EC 6501 DIGITAL COMMUNICATION

Transcription

1 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 draw the unipolar and RZ signal. [N/D-16].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

2 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 t filters are changed continuously according to the received data. The filter coefficients are chan 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 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 I To determine the sensitivity of the system to timing error The margin over the noise is determ 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.

3 14. State Nyquist criterion for zero ISI. [N/D 11] Zero ISI can be obtained if the transmitted pulse satisfies the following condition

4 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]

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6 2. Describe how eye pattern is helpful to obtain the performance of the System in detail with a neat sketch. (16) [N/D-16]

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

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10 4. Draw the block diagram of duo binary signaling scheme without and with precoder and explain. (9) [M/J-16]

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

12 6.For the binary sequence 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