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Chapter: Digital Communication - Baseband Transmission

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Important Short Questions and Answers: Baseband Transmission

Digital Communication - Baseband Transmission - Important Short Questions and Answers: Baseband Transmission

BASEBAND TRANSMISSION

 

1.    What is meant by transparency with respect to line codes

 

The line code is said to be transparent if the synchronization between the transmitter and receiver is maintained for any type of input data sequence.

 

2. Draw the NRZ and RZ code for the digital data 10110001

 [OR]

 

Draw the RZ bipolar line code format for the information {10110}


 

3. What is Manchester code? Draw the Manchester format for the data stream 10110?

 

In Manchester code each bit of data is signified by at least one transition. Manchester encoding is therefore considered to be self-clocking, which means that accurate clock recovery from a data stream is possible. In addition, the DC component of the encoded signal is zero. Although transitions allow the signal to be self-clocking, it carries significant overhead as there is a need for essentially twice the bandwidth of a simple NRZ or NRZI encoding


 

4. State any four desirable properties of line code

·    The PAM signal should have adequate timing content,

·              The PAM signal should immune to channel noise and interference

·              The PAM signal should allow error detection and error correction

·              The PAM signal should be transparent to digital data being transmitted.

 

5.   What is intersymbol interference in baseband binary PAM systems?

 

In baseband binary PAM, symbols are transmitted one after another. These symbols are separated by sufficient time durations. The transmitter, channel and receiver acts as a filter to this baseband data. Because of the filtering characteristics, transmitted PAM pulses are spread in time.

 

6. What is correlative coding?

 

Correlative level coding is used to transmit a baseband signal with the signalling 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 .signalling .and modified duobinary signalling.

 

7. 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.

 

8. What are eye pattern?

Eye pattern is used to study the effect of ISI in baseband transmission.

 

1) Width of .eye .opening defines the .interval over .which the .received wave can be sampled without error from ISI.

 

2.) The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied.

 

3) Height of the eye opening at sampling time is called margin over noise.

 

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 Tb seconds.

 

10.      Why do you need adaptive equalization in a switched telephone network.

 

In switched telephone network the distortion depends upon 1) Transmission characteristics of individual links.

 

2) Number of links in connection.

 

Hence fixed pair of transmit and receive filters will not serve the equalization problem. The transmission characteristics keep on changing. Therefore adaptive equalization is used.

 

11.      What are the necessity of adaptive equalization?

 

Most of the channels are made up of individual links in switched telephone network,the distortion induced depends upon

 

1 ) transmission characteristics of individual links

2) number of links in connection

 

12. Define the principle of adaptive equalization?

 

The filters adapt themselves to the dispersive effects of the channel that is .the .cofficients of the filters are changed contineously according to the received data .The filter cofficients are changed in such a way that the distortion in the data is reduced.

 

13. Define the term ISI?

 

Ans. The presence of outputs due to other bits interference with the output of required bit . This effect is called inter symbol interference (ISI).

 

14. 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 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.

 

 

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

 

 

16.What is raised cosine spectrum?

 

In the raised cosine spectrum, the frequency response P(f) decreases towards zero gradually That is there is no abrupt transition.

 

17. What is nyquist Bandwidth?

 

The B.is called nyquist bandwidth. .The nyquist bandwidth is the minimum transmission bandwidth for zero ISI.

 

18.      Give two applications for Eye pattern.

 

·              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

 

19.What are the information that can be obtained from eye pattern regarding the signal quality?

 

·              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

 

20. A 64 kbps binary PCM polar NRZ signal is passed through a communication system with a raised-cosine filter with roll-off factor 0.25. Find the bandwidth of a filtered PCM signal.

 

Fb=64lpbs

B0=FB/2=32kpbs

α=0.25

B=B0(1+α)=30*103(1+0.25)=40kHz

 

 



BASEBAND TRANSMISSION

 

Line codes:

 

The line code is said to be transparent if the synchronization between the transmitter and receiver is maintained for any type of input data sequence.

 

Manchester code:

 

In Manchester code each bit of data is signified by at least one transition. Manchester encoding is therefore considered to be self-clocking, which means that accurate clock recovery from a data stream is possible. In addition, the DC component of the encoded signal is zero. Although transitions allow the signal to be self-clocking, it carries significant overhead as there is a need for essentially twice the bandwidth of a simple NRZ or NRZI encoding

 

Properties of line code:

 

The PAM signal should have adequate timing content,

The PAM signal should immune to channel noise and interference

The PAM signal should allow error detection and error correction

The PAM signal should be transparent to digital data being transmitted.

 

Intersymbol interference:

 

In baseband binary PAM, symbols are transmitted one after another. These symbols are separated by sufficient time durations. The transmitter, channel and receiver acts as a filter to this baseband data. Because of the filtering characteristics, transmitted PAM pulses are spread in time.

 

correlative coding:

 

Correlative level coding is used to transmit a baseband signal with the signalling 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 .signalling .and modified duobinary signalling.

 

Duobinary baseband PAM system

 

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.

 

Eye pattern:

 

Eye pattern is used to study the effect of ISI in baseband transmission. 1) Width of .eye .opening defines the .interval over .which the .received

wave can be sampled without error from ISI.

 

2.) The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied.

 

3) Height of the eye opening at sampling time is called margin over noise.

 

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 Tb seconds.

 

Adaptive equalization:

 

In switched telephone network the distortion depends upon

1) Transmission characteristics of individual links.

2) Number of links in connection.

 

Hence fixed pair of transmit and receive filters will not serve the equalization problem. The transmission characteristics keep on changing. Therefore adaptive equalization is used.

 

Most of the channels are made up of individual links in switched telephone network,the distortion induced depends upon 1 ) transmission characteristics of individual links.2) number of links in connection

 

Principles of adaptive equalization:

 

The filters adapt themselves to the dispersive effects of the channel that is .the .cofficients of the filters are changed contineously according to the received data .The filter cofficients are changed in such a way that the distortion in the data is reduced.

 

ISI:

 

The presence of outputs due to other bits interference with the output of required bit . This effect is called inter symbol interference (ISI).

 

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

 

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

 

 

Raised cosine spectrum:

 

In the raised cosine spectrum, the frequency response P(f) decreases towards zero gradually That is there is no abrupt transition.

 

Nyquist Bandwidth:

 

The B.is called nyquist bandwidth. .The nyquist bandwidth is the minimum transmission bandwidth for zero ISI.

 

Applications for Eye pattern.

 

·              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.

 

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