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EC 2311 COMMUNICATION ENGINEERING

DEPARTMENT OF EEE

EC2311 COMMUNICATION ENGINEERINGPART A –ANSWERS

UNIT IANALOG COMMUNICATION

1. With reference to AM define modulation index. It is defined as the ratio of maximum amplitude of the modulating voltage to the maximum amplitude of carrier voltage =Am/Ac (or) =ka Am

2. Why is modulation necessary? Ease of radiation Multiplexing To overcome hardware limitation To reduce noise and interference Frequency Assignment

3. The carrier of a 100% modulated wave is suppressed Find the percentage of power saving Power saving in DSBSC compared to AM is 66.6%

4. What is meant by heterodyning action? A device that carries out the frequency translation of a modulated wave is called a mixer . Converting AF signal to a fixed frequency called intermediate frequency is called heterodyning action IF for AM is 455KHZ. IF for FM is 10 KHZ

5. Mention the advantages of SSB transmission It reduces bandwidth requirement It eliminates high power carrier wave and hence power saving is high compared with DSBSC 9 to 12 db improvement in S/N Eliminates distortion due to selective fading Reduction of carrier interference with other station Some privacy is automatically produced Reduction in operating cost

6. Classify modulators and detectors Modulators are classified into Linear and non-linear Linear modulators:1.Transistor 2.Switching modulators 3.Balanced modulatorsNon-Linear :1.Square law 2.Product Modulator Detectors are classified into 1.Square law detector 2.Envelope detector

7. State the advantages of SHR over TRF receiver. Improved selectivity in terms of adjacent channel More uniform selectivity over the complete frequency range Improved receiver stability Uniform bandwidth because of fixed IF

8. Define selectivity and sensitivity of a radio receiver Selectivity: It is defined as the ability to select a particular frequency from different frequency Sensitivity: It is defined as the ability of receiver to give maximum output even though i/p is very low

9. Distinguish b/w SSB and VSB signals.

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SSB: As the transmission of information is concerned only one side band is necessary and if the carrier

and the other side band are suppressed at the transmitter no information is lost When only one side band is transmitted the signal is called SSB signal

SSB reduces bandwidth requirement It eliminates high power saving, saving is 83% The principal disadvantage of SSB is its Cost and Complexity Applied in police, wireless mobile etc

VSB: In VSB one sideband is passed almost completely whereas just a trace or vestige of other side band is

retained The transmitted vestige of the unwanted sideband compensates for the amount removed from the desired side band

Transmission bandwidth BT=w+fv where w-message bandwidth fv –width of vestigial sideband Power saving in VSB is75% Applied in television and high speed data transmission

10. What is vestigial side band transmission of a signal? In VSB one sideband is passed almost completely whereas just a trace or vestige of other side band is retained The transmitted vestige of the unwanted sideband compensates for the amount removed from the desired side bandApplied in television and high speed data transmission

11. A Tx supplies 8KW to the antenna when unmodulated. Determine the total power when amplitude modulate to 30%.

Pt=Pc(1+ma2 /2)

=8x103 (1+0.32/2)

12. What is the main difference b/w Linear modulation and non-linear modulation When a modulated wave is a linear function of a modulating wave it is called as linear modulation When a modulated wave is a non linear function of a modulating wave it is called as a non linear modulation

13. What is quadrature null effect ? Demodulated wave of DSBSC wave Vo(t)=1/2 AC AC’ cosm(t) Demodulated signal Vo(t) is proportional to m(t) when the phase error is constant .The amplitude of demodulated signal is maximum when =0 and it is minimum when =/2.The zero demodulated signal when occurs for =/2.represents the quadrature null effect of coherent detector.

14. Define fidelity ?The ability of the receiver to reproduce all the range of modulating frequencies quality is called fidelity the receiver.

15. What is the main difference b/w frequency modulation and phase modulation Frequency modulation :It is the form of angle modulation in which instantaneous frequency fI(t) is varied linearly with the base band signal m(t) Where,fI (t)=fc+kf m(t) fc unmodulated carrier kf –Frequency sensitivity of the modulator m(t)-Base band signalintegrating above equation w. r .t time and multiplying with 2 i(t)= 2fc t+2Kf m(t) dt s(t)=Ac cos i (t)

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s(t)= Ac cos(2fc t+2Kf m(t) dt) Phase modulation : It is that form of Angle modulation in which angle i(t) is varied linearly with the base band signal m(t) as as shown by i(t)= 2fc t+Kpm(t) s(t)=Ac cos i (t) s(t)= Ac cos(2fc t+Kpm(t)

16. Define modulation index for AM and FM =Am/Ac---------------------------AM =f/fM

Ratio of frequency deviation and modulating signal frequency

17. What are the constituents of a super heterodyne receiver? RF amplifier, Mixer, Local oscillator, IF amplifier, Detector. AF amplifier, Loud Speaker

18. With reference to FM define modulation index and frequency deviation modulation index =f/fM

Frequency deviation :It represent the maximum departure of instantaneous frequency of FM wave from the carrier frequency fc .Frequency deviation is proportional to the amplitude of modulating wave and is independent of the modulation frequency f=Kf Am

19. Determine the modulation depth of FM system with a maximum frequency deviation of 75 KHz and the maximum modulating frequency of 10 KHz

