(Aktu Btech) Analog and Digital Communication Important Unit-4 Digital Modulation Techniques

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Important Questions For Analog and Digital Communication:
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Q1. Explain digital modulation.

Ans. Digital modulation: 

• 1. Modulation is the process of altering a carrier’s characteristic in accordance with a modulating wave.
• 2. The modulating wave in digital transmission is made up of binary data.
• 3. A sinusoidal wave must be used as the carrier.
• 4. With a sinusoidal carrier, the modulator uses a step change in the amplitude, frequency, or phase of the carrier to identify one signal from another.

Q2. Write short note on ASK, FSK and PSK. 

Ans. Need: 

• 1. Used in image processing for pattern recognition robotic version and image enhancement, 
• 2. Used in digital signal processing 
• 3. In telephone for text messaging 

Types of digital modulation:

• a. Amplitude shift keying: 
  • 1. Depending on the carrier frequency, the input data determines whether the output amplitude should be zero or a range of positive and negative values.
  • 2. It displays the binary data as fluctuations in the signal’s amplitude.
  • 3. When ASK is modulated, the binary signal outputs the carrier signal for HIGH input and zero for LOW input.
• b. Frequency shift keying: 
  • 1. Depending on the input data used, the output signal’s frequency will either be high or low.
  • 2. In this, the carrier signal’s frequency changes in accordance with the discrete digital modifications.
  • 3. A FSK modulated wave has a high frequency output for binary HIGH inputs and a low frequency output for binary LOW inputs.
• c. Phase shift keying: The phase of the output signal gets shifted depending upon the input. 

Waveforms of ASK, PSK and FSK :  

Q3. Define the method of generation of ASK signals.

Ans. Amplitude shift keying: Amplitude shift keying (ASK) is the simplest digital modulation technique. The ASK waveform may be represented as 

Generation of ASK signal: Just applying the incoming binary data and sinusoidal carrier to the two inputs of a product modulator or balanced modulator will yield an ASK signal. The ASK waveform will be the output as a result.

Coherent detection of Binary ASK: 

• 1. The demodulation of binary ASK can be achieved by using coherent detector.
• 2. The decision making device compares the output of integrator with a preset threshold.

• 3.The linear operation is used by the coherent detector. The locally generated carrier in this manner has the same frequency and phase as the carrier utilized at the transmitter. This detection technique is referred to as coherent detection for this reason.

Non-coherent detection of Binary ASK: 

• 1. Binary ASK can also be achieved by using envelope detector.
• 2. This method avoids the use of synchronization of local carrier. 
• 3. This method uses some form of rectification and low pass filtering at the receiver. The diagram of such detector is shown in Fig. 

Q4. Discuss the method for detection of DPSK with the help of neat diagram.

Ans. Detection of DPSK: 

• 1. At the receiver input, the received DPSK signal plus noise is passed through a bandpass filter centered at the carrier frequency f_c so as to limit the noise power. 
• 2. A correlator is used to compare the output of the filter and its delayed version.
• 3. The cosine of the difference between the carrier phase angles in the two correlator inputs determines the output of the correlator that results.
• 4. The correlator output is then judged in favour of symbols 1 and 0 by comparison to a threshold of zero volts.
• 5. If the output of the correlation device is positive, the receiver chooses the symbol “1.” The receiver selects symbol 0 if the correlator output is negative.

Q5. Write a short note on the QPSK receiver.

Ans.

• 1. The QPSK receiver consists of a pair of correlators with a common input and supplied with a locally generated pair of coherent reference signal ɸ₁(t) and ɸ₂(t). 
• 2. The corresponding output of correlators x₁ and x₂ produced in response to the received signal x(t), are each compared with a threshold of zero. 
• 3.  If x₁ > 0, a decision is made in favor of symbol 1 for in-phase channel output, but if x₁ < 0, a decision is made in favor of symbol 0.
• 4. Similarly if x₂ > 0, a decision is made in favor of symbol 1 for quadrature channel output, but if x₂ < 0 a decision is made in favor of symbol 0.

• 5. Finally, these two binary sequences at the in-phase and quadrature channel outputs are combined in a multiplexer to reproduce the original binary sequence at the receiver output.

Q6. Explain with the help of diagram, a method of generating and demodulating phase shift keying signal.

Ans. 1. In a coherent binary PSK system, the pair of signals, S₁(t) and S₂(t) are used to represent binary symbols 1 and 0, respectively.

2. It is given by 

where, 0 ≤ t< T_b and E_b is transmitted signal energy per bit. 

3. In order to ensure that each transmitted bit contains an integral number of cycles of the carrier wave, the carrier frequency f_c is chosen equal to n_c/T_b for some fixed integer n_c.

Generation: 

• 1. To generate a binary PSK wave, the input binary sequence is represented in polar form with symbols 1 and 0 represented by constant amplitude levels of

• 2. The binary wave and sinusoidal carrier wave ɸ₁(t) are applied to product modulator.
• 3. The carrier and the timing pulses used to generate the binary wave are usually extracted from a common master clock. The desired PSK wave is obtained at output.  

Detection of coherent Binary PSK: 

• 1. To detect the original binary sequence of 1’s and 0’s, the noisy PSK wave x(t) is applied to a correlator, which is also supplied with a locally generated coherent reference signal ɸ₁(t). 
• 2. The correlator output x₁, is compared with a threshold of zero volts. 3. If x₁ > 0, the receiver decides in favor of symbol 1. On the other hand, if x₁ < 0, it decides a favor of symbol 0.

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