EC 6501 – DIGITAL COMMUNICATION

UNIT - IV

PART – A

1. Distinguish coherent vs non coherent digital modulation techniques. [N/D-16]

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.

2. Draw a block diagram of a coherent BFSK receiver. [N/D-16]

3. What is signal constellation diagram? (Dec-08)

The signal constellation diagram is similar to the phasor diagram but the entire phasor is not drawn. The

signal constellation diagram shows only relative positions of the peaks of the phasors. The signal

constellation diagram is also called state space diagram.

4. Distinguish between coherent and non-coherent reception. [M/J-16]

5.Differentiate between coherent and non-coherent detection [N/D -11]

6. Differentiate between coherent and non-coherent detection [A/M -12]

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 is QPSK? Write the expression for the signal set of QPSK. [M/J-16]

8. Define QPSK. [N/D-09]

QPSK is Quadriphase –shift keying. In QPSK the phase of the carrier takes on one of the four

equally spaced values Such as π/4, 3π/4, 5π/4 and 7π/4.

In QPSK two successive bits in the data sequence are grouped together. This combination of

two bits forms four distinct symbols. When symbols are changed to next symbol the phase of

the carrier is changed by 450

.

9. What is the error probability of MSK & DPSK?

Error probability of MSK: Pe = 1/2erfc√E/No

Error probability of DPSK: Pe = 1/2e -Eb/No

10. What are the drawbacks of binary PSK system? [A/M -12]

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.

11. Draw the PSK waveform for 011011. . [A/M - 11]

12 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?[N/D-12]

13. What is correlator? Correlator is the coherent receiver. It correlates the received noisy signal f(t) with the locally

generated replica of the unknown signal x(t). Its output is denoted as r(t).

14. List the advantages of Pass band transmission.

a. Long distance.

b. Analog channels can be used for transmission.

c. Multiplexing techniques can be used for bandwidth conservation.

d. Transmission can be done by using wireless channel also.

PART – B

1. Describe the generation and detection of coherent binary PSK Signals. (10) [N/D-16]

2. Explain the concept of coherent BPSK with transmitter and receiver block diagrams and obtain the expression for probability of error. [ N/D -11]

3. Explain the generation and detection of coherent binary PSK signal and derive The power

spectral density of binary PSK signal and plot it, (16) [M/J-16]

4. Illustrate the power spectra of binary PSK signal. (6)

5. Describe the generation and detection of coherent QPSK signals. (12)

6. (i) Discuss QPSK signaling. (ii) Derive the bit error probability due to QPSK receiver. Compare the performance of QPSK receiver with that of PSK receiver. . [A/M -11]

7. Discuss QPSK signaling. Derive the bit error probability due to QPSK receiver. [A/M -13] 8. Discuss QPSK signaling. Derive the bit error probability due to QPSK receiver. [M/J -13]

In a sense, QPSK is an expanded version from binary PSK where in a symbol consists of two bits and

two orthonormal basis functions are used. A group of two bits is often called a „dibit‟ . So, four dibits are

possible. Each symbol carries same energy. Let, E: Energy per Symbol and T: Symbol Duration = 2.*

Tb, where Tb: duration of 1 bit.

9. Draw the functional block diagram of modulator for QAM and explain its operation. (8)

[A/M – 12]

10. Explain the non-coherent detection of FSK signal and derive the expression for Probability

of error. (16) [N/D – 13]