Mostly Asked Communication Systems MCQs with Answers & Explanations

1. Nyquist Sampling Theorem

The minimum sampling rate for a signal of bandwidth $B$ Hz is:
A) $B$ B) $1/B$ C) $2B$ D) $B/2$
Answer: C) $2B$
Explanation: According to Nyquist, sampling rate must be at least twice the highest frequency to avoid aliasing.

2. Shannon Capacity

For an AWGN channel of bandwidth $B$ and SNR $S/N$, capacity is:
A) $B\log_{10}(1+S/N)$ B) $2B\log_2(1+S/N)$ C) $B\log_2(1+S/N)$ D) ${\log }_2(B)(1+S\mathrm{/}N)$ 

Answer: C) $B\log_2(1+S/N)$
Explanation: Shannon’s theorem gives channel capacity in bits/sec.

3. Thermal Noise

Thermal noise power over bandwidth $B$ at temperature $T$:
A) $kTB$ B) $2kTB$ C) $kT/B$ D) $k/B$
Answer: A) $kTB$
Explanation: Johnson–Nyquist noise formula.

4. Noise Figure

Noise figure $F$ is defined as:
A) $SNR_{out}/SNR_{in}$ B) $SNR_{in}/SNR_{out}$ C) $N_{out}/N_{in}$ D) $G/N$
Answer: B) $SNR_{in}/SNR_{out}$
Explanation: Noise figure measures degradation of SNR through a device.

5. AM Modulation Index

From envelope maxima/minima:
A) $(V_{max}+V_{min})/(V_{max}-V_{min})$
B) $(V_{max}-V_{min})/(V_{max}+V_{min})$
C) $V_{max}/V_{min}$
D) $V_{min}/V_{max}$
Answer: B
Explanation: $m=(V_{max}-V_{min})/(V_{max}+V_{min})$

6. Bandwidth of DSB-SC

For baseband message bandwidth $W$:
A) $W$ B) $2W$ C) $W/2$ D) $4W$
Answer: B) $2W$
Explanation: Two symmetric sidebands.

7. Detector for AM

Which is used for $0<m<1$:
A) Synchronous detector B) Envelope detector C) PLL D) Phase discriminator
Answer: B) Envelope detector
Explanation: Simple, low-cost detection.

8. Bandwidth of SSB

SSB bandwidth equals:
A) $2W$ B) $W$ C) $W/2$ D) Carrier frequency
Answer: B) $W$
Explanation: Only one sideband.

9. FM Bandwidth

Carson’s rule:
A) $2(\Delta f + f_m)$ B) $\Delta f + f_m$ C) $2\Delta f$ D) $2f_m$
Answer: A
Explanation: Approximate FM bandwidth.

10. Pre-emphasis in FM

Pre-emphasis:
A) Boosts low freq. B) Boosts high freq. C) Attenuates highs D) Removes carrier
Answer: B) Boosts high frequencies
Explanation: To counteract high-frequency noise.

11. Capture Effect

Capture effect is prominent in:
A) AM B) FM C) PM D) DSB-SC
Answer: B) FM
Explanation: Receiver locks to stronger FM signal.

12. PCM Bit Rate

Bit rate $R_b$:
A) $n f_s$ B) $f_s/n$ C) $2nf_s$ D) $n/f_s$
Answer: A) $n f_s$
Explanation: $n$ bits per sample × sampling frequency.

13. SQNR in PCM

For $n$-bit PCM:
A) $3n$ dB B) $6.02n+1.76$ dB C) $10n$ dB D) $20n$ dB
Answer: B
Explanation: Approximate formula for sinusoidal input.

14. Aliasing

Occurs when:
A) $f_s=B$ B) $f_s<2B$ C) $f_s>2B$ D) $f_s=\infty$
Answer: B) $f_s<2B$
Explanation: Nyquist criterion violated.

15. Delta Modulation Slope Overload

Occurs when:
A) Step size too large B) Step too small for steep slope C) Sampling too high D) No companding
Answer: B
Explanation: Can’t track signal slope.

16. Digital Modulation Types

ASK, FSK, PSK are:
A) Analog baseband B) Digital passband C) Analog passband D) Line codes
Answer: B
Explanation: Digital info on carrier.

17. Bits per M-QAM Symbol

A) $\log_2 M$ B) $M$ C) $2M$ D) $1/M$
Answer: A
Explanation: Each symbol encodes $\log_2 M$ bits.

18. Gray Coding in QAM

Purpose:
A) Reduce bandwidth B) Reduce power C) Reduce bit errors per symbol error D) Reduce latency
Answer: C
Explanation: Adjacent symbols differ by 1 bit.