=f /fM

=75 x103 /10 x103

=7.5

20. Distinguish between narrow band FM and Wideband FM NARROW BAND WIDE BAND

1.Modulation index is small 1.Modulation index is large

2.Bt=2w 2.Bt =2f

3.FM wave composed of carrier and single pair of side frequencies

large FM wave contain carrier and infinite number of side frequencies

21. Write down the expression for FM signal with sinusoidal modulation Frequency modulation :It is the form of angle modulation in which instantaneous frequency fI(t) is varied linearly with the base band signal m(t) Where,fI (t)=fc+kf m(t) fc unmodulated carrier kf –Frequency sensitivity of the modulator m(t)-Base band signalintegrating above equation w. r .t time and multiplying with 2 i(t)= 2fc t+2Kf m(t) dt s(t)=Ac cos i (t) s(t)= Ac cos(2fc t+2Kf m(t) dt)

22. What is multi tone modulation

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Modulating wave m(t) in practice usually consists of group of sine waves of different frequencies which may be completely unrelated or harmonically unrelated. Modulating such i/p signal is called multi tone modulation.

23. Mention the nature of FM detector FM detector produces the o/p voltage whose instantaneous amplitude is directly proportional to the instantaneous frequency of i/p FM wave i.e it recovers the original modulating wave from the i/p FM wave

24. What are the main difference b/w AM Rx and FM RxThe basic difference is AM demodulator replaced by FM demodulator such as limiter and discriminator.

Typical freq parameters of commercial AM value RF carrier range 0.535-1.605 MHz Mid band freq of IF station =.455 MHz IF band width =10 KHz Typical freq parameters of commercial FM value RF carrier range 88-108 MHz Mid band freq of IF station =.10.7 MHz IF band width =.2 MHz In an FM system the message information is transmitted by variation of the instantaneous frequency

of sinusoidal carrier wave and its amplitude is maintained constant .Therefore any variation of the carrier amplitude at the R x i/p must result from the noise or interference The amplitude limiter before discriminator remove amplitude variation.

25. Distinguish b/w direct method and indirect method of producing FM wave IN indirect method the instantaneous frequency of the carrier wave is varied directly in accordance with the base band signal by means of a device known as vco e.g Hartley oscillatorDisadvantage is that carrier frequency is not obtained from a highly stable oscillatorIn indirect method the base band signal m(t) is first integrated used to phase modulate a crystal controlled oscillator .Hence stable carrier frequency is obtained

26. Why is FM system preferred over AM system. PM system can provide better discrimination against noise and interference than AM

27. What is pre-emphasis and de-emphasis A more satisfactory approach to the efficient utilization of the allowed freq band is based on the use of pre emphasis in the transmitter and de-emphasis in the receiverPre emphasis is boosting or emphasize the high freq components of the message signal prior to modulation in transmitterDe-emphasis is compensation in the receiver section

28. What is the need for AGC in a receiver AGC is used to control the gain of RF and IF stages in the receiver to keep the o/p of the detector almost constant despite changes in the signal to the tuner

29. What is the need for amplitude limiter in FM receiver? Amplitude limiter prevents the variation in amplitude due to noise or interference since the modulation depends only on the frequency of carrier with respect to modulating signal

30. Define FM threshold effect. It is defined as the minimum carrier to noise ratio yielding an FM improvement which is not significantly deteriorated from the value predicted by the usual signal to noise formula assuming small noise.

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31. What is single tone modulation Frequency modulation with single modulating frequency fm and carrier frequency fc is called single tone modulation m(t)=Am cos 2fm t fI(t)=fc +kf AM cos 2fm t

32. Define noise figure of a radio receiver.It is defined as ratio of available SNR power supplied to the input terminal of a receiver or amplifier to the available SNR supplied to the output or load resistor

F=Available SNR Power at input/ Available SNR Power at output

33. Define SNR of a radio receiver It is defined as the ratio of signal power to noise power at the input of a receiver is called (SNR)c &the ratio of signal power to noise power at the output of a receiver is called (SNR)o

34. Define Figure of merit for receiver It is defined as the ratio of output signal to noise ratio to channel signal to noise ratio F=(SNR)o/(SNR)c

35. Compare DSB and SSB in terms of SNR. Comparing figure of merit for DSB & SSB we conclude that for the same average transmitted signal power and same average noise power in the message bandwidth an SSB receiver will have exactly the same output signal to noise ratio as DSBSC receiver when both receivers use coherent detection for the recovery of message signal

36. What are the characteristics of FM detectors?1. Better linearity and efficiency; 2. Insensitive to amplitude changes 3.Flexible frequency adjustment

37. Name few FM detectors. 1. Balanced slope detector; 2.Foster-Seely Discriminator; 3.Ratio Detector

38. List the disadvantages of Balanced Slope Detector.No amplitude limiting; triple tuning is difficult; Linearity is not sufficient

39. What is the main difference between natural sampling and instantaneous sampling Instantaneous sampling: Consider an arbitrary signal g(t) of finite energy which is specified for all time as shown in fig a .Suppose that we sample the signal g(t) instantaneously and at uniform rate once every T seconds .As a result of this sampling process we obtain an infinite sequence of numbers spaced Ts seconds apart and denoted by g(nTs) where n takes all a possible integer values .Ts is the sampling period.Natural sampling: This is applied in PDM(pulse duration modulation ).Samples of message signal are used to vary the duration of individual pulses .

40. What is the purpose of Pre-emphasis and De-emphasis network?Pre-emphasis

It is used to increase the frequency deviation which in turn increases the noise immunity.De-emphasis

It is used to decrease the amplitude of noise side-band’s and thereby the effect of noise is reduced.

