# Solved Sample Paper of DIGITAL COMMUNICATION TECHNIQUE

DIGITAL COMMUNICATION TECHNIQUE

1.(a)what is communication ?

Ans-The way in which data transferring information from one place to another another place is known as communication.

(b).What is sampling ?

Ans- The process by which a signal is divided into number of samples which carries the characteristics of the original signal is called sampling.

(c).what is white noise ? where is it encountered ?

AnsWhite noise:

*White noise is noise whose power spectral density is uniform over the entire frequency range of interest.

*It occurs due to the superposition of all visible light spectral components.

*Gn (f)=ή/2

Where ή= constant.

*It is encountered in the communication channel.

*This noise is always additive in nature. That why the channel is called “AWGN channel” “Additive white Guassian Noise“.

(d)Difine “code efficiency”?

Ans-code efficiency , n = Lmin/L

As per source coding theorem :

L ≥ H (x)

Lmin = H (x)

N = H(x)/L

(e). Define “code redundancy” ?

Ans- code redundancy = 1 – n = 1 – code efficiency.

Y = 1 – n.

(f). What is scrambling ?

Ans-*It is a process by which data is randomized.

*in general , a scrambler  tends to make the data more random by removing long strings of 1’s or 0’s. Scrambling can be helpful  in timing extraction by removing long strings of 0’s in binary data.The digital network must be able to cope with these long zero strings using zero suppression techniques.

(g). What is QPSK ?

Ans– QPSK is known as Quadrature Phase Shift Keying.

*The channel bandwith depends upon the bit rate or signaling rate fb .In digital bandpass transmission , a carrier is used for transmission . This carrier is transmitted over a channel.

*If two or more bits are combined in some symbols, then the signaling rate will be reduced.

(h).What is Carrier Recovery ?

Ans– *Carrer recovery is the process  of extracting a phase coherent reference carrier from a received signal. So it is also called as phase referencing.

*There are two loop in carrier recovery.

i. Squaring loop.

ii. Costas loop.

(i). Difference between BPSK and DPSK.

Ans-

BPSK

i. Low error of probability.

ii. complexity is lower.

iii. Low interface of Noise.

iv. Synchronous carrier is needed.

v. Bit determination at the receiver based on signal bit interval.

DPSK

i. error of probability higher than BPSK.

ii.Complexity higher than BPSK.

iii. Interference of Noise is high.

iv. Synchronous carrier is not needed.

v. Bit determination at the receiver based on signal received in two successive bit intervals.

(j). What is need of signaling in a PCM system?

Ans– i. Increase in transmission bandwith.

ii. Increase the power efficency.

iii. Better Transparency.

iv. Increase the erorr detection capability.

2.What is Shannon’s Theorem? What is the mean of Capacity of a Gaussian Channel?

*It is the most fundamental theorem of communication, is concerned with the rate of transmission of information over a communication channel.

*It based on the principles that a communication system will transmit information with an arbitary small probability of erorr provided that the information rate R is less than or equal to a rate c called the “channel capacity” ; this technical approach is called “coding” .

*Defination: This states that , A source M equally likely messages, with M >>1, which is generating information at a rate R .Given by a channel capacity C, IFR<=C, then there exist a coding techniques such that the output of the source may be transmitted over the channel with a probability of error in the received message which may be made arbitrarily small.’

capacity of Gaussian channel

A theorem which is complementary to shannon’s theorem and applies to a channel in which the noise  is gaussian is known as the “shannon’s – Hartly theorem”.

Theorem: The channel capacity of a white , band limited Gaussian is

C = Blog2(1+S/N)bits / sec

B = band width of the channel

where

S=signal power

N= Total noise within the channel bandwith =ήB

ή/2=double sided power spectral density.

*firstly , we find that channels are encountered in physical systems generally,are at least approximately,Gaussian.

* second , it turns out that the results obtained for a Gaussian often provide a lower bound on the performance of a system operating over a non Gaussian channel.

3.What is pulse Width Modulation(PWM) and describe it ?

Defination:

PWM (pulse width modulation) is a modulation system in which the width of the pulses is varied according to modulating signal where as Amplitude and position of pulses remains unchanged.

*The other type of a analog modulation is the pulse width modulation (pwm).

*In PWM , the width of the modulated pulses varies in proportion with the amplitude of modulating signal .

*As seen from the waveforms , the amplitude and the frequency of the PWM wave  remains constant.

*Only the width changes .That is why the “information” is contained in the width variation.

*This is similar to FM.As the noise is normally “additive”noise,it changes the amplitude of the PWM signal.

*At the receiver , it is possible  to remove these unwanted amplitude variations very easily by the means of a limiter circuit.

Generation of PWM: * As the information is contained width variation .It is unaffected by the amplitude variations introduced by the noise.Thus the PWM system is more immune to noise than the PAM signal.