19. Minimum Bandwidth (Binary Baseband)

With ideal sinc pulses:
A) $R_b$ B) $R_b/2$ C) $2R_b$ D) $R_b/4$
Answer: B) $R_b/2$
Explanation: Nyquist baseband criterion.

20. Raised-Cosine Roll-off

Bandwidth needed:
A) $R_s/2$ B) $R_s(1+\alpha)/2$ C) $R_s(1-\alpha)$ D) $R_s\alpha$
Answer: B
Explanation: Depends on symbol rate $R_s$ and roll-off $\alpha$.

21. Matched Filter

A matched filter maximizes:
A) Bit rate B) SNR at sampling instant C) Bandwidth efficiency D) Transmit power
Answer: B
Explanation: Matched filter is optimal for maximizing SNR in presence of noise.

22. Eye Diagram

Closing of the eye vertically indicates:
A) Timing jitter B) Noise C) ISI D) Clock drift only
Answer: C) ISI
Explanation: ISI reduces eye opening vertically.

23. Hamming Code

With minimum distance 3, it can:
A) Detect 2, correct 0 B) Detect 1, correct 1 C) Detect 2, correct 1 D) Correct 2
Answer: C
Explanation: Distance 3 allows correction of 1 error, detection of 2.

24. Viterbi Algorithm

Used for:
A) Source coding B) Convolutional decoding C) Modulation D) Equalization
Answer: B
Explanation: Viterbi is maximum likelihood decoder for convolutional codes.

25. Processing Gain in CDMA

Defined as:
A) Data rate / chip rate B) Chip rate / data rate C) $E_b/N_0$ D) Spreading factor × bandwidth
Answer: B
Explanation: Higher chip rate spreads signal, increases gain.

26. OFDM Orthogonality

Subcarrier spacing is:
A) $1/T$ B) $T$ C) $2/T$ D) $1/2T$
Answer: A
Explanation: Orthogonality ensured by spacing $1/T$, where $T$ is symbol period.

27. Cyclic Prefix in OFDM

Primarily combats:
A) AWGN B) Phase noise C) ISI from multipath D) Frequency offset
Answer: C
Explanation: CP absorbs multipath delay.

28. GSM Modulation

GSM uses:
A) QPSK B) GMSK C) 16-QAM D) OFDM
Answer: B
Explanation: Gaussian Minimum Shift Keying.

29. GSM Bandwidth

Nominal carrier bandwidth:
A) 25 kHz B) 100 kHz C) 200 kHz D) 1.25 MHz
Answer: C
Explanation: Standard GSM channel = 200 kHz.

30. LTE Multiple Access

Downlink / Uplink are:
A) SC-FDMA, OFDMA B) OFDMA, SC-FDMA C) OFDMA, CDMA D) TDMA, OFDMA
Answer: B
Explanation: Downlink → OFDMA, Uplink → SC-FDMA.

31. 5G NR Subcarrier Spacing

Is:
A) Fixed 15 kHz B) $15 \times 2^\mu$ kHz C) 7.5 kHz only D) 240 kHz only
Answer: B
Explanation: Flexible numerology in 5G.

32. MIMO Spatial Multiplexing

Capacity grows proportional to:
A) $\min(N_t,N_r)$ B) $N_t+N_r$ C) $N_tN_r$ D) $|N_t-N_r|$
Answer: A
Explanation: Limited by smaller of transmit/receive antennas.

33. Superheterodyne Receiver

Mixer converts RF to:
A) Baseband B) Image frequency C) Intermediate frequency D) LO frequency
Answer: C
Explanation: Fixed IF simplifies filtering.

34. Image Frequency (High-Side LO)

With $f_{LO}=f_s+IF$, image is:
A) $f_s-IF$ B) $f_s+IF$ C) $f_s+2I\mathrm{F\hspace{1em}D) }$$f_s-2IF$
Answer: C
Explanation: Image = $f_{LO}+IF = f_s+2IF$

35. AGC

Automatic gain control is used to:
A) Reduce noise figure B) Maintain constant output level C) Change bandwidth D) Shift frequency
Answer: B
Explanation: Compensates for signal fading.

36. VSWR

In terms of $|\Gamma|$:
A) $(1-|\Gamma|)/(1+|\Gamma|)$ B) $1/|\Gamma|$ C) $(1+|\Gamma|)/(1-|\Gamma|)$ D) $1-|\Gamma|$
Answer: C
Explanation: Standard formula.

37. Doppler Shift

Approx value:
A) $v\lambda$ B) $v/\lambda$ C) $\lambda/v$ D) $2v/\lambda$
Answer: B
Explanation: $f_d=v/\lambda$

38. Coherence Time

Approx:
A) $1/B$ B) $1/(2f_D)$ C) $1/f_c$ D) $1/\sqrt{SNR}$
Answer: B
Explanation: Inverse of Doppler spread.

