Digital Communications Questions and Answers – Bandwidth Efficiency Plane and Bandwidth Efficient Modulation

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This set of Digital Communication online test focuses on “Bandwidth Efficiency Plane and Bandwidth Efficient Modulation”.

1. QPSK is a composite of
a) Two BPSK
b) Three BPSK
c) Two FSK
d) Two M-ary PSK

Explanation: QPSK is effectively a composite of two BPSK signals transmitted on orthogonal components of the carrier.

2. The IF transmission bandwidth of MFSK is
a) M/T
b) T/M
c) 2M/T
d) M/2

Explanation: In non coherent orthogonal MFSK modulation, the IF transmission bandwidth is WIF = M/T.

3. The bandwidth efficiency of MFSK is
a) M/log2M
b) log2M/M
c) M/log10M
d) log10M/M

Explanation: For non coherent orthogonal MFSK modulation the bandwidth efficiency is R/W=log2M/M.
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4. The bandwidth efficiency increases as M increases.
a) True
b) False

Explanation: The bandwidth efficiency decreases as M increases.

5. The bandwidth efficiency of QFSK is _______ that of BFSK.
a) Greater than
b) Less than
c) Equal to
d) None of the mentioned

Explanation: The bandwidth efficiency of QPSK is equal to that of the BFSK.

6. Which modulation is the most efficient one?
a) BPSK
b) BFSK
c) QPSK
d) QAM

Explanation: Of all the modulation showed, QAM is the most bandwidth efficient one.

7. The primary communication resource is
a) Transmitted power
c) Efficiency
d) None of the mentioned

Explanation: The primary communication resources are transmitted power and channel bandwidth.

8. Greater the redundancy lesser is the bandwidth expansion.
a) True
b) False

Explanation: Within the same block size, the code with greater redundancy requires more bandwidth expansion.

9. Which modulation scheme uses nonlinear transponders?
a) MSK
b) Offset QPSK
c) MSK & Offset QPSK
d) None of the mentioned

Explanation: Offset QPSK and MSK are two examples of constant envelope modulation schemes that are attractive for systems using non linear transponders.

10. Which modulation requires more bandwidth?
a) QPSK
b) OQPSK
c) BPSK
d) BFSK

Explanation: BPSK requires more bandwidth than the other modulation techniques for the given level of spectral density.

11. Which modulation has lower side lobe levels?
a) QPSK
b) OQPSK
c) BPSK
d) MSK

Explanation: MSK spectrum has wider main lobe level and lower side lobe levels than the other modulation spectrum.

12. Which modulation is spectrally more efficient?
a) BPSK
b) MSK
c) QPSK
d) OQPSK

Explanation: MSK is spectrally more efficient than the other modulation schemes. It has wider main lobe level and lower side lobe levels.

13. Which modulation spectrum has narrow main lobe?
a) QPSK
b) OQPSK
c) BPSK
d) MSK

Explanation: QPSK has narrower main lobe than MSK because for a given rate the QPSK symbol rate is half the MSK symbol rate.

14. Which technique can be used for bandwidth reduction?
a) BPSK
b) QPSK
c) MPSK
d) MFSK

Explanation: Coherent M-ary PSK modulation is a well known technique for achieving bandwidth reduction.

15. QPSK amplitude modulates
a) Sine function
b) Cosine function
c) Sine & Cosine function
d) None of the mentioned

Explanation: QPSK modulation consists of two independent streams. One stream amplitude modulates cosine wave and the other amplitude modulates sine wave.

16. QAM is a combination of
c) PSK and FSK
d) None of the mentioned

Explanation: QAM is a combination of both ASK and PSK and is also called as amplitude phase keying.

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