* A sawtooth generates a  sawtooth signal of frequency fs, therefore the sawtooth signal in this case is a sampling signal .It is applied to the inverting terminal of a comparator.

* The modulating signal x(t) is applied to the non inverting terminal of the same comparator .

*The comparator output will remain high as long as the instantaneous amplitude of x(t) is higher than that of the ramp signal.

* Note that the leading edges of the PWM wave from coincide with the falling edges of the ramp signal.

Detection Of PWM Signal:

*The pwm signal received at the input of the  detection circuit is contaminated with noise.This signal is applied to pulse generator circuit which regenerates the pwm signal.Thus some of the noise is recovered and the pulse are squared up.

*The generated pulse are applied to a reference pulse generator .It produces a train of constant amplitude , constant width pulses. These pulses are synchronized to the leading edges of the regenerated PWM pulses but delayed by a fixed interval.

i. Less effect of noise.

ii. Very good noise immunity.

iii. Synchronization between the transmitter and receiver is not essential.

I. Due to the variable pulse width, the pulses have variable power contents. So the transmission must be powerful enough to handle the maximum width, pulse, though the average power transmitted can be as low as 50% of this maximum power.

4. Write short notes:

(a). Multiplexer:

* Multiplexing may be defined as , ” a technique which allows many users to share a common communication channel simultaneously.”

i. There are two major types of multiplexing techniques ,they are

a. Time Division Multiplexing (TDM)

B. Frequency Division Multiplexing (FDM)

ii. The digital  multiplexing of digital  signals can be accomplished by using a bit by bit interleaving procedure with a selector switch that sequentially takes a bit from each incoming line and applies it to the high speed common the output of this common line separated out into it ‘s low speed individual components and then delivered to their respective destination.

iii. Digital multiplexer is of 2 types.

a. Low speed multiplexer

b. High speed multiplexer

iv. Low speed multiplexer is designed to combine relatively low speed digital signals upto a maximum rate of 4800bit/ sec into a high speed  multiplexed signal at a rate of 9600bit/ sec.

*It is used in voice grade channel and modern.

(b) Regenerative Repeater:

*It is used at regularly spaced interval along a digital transmission line to detect the incoming digital signal and regenerate new clean pulses for further transmission.

* Normally it performs three major functions

a. Reshaping incoming pulses by using equalizer.

b. The extraction of information required to sample incoming pulse at optimum level.

c.The last part is decision making which is based on the pulses samples.

Operation:

i. The input pulses is attenuated and distorted by the transmitting system.So this distortion results dispersion which is caused due to high frequency components are attenuated.

ii. To restrict it equalizer is used which frequency characteristics is the inverse of transmitting medium.Actually equalizer restore higher frequency components and eliminates the dispersion.

5. short notes

(a). Linear equalizer:

* Channel equalizer approaches to compensate for the ISI means to minimize the ISI.

* It is used to regain the pulse shape by filtering the signal by a filter, the characteristics which is close approximationof inverse of the channel.

*This is usually done through transverse filtering which uses weighted tap delay lines.The weights on the tap are adjusted in a such manner that the pulse shape is best and isi is minimized.

* The filter structure has a computation complexity which is a linear function of the channel dispersion length L.

* If the received pulse side lobes dont go through zero at all sample time.

* since we are interested in sampling the equalized waveform at any predefined sample times,so we need a equalizing circuit called transversal filter a each sample point of the main lobe

* the main contribution is from the central tap with “Co” coefficient and other taps contributing echoes of main signal at symbol intervals on either side of main signal.

(b). Timing Extraction:

* The received signal need to be sampled at precise instants, which requires a clock signal at the receiver in synchronisim with the clock signal at the transmitter. This process is known as bit or symbol synchronization ,for this purpose different methods are followed.

* Master slave concept is suitable for large number of data and high speed communication systems.

* Transmitting a separate synchronizing signal i.e , pilot clock.

* Self synchronization is very efficient method because the timing is derived from the digital signal it self.

6(a). Timing Jitter:

* Small random deviations of the incoming pulses from their ideal solution known as timing jitters which are always present even in the sophisticated system.

* Variations of pulse position or sampling instant cause timing jitters.

* This results from the several reasons as some of which are dependent on the pulse pattern being transmitted where as others are not .

* The random forms of jitters are caused by noise, interference and mistuning   of the clock circuit  for timing extraction must be large enough to provide an adequate suppression of timing jitters.

* The pattern  jitters results from clock mistuning , amplitude to phase conversion in the clock circuit and ISI which alters the position of the picks of the input signal according to pattern .

* The rms value of jitter over a long chain of ‘N’ repeaters is proportional to N.

* Jitter accumulation over a digital link can be reduced by buffering the link with an elastic store and clocking out the digit stream under the control of highly stable PLL.

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