41. Define amplitude Modulation.Amplitude Modulation is the process of changing the amplitude of arelatively high frequency carrier signal in proportion with the instantaneous value

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of the modulating signal.

42. Define Modulation index and percent modulation for an AM wave.Modulation index is a term used to describe the amount of amplitudechange present in an AM waveform .It is also called as coefficient of modulation.Mathematically modulation index ism = Em/EcWhere m = Modulation coefficientEm = Peak change in the amplitude of the output waveform voltage.Ec = Peak amplitude of the unmodulated carrier voltage.Percent modulation gives the percentage change in the amplitude of the outputwave when the carrier is acted on by a modulating signal.

43. Define Low level Modulation.In low level modulation, modulation takes place prior to the outputelement of the final stage of the transmitter. For low level AM modulator class Aamplifier is used.

44. Define High level Modulation.In high level modulators, the modulation takes place in the final elementof the final stage where the carrier signal is at its maximum amplitude. For highlevel modulator class C amplifier is used.

45. What is the advantage of low level modulation?An advantage of low level modulation is that less modulating signalpower is required to achieve a high percentage of modulation.

46. Distinguish between low level and high level modulation.In low level modulation, modulation takes place prior to the outputelement of the final stage of the transmitter.It requires less power to achieve ahigh percentage of modulation.In high level modulators, the modulation takes place in the final elementof the final stage where the carrier signal is at its maximum amplitude and thus,requires a much higher amplitude modulating signal to achieve a reasonablepercent modulation.

47. Define image frequency.An image frequency is any frequency other than the selected radiofrequency carrier that ,if allowed to enter a receiver and mix with the localoscillator ,will produce a cross product frequency that is equal to the intermediatefrequency.

48. Define Local Oscillator tracking.Tracking is the ability of the local oscillator in a receiver to oscillate eitherabove or below the selected radio frequency carrier by an amount equal to theintermediate frequency throughout the entire radio frequency band.

49. Define High side injection tracking.In high side injection tracking , the local oscillator should track above theincoming RF carrier by a fixed frequency equal to fRF +fIF .

50. Define Low side injection tracking.In low side injection tracking ,the local oscillator should track below the

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RF carrier by a fixed frequency equal to fRF -fIF .

51. Define tracking error. How it is reduced.The difference between the actual local oscillator frequency and thedesired frequency is called tracking error.It is reduced by a technique called threepoint tracking.

52. Define image frequency rejection ratio.The image frequency rejection ratio is the measure of the ability ofpreselector to reject the image frequency.Mathematically ,IFRR isIFRR =(1+Q22)1/2Where = (fim/fRF)-(fRF/fim)

53. Define Heterodyning.Heterodyne means to mix two frequencies together in a nonlinear deviceor to translate one frequency to another using nonlinear mixing.

54. What are the disadvantages of conventional (or) double side band full carrier system?In conventional AM ,carrier power constitutes two thirds or more of thetotal transmitted power. This is a major drawback because the carrier contains noinformation ;the sidebands contain the information .Second ,conventional AM systems utilize twice as much bandwidth as neededwith single sideband systems.

55. Define Single sideband suppressed carrier AM.AM Single sideband suppressed carrier is a form of amplitude modulationin which the carrier is totally suppressed and one of the sidebands removed

56. Define AM Vestigial sideband.AM vestigial sideband is a form of amplitude modulation in which thecarrier and one complete sideband are transmitted, but only part of the secondsideband is transmitted.

What are the advantages of single sideband transmission?The advantages of SSBSC arePower conservation: Normally ,with single side band transmission ,only onesideband is transmitted and the carrier is suppressed. So less power is required toproduce essentially the same quality signal.Bandwidth conservation: Single sideband transmission requires half as muchbandwidth as conventional AM double side band transmission.Noise reduction: Because a single side band system utilizes half as muchbandwidth as conventional AM,the thermal noise power is reduced to half that ofa double side band system.

57. What are the disadvantages of single side band transmission? Complex receivers: Single side band systems require more complex and

expensive receivers than conventional AM transmission . Tuning Difficulties: Single side band receivers require more complex and

precise tuning than conventional AM receivers.

58. Define direct frequency modulation.In direct frequency modulation , frequency of a constant amplitude carrier

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signal is directly proportional to the amplitude of the modulating signal at a rateequal to the frequency of the modulating signal.

Define indirect frequency Modulation.In indirect frequency modulation phase of a constant amplitude carrier isdirectly proportional to the amplitude of the modulating signal at a rate equal tothe frequency of the modulating signal.

59. Define instantaneous frequency deviation.The instantaneous frequency deviation is the instantaneous change in thefrequency of the carrier and is defined as the first derivative of the instantaneousphase deviation.

60. Define frequency deviation.Frequency deviation is the change in frequency that occurs in the carrierwhen it is acted on by a modulating signal frequency.Frequency deviation istypically given as a peak frequency shift in Hertz(f).The peak to peak frequencydeviation (2f) is sometimes called carrier swing.The peak frequency deviation issimply the product of the deviation sensitivity and the peak modulating signalvoltage and is expressed mathematically as f=K1 Vm Hz

61. State Carson rule.Carson rule states that the bandwidth required to transmit an anglemodulated wave as twice the sum of the peak frequency deviation and the highestmodulating signal frequency. Mathematically carson’s rule isB=2(f +fm) Hz.

