# Digital Communication's Quetion Bank - Mid 1

Unit-1

1. Explain the steps in converting a continuous signal into a digital signal.

2. Discuss the advantages of digital communications over analog communications.

3. A bandlimited signal of 4KHz bandwidth is to be sampled using an ideal sampling circuit. What

should be the maximum time gap between two successive samples so that a low pass filter can

reconstruct the original signal?.

4. A PCM system uses a uniform quantizer followed by a 7-bit binary encoder. The bit rate of the

systm is equal to 50x106 bits/sec. (a) What is the maximum message bandwidth for which the

system operates satisfactorily? (b) Determine the output SNR when a sinusoidal modulating wave

of frequency 1MHz is applied to the input.

5. Draw the block diagram of DPCM and ADM.

6. What is aliasing and how it is reduced?

**Part-b**

1. a) Explain the block diagram of a digital communication system.

b) The data to be transmitted at the rate of 10000 bits/sec over a channel having bandwidth

B=3000 Hz. Determine the signal to noise required. If the bandwidth is increased to 10000 Hz,

then determine the signal to noise ratio.

c) Discuss various sampling techniques.

2. a) State sampling theorem and give its proof.

b) What is Nyquist rate and nyquist interval?

c) Determine Nyquist rate and Nyquist interval for the following signal.

(i) x(t) = sinc 200t (ii) x(t) = sinc2200t and (iii) x(t) = sinc 200t + sinc2200t.

3. a) State Heartly shannon’s law for channel capacity and explain (Bandwidth – SNR Trade off).

b) For an AWGN channel with 4 kHz bandwidth, the noise spectral density N0/2 is 1 pico

Watts/Hz and the signal power at the reiceiver is 0.1 mW. Determine the capacity of the channel.

Also determine its maximum capacity.

c) Explain Natural sampling and Flat top sampling with necessary expression and waveforms.

4. a) Draw the block diagram of PCM and explain.

b) Derive the expression for signal to quantization noise ratio (SNR) for PCM system that

employs linear quantization technique. Assume that the input to the PCM system is a sinusoidal

signal.

c) What is Companding process. State A-law and µ-law.

5. a) The bandwidth of signal input to the PCM is restricted to 4kHz. The input varies from -3.8V to

+3.8V and has the modulator produces binary output. Assume uniform quantization. Then (i)

Calculate the number of bits required per sample. (ii) Outputs of 30 such PCM coders are time

multiplexed. What is the minimum required transmission bandwidth for the modulated signal

b). A DM system is designed to operate at 3-times the Nyquist rate for a signal with a 3-kHz

bandwidth. The quantizing step size is 250 mV. (i) Determine the maximum amplitude of a 1kHz

input sinusoid for which the modulator does not show slope overload. (ii) Determine the post

filtered output SNR for the signal above.

c) Give the differences between CM and DM.

6. a) Explain the block diagram of DM.

b) Consider a sine wave of frequency fm and amplitude Am applied to a data modulator of step size

Δ. Show that the slope overload distortion will occur if Am > Δ/(2πfmTs).

c) Derive the expression for maximum SNR of DM system.

Unit-2

1. Distinguish Baseband data transmission and Passband data transmission.

2. For the sequence 1111000001010110 find DPSK Phase and verify at the receiver.

3. In a digital CW communication system, the bit rate of NRZ data stream is 1 Mbps and carrier

frequency is 100 MHz. Find the symbol rate of transmission and bandwidth requirement of

the channel in the following cases of different techniques used. i) BPSK system ii) QPSK

system iii) 16-Ary PSK sytem and iv) 8-PSK system.

4. Define Ecludien distance. What is its significance?

5. Which modulation scheme is widely used for wireless mobile communications? Why?

6. Define coherent detection of carrier signal and define non-coherent detection of carrier signal.

Part-b:

1. a) Explain the transmitter and receiver of ASK with necessary block diagrams. Derive

expressions for power spectrum and bandwidth.

b) Derive expression for probability of error in Binary Amplitude shift keying modulation

technique.

2. a) Explain the transmitter and receiver of BFSK with necessary block diagrams. Derive

expressions for power spectrum and bandwidth.

b) ) Derive expression for probability of error in Binary Frequency shift keying modulation

technique.

3. a) Explain the transmitter and receiver of BPSK with necessary block diagrams. Derive

expressions for power spectrum and bandwidth.

b) ) Derive expression for probability of error in Binary Phase shift keying modulation

technique.

4. a) Explain the transmitter and receiver of DPSK with necessary block diagrams. Derive

expressions for power spectrum and bandwidth.

b) ) Derive expression for probability of error in Differential Phase shift keying modulation

technique.

5. a) ) Explain the transmitter and receiver of QPSK with necessary block diagrams. Derive

expressions for power spectrum and bandwidth.

b) ) Derive expression for probability of error in Quadrature Phase shift keying modulation

technique.

6. a) Distinguish among ASK, BFSK, BPSK and QPSK.

b) If the bit rate is 200Mbps and the carrier frequency is 50MHz then find (i) Symbol rate

(ii) Bandwidth for transmission (iii) Baseband Power spectrum and (iv) Passband power

Unit-3

3. Define Information, State the properties of Information.

4. Define the terms: i) Information rate ii) Redundancy iii) Coding efficiency and iv) Symbol rate.

5. Define Binary Symmetric Channel and Binary Erasure channel.

6. State the Source coding theorem and State the channel coding theorem.

1. a) Explain Baseband signal receiver with a necessary block diagram.

2. a) Explain correlation receiver with a neat block diagram. Explain the function of each block.

Give the reason why correlation receiver is called as integrated and dump filter.

b) Explain eye diagrams. What is its significance?

3. a) Define Mutual information. Prove any 4 propertis of Mutual information.

b) Explain matched filter? Derive the expression for its output SNR.

4. a) Define Entropy. Prove any 4 properties of Entropy.

b) A DMS has an alphabet of eight letters xi, i=1,2,3,8 with probabilities 0.25, 0.2 , 0.15, 0.12,

0.1, 0.08, 0.05, 0.05. (a) Determine the average number of binary digits per source symbol (b)

b) A discrete memoryless source consists of three symbols x1, x2, x3, x4, x5 and x6 with

probabilities 0.3, 0.2, 02, 01, 01, and 0.1 respectively. Determine the minimum variance of

6. a) Explain Shannon-Fano coding algorithm.

b) A discrete memoryless source consists of three symbols x1, x2, x3, x4, x5 and x6 with

probabilities 0.25, 0.3, 02, 0.12, 0.08, and 0.05 respectively. Apply Shannon-Fano coding.

UNIT IV

1. Define Linear block codes and Convolutional codes.

2. State the advantages and disadvantages of cyclic codes.

3. Discuss the error detection and correction capabilities of linear block codes.

4. Discuss the properties of Convolution codes.

5. State and prove the condition for entropy to be maximum.

6. Prove that H(X,Y) = H(X) + H(Y/X) = H(Y) + H(X/Y).

1. a) Explain Matrix description of linear block codes.

b) An error control code has the following parity check matrix. Determine (i) Generator matrix

(ii) The codeword starting with 100… (iii) Comment on error detection capability and (iv)

Decode 110101

2. a) Explain syndrome calculation and decoding.

b) An error control code has the following matrix. Determine (i) All codes (ii) Parity check

matrix (iii) Minimum distance & (iv) Decode 1001

3. a) State and explain the properties of cyclic codes.

b) Construct the (7, 4) linear code word for the generator polynomial G(D) = 1+D3+D2 for the

message bits 1001.

4. (a) Explain with neat diagram Cyclic encoder and Decoder

(b) The generator polynomial of a (7, 4) cyclic code is x3+x+1. Construct the generator matrix for a

systematic cyclic code and find the code word for the message (1101) using the generated matrix.

5. a) Briefly describe the Viterbi algorithm for maximum-likelihood decoding of convolutional codes.

b) For the convolutional encoder shown in figure draw the state diagram and the trellis diagram.

6. a) Explain how you would draw the trellis diagram of a convolutional encoder given its state

diagrams.

b) Find the generator matrix G(D) for the (2, 1, 2) convolutional encoder of figure shown.

UNIT V

1. Define spread spectrum modulation.

2. Discuss the advantages of Spread spectrum Modulation.

3. Comment on source coding methods.

4. Comment on Pulse coding techniques.

5. Comment on Information theory.

6. Comment on sampling process.

**Part B**

1. a) Explain direct sequence Spread spectrum modulation with a block diagram.

b) Explain ranging using DSSS modulation technique.

2. a) Explain Code division Multiple access technique.

b) Define channel matrix. Explain various channel types.

3. a) Explain Frequency hopping spread spectrum technique.

b) Explain tracking in FHSS modulation technique.

4. a) Explain the generation and characteristics of PN-Sequences.

b) Define AWGN channel. Discuss channel capacity theorem.

5. a) Distinguish DSSS modulation and FHSS modulation.

b) Comment on error control coding.

6. a) Explain the Synchronization in spread spectrum systems.

b) Explain about maximal PN Codes.