This document discusses digital modulation techniques. It describes how digital modulation encodes digital signals into analog carrier signals by varying the amplitude, frequency, or phase. It explains the main digital modulation types: ASK, FSK, and PSK. ASK varies amplitude, FSK varies frequency, and PSK varies phase. BPSK and QPSK are described as phase shift keying techniques. QPSK encodes 2 bits per symbol by shifting the phase by 45 degree increments. The document provides examples and diagrams to illustrate digital modulation encoding.

1. Quaid-e-Awam University Of Engineering Science
And Technology Nawabshah
Subject: Advanced Communication System
Topic: Digital Modulation
Presented By:
Ghullam Muaeed (19ES28)
Department:
Electronic Engineering

2. Digital Modulation
• Digital modulation is the process of encoding a digital
information signal into the amplitude, phase, or frequency
of the transmitted signal.
• The encoding process affects the bandwidth of the
transmitted signal and its robustness to channel
impairments.
• This modulation technique encodes several bits into one
symbol, and the rate of symbol transmission determines
the bandwidth of the transmitted signal.

3. Digital Modulation
• Since the signal bandwidth is determined by the
symbol rate, having a large number of bits