62. Define Deviation ratio.Deviation ratio is the worst case modulation index and is equal to themaximum peak frequency deviation divided by the maximum modulating signalfrequency. Mathematically, the deviation ratio isDR= f (max)/fm(max)

UNIT IIDIGITAL COMMUNICATION

1.State sampling Theorem.If a finite energy signal g(t) contains no frequency component higher than W Hz, it is completely determined by specifying its ordinates at a separation of points spaced 1/ 2W seconds apart.

If a finite energy signal g(t) contains no frequency component higher than W Hz, it is completely recovered from its ordinates at a separation of points spaced 1/ 2W seconds apart.

2.Methods of Sampling1. Ideal Sampling (or) Instantaneous Sampling2. Natural Sampling3. Flat-top Sampling

3.What is Aliasing or Foldover?Aliasing occurs when the signal is sampled at a rate less than Nyguist rate(2W samples/ sec). It is prevented by using

Guard BandsPre-alias Filter

4.What is PAM?It is a process in which amplitudes of regularly spaced pulses are varied in proportion to the corresponding sample values of continuous message signal.

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5.What do you mean by Aperture Effect?It is nothing but amplitude distortion occurring at PAM due to the sinc function. It is overcome by using a Equalizer whose transfer function is | H(f)| = T –1 sinc(fT)

6.What is PPM?It is the process in which the position of a pulse relative to its unmodulated time of occurrence is varied in accordance with message signals.

7. What is PWM?It is the process in which the samples of message signal are used to vary the duration of individual pulses in the carrier.

8.What is Quantization?It is the process in which the analog sample of the original signal is converted into a digital form.

9.What is Sampling?It is the process in which the original analog signal is converted into a discrete time and continuous amplitude signal.

10.Classify Quantizers.Uniform Quanatizer – Representation levels are uniformly spacedNon-Uniform Quanatizer – Representation levels are non-uniformly spaced

11.What is Quantization Noise?The difference between the output analog sample and the discrete output quantized signal gives raise to an error called Quantization Noise.

12.What are the two types of Quantization Noise?1. Slope – overload distortion2. Granular Noise

13.What is Slope- Overload Distortion?When the message signal varies steeply, the quantized approximation cannot follow the message signal and this results in slope-overload distortion. It can be reduced by increasing the step-size. To minimize the distortion, we need to have

/ Ts ≥ max | d(x(t)) / dt |

14.What is Granular Noise?When the input waveform has a flat segment, then the step-size is larger when compared to

the input. Therefore the approximation hunts around the segment resulting in Granular noise.This noise can be reduced by decreasing the step-size.

15.What is Companding?It is the combined process of compressing and Expanding used for improving the dynamic range of signal and also to increase the SNR of low level signals.

16.List the advantages of Digital pulse modulation systems.Immunity to

3. Transmission distortion.4. Interference5. Noise

17.Give the expression for the capacity of a Gaussian channel.

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Capacity of a white Gaussian channel is C=B log 2 (1+S/N) bits /sec. Here B is the channel bandwidth ,S is the signal power, N is the total noise power within the channel bandwidth.

18.Define Time division multiplexing. In time division multiplexing common cable/medium is shared by different channels on time basis Each channel is assigned a fixed rtime slot .Every Channel transmits in its own time slot.In TDM

19. Define the matched filter. A filter that maximizes the output signal to noise ratio is called matched filter.

20.What are the advantages of digital transmission?The advantage of digital transmission over analog transmission is noise

immunity. Digital pulses are less susceptible than analog signals tovariations caused by noise. Digital signals are better suited to processing and multiplexing than analog

signals. Digital transmission systems are more noise resistant than the analog

transmission systems. Digital systems are better suited to evaluate error performance.

21 What are the disadvantages of digital transmission?_ The transmission of digitally encoded analog signals requiressignificantly more bandwidth than simply transmitting the originalanalog signal._ Analog signal must be converted to digital codes prior totransmission and converted back to analog form at thereceiver, thus necessitating additional encoding anddecoding circuitry.

22 Define pulse code modulation.In pulse code modulation, analog signal is sampled and converted to fixed length,serial binary number for transmission. The binary number varies according tothe amplitude of the analog signal.

23 What is the purpose of the sample and hold circuit?The sample and hold circuit periodically samples the analog input signal and

converts those samples to a multilevel PAM signal.

24 What is the Nyquist sampling rate?Nyquist sampling rate states that, the minimum sampling rate is equal to twice

the highest audio input frequency.

25 Define and state the causes of fold over distortion.The minimum sampling rate(fs) is equal to twice the highest audio inputfrequency(fa).If fs is less than two times fa ,distortion will result. The distortion isCalled aliasing or fold over distortion.The side frequencies from one harmonic fold over into the sideband of anotherharmonic. The frequency that folds over is an alias of the input signal hence ,thenames “aliasing” or “fold over distortion” .26 Define overload distortion.

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If the magnitude of sample exceeds the highest quantization interval, overloaddistortion occurs.

27 Define quantization.Quantization is a process of approximation or rounding off. Assigning PCM codesto absolute magnitudes is called quantizing.

28 Define dynamic range.Dynamic range is the ratio of the largest possible magnitude to the smallestpossible magnitude. Mathematically, dynamic range isDR= Vmax/Vmin

29 Define coding efficiency.Coding efficiency is the ratio of the minimum number of bits required to achievea certain dynamic range to the actual number of PCM bits used. Mathematically,Coding efficiency= Minimum number of bits (including sign bit) X 100Actual number of bits(including sign bit)

30 Define companding.Companding is the process of compressing, then expanding. With compandedsystems, the higher amplitude analog signals are compressed prior totransmission, then expanded at the receiver.

