# Digital Communications Questions and Answers – Spread Spectrum, TDMA and FDMA

This set of Digital Communication Questions for entrance exams focuses on “Spread Spectrum, TDMA and FDMA”.

1. In which modulation schemes, the spreading of spectrum occurs?
a) Frequency modulation
b) Amplitude modulation
c) Frequency & Amplitude modulation
d) None of the mentioned

Explanation: In modulation schemes such as frequency modulation and pulse code modulation spreading of spectrum occurs but it cannot be considered as types of spread spectrum technique.

2. Gaussian noise has _____ power spread _____ over all frequencies.
a) Zero, uniformly
b) Zero, non uniformly
c) Infinite, uniformly
d) Infinite, non uniformly

Explanation: Gaussian noise is a mathematical model which has infinite power spread uniformly over all frequencies.

3. Jamming is caused by
a) Multipath
b) Natural phenomena
c) Multipath & Natural phenomena
d) None of the mentioned

Explanation: Jamming can be caused by natural phenomenon and also is caused by multipath.

4. Which system makes the detection difficult for all users other than the intended users?
a) Low probability of intercept
b) Low probability of detection
c) Low probability of intercept & detection
d) None of the mentioned

Explanation: Low probability of detection or low probability of intercept is a system which makes the detection difficult for anyone but the intended users.

5. The radio meter consists of
a) Low pass filter
b) High pass filter
c) Band pass filter
d) Band reject filter

Explanation: The radio meter consists of band pass filter, squaring circuit and integrating circuit.

6. Spread spectrum signals are used for
a) Ranging
b) Determination of position
c) Ranging & Determination of position
d) None of the mentioned

Explanation: Spread spectrum signals are used for ranging or determination of position location.

7. Uncertainty in the delay measurement is _________ to the bandwidth of signal pulse.
a) Directly proportional
b) Inversely proportional
c) Not related
d) None of the mentioned

Explanation: Uncertainty in the delay measurement is inversely proportional to the bandwidth of the signal pulse.

8. Uncertainty in the delay measurement is _______ to rise time of the pulse.
a) Directly proportional
b) Inversely proportional
c) Not related
d) None of the mentioned

Explanation: Uncertainty in the delay measurement is directly proportional to the rise time of the pulse and inversely proportional to the bandwidth of the signal pulse.

9. In spread spectrum technique, the desired signal is multiplied _____ and interference signal is multiplied _____
a) Once, twice
b) Twice, once
c) Twice, thrice
d) Thrice, twice

Explanation: In spread spectrum technique, the desired signal is multiplied twice and interference signal is multiplied once.

10. Each satellite has ______ transponders.
a) 10
b) 12
c) 15
d) 18

Explanation: Each satellite has 12 transponders with a bandwidth of 36 Mhz each.

11. The preamble consists of
a) Synchronization
c) Error control sequence
d) All of the mentioned

Explanation: The preamble portion consists of synchronization, addressing and error control sequences.

12. Which method is efficient?
a) Fixed assignment
b) Dynamic allocation
c) Fixed assignment & Dynamic allocation
d) None of the mentioned

Explanation: Dynamic allocation method is more efficient than fixed assignment as any slot won’t be wasted unnecessarily.

13. Which has a lesser average packet delay?
a) FDMA
b) TDMA
c) Dynamic FDMA
d) None of the mentioned

Explanation: Both TDMA and FDMA has equivalent performance but when it comes to average packet delay TDMA has lesser delay than FDMA.

14. The maximum waiting time before transmission of a packet is
a) (M-1)T/M
b) (M-1)M/T
c) (M+1)T/M
d) (M+1)M/T

Explanation: The maximum waiting time before transmission of a packet is given by (M-1)T/M.

15. CDMA is
a) Similar to FDMA
b) Similar to TDMA
c) Combination of both
d) None of the mentioned

Explanation: Code division multiple access is the technique which is the combination of both TDMA and FDMA.

16. Frequency modulation technique involves
a) Data modulation
b) Frequency hopping modulation
c) Data & Frequency hopping modulation
d) None of the mentioned

Explanation: The FH modulation is considered as a two step process. It includes data modulation and frequency hopping modulation.

17. Which needs precise time coordination?
a) CDMA
b) TDMA
c) CDMA & TDMA
d) None of the mentioned

Explanation: CDMA when compared to TDMA does not need precise time coordination among various simultaneous transmitters.

Sanfoundry Global Education & Learning Series – Digital Communications.

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