Analog and Digital Communication

1. UNIT 5
DIGITAL BAND PASS
MODULATION TECHNIQUES

2. Contents
• Preliminaries
• Binary Amplitude Shift Keying
• Phase Shift Keying: Binary Phase Shift Keying (BPSK), Generation and
Coherent Detection of BPSK signals, Quadrature Phase Shift Keying,
Generation and Coherent Detection of QPSK signals, Offset Quadrature
Phase Shift Keying
• Frequency Shift Keying: Binary Frequency Shift Keying, Continuous Phase
Frequency Shift Keying, Minimum Shift Keying
• Non coherent Digital Modulation Schemes: Differential Phase Shift Keying
• Orthogonal Frequency Division Multiplexing.

3. Preliminaries
• Digital modulation techniques dealing with band-pass data
transmission in which a modulation strategy configured around a
sinusoidal carrier whose amplitude, phase, or frequency is varied in
accordance with the information-bearing data stream
• Three basic modulation schemes:
• Amplitude-Shift Keying (ASK)
• Phase-Shift Keying (PSK)
• Frequency-Shift Keying (FSK)

4. • A digital communication system is said to be coherent if the receiver
is synchronized to the transmitter with respect to carrier phase;
otherwise, the system is said to be noncoherent.
• Naturally, a noncoherent system offers the practical advantage of
reduced complexity but at the cost of degraded performance.

5. • Given a binary source that emits symbols 0 and 1, the modulation
process involves switching or keying the amplitude, phase, or
frequency of a sinusoidal carrier wave between a pair of possible
values in accordance with symbols 0 and 1. To be more specific,
consider the sinusoidal carrier,

6. Three distinct forms of binary modulation are defined as follows:
Binary amplitude shift-keying (BASK), in which the carrier frequency and carrier phase
are both maintained constant, while the carrier amplitude is keyed between the two
possible values used to represent symbols 0 and 1.
Binary phase-shift keying (BPSK), in which the carrier amplitude and carrier frequency
are both maintained constant, while the carrier phase is keyed between the two possible
values (e.g., 0° and 180°) used to represent symbols 0 and 1.
Binary frequency-shift keying (BFSK), in which the carrier amplitude and carrier phase
are both maintained constant, while the carrier frequency is keyed between the two
possible values used to represent symbols 0 and 1.

7. The carrier amplitude is given by,
Thus the sinusoidal carrier is given by,
Consider a linear modulation scheme for which the modulated wave is defined by,
where b(t) denotes an incoming binary wave.

8. The transmitted signal energy per bit is given by,

10. Binary Amplitude Shift Keying
• ON–OFF signaling

11. Binary Phase Shift Keying (BPSK)

12. Signal Space Diagram Bit Error Rate

13. Generation
Detection

14. Quadrature Phase Shift Keying

15. Signal Space Diagram
Bit Error Rate

16. Generation

17. Detection

18. OQPSK
• The extent of amplitude fluctuations exhibited by QPSK signals may
be reduced by using a variant of quadriphase-shift keying known as
the offset quadriphase-shift keying (OQPSK).
• However, the ±90° phase transitions in OQPSK occur twice as
frequently but with a reduced range of amplitude fluctuations,
compared with QPSK.
• In addition to the ±90° phase transitions, there are also ±180° phase
transitions in QPSK.
• Therefore amplitude fluctuations in OQPSK due to filtering have a
smaller amplitude than in OQPSK.

19. Non coherent Digital Modulation Schemes:
Differential Phase Shift Keying
• Both amplitude-shift keying and frequency-shift keying lend
themselves naturally to noncoherent detection whenever it is
impractical to maintain carrier-phase synchronization of the receiver
to the transmitter.
• But in the case of phase-shift keying, we cannot have noncoherent
detection in the traditional sense because the term “noncoherent”
means having to do without carrier-phase information.
• To get around this difficulty, we employ a “pseudo PSK” technique
known as differential phase-shift keying (DPSK), which, in a loose
sense, does permit the use of noncoherent detection.

20. • DPSK eliminates the need for a coherent reference signal at the
receiver by combining two basic operations at the transmitter:
• Differential encoding of the input binary wave.
• Phase-shift keying.
• It is because of this combination “differential phase-shift keying.” In
effect, to send symbol 0, we phase advance the current signal
waveform by 180 degrees, and to send symbol 1 we leave the phase
of the current signal waveform unchanged.

22. Generation

23. Detection

24. OFDM

27. BFSK

28. Bit Error Rate

31. MSK

35. Bit Error Rate