31 Define slope overload. How it is reduced.The slope of the analog signal is greater than the delta modulator can maintain,and is called slope overload. Slope overload is reduced by increasing the clockfrequency and by increasing the magnitude of the minimum step size.

32 Define granular noise.How it is reduced.When the original input signal has relatively constant amplitude, thereconstructed signal has variations that were not present in the original signal.This is called granular noise.Granular noise can be reduced by decreasing the step size.

33 Define adaptive delta modulation.Adaptive delta modulation is a delta modulation system where the step size of theAC is automatically varied depending on the amplitude characteristics of theanalog input signal

34 Define peak frequency deviation for FSK.Peak frequency deviation (_f) is the difference between the carrier rest frequencyand either the mark or space frequency and either the mark or space frequency.

35 Define bit rate.In digital modulation, the rate of change at the input to the modulator is called thebit rate (fb) and has the unit of bits per second (bps).

36 Define Baud rate.The rate of change at the output of the modulator is called baud.

37 Define QAM.Quadrature amplitude modulation is a form of digital modulation where thedigital information is contained in both the amplitude and phase of the transmitted

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carrier.

38 Write the relationship between the minimum bandwidth required for anFSK system and the bit rate.The minimum bandwidth can be approximated asB=2_f +2fbWhere B=minimum bandwidth (hertz)_f=minimum peak frequency deviation (hertz)Fb=bitrate

39 What is PAM?It is a process in which amplitudes of regularly spaced pulses are varied in proportion to the corresponding sample values of continuous message signal.

40 What do you mean by Aperture Effect?It is nothing but amplitude distortion occurring at PAM due to the sinc function. It is overcome by using a Equalizer whose transfer function is

| H(f)| = T –1 sinc(fT)

41 What is PPM?It is the process in which the position of a pulse relative to its unmodulated time of occurrence is varied in accordance with message signals.

42 What is PWM?

It is the process in which the samples of message signal are used to vary the duration of individual pulses in the carrier.

43 Classify Quantizers.Uniform Qunatizer – Representation levels are uniformly spaced

Non-Uniform Qunatizer – Representation levels are non-uniformly spaced

44 List the advantages of Digital pulse modulation systems.Immunity to--- Transmission distortion, Interference & Noise

45 What is the minimum bandwidth requirement of a PAM system?Nfm N:No.of signals to be multiplexed fm:Highest frequency of the signal

46 How is synchronization achieved in PAM system?In a PAM system,marker pulses are introduced in each frame.These pulses are identified by making its amplitude greater than all possible message pulsese and the marker pulse is identified at the receiver by applying the pulses to the slicer network.

47 What are the corrective measures taken to avoid aliasing effect?A low pass anti-alias filter is used to attenuate high frequency of the signal that lie outside the frequency band of interest prior to sampling. The filtered signal is sampled at a slightly higher rate than Nyquist rate.

48 Mention the advantages of digital communication over analog communication.

i)It gives ruggedness to transmission noise and interference.

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ii) Efficient regeneration of coded signal along transmission path.iii)Possible to have a uniform format for different kinds of baseband signal.

49 How many bits required to sample audio signals with 128 quantization levels ? 7.

50 What is hunting as related to delta modulation?

The initial large discrepancy between the message signal and its stepwise approach is called hunting or startup response resulting in redundancy (repetition of same pattern of bits for almost nil variation in slope in message signal).

51 State the differences between DPCM and DM. DM uses only one bit information for transmission. Replacement of the prediction filter in DPCM by a single delay element constitutes DM system.

52 PCM requires larger BW as compared to DM for the same performance. State true or false.False.

53 Define PSK.

PSK is a modulation technique achieved by keying the phase of the carrier between either two possible values corresponding to the binary symbols 0,1 withfixed limits set by the channel.

54 What is meant by DPSK method?It is a non-coherent binary modulation technique. This is a modified technique of binary phase shift keying. DPSK avoids the need to provide a synchronous carrier required at the demodulator for detecting BPSK signal.

55 Differentiate coherent and non-coherent detection methods.In coherent method, carrier is regenerated at the receiver.In non-coherent method, carrier need not be regenerated at the receiver side.

56 What is meant by Binary – phase- shift keying?If the transmitted signal is sinusoid of fixed amplitude then it is called as Binary – phase- shift keying. It has one fixed phase when the data is at one level & when the data is at another level the ‘phase’ is different by 180

57 How the BPSK signal is generated?The BPSK signal is generated by applying the waveform Cos(uo t) as a carrier to a ‘balanced modulator’ and applying the base band signal b(t) as the modulating waveform, in this sense BPSK can be thought of as an AM signal.

58 What is the significance of DPCM?In DPCM instead of quantizing the individual sample values, the difference between the successive samples is quantized. As the difference between sample values {M(k) – M(k-1)} will be smaller than the sample value themselves fewer levels will be required to quantize the difference than that required to quantize M(k) and correspondingly fewer bits will be needed to encode the levels.

59 State the difference between DPCM and DM.

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Parameter DPCM DM

Number of Bits

Levels, Step size.

Bandwidth of transmission channel.

Bits can be more than one but less than PCM.

Fixed numbers of levels are used.

Bandwidth required is lesser than PCM.

It can use 4, 8 or 16 bits per sample.

Step size is fixed and cannot be varied.

Lower bandwidth is required.

60 Define slope overload error?The signal m (t) over a extended time has a very large slope which cannot be approached by the approximated signal m’ (t) In short time.as a result the error m (t)-m’ (t) becomes progressively larger i.e., exceeding s/2 this is known as slope overload error.

