Passband Transmission MCQ Quiz - Objective Question with Answer for Passband Transmission - Download Free PDF

QPSK modulation represents symbols by a constellation of four-phase angles of the carrier signal, orthogonal to each other.

There are two bits per symbol. So for example 00 = 0°, 01 = 90° , 10 = 180° and 11= 270°. 

QPSK transmits twice the data rate in a given bandwidth compared to BPSK, at the same BER. 

The constellation diagram for 4 different symbols is as shown:

In Frequency Shift Keying (FSK) modulation, the carrier frequency is switched between two frequencies based on:

The correct answer is option 2: whether a binary 1 or 0 is sent.

Additional Information 

Frequency Shift Keying (FSK) is a method of digital signal modulation in which the frequency of a carrier signal is varied in accordance with the digital signal data. In simpler terms, FSK uses different frequencies to represent different data values. This method is particularly useful for transmitting digital data over analog communication channels. The primary reason for using FSK modulation is its robustness in the presence of noise compared to other modulation techniques like amplitude modulation (AM) or phase modulation (PM).

In FSK modulation, the carrier frequency is switched between two or more discrete frequencies based on whether a binary '1' or '0' is being transmitted. This means:

• When the digital signal is a binary '1', the carrier frequency shifts to a higher frequency, typically referred to as the mark frequency.
• When the digital signal is a binary '0', the carrier frequency shifts to a lower frequency, typically referred to as the space frequency.

This frequency variation allows the transmission of digital data over analog systems efficiently. The receiver must be capable of distinguishing between the different frequencies to decode the transmitted binary information. One of the most common applications of FSK is in modem technology, where it is used to transmit data over telephone lines.

FSK modulation has several advantages, including:

• FSK signals are less susceptible to noise and distortion compared to amplitude-based modulation techniques.
• The implementation of FSK modulation and demodulation is straightforward and does not require complex hardware.
• FSK provides reliable data transmission, especially in environments where signal integrity is a concern.

The correct answer is 1) QPSK (Quadrature Phase Shift Keying).

Explanation

• QPSK (Quadrature Phase Shift Keying):
  • QPSK is a multilevel modulation technique that uses four distinct phase shifts to represent four different symbols.
  • These four phases are typically separated by 90 degrees (e.g., 45°, 135°, 225°, and 315°).
  • Each symbol represents two bits of information, allowing for twice the data rate compared to BPSK.
• Other options:
  • BFSK (Binary Frequency Shift Keying): Uses two different frequencies.
  • BPSK (Binary Phase Shift Keying): Uses two phase shifts.
  • 8-PSK: Uses eight phase shifts.

The correct option is 2

Concept:

The bandwidth of a communication system is broadly determined by the fastest rate at which it can transmit information, which, in terms of signal processing, can be related to the minimum pulse width it can reliably handle. Looking at the options provided:

• PPM (Pulse Position Modulation): In PPM, the position of a pulse within a fixed time frame is varied according to the amplitude of the modulating signal. The minimum pulse width in PPM would essentially impact the maximum number of distinct positions (and thus the modulation depth) that can be discriminated within that frame, which in turn affects the bandwidth. However, PPM's bandwidth efficiency is more complex, involving the timing accuracy and how finely the pulse position can be adjusted and detected rather than just the pulse width.
• PDM (Pulse Density Modulation) or PWM (Pulse Width Modulation): In the context of pulse width as a criterion (as "Pulse Density Modulation" might have been confused with "Pulse Width Modulation," PDM is often used for the latter), PWM is where the modulation is achieved by varying the width of the pulses according to the amplitude of the analog signal. Here, the minimum pulse width directly impacts the bandwidth as it defines the highest frequency component that the system needs to faithfully reproduce.
• QAM (Quadrature Amplitude Modulation): QAM is a modulation scheme that conveys two analog signal or digital bit streams by modulating the amplitudes of two carrier waves, using amplitude-shift keying (ASK) modulated with phase-shift keying (PSK). While it's a mixed modulation scheme and has parameters affecting bandwidth, the concept of "minimum pulse width" doesn't straightforwardly apply as it does with pulse-based modulation schemes like PPM or PWM because QAM fundamentally relies on the modulation of the amplitude and phase of a carrier wave, not the timing of pulses.
• All of these: Given the above explanations, saying that the "minimum pulse width" determines the bandwidth in all these technologies isn't accurate. While the minimum pulse width is a critical factor in pulse-based modulation techniques (like PWM, assuming PDM here refers to PWM), and has an impact on the signaling rate and, by extension, the bandwidth, QAM's bandwidth considerations are more influenced by the symbol rate and the efficiency of amplitude and phase use rather than pulse width.

