Question Bank

Digital Communications

UNIT I

Objective questions

1. Define the term Signals.2. Define the term Systems.3. Define the term Signal Space.4. Define Bandpass signals and system.5. Define Baseband signal &system.6. Give the basic difference between Analog and Digital communication.7. State Sampling Theorem.8. What is impulse sampling, Natural sampling9. Define Pulse code Modulation.10. In what way PCM is different from other Modulation?11. What is the meaning of Quantization?12. Define uniform Quantization 13. Define nonuniform Quantization.14. What is meant by Quantizing Noise?15. How Delta modulation is different from PCM?16. Which two noises are occurred in Delta modulation?17. What is meant by coherent?18. What is meant by non coherent?19. Write the advantages and disadvantages of a digital communication system.20. Explain the difference between baseband transmission and bandpass transmission.

Subjective questions

1. Draw and explain the block diagram of Delta modulation2. What are the different types of quantizer?3. Explain the concept of companding.4. Explain coherent binary PSK with the help of transmitter & receiver.5. Explain coherent binary FSK with the help of transmitter & receiver.6. Derive the expression for probability of error in coherent FSK7. Explain the properties of matched filter.8. Explain the concept of matched filter. 9. Derive the expression for probability of error in QPSK10. Draw the signal space diagram for QPSK and find Euclidean distance.11. Derive the expression for probability of error in coherent BPSK12. Explain the difference between correlator and matched filter.13. Draw the block diagram of DPSK system. Explain the working by means of suitable diagram 14. Write short note on QPSK.15. Explain the PCM system. 16. Explain concept of Quantizing in PCM.17. Explain concept of multiplexing in PCM.18. Explain concept of repeaters in PCM.19. Explain use of TDM in PCM20. Give the expression for S to quantization noise ratio for PCM system. Hence prove that (S/N)dB =(4.8+6v)dB 21. Explain coherent MSK with the help of transmitter & receiver. 22. Derive the expression for probability of error in coherent MSK.23. Draw the signal space diagram for MSK24. Draw the waveforms related to MSK.25. Derive the expression for probability of error in coherent QPSK

UNIT II

Objective questions

1. Define the Information Theory.2. Give the name of different types of Compact code.3. Give the formula for entropy?4. Explain discrete communication channel and its expression for information content?5. Explain continues communication channel and its expression for information content.?6. What is p (xi / yj)?7. How will you write mutual information exchange?8. Write expression for discrete channel capacity?9. The formula for information measure is ---?10. Define the term Rate.11. Define the term Distortion.12. Define the term Rate distortion.13. What is the difference between scalar and vector quantization?14. Explain binary symmetric channel.15. Derive expression for discrete channel capacity?16. What are the conditions to be satisfied by hamming codes?17. Avg. information content per symbol is called --?18. For long distance communication it is necessary to generate ---- signals to suit to transmission medium?19. The information content is always a ------ quantity?20. Explain information from Scientific point of View.

Subjective questions 1. What is prefix?2. Explain prefix code with the help of example3. Derive Shannons channel capacity theorem4. Give the significance of Shannons channel capacity theorem.5. Write short note on: LZ coding6. Consider the binary sequence: 101011010010011101010000110011101010001101.Using LZ coding Encode this sequence.7. Consider the binary sequence: 10011101010000110011101010001101.Using LZ coding Encode this sequence if 1 and 0 are already in dictionary.8. Write short note on: Scalar Quantization.9. Explain the concept of vector Quantization with mathematical expression.10. (1-p)(1-p)ppConsider the BSC with P(x1)= as shown in fig. Show that mutual information is I(X,Y)=H(Y)+p log p+(1-p)log(1-p)

11. A zero memory source emits messages x1, x2,x3 with probabilities 0.9,0.02 and 0.08 resp. Find the optimum binary code for this source as well as for its 2ndOrder extension (i.e. N=2). Determine code efficiency in each case.12. Explain distortion, Rate-Distortion R(D) and Distortion rate D(R) functions.13. Calculate the limiting value for a discrete memoryless Gaussian source14. Interpret the following entropies H(X),H(Y),H(X|Y),H(Y|X)and H(XY)15. PePePePeX1Y2Y1X2Find the channel capacity of the BSC shown in figure:

16. 0.80.20.3X1Y2Y1X21Y3X30.7Consider a binary input ,output channel shown below

Find H(X),H(Y),H(X|Y),H(Y|X)and H(XY)

17. A discrete memoryless source has an alphabet of seven symbols with probabilities 0.25,0.25,0.125,0.125,0.125,0.0625,0.0625 resp. Compute Huffman code for this source, calculate entropy, avg. codeword length & variance of this code. 18. Explain the source coding theorem.19. A discrete memoryless source has an alphabet of seven symbols with probabilities 0.3,0.20,0.16,0.14,0.10,0.02,0.08 resp. Compute Huffman code for this source, calculate entropy, avg. codeword length & variance of this code.20. Derive the expression for condition of maximum entropy.21. A zero memory source emits messages x1, x2with probabilities 0.8,0.2 resp. Find the optimum binary code for this source as well as for its 2nd and 3rdOrder extension (i.e. N=2 & N=3). Determine code efficiency in each case.22. A zero memory source emits messages x1, x2,x3 with probabilities 0.45,0.35 and 0.20 respectively. Find the optimum (Huffman) binary code for this source as well as for its 2ndOrder extension (i.e. N=2). Determine code efficiency in each case.