61 Define granular noise?Even though the signal m(t) remains unvarying the approximated signal m’(t) swinging up or down above & below m(t) . It is because of step-wise approach of m’ (t) to the m(t).This process is referred as hunting & it gives the noise known as granular noise.

62 What is meant by hunting with reference to delta modulation?

The up & down movement of the approximated signal m’ (t) above & below the input signal m (t) because of the step-wise approach during delta modulation is called hunting.

63 What is the principle of adaptive delta modulation?The principle underlying ADM:

a) If successive errors are of opposite polarity, then the delta modulator is operating in its granular mode. In this case the step size is reduced.

b) If successive errors are of same polarity, then the delta modulator is operating in its slope overload mode. In this case the step size is increased.

64 What are the limitations of delta modulation? The major limitations of delta modulation are: a) Slope overload error b) Granular noise

65 What is the channel Bandwidth of standard PCM?PCM has 256 levels of quantization and a sampling rate of 8000 samples per second. Hence the channel bandwidth of standard PCM is 64kbps.

66 What are the steps involved in the design of the coder?The steps involved are

i. To remove redundancies from speech signals as far as possible.ii. To assign the available bits to code the non-redundant parts of the speech signals in

an efficient manner

67 Why is bit reduction needed while coding of the speech signal?

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Bit reduction is needed while coding because the channel bandwidth required for a standard PCM is 64kbps.But in certain applications the channel bandwidth is at premium, then the speech signal must be coded at low bit rates without affecting the quality of reproduction.

UNIT IIISOURCE CODES, LINE CODES & ERROR CONTROL

1) Give the factors which influence reliable transmission? Transmitted signal power. channel bandwidth.

2) List out the advantages of error control coding. Reduces the required transmitted power. Reduces the size of antennas. Reduces the hardware cost.

3) What are the disadvantages of error control coding?o Increases the transmission bandwidth.o Increases the complexity of decoder.

4) Give the types of error control codes.o Block codes.o Convolutional codes.

5) List the types of block codes. linear block codes. cyclic codes.

6) Define block codes.The codes which consists of (n-k) parity bits for every k bit message block are known as block codes

7) Define linear block codes.Block code is the code in which every ‘k-bit’ message block (n-k) parity bits are appended to

produce ‘n’ bit codeword.If the parity bits are the linear combination of ‘k’ message bits then the code is referred as linear block codes.

8) What are the systematic codes?Block codes in which the message bits are transmitted in unaltered form are called systematic codes.

9) Define generator matrix.Generator matrix Gkxn is used in the encoding operation and its k rows are linearly independent the

encoding operation and its k rows are linearly independentGkxn=[Pkx(n-k) | I kxk]

Where P-parity matrix I-identity matrix

10) What is parity check matrix?Parity check matrix H(n-k)xn is formed using the expression

H=[I(n-k)|PT(n-k)x k]and is used in decoding operation to generate syndrome.

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11) State closure property.Closure property states that the sum of any 2 codewords is another codeword in the code.

12) Define hamming distance Hamming distance d(c1,c2) is the no of locations in which the elements of 2 code vectors differ.

13) Define hamming weight Hamming weight w(c) – no of non zero elements in the code vector.

14) Define minimum distance.Minimum distance d min–smallest hamming weight of the non zero codevector.

15)15) What is dual code?What is dual code?An (n An (n ,k) linear block code with generator matrix g k*nand parity check matrix h(n-k)*n has a dual code

(n,n-k) size generator matrix and parity matrix g and this code satisfies the condition GH T=0.

16) What is Hamming code?Hamming codes are the important class of block codes.Hamming codes have the minimum distance d min = 3Hamming codes satisfy the condition t ≤ ( ½ ( d min-1))Hamming codes are single error –correcting code.

17) Define syndrome.Syndrome contains information about the error pattern ‘e ‘ and may therefore be used for error

detection . S is a 1 x (n-k0 vector and is used to decode the vector C from the received vector ‘r ‘ S = r H

T where r = C + e .

18) Give the properties of syndrome. The syndrome depends only on the error pattern and not on the transmitted code word. All error patterns that differ by a codeword have the same syndrome. S = e H T

19) Write short notes on Decoding table.Decoding table has 2 (n-k) rows and two columns. Each column of the H matrix form the row of the

table . The error pattern corresponding to the i th row indicates the error occurred at the i th position of the code vector that is corresponding to the syndrome calculated ,the error pattern from the decoding table.

20) Define: Cyclic codesCyclic codes is a sub-class of linear block codes .It posses a well defined mathematical structure

and which provides efficient decoding.

21) What is generator polynomial?g(x) is a polynomial of degree n-k that is a factor of xn +1. It is the polynomial of least degree in

the code. the coefficients hi is 0 or 1

22) What is parity check polynomial?It is the polynomial of degree k the coefficients hi is 0 or 1

23) What is Syndrome?The transmitted code word c={c,c,c,……c} over the noisy channel results a received word

r={r,r,r…….r} then the received polynomial r(x)=r

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24) Give any two properties of cyclic codes?Give any two properties of cyclic codes? Linearity Cyclic property25) State cyclic porperty.

Cyclic property states that any cyclic shift of a codeword inb the code is also a codeword.

26) State linearity property.Linearity property states that the sum of any 2 codewords in the code is also a code.

27)27) List the other cyclic codes?List the other cyclic codes? Cyclic redundancy codes(crc) Bose- chaudhuri- hocquenghem(bch) codes. Reed-solomon (rs) codes.