Therefore, focusing strictly on the question of "minimum pulse width," it is most directly relevant to Pulse Width Modulation (PWM). However, since PWM isn't explicitly listed as an option and assuming a mix-up in terminology with PDM being intended to refer to PWM, it would be the most affected by minimum pulse width limitations in terms of bandwidth. QAM doesn't fit the context of the question as its bandwidth is not determined by pulse width. PPM does involve pulse timing but is less about pulse width and more about position within a given time frame.

The correct option is 2

Concept:

QPSK modulation represents symbols by a constellation of four-phase angles of the carrier signal, orthogonal to each other.

There are two bits per symbol. So for example 00 = 0°, 01 = 90° , 10 = 180°m and 11= 270°. 

QPSK transmits twice the data rate in a given bandwidth compared to BPSK - at the same BER. 

The constellation diagram for 4 different symbols is as shown:

Analogue modulation techniques:

1. Amplitude Modulation
2. Frequency Modulation
3. Phase Modulation

Digital Modulation Techniques:

1. Amplitude shift key
2. Frequency shift key
3. Phase Shift key
4. QAM

Notes:

ASK, PSK and FSK are signaling schemes used to transmit binary sequences through free space. In these schemes, bit-by-bit transmission through free space occurs.

FSK (Frequency Shift Keying):

In FSK (Frequency Shift Keying) binary 1 is represented with a high-frequency carrier signal and binary 0 is represented with a low-frequency carrier, i.e. In FSK, the carrier frequency is switched between 2 extremes.

For binary ‘1’ → S1 (A) = Acos 2π fHt

For binary ‘0’ → S2 (t) = A cos 2π fLt . The constellation diagram is as shown:

ASK(Amplitude Shift Keying):

In ASK (Amplitude shift keying) binary ‘1’ is represented with the presence of a carrier and binary ‘0’ is represented with the absence of a carrier:

For binary ‘1’ → S1 (t) = Acos 2π fct

For binary ‘0’ → S2 (t) = 0

The Constellation Diagram Representation is as shown:

where ‘I’ is the in-phase Component and ‘Q’ is the Quadrature phase.

PSK(Phase Shift Keying):

In PSK (phase shift keying) binary 1 is represented with a carrier signal and binary O is represented with 180° phase shift of a carrier

For binary ‘1’ → S1 (A) = Acos 2π fct

For binary ‘0’ → S2 (t) = A cos (2πfct + 180°) = - A cos 2π fct

The Constellation Diagram Representation is as shown:

Concept:

BPSK transfers one bit per symbol. It either transmits 0 or 1 at a time.

QPSK is a modulation scheme that allows one symbol to transfer two bits of data. There are four possible two-bit numbers (00, 01, 10, 11).

Calculation:

For BPSK M = 2 bandwidth = 2Rb

For QPSK M = 4 bandwidth = Rb

The bandwidth required for QPSK modulated channel is half of the bandwidth of BPSK.

In ASK modulation each symbol is transmitted using a single bit

Hence, the bit rate = baud rate

Baud rate = 800 baud

If the question would have asked QPSK in case of ASK

Then QPSK transfers 2 bit for 1 symbol

Baud rate = 800/2 = 400 baud

Digital to Analog Modulation technique is as shown.