UNIT IIIObjective questions

1. In ADPCM, give the types of adaptive quantizer.2. In ADPCM, AQF required which three things.3. Give the name of encoding techniques in frequency subband.4. In subband coding, what is meant by quadrature mirror filter(QMF)?5. What is use of adaptive transform coding(ATC)?6. Define the term White Gaussian noise.7. In LPC, what is the use of speech synthesizer?8. Explain the term Image compression .9. Explain the term data compression.10. In Image compression ,explain lossless technique.11. In Image compression ,explain lossy technique.12. Define the term data redundancy.13. Give the name of three types of data redundancy in image compression.14. Define the term data compression.15. Which two different data compression algorithms are used ?16. Explain the meaning of JPEG.17. Explain the Meaning of MPEG18. Differentiate between subband coding & transform coding.19. Differentiate between Lossy & Lossless compression.20. Explain the concept of Quadrature mirror filter.

Subjective questions

1. Discuss in detail image compression model2. Explain JPEG standard for Image compression in detail.3. Explain MPEG standard for Image compression in detail.4. Explain adaptive sub-band coding methods5. Write short notes on:-Linear predictive coding 6. Write short notes on:-ADPCM7. Explain the LP coding with mathematical analysis8. Explain CELP coding9. Compare LP and CELP coding10. Explain the advantages of MPEG coding11. Explain transform coding methods12. Give the model of digital communication system.

UNIT IVObjective questions

1. Give the concept of Basis Function in Digital communication.2. What do you mean by Orthogonal Basis function?3. What do you mean by Orthonormal Basis Function?4. Explain the term optimum in digital communication.5. Explain the term correlators.6. What is the application of Gram Schmidth orthogonalization procedure?7. What is the meaning of signal space representation?8. State optimum receiver Algorithm.9. Spectral density of PSK, FSK & ASK can be improved using which factor?10. Non coherent scheme do not require a ----- local oscillator?11. Band width required for ASK and PSK is same or not?12. FSK signal can be demodulated using ---detector?13. For each symbol 1 & 0 in PSK phase of carrier differs by --- degrees?14. In ASK symbol 1 is represented by transmitting a --- wave?15. Phase synchronization ensures these two waves in phase in the Ask receiver transmitter?16. PSK transmission is polar or non polar?17. PSK is liner or non-linear modulation scheme?

Subjective questions

1. Explain the geometrical representation of signals.2. Explain the vector space concept for signal space representation3. Explain Gram-Schmidt orthogonalization procedure with block diagram 4. Give the mathematical analysis for Gram-Schmidt orthogonalization.5. Explain the concept of basis function6. State the maximum likelihood criteria.7. Explain signal representation of a signal N=2and M=3.8. Give the condition for Orthogonality for basis function9. Explain the signal space concept for representation of a signal.10. Explain probability of errors in AWGN channel11. Explain optimum receiver for AWGN channel12. Write short note on QAM13. consider the signals s1(t), s2(t), s3(t), s4(t), shown in fig. Find the orthogonal basis function using Gram Schmidt orthogonalization procedure

14. consider the signals s1(t), s2(t), s3(t), s4(t), shown in fig. Find the orthogonal basis function using Gram Schmidt orthogonalization procedure

-.51 2 t

1 2 t -.5

11 S1(t) S2(t)

1 2 t .51 S4(t)1 2 t -1 S3(t)

UNIT V

Objective questions

1. How many coding techniques are used for transmission the digital data?2. Define Parity & Parity check code.3. Define the term Hamming distance in coding.4. What is meant by code efficiency ?5. What are convolution codes?6. Write about code tree and trellis diagram for convolution encoder.7. Define the term Constraint length(K) in convolutional code.8. Define the term Metric in Viterbi Algorithm.9. Define the term Surviving path in Viterbi Algorithm10. Define the term Metric Divergence effect in sequential decoding method.11. What is meant by signal flow graph?12. Define the term Free distance.13. What is the meaning of distance transfer function?14. Define the term Redundancy.15. What do you mean by constellation?16. Define the term mapping.17. Give the relation between Euclidean distance and partition in trellis code modulation.

