About this previous year paper
COMMUNICATION ENGINEERING (NEC609)
B.Tech – Semester VI | Time: 3 Hours | Max Marks: 100
SECTION – A (Short Answer Questions)
(10 × 2 = 20 Marks)
(a) Communication Process
The communication process involves information source → transmitter → channel → receiver → destination. Noise may affect the signal while it travels through the channel.
(b) Modulation Process
Modulation is the process of varying a parameter of a high-frequency carrier (amplitude, frequency, or phase) in accordance with the message signal to enable efficient transmission.
(c) Nonlinear effects in FM systems
Nonlinearities cause harmonic distortion and intermodulation, resulting in spectral spreading and distortion of the FM signal.
(d) White Noise
White noise is a random signal having constant power spectral density over all frequencies.
(e) Sampling Process
Sampling converts a continuous-time signal into a discrete-time signal by taking values at uniform intervals. According to Nyquist, sampling rate ≥ 2B.
(f) Probability of error due to noise
It is the probability that noise causes incorrect detection of symbols, commonly expressed as PeP_ePe.
(g) Band-pass transmission model
A band-pass model represents signals centered around a carrier frequency, useful for analyzing modulated signals.
(h) Uncertainty
Uncertainty measures the randomness or unpredictability of information and is quantified using entropy.
(i) Channel Capacity
Channel capacity is the maximum reliable data rate of a channel:
C=Blog2(1+S/N)C = B \log_2(1 + S/N)C=Blog2(1+S/N)
(j) Lossless Data Compression
Lossless compression reduces data size without losing information, allowing perfect reconstruction (e.g., Huffman coding).
SECTION – B (Long Answer Questions)
(Attempt any FIVE – 5 × 10 = 50 Marks)
2(a) Effective modulation index of multi-tone AM signal
For an AM signal modulated by n tones:
meff=m12+m22+⋯+mn2m_{eff} = \sqrt{m_1^2 + m_2^2 + \dots + m_n^2}meff=m12+m22+⋯+mn2
It ensures total modulation does not exceed unity to avoid distortion.
2(b) Quantization and quantization error
Quantization maps continuous amplitude values to discrete levels.
Quantization error = actual value − quantized value.
Minimization methods:
Increase number of levels
Use non-uniform quantization
Apply companding
Quantizing & coding:
Sample → Quantize → Encode into binary.
2(c) Noise in AM system and SNR; Figure of Merit (DSB-SC)
Noise in AM includes additive white noise affecting amplitude.
For DSB-SC:
FOM=(S/N)out(S/N)in=1\text{FOM} = \frac{(S/N)_{out}}{(S/N)_{in}} = 1FOM=(S/N)in(S/N)out=1
Thus, DSB-SC has unity figure of merit.
2(d) Pre-emphasis & De-emphasis; FOM of SSB-SC
Pre-emphasis: Boosts high-frequency components before transmission.
De-emphasis: Attenuates boosted components at receiver.
This improves SNR for high-frequency signals.
SSB-SC FOM: Greater than AM and DSB due to reduced bandwidth and noise.
2(e) Digital PLL, Ex-OR comparator, FDM, TDM, Bandwidth
Digital PLL: Synchronizes phase/frequency digitally.
Ex-OR comparator: Output duty cycle proportional to phase difference.
FDM: Users share channel using different frequencies.
TDM: Users share channel using different time slots.
Bandwidth: Range of frequencies occupied by a signal.
2(f) FSK transmitter and receiver
FSK Transmitter: Binary data selects one of two carrier frequencies.
Receiver: Uses band-pass filters and detectors to recover data.
FSK offers better noise immunity than ASK.
2(g) Superheterodyne receiver vs TRF receiver
Superheterodyne receiver: Converts RF to fixed IF
High selectivity and sensitivity Stable operation
TRF receiver: Amplifies RF directly
Poor selectivity Difficult tuning
Superheterodyne performs much better.
2(h) Power Spectral Density (PSD) & Noise in AM/FM receivers
PSD describes power distribution over frequency.
AM receiver: Noise directly affects amplitude.
FM receiver: Uses limiters; better noise immunity.
SECTION – C (Very Long Answer Questions)
(Attempt any TWO – 2 × 15 = 30 Marks)
3. FM parameters & numerical
Instantaneous frequency: Time-varying carrier frequency
Frequency deviation: Maximum change in carrier frequency
Bandwidth (Carson’s rule):
BW=2(Δf+fm)BW = 2(\Delta f + f_m)BW=2(Δf+fm)
Given:
fc=100 MHzf_c = 100\,MHzfc=100MHz
Am=20 VA_m = 20\,VAm=20V
fm=100 kHzf_m = 100\,kHzfm=100kHz
Frequency sensitivity kf=25 kHz/Vk_f = 25\,kHz/Vkf=25kHz/V
Δf=kfAm=25×20=500 kHz\Delta f = k_f A_m = 25 \times 20 = 500\,kHzΔf=kfAm=25×20=500kHz BW=2(500+100)=1.2 MHzBW = 2(500 + 100) = 1.2\,MHzBW=2(500+100)=1.2MHz
4(a) ASK & PSK transmitter and receiver
ASK: Carrier amplitude varies with data
PSK: Carrier phase varies with data
Receiver: Uses coherent detection and decision circuits.
4(b) Why QPSK is better than PSK
Transmits 2 bits/symbol
Same bandwidth as BPSK
Better power efficiency
5. Short Notes
(a) OFDM & Source Coding Theorem
OFDM: Uses multiple orthogonal sub-carriers for high data rates.
Source coding theorem: Minimum average code length ≥ entropy.
(b) PPM & TDM
PPM: Information in pulse position.
TDM: Multiple signals share time slots.
(c) ISI & Eye Pattern
ISI: Overlapping of symbols due to channel distortion.
Eye pattern: Visual tool to analyze ISI and noise effects.