As shown QAM is the mixture of both ASK and PSK.

Hence, amplitude and the phase of the carrier frequency both vary with the message signal.

Constellation diagram of a QAM signal with 2 different amplitude levels and 8 different phases is shown.

Concept:

QPSK is a modulation scheme that allows one symbol to transfer two bits of data. There are four possible two-bit numbers (00, 01, 10, 11).

For bandpass transmission,

Bandwidth for M-ary PSK  

Given:

BW = 36 × 10⁶ Hz

Roll-off factor = 0.2

Calculation:

Bandwidth for QPSK  

2 × 36 × 10⁶ = 1.2 R_b

R_b = 72/1.2 Mbps 

= 60 Mbps

Concept:

QPSK is a modulation scheme that allows one symbol to transfer two bits of data. There are four possible two-bit numbers (00, 01, 10, 11).

Analysis:

Transmission bandwidth

Maximum required bandwidth

40% of

Total = (0.2 + 0.5) R_b = 0.7 R_b

No. of channels required

Concept:

Probability of symbol error for coherently detected M-ary FSK:

Where

Probability of bit error for coherently detected M-ary FSK-

Calculation:

Given: M = 32

ASK, PSK and FSK are signaling schemes used to transmit binary sequences through free space. In these schemes, bit-by-bit transmission through free space occurs.

FSK (Frequency Shift Keying):

In FSK (Frequency Shift Keying) binary 1 is represented with a high-frequency carrier signal and binary 0 is represented with a low-frequency carrier, i.e. In FSK, the carrier frequency is switched between 2 extremes.

For binary ‘1’ → S1 (A) = Acos 2π fHt

For binary ‘0’ → S2 (t) = A cos 2π fLt . The constellation diagram is as shown:

ASK(Amplitude Shift Keying):

In ASK (Amplitude shift keying) binary ‘1’ is represented with the presence of a carrier and binary ‘0’ is represented with the absence of a carrier:

For binary ‘1’ → S1 (t) = Acos 2π fct

For binary ‘0’ → S2 (t) = 0

The Constellation Diagram Representation is as shown:

where ‘I’ is the in-phase Component and ‘Q’ is the Quadrature phase.

PSK(Phase Shift Keying):

In PSK (phase shift keying) binary 1 is represented with a carrier signal and binary O is represented with 180° phase shift of a carrier

For binary ‘1’ → S1 (A) = Acos 2π fct

For binary ‘0’ → S2 (t) = A cos (2πfct + 180°) = - A cos 2π fct

The Constellation Diagram Representation is as shown:

ASK, PSK and FSK schemes are signaling schemes used to transmit binary sequences through free space. In these schemes, bit-by-bit transmission through free space occurs.

In ASK (Amplitude shift keying) binary ‘1’ is represented with the presence of a carrier and binary ‘0’ is represented with the absence of a carrier:

For binary ‘1’ → S₁ (t) = Acos 2π f_ct

For binary ‘0’ → S₂ (t) = 0

In PSK (phase shift keying) binary 1 is represented with a carrier signal and binary O is represented with 180° phase shift of a carrier

For binary ‘1’ → S₁ (A) = Acos 2π f_ct

For binary ‘0’ → S₂ (t) = A cos (2πf_ct + 180°) = - A cos 2π f_ct

The constellation diagram for binary PSK is given as:

Concept:

Resolution is given as:

Resolution        ---(1)

n - number of bits

Transmission B.W = R_b       ---(2)

Bit Rate(R_b) = m ⋅ n ⋅ f_s

m – number of message signals

n – number of bits

f_s – sampling frequency

Calculation:

Given:

m = 8,

f_m = 5 kHz,

Range = 0 to +2 V

From equation (1)

n – 1 = 8

n = 9

from equation (2);

BW = R_b = m ⋅ n ⋅ f_s

∴ f_s = 2 f_m = 10 kHz

BW = 8 × 9 × 10

BW = 720 kHz