Subjective questions

1. Explain the difference between convolution codes and block codes.2. A rate 1/3 convolution code is described by; g1=[1 0 1], g2=[1 0 0],g3=[1 1 1] i. Draw the encoder corresponding to this codeii. Draw code trellis diagram.iii. Draw state diagram for this code.iv. O/P of demodulator detector is (101001011110111),using viterbi algorithm, find surviving path.3. Explain FANO metric 4. Explain FANO algorithm with the help of flowchart.5. A rate 1/3 convolution code is described by; g1=[1 1 1], g2=[1 0 0],g3=[1 0 1] 6. Draw the encoder & code tree corresponding to this code.7. What is constraint length of convolution code.8. A rate 2/3 convolution code is described by; g1=[1 0 1 1, g2=[1 1 0 1], g3=[1 0 1 0] Draw the encoder ,code tree ,code trellis & state diagram corresponding to this code9. A convolutional code is described by g1=[1 0 0], g2=[1 0 1],g3=[1 1 1] Find the advance transfer function and the free distance of this code 10. Write short note on Sequential decoding11. Write short note on viterbi decoding12. A rate 2/3 convolution code is described by; g1=[1 0 1 1, g2=[1 1 0 1], g3=[1 0 1 0] . O/P of demodulator detector is (01011111110111),using viterbi algorithm, find surviving path.13. A convolutional code is described by g1=[1 0 1], g2=[1 0 0],g3=[1 1 1] Find the advance transfer function and the free distance of this code 14. Explain free distance & coding gain15. Give advantages of convolution code16. Explain trellis coded modulation17. Explain set partitioning of 8PSK signal constellation.18. Explain set partitioning of 16 QAM signal constellations.19. Draw and explain undgerboeck modulation technique.20. Draw trellis diagram for ungerboeck modulation.

UNIT VI

Objective questions

1. Define spread spectrum modulation.2. How spread spectrum is differ from FM/PCM?3. What is the use of Spread spectrum technique in military ?4. Define the term Averaging system in Spread spectrum.5. Define the term Averaging system in Spread spectrum .6. Give the name of types under spread spectrum techniques.7. Define the term Pseudo noise sequence.8. What is the difference between PN sequences and Maximum length sequence?9. What is Processing Gain in DSSS?10. What is Jamming margin in DSSS?11. What do you mean by Frequency Hopping in Spread spectrum?12. What do you mean by Fast Frequency Hopping in Spread spectrum?13. What do you mean by Slow Frequency Hopping in Spread spectrum?14. Define the term chip rate, chip duration in PN sequence.15. What is the meaning of Interference rejection?16. Explain the term Antijam capability.17. Explain the term multipath protection.18. Explain balance property of PN sequence.19. Explain run property of PN sequence.20. Explain auto correlation property of PN sequence

Subjective questions

1. What is spread spectrum? 2. Define processing gain & jamming margin 3. List the applications of spread spectrum system4. Explain the applications of DS-SS signals. 5. Explain DS-SS system with coherent BPSK. 6. Give expression for Probability of error and derive the expression for jamming margin for DSSS system. 7. Explain balance property of PN sequence8. Explain run property of PN sequence9. Explain auto correlation property of PN sequence10. Tabulate and plot autocorrelation function of all shift of an L=4 stage shift register Generator, generating a 15 bit (N=2L-1) maximal length PN sequence and verify its properties. Assume initial stage of shift register 100011. Write short notes on: - Slow Frequency Hopping 12. Write short notes on: - Fast Frequency Hopping13. A recorded conversation is to be transmitted by PN spread spectrum. Assuming the spectrum of speech waveform is bandlimited to 3kHz.Fing chip rate required to obtain a PG of 20 db.14. A slow FH/MFSK system has the following the following parameters. No. of bits MFSK symbol=4 No. of MKSK symbol /hop=4 Calculate PG.15. Tabulate and plot autocorrelation function of all shift of an L=3 stage shift register Generator, generating a 7 bit (N=2L-1) maximal length PN sequence and verify its properties. Assume initial stage of shift register 100. 16. A slow FH/MFSK system has the following the following parameters. No. of bits MFSK symbol=4 No. of MKSK symbol /hop=5 Calculate PG.17. A PN sequence is generated using a feedback shift of length m=4.The chip rate is 107chips /second. Find the following PN sequence length, chip duration. 18. Write short note on CDMA19. A conversation is to be transmitted by PN spread spectrum. Assuming the spectrum of speech waveform is bandlimited to 4kHz.Fing chip rate required to obtain a PG of 20 db. Give that sequence length is grater than 5 hrs, find no. of shift register stages required.20. Explain the properties of maximum length sequence