28)28) Give the properties of syndrome polynomial?Give the properties of syndrome polynomial? the syndrome of a received word polynomial r(x) is also the syndrome of the corresponding error

polynomial e(x). if s(x) be the syndrome of the received word polynomial r(x), xr(x) is x s(x). the syndrome polynomial s(x) is identical to the error polynomial e(x) if the errors are confined to

the (n-k) parity check bits of the received word polynomial r(x).

29)29) What is the need for convolutional coding?What is the need for convolutional coding?Convolutional coding may be the preferred method in applications where the message bits come in

serially rather in large blocks in which case the use of buffer may be undesirable.

30)30) What is the convolutional coder?What is the convolutional coder?Convolutional coder generates redundant bits using modulo-2 convolution.

31)31) Define constraint length?Define constraint length?

The constarint length of a convolutional codes expressed in message bits is defined as the no of shifts over which a single message bit can influence the encoder output for a m stage shift register .the constarint length k=m+1.

32)32) What is trellis?What is trellis?Trellis is a tree like structure with remerging branches. The code branch with an input ‘o’ is drawn by a

solid line and a branch by an input ‘1’ is drawn as a dashed line.each input sequence corresponds to a specific path through the trellis.

trellis contains (l+k) levels where l - length of the message and k - constraint length level j - is the depth of the trellis

33)33) What is code tree ?What is code tree ?Each branch of the tree responds an input symbol with the corresponding pair of input binary

symbols indicated on the branch.the input ‘o’ specifies the upper branch of the tree and the input ‘1’ specifies the lower branch of the tree.a specific path is traced from left to right in accordance with the input sequence .the corresponding coded symbols on the branches of that path constitute the output sequence .

34)34) Give the graphical representation of convolutional encoder?Give the graphical representation of convolutional encoder?o Code treeo Trelliso State diagram

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35)35) What is meant by tail of the message ?What is meant by tail of the message ?To restore m-stage shift register of convolutional encoder to its zero initial state , the terminating

sequence of ‘m’ zeros are to be appended to last input bit of the message sequence .this terminating is known as the tail of the message.

36)36) Define non systematic codes?Define non systematic codes?Codes in which message bits and parity bits cannot be identified are reffered as non systematic

codes.

37)37) Distinguish block codes and convolution codes?Distinguish block codes and convolution codes?

block codes

convolutional codes

1) code the block of k Msg bits. 1) code each msg bit individually.

2) needs the buffer to store msg block. 2) does not need the buffer since the bits are arriving in serial fashion.

UNIT 1V MULTIPLE ACCESS TECHNIQUES

1. What is meant by spread spectrum?Spread spectrum is a means of transmission in which the data of interest occupies a bandwidth in excess of the minimum bandwidth necessary to send the data

2. What are the applications of spread spectrum? It is used in military communications systems. It allows the transmitter to transmit a message to a receiver without the message being detected by a

receiver for which it is not intended. It decreases the transmitted power spectral density so that it lies well below the thermal noise level of

any unfriendly receiver. It turns out not to be possible to conceal the transmission.

3.What is DS spread spectrum?DS spread spectrum signal is one in which the amplitude of an already modulated signal is AM modulated by a very high rate NRZ binary stream of digits.

4.Define FH spread spectrum.FH spread spectrum is an FM or FSK technique while DS is a AM or PSK technique. The signal to be FH is usually a BFSK signal although M-ary FSK, MSK or TFM can be employed.

5. What is meant by pseudo random noise?The pseudo random noise is the nose which presents in the DS spread spectrum, if the DS spread spectrum signal is V(t)=g(t)S(t)=2Psd(t)cosot. Here g(t) is a pseudo random noise binary sequence having the values 1. A pseudo noise sequence is defined as a coded sequence of 1s and 0s with certain autocorrelation properties. The class of sequences used in spread spectrum communication is usually periodic in that a sequence of 1s and 0s repeats itself exactly with a known period

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6.Write the advantages of FH over direct sequence spread spectrum. Processing gain is more Spread of the transmitted signal is spread sequentially rather than instantaneously Greater transmission bandwidth and more chips per bit.

UNIT VSATELLITE AND OPTICAL FIBRE COMMUNICATIONS

1.State Kepler’s first law. Kepler’s first law states that the satellite will follow an elliptical path in its orbit around the primary body. An ellipse has two focal points as F1 and F2.

2. State Kepler’s second & third law. It states that for equal time intervals the satellite sweeps out equal areas in the orbital plane, focused at the baycenter. Third law states that the square of the periodic time of orbit is proportional to the cube of the mean distance between the two bodies.

3.What is apogee and perigee? Apogee is the point farthest from the earth. Perigee is the point of closest approach to the earth.

4.What are the classification of satellites? Satellites are generally classified as a low earth orbit (LEO),Medium earth orbit (MEO),or Geosynchronous earth orbit(GEO) .

5.What is major axis?The line joining the perigee and the apogee through the centre of earth.

6.What is minor axis?

4.What is geo-stationary satellite? Geo-stationary satellite is one that appears to be stationary relative to the earth.

5.What is ascending node? The point where the orbit crosses the equatorial plane from south to north. The point where a polar or inclined orbit crosses the equatorial plane traveling from north to south is called ascending node.

6.What are the advantages of optical fibre? Optical fibers are free from electromagnetic radiation and other electrical interference. Optical fibres have a very small attenuation .Optical fibers have large bandwidth capacity.

