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Chapter: Digital Communication - Digital Modulation Scheme

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Important Short Questions and Answers: Digital Modulation Scheme

Digital Communication - Digital Modulation Scheme - Important Short Questions and Answers: Digital Modulation Scheme

DIGITAL MODULATION SCHEME

 

 

1.Define QAM and draw its constellation diagram. ?




2. A binary frequency shift keying system employs two signaling frequencies 1f and 2f. The lower frequency 1f is 1200 Hz and signaling rate is 500 Baud. Calculate 2f. ?

 

For binary FSK baud=fb

 

Fb=500Hz

Consider the FN modulation index(h) of 1 in FSK

Fm-fs/Fb =h=1

Fm-fs=fb

Fs=f1=1200HZ

Fm-1200Hz=500Hz

Fm=1700Hz,f2=fm=1700Hz.

 

3. Draw the PSK waveform for 011011.


 

4. What is meant by coherent detection system?

 

In coherent ASK, correlation receiver is used to detect the signal. Locally generated carrier is correlated with incoming Ask signal. The locally generated carrier is in exact phase either transmitted carrier. Coherent Ask is also called synchronous ASK.

 

5. Why is PSK always preferable over ASK in coherent detection?

 

ASK is on-off signaling where as the modulated carrier is continuously transmitted in PSK. Hence peak power requirement is more in ASK, where it is reduces in PSK.

 

6. Differentiate between coherent and non-coherent detection

In coherent detection the local carrier generated at the receiver is phase locked with the carrier at the transmitter. Hence it is also called synchronous detection. In non coherent detection the local carrier generated at the receiver not be phase locked with the carrier at the transmitter. It is simple, but it has higher probability of error.

 

7. What are the drawbacks of binary PSK system?

It is difficult to detect +b(t) and –b(t) because of squaring in the receiver Problem, of ISI and inter channel interference are present.

 

8. A BPSK system makes errors at the average rate of 1000 errors per delay. Data rate is 1 kbps . The single-sided noise power spectral density is 10-20 W/Hz. Assuming the system to be wide sense stationary,

what is the average bit error probability?

 

 

24*60*60=86400sec 86.4*106

 

Bir error probability Pe=100/86.4*106 =1.1157*10-6

 

9. What is meant by DPSK?

 

In DPSK, the input sequence is modified. Let input sequence be d(t) and output Sequence be b(t). Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes level when d(t)=0.

 

When b(t) changes level, phase of the carrier is changed. And as stated above, b(t) changes t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1. Hence the technique is called Differential PSK.

 

10. Explain coherent detection?

 

In coherent detection, the local carrier generated at the receiver is phase locked with the carrier at the transmitter. The detection is done by correlating received noisy signal and locally generated carrier. The coherent detection is a synchronous detection.

 

11. Bring out the difference between coherent & non coherent binary modulation scheme.

 

a. Coherent detection:

 

In this method the local carrier generated at the receiver is phase locked with the carrier At the transmitter. Hence it is called synchronous detection

 

b. Non coherent detection:

 

In this method, the receiver carrier need not be phase locked with transmitter carrier. Hence it is called envelope detection.

 

12.      Write the expression for bit error rate for coherent binary FSK. Bit error rate for coherent binary FSK is given as,

 

Pe = 1/2erfc√0.6E/No

 

13.      What is Signal constellation diagram?

Suppose that in each time slot of duration T seconds, one s2(t), . . sM(t) is transmitted with equal probability, 1/M For geometric representation, the signal si (t), = 1, 2, ..., M, is applied to a bank of correlators. The correlator outputs define the signal vector si. The set of message points corresponding to the set of transmitted signals {si(t))} i=1..M is called a signal constellation.

 

14.      What is meant by memory less modulations?

 

When the digital symbol modulates amplitude, phase or frequency of the carrier without any reference to previous symbol, it is called memory less modulations. Eg.:ASK,PSK,FSK,QPSK etc.

 

15.Define QPSK.

 

·              In QPSK two successive bits in the data sequence are grouped together.This combination of two bits forms four distinct symbols.When the symbol is changed to next symbol the phase of the carrier is changed by 450(or π/4).

 

·              Because of combination of two bits there will be four symbols.Hence the phase shift will be π/4, 3π/4, 5π/4 or 7π/4.

QPSK reduces amplitude variations and required transmission bandwidth.

 

DIGITAL MODULATION SCHEME – IMPORTANT TERMS

 

QAM :

 

QAM is a combination of ASK and PSK



Coherent detection system:

 

In coherent ASK, correlation receiver is used to detect the signal. Locally generated carrier is correlated with incoming Ask signal. The locally generated carrier is in exact phase either transmitted carrier. Coherent Ask is also called synchronous ASK.

 

ASK

 

ASK is on-off signaling where as the modulated carrier is continuously transmitted in PSK. Hence peak power requirement is more in ASK, where it is reduces in PSK.

 

Binary PSK system:

 

It is difficult to detect +b(t) and –b(t) because of squaring in the receiver Problem, of ISI and inter channel interference are present.

 

DPSK:

 

In DPSK, the input sequence is modified. Let input sequence be d(t) and output Sequence be b(t). Sequence b(t) changes level at the beginning of each interval in which d(t)=1 and it does not changes level when d(t)=0.

 

When b(t) changes level, phase of the carrier is changed. And as stated above, b(t) changes t=its level only when d(t) =1. This means phase of the carrier is changed only if d(t)=1. Hence the technique is called Differential PSK.

 

Coherent detection:

 

In coherent detection, the local carrier generated at the receiver is phase locked with the carrier at the transmitter. The detection is done by correlating received noisy signal and locally generated carrier. The coherent detection is a synchronous detection.

Non coherent detection:

 

In this method, the receiver carrier need not be phase locked with transmitter carrier. Hence it is called envelope detection.

 

Bit error rate for coherent binary FSK.

 

Bit error rate for coherent binary FSK is given as,

 

Pe = 1/2erfc√0.6E/No

 

Signal constellation diagram:

 

Suppose that in each time slot of duration T seconds, one s2(t), . . sM(t) is

 

transmitted with equal probability, 1/M For geometric representation, the signal si (t), i = 1, 2, ..., M, is applied to a bank of correlators. The correlator outputs define the signal vector si. The set of message points corresponding to the set of transmitted signals {si(t))} i=1..M is called a signal constellation.

 

Memory less modulations:

 

When the digital symbol modulates amplitude, phase or frequency of the carrier without any reference to previous symbol, it is called memory less modulations. Eg.:ASK,PSK,FSK,QPSK etc.

 

QPSK.

 

·              In QPSK two successive bits in the data sequence are grouped together.This combination of two bits forms four distinct symbols.When the symbol is changed to next symbol the phase of the carrier is changed by 450(or π/4).

 

·              Because of combination of two bits there will be four symbols.Hence the phase shift will be π/4, 3π/4, 5π/4 or 7π/4.

QPSK reduces amplitude variations and required transmission bandwidth.

 

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