3.3. What are line codes? Name some popular line codes. [M/J-16]

A line code is a code chosen for use within a communications system for transmitting a

digital signal down a line. Line coding is often used for digital data transport. Some line

codes are digital baseband modulation or digital baseband transmission methods, and

these are baseband line codes that are used when the line can carry DC components.

NRZ–L, NRZ–M, RZ, Bipolar,Unipolar

4. What is ISI and what are the causes of ISI? [M/J-16]

The presence of outputs due to other bits interference with the output of required bit.

This effect is called inter symbol interference (ISI).

5. Write the performance of data transmission system using eye pattern technique.

The width of the eye opening defines the interval over which the received wave can be

sampled without error from inter symbol interference. The sensitivity of the system to

timing error is determined by the rate of closure of the eye as the sampling time is

varied

6. Define duo binary encoding.

Duo binary encoding reduces the maximum frequency of the base band signal the “word duo”

means to the double transmission capacity of the binary system.

7. What is the necessity of equalization?

When the signal is passed through the channel distortion is introduced in terms of

1) Amplitude

2) Delay this distortion creates problem of ISI. The detection of the signal also becomes

difficult this distraction can be compensated with the help of equalizer.

8. Define the principle of adaptive equalization. [N/D 08]

The filters adapt themselves to the dispersive effects of the channel that is .the coefficients of the

filters are changed continuously according to the received data. The filter coefficients are changed

in such a way that the distortion in the data is reduced,

9. How is eye pattern obtained on the CRO?

Eye pattern can be obtained on CRO by applying the signal to one of the input channels and

given an external trigger of 1/Tb Hz. This makes one sweep of beam equal to Tbseconds.

10. Define Duobinary baseband PAM systemK

Duobinary encoding reduces the maximum frequency of the baseband signal. The word „duo‟

means to double the transmission capacity of the binary system.

Let the PAM signal ak represents kth bit. Then the encoder the new waveform as

Ck =ak + ak-1

Thus two successive bits are added to get encoded value of the kth bit. Hence Ck becomes a

correlated signal even though ak is not correlated. This introduces intersymbol interference in the

controlled manner to reduce the bandwidth.

11. How does pulse shaping reduce inter symbol interference? [N/D10]

The shape of the pulse is selected such that the instant of detection, the interference due

to all other symbol is zero. The effect of ISI totally eliminates if the signal is sampled at

Tb,2Tb,3Tb… and so on.

12. Give two applications for Eye pattern. [A/M 11]. [N/D 2012]

To determine an interval over which the received wave van be sampled without error due ot ISI.

To determine the sensitivity of the system to timing error The margin over the noise is determined

from eye pattern.

13. How is the best time for sampling determined from eye pattern? [N/D – 07]

The width of eye opening from the eye pattern defines the best sampling time interval for sampling.

14. State Nyquist criterion for zero ISI. [N/D – 11]

Zero ISI can be obtained if the transmitted pulse satisfies the following condition

PART – B

1. Explain how Nyquist’s Criterion eliminates interference in the absence of noise for distortion

less baseband binary transmission. (16) [N/D-16]

2. Describe how eye pattern is helpful to obtain the performance of the System in

detail with a neat sketch. (16) [N/D-16]

3. Sketch the power spectra of (a|) polar NRZ and (b) bipolar RZ signals.(8) [M/J-16]

4. Draw the block diagram of duo binary signaling scheme without and with precoder

and explain. (9) [M/J-16]

5. Explain the various properties of line coding techniques. (8)

6.For the binary sequence 101011100 draw the digital format waveforms

corresponding to 1) Unipolar NRZ 2)Polr NRZ 3)Bipolar NRZ 4)Manchester coding

5) Polor RZ 6) Bipolar RZ 7) Uni polar RZ