39. Coherence Bandwidth

Relation with RMS delay spread:
A) $1/\tau_{rms}$ to $1/(5\tau_{rms})$ B) $=\tau_{rms}$ C) $=2\tau_{rms}$ D) $=\sqrt{\tau_{rms}}$
Answer: A
Explanation: Approximate inverse relationship.

40. Rayleigh Fading

Occurs when:
A) LOS dominant B) Many scatterers, no LOS C) Free-space only D) Indoors only
Answer: B
Explanation: Rayleigh assumes no direct path.

41. Rician Fading Factor (K)

Represents:
A) Path loss B) LOS-to-scatter power ratio C) Doppler ratio D) Delay spread
Answer: B
Explanation: $K=\frac{P_{LOS}}{P_{scatter}}$.

42. Diversity Combining

Best SNR:
A) Selection B) Equal-gain C) Maximal ratio combining D) Alamouti
Answer: C
Explanation: Weights proportional to SNR.

43. Zero-Forcing Equalizer

Characteristic:
A) Minimizes MSE B) Eliminates ISI but amplifies noise C) Maximizes SNR D) Nonlinear
Answer: B
Explanation: Ignores noise, cancels ISI.

44. Scramblers

Purpose:
A) Compress B) Encrypt C) Randomize sequence/DC balance D) Increase power
Answer: C
Explanation: Prevents long runs, ensures synchronization.

45. Line Coding

Which has zero DC and good timing?
A) NRZ-L B) RZ C) Manchester D) Unipolar
Answer: C
Explanation: Self-clocking, balanced.

46. Bit Rate vs Symbol Rate

Binary signaling:
A) $R_s$ B) $2R_s$ C) $R_s/2$ D) $R_s\log_{10}2$
Answer: A
Explanation: One bit per symbol.

47. Balanced Modulator

Produces:
A) AM B) DSB-SC C) SSB D) VSB
Answer: B
Explanation: Carrier suppressed.

48. PLL

Used for:
A) AM detection B) FM/FSK demodulation & carrier recovery C) Sampling D) Time-division
Answer: B
Explanation: PLL tracks frequency/phase.

49. Companding

Purpose in PCM:
A) Increase dynamic range to quantizer B) Change bandwidth C) Reduce Nyquist rate D) Remove noise
Answer: A
Explanation: Improves SNR for weak signals.

50. Nyquist Criterion

For zero ISI:
A) Peaks at $T/2$ multiples B) Zero at $T$ multiples (except origin) C) Random timing D) Always max amplitude
Answer: B
Explanation: Ensures no overlap between symbols.

51. AM Power Efficiency

Highest for:
A) DSB-SC B) AM with carrier C) SSB D) PM
Answer: C
Explanation: Only one sideband = most efficient.

52. Carson’s Rule

Increasing $\Delta f$ in FM:
A) Reduces noise effects B) Increases ISI C) Lowers capture D) Reduces bandwidth usage
Answer: A
Explanation: Larger deviation improves noise immunity.

53. BFSK BER

Noncoherent detection BER compared to BPSK:
A) Lower B) Same C) Higher D) Zero
Answer: C
Explanation: BFSK is less power-efficient than BPSK.

54. M-PSK Bandwidth

Nyquist bandwidth :
A) $R_s$ B) $R_s/2$ C) $2R_s$ D) $R_s\log_2 M$
Answer: A
Explanation: Symbol rate decides minimum bandwidth.

55. OFDM IFFT

At transmitter:
A) Create orthogonal subcarriers B) Filter noise C) Demodulate FSK D) Add redundancy
Answer: A
Explanation: IFFT maps data onto subcarriers.

56. Guard Band in FDM

Used to:
A) Increase data rate B) Reduce adjacent-channel interference C) Reduce noise figure D) Provide sync
Answer: B
Explanation: Prevents overlap between channels.

57. TDM Framing Bits

Used for:
A) Error correction B) Clock recovery only C) Slot boundary identification D) Encryption
Answer: C
Explanation: Helps receiver align slots.

58. BER of BPSK

Depends on:
A) $E_b/N_0$ via Q-function B) Channel bandwidth only C) Carrier frequency only D) Modulation index
Answer: A
Explanation: Performance curve determined by $E_b\mathrm{/}{N}_0$

59. Free-Space Path Loss

FSPL increases with:
A) Lower frequency B) Shorter distance C) Higher frequency and distance D) Antenna gain
Answer: C
Explanation: Proportional to $(d/\lambda)^2$

60. Image Rejection in Superhet

Primarily improved by:
A) IF filter only B) RF pre-selector (high Q) C) Audio filter D) AGC loop
Answer: B
Explanation: RF filtering removes image before mixing.