# Wireless & Mobile Communications Questions & Answers – Linear Modulation Techniques

This set of Wireless & Mobile Communications Multiple Choice Questions & Answers (MCQs) focuses on “Linear Modulation Techniques”.

1. In linear modulation technique __________ of transmitted signal varies linearly with modulating digital signal.
a) Amplitude
b) Frequency
c) Phase
d) Angle

Explanation: In linear modulation technique, the amplitude of transmitted signal varies linearly with modulating digital signal. It is a form of digital modulation technique.

2. Linear modulation techniques are not bandwidth efficient.
a) True
b) False

Explanation: Linear modulation techniques are bandwidth efficient. They are used in wireless communication systems when there is an increasing demand to accommodate more and more users within a limited spectrum.

3. Which of the following is not a linear modulation technique?
a) OQPSK
b) π/4 QPSK
c) FSK
d) BPSK

Explanation: OQPSK, π/4 QPSK and BPSK are the most popular linear modulation techniques. They have very good spectral efficiency. However, FSK is an non-linear modulation technique.

4. In BPSK, the ________ of constant amplitude carrier signal is switched between two values according to the two possible values.
a) Amplitude
b) Phase
c) Frequency
d) Angle

Explanation: In binary phase shift keying (BPSK), the phase of a constant amplitude carrier signal is switched between two possible values m1 and m2. These two values corresponds to binary 1 and 0 respectively.

5. By applying cos(2πft), BPSK signal is equivalent to ________
a) Double sideband suppressed carrier amplitude modulated waveform
b) Single sideband suppressed carrier amplitude modulated waveform
c) Frequency modulated waveform
d) SSB amplitude waveform

Explanation: The BPSK signal is equivalent to a double sideband suppressed carrier amplitude modulated waveform, where cos(2πft) is applied as the carrier. Hence, a BPSK signal can be generated using a balanced modulator.
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6. BPSK uses non-coherent demodulator.
a) True
b) False

Explanation: BPSK uses coherent or synchronous demodulation. It requires the information about the phase and frequency of the carrier be available at the receiver.

7. DPSK uses coherent form of PSK.
a) True
b) False

Explanation: Differential phase shift keying uses noncoherent form of phase shift keying. Noncoherent form avoids the need for a coherent reference signal at the receiver. Noncoherent receivers are also easy and cheap to build.

8. In DPSK system, input signal is differentially encoded and then modulated using a ________ modulator.
a) Amplitude
b) Frequency
c) BPSK
d) QPSK

Explanation: In DPSK system, input binary sequence is first differentially encoded and then modulated using a BPSK modulator. The differentially encoded sequence is generated from input binary sequence by complimenting their modulo-2 sum.

9. The energy efficiency of DPSK is _________ to coherent PSK.
a) Superior
b) Same
c) Zero
d) Inferior

Explanation: The energy efficiency of DPSK is inferior to that of coherent PSK by about 3 dB. But, it has an advantage of reduced receiver complexity.

10. QPSK has ________ the bandwidth efficiency of BPSK.
a) Twice
b) Same
c) Half
d) Four times

Explanation: Quadrature phase shift keying (QPSK) has twice the bandwidth of BPSK. It is because two bits are transmitted in a single modulation symbol. The phase of the carrier takes on one of the four equally spaced values, where each value of phase corresponds to a unique pair of message bit.

11. QPSK provides twice the bandwidth efficiency and _______ energy efficiency as compared to BPSK.
a) Twice
b) Half
c) Same
d) Four times

Explanation: The bit error probability of QPSK is identical to BPSK but twice as much data can be sent in the same bandwidth. Thus, when compared to BPSK, QPSK provides twice the spectral efficiency with exactly the same efficiency.

12. What is the full form of OQPSK?
a) Optical Quadrature Phase Shift Keying
b) Orthogonal Quadrature Pulse Shift Keying
c) Orthogonal Quadrature Phase Shift Keying
d) Offset Quadrature Phase Shift Keying

Explanation: OQPSK stands for offset quadrature phase shift keying. It is a modified form of QPSK which is less susceptible to deleterious effects and supports more efficient amplification. OQPSK is sometimes also called staggered QPSK.

13. The bandwidth of OQPSK is _______ to QPSK.
a) Identical
b) Twice
c) Half
d) Four times

Explanation: The spectrum of an OQPSK signal is identical to that of QPSK signal. Hence, both signals occupy the same bandwidth. The staggered alignment of the even and odd bit streams in OQPSK signal does not change the nature of spectrum.

14. QPSK signals perform better than OQPSK in the presence of phase jitter.
a) True
b) False

Explanation: OQPSK signal perform better than QPSK in the presence of phase jitter. It is due to the presence of noisy reference signal at the receiver.

15. Which of the following is not a detection technique used for detection of π/4 QPSK signals?
a) Baseband differential detection
b) IF differential detection
c) FM discriminator detection
d) Envelope detection

Explanation: There are various types of detection techniques used for the detection of π/4 QPSK signals. They include baseband differential detection, IF differential detection and FM discriminator detection.

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