About this previous year paper
BEC601 – DIGITAL COMMUNICATION
Time: 3 Hours | Max Marks: 70
SECTION A – Short Answer Questions
(2 × 7 = 14 marks | Attempt ALL)
Write definition / formula + 1–2 crisp points
a. Random Variable & Probability in Communication
A random variable assigns numerical values to outcomes of a random experiment.
In communication systems, noise and signals are modeled as random variables to analyze probability of error and performance.
b. Mean and Variance of a Random Variable Mean (μ): Average value (expected value)
Variance (σ²): Measure of spread around the mean
They describe signal power and noise fluctuations.
c. Comparison of PSK and FSK
| PSK | FSK |
|---|---|
| Phase varies | Frequency varies |
| Better BER performance | More bandwidth required |
| Power efficient | Less power efficient |
d. Digital Communication System & Advantages
A system that transmits information using discrete symbols (bits).
Advantages over analog: Better noise immunity
Easier encryption and error control
e. BER of BPSK in AWGN Channel
Pb=Q(2EbN0)P_b = Q\left(\sqrt{\frac{2E_b}{N_0}}\right)Pb=Q(N02Eb)
BPSK has minimum BER among binary schemes.
f. Applications of DS-SS Systems
GPS
Military communication (anti-jamming)
g. Signal Space Representation
It represents signals as vectors in orthogonal basis functions, simplifying detection and error probability analysis.
SECTION B – Medium Answer Questions
(7 × 3 = 21 marks | Attempt ANY THREE)
Write definition → explanation → diagram/example
a. Statistical Averages & Properties
Statistical averages describe long-term behavior of random processes.
Properties:
Linearity Time average = ensemble average (ergodic process)
Used in noise and signal analysis.
b. Scrambler and Unscrambler
A scrambler randomizes data to avoid long runs of 0s or 1s.
An unscrambler recovers original data.
Draw block diagram (LFSR based).
c. ASK, FSK, PSK – Working & Constellation
ASK: Amplitude varies FSK: Frequency varies PSK: Phase varies
Draw constellation diagrams and compare bandwidth & noise performance.
d. Spread Spectrum Communication
Spreads signal over wide bandwidth → low power density.
Types: Direct Sequence (DS-SS): PN code multiplication
Frequency Hopping (FH-SS): Rapid carrier frequency changes
Characteristics: Anti-jamming, security, interference resistance.
e. Systematic (7,4) Cyclic Code (Numerical)
Given generator polynomial:
g(x)=x3+x2+1g(x)=x^3+x^2+1g(x)=x3+x2+1
Construct all codewords using message bits + parity bits.
SECTION C – Attempt ANY ONE
(7 marks)
a. Power Spectral Density (PSD)
PSD describes power distribution over frequency.
Relation with autocorrelation:
S(f)=∫−∞∞R(τ)e−j2πfτdτS(f) = \int_{-\infty}^{\infty} R(\tau)e^{-j2\pi f\tau} d\tauS(f)=∫−∞∞R(τ)e−j2πfτdτ
b. Random Process in Communication
A random process is a collection of random variables indexed by time.
Used to model noise, fading, and information signals.