7.What is meant by the term numerical aperture of an fiber? Numerical aperture is used as figure merit for the fiber .It is given as N A

=n12 –n22 /no

Where no is refractive index of medium through which light is launched. n1 is refractive index of fiber core glass.n2 is refractive index of fiber cladding.

8.What is the difference between TDM and FDM?In TDM ,fixed time slot is given to each channel .In FDM fixed frequency slot is given to each channel.Thus in FDM ,bandwidth is shared ,whereas in TDM time is shared.

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9.What is meant by dispersion and what is it unit of measurement?The time dispersion of the pulse takes place during transmission. The changes timing characteristics of the pulse. Because of dispersion ,the pulse is widened and amplitude is further reduced, This may cause overlapping of adjacent pulses. Dispersion is measured in nanoseconds per kilometers.

10.Distinguish between graded index and step index multimode fiber? In step index fiber the refractive index of the fiber core remains same throughout its bulk. There is sudden transition of refractive index at the interface at the interface of core and cladding .Because of the constant refractive index of the core material the light rays travel straight and reflected from the walls of the core. Hence the light path is zig zag straight segments. In graded index fiber the refractive index is maximum at the core axis and goes decreasing gradually towards the wall of the core. The acceptance cone of the graded index fiber is more than the step index fibers.

11.What is acceptance cone half angle for an optical fiber with core and cladding refractive index of n1=1.48 and n2=1.46 respectively. The acceptance cone angle is given by Qo=sin-1 [sqrt(n12 –n22 /no ] (no=1) =14

12) Define satellite.Satellite is a celestial body that orbits around a planet.In aerospace terms,a satellite is a space vehicle launched by humans and orbits earth or anothercelestial body.

13) State Kepler’s third law.The third law states that the square of the periodic time of orbit isproportional to the cube of the mean distance between the primary and thesatellite.14) Define orbital satellite.Orbital satellites are also called as nonsynchronous satellite. Nonsynchronous satellites rotate around earth in an elliptical or circular pattern. In a circular orbit, the speed or rotation is constant however in elliptical orbits the speed depends on the height the satellite is above the earth.

15) Define posigrade orbit.If the satellite is orbiting in the same direction as earth’s rotation and at anangular velocity greater than that of earth, the orbit is called a prograde (or)posigrade orbit.

16) Define retrograde orbit.If the satellite is orbiting in the opposite direction as the earth’s rotation orin the same direction with an angular velocity less than that of earth, the orbit iscalled a retrograde orbit.

17) Define Geo synchronous satellite.Geo synchronous or geo stationary satellites are those that orbit in acircular pattern with an angular velocity equal to that of Erath. Geosynchronoussatellites have an orbital time of approximately 24 hours, the same as earth; thusgeosynchronous satellites appear to be stationary as they remain in a fixedposition in respect to a given point on earth.

18) Define angle of inclination.The angle of inclination is the angle between the earth’s equatorial planeand the orbital plane of a satellite measured counterclockwise at the point in theorbit where it crosses the equatorial plane traveling from south to north.

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19) Define Descending node.The point where a polar or inclined orbit crosses the equatorial planetraveling from south to north. This point is called descending node.

20) Define line of nodes.The line joining the ascending and descending nodes through the center ofearth is called line of nodes.

21) Define angle of elevation.Angle of elevation is the vertical angle formed between the direction oftravel of an electromagnetic wave radiated from an earth station antenna pointingdirectly toward a satellite and the horizontal plane. 22) Define Azimuth angle.Azimuth is the horizontal angular distance from a reference direction,either the southern or northern most point of the horizon.

23) What are the advantages of optical fiber communication? Greater information capacity Immunity to crosstalk Immunity to static interference Environmental immunity Safety & Security

24) Define a fiber optic system.An optical communications system is an electronic communication systemthat uses light as the carrier of information. Optical fiber communication systemsuse glass or plastic fibers to contain light waves and guide them in a mannersimilar to the way electromagnetic waves are guided through a waveguide.

25) Define refractive index.The refractive index is defined as the as the ratio of the velocity ofpropagation of light ray in free space to the velocity of propagation of a lightray in a given material.

26) Define critical angle.Critical angle is defined as the minimum angle of incidence at which alight ray may strike the interface of two media and result in an angle of refractionof 90°or greater.

27) Define single mode and multi mode propagation.If there is only one path for light to take down the cable, it is called singlemode. If there is more than one path ,it is called multimode.

28) Define acceptance angle.It defines the maximum angle in which external light rays may strike theair/fiber interface and still propagate down the fiber with a response that is nogreater than 10 dB below the maximum value.

29) Define numerical aperture.Numerical aperture is mathematically defined as the sine of the maximumangle a light ray entering the fiber can have in respect to the axis of the fiber andstill propagate down the cable by internal reflection.

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30) Define modal dispersion.Modal dispersion or pulse spreading is caused by the difference in thepropagation times of light rays that take different paths down a fiber. Modaldispersion can occur only in multimode fibers. It can be reduced by using singlemode step index fibers and graded index fibers.31) What are the advantages of heterojunction LEDs?a. The increase in current density generates a more brilliant light spot.b. The smaller emitting area makes it easier to couple its emitted light intofiber.c. The small effective area has a smaller capacitance, which allows theplanar heterojunction LED to be used at higher speeds.

32) What are the disadvantages of injection laser diode?_ ILDs are typically on the order of 10 times more expensive than LEDs_ Because ILDs operate at higher powers, they typically have a much shorterlife time than LEDs._ ILDs are more temperature dependent than LEDs.

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