IT-404 Analog & Digital Communication - Nov 2023 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDraw the block diagram of communication system and explain the function of each block. Define a signal and discuss the different types of signals, including continuous and discrete signals, deterministic and non-deterministic signals, periodic and non-periodic signals.
   How to answer

   • Block diagram: source, transmitter, channel, receiver, destination with noise
   • Function of each block in communication chain
   • Definition of signal
   • Types: continuous/discrete, deterministic/random, periodic/non-periodic with examples
2. 7 Marks
   bConsider a causal LTI system with the impulse response $h(t) = e^{-2t}u(t)$, where $u(t)$ is the unit step function. Calculate and plot the frequency response $H(\omega)$ of the system using Fourier transform.
   How to answer

   • Fourier transform of $e^{-2t}u(t)$ gives $H(\omega)=1/(2+j\omega)$
   • Magnitude $|H(\omega)|=1/\sqrt{4+\omega^2}$ and phase $-\tan^{-1}(\omega/2)$
   • Low-pass frequency response calculation steps
   • Sketch of magnitude and phase vs $\omega$
3. 7 Marks
   aDiscuss the power relation in AM and calculate the power efficiency of an AM system. Explain the concept of transmission efficiency in AM and how it can be improved.
   How to answer

   • AM power: $P_t = P_c(1+\mu^2/2)$ derivation
   • Sideband vs carrier power and efficiency $\eta = \mu^2/(2+\mu^2)$
   • Concept of transmission efficiency
   • Methods to improve: suppress carrier, SSB, high modulation index
4. 7 Marks
   bDerive the expression for the modulation index in AM. Explain how the modulation index affects the amplitude and frequency spectrum of an AM wave. Discuss the concept of linear and over modulation in AM.
   How to answer

   • AM expression and definition $\mu = A_m/A_c$
   • Derivation using $V_{max}, V_{min}$: $\mu=(V_{max}-V_{min})/(V_{max}+V_{min})$
   • Effect on envelope, sideband amplitude and spectrum
   • Under, critical and over-modulation with distortion
5. 7 Marks
   aDiscuss the direct and indirect method of frequency Modulation (FM) generation. Explain the working principle and advantages of each method.
   How to answer

   • Direct FM: VCO/varactor, working and stability
   • Indirect FM: Armstrong phase modulation + frequency multiplication
   • Block diagrams of both methods
   • Advantages comparison of each method
6. 7 Marks
   bExplain the concept of deviation ratio and its significance in FM systems. In frequency modulation, the frequency deviation is given as $10 \text{ KHz}$ and the maximum modulating frequency is $5 \text{ KHz}$. Determine the deviation ratio for the FM signal.
   How to answer

   • Definition deviation ratio $D = \Delta f / f_m$
   • Significance for bandwidth and Carson's rule
   • Substitution $\Delta f=10$ kHz, $f_m=5$ kHz
   • Result $D=2$ and interpretation
7. 7 Marks
   aDefine Pulse Code Modulation (PCM) and discuss the quantization process and quantization error.
   How to answer

   • Definition of PCM: sampling, quantization, encoding
   • Quantization process: uniform/non-uniform, step size
   • Quantization error and noise
   • Block diagram of PCM system
8. 7 Marks
   bA Pulse Code Modulation (PCM) system operates with quantization step size of $0.1 \text{V}$. If the signal being quantized has maximum amplitude of $2 \text{V}$, calculate the quantization error. Determine the signal to quantization noise ratio for the PCM system.
   How to answer

   • Quantization error $= \Delta/2 = 0.05$ V
   • Number of levels from $V_{max}$ and step size
   • SQNR formula $(S/N_q)$ calculation
   • Interpretation for PCM system performance
9. 7 Marks
   aDescribe the Binary Phase Shift Keying (BPSK) modulation technique. Derive the equation for BPSK modulation and discuss its bandwidth. Explain coherent detection of BPSK.
   How to answer

   • BPSK principle: 0/180 degree phase shift for binary 1/0
   • Derivation $s(t)=A_c\cos(2\pi f_c t+\pi d(t))$
   • Bandwidth $B=2R_b$ discussion
   • Coherent detection using balanced modulator and LPF
10. 7 Marks
    bConsider a Binary Phase Shift Keying (BPSK) System with carrier frequency of $1 \text{MHz}$ and a bit rate of $100 \text{kbps}$. Determine the phase shift used for modulation. Calculate the bandwidth required for BPSK signal.
    How to answer

    • BPSK uses 180 degree (0 and pi) phase shift
    • Bit duration $T_b=1/R_b$ from 100 kbps
    • Bandwidth $BW \approx 2R_b = 200$ kHz
    • Relation of carrier 1 MHz to data rate
11. 7 Marks
    aDescribe the principles of Adaptive Delta Modulation (ADM) in digital communication. Discuss its advantage over conventional delta modulation.
    How to answer

    • Principle of ADM with variable step size
    • Adaptation logic based on past bits
    • Block diagram of ADM transmitter/receiver
    • Advantages: reduced slope overload and granular noise
12. 7 Marks
    bExplain the concept of Nyquist criterion in sampling. Discuss the condition required to avoid aliasing in sampled signal.
    How to answer

    • Nyquist criterion $f_s \ge 2f_m$ statement
    • Sampling theorem and reconstruction
    • Aliasing effect when undersampled
    • Condition to avoid aliasing with guard band/filter
13. 7 Marks
    aExplain the concept of indirect frequency synthesis using Phase Locked Loop (PLL). Discuss the working principle of PLL in FM generation.
    How to answer

    • Concept of indirect synthesis via PLL frequency multiplication
    • PLL blocks: phase detector, LPF, VCO, divider
    • Working in FM generation and frequency stability
    • Advantages of PLL method
14. 7 Marks
    bDiscuss angle modulation and explain its significance in communication systems. Discuss the types of angle modulation techniques.
    How to answer

    • Angle modulation definition: phase/frequency variation
    • Significance: noise immunity, constant envelope
    • Types: FM and PM with expressions
    • Comparison of FM vs PM applications
15. 7 Marks
    aDescribe the concept of Quadrature Amplitude Modulation (QAM). Explain how QAM combines both amplitude and phase modulation.
    How to answer

    • QAM definition combining ASK and PSK
    • In-phase and quadrature carriers
    • Signal expression with amplitude and phase states
    • Advantage of high spectral efficiency
16. 7 Marks
    bDiscuss the generation and detection of QAM signals. Explain constellation diagram and the relationship between the number of bits and constellation size in QAM.
    How to answer

    • QAM transmitter: I/Q mapping, modulators, summer
    • QAM receiver: coherent demodulation and detection
    • Constellation diagram explanation with examples
    • Relation $M=2^k$, bits per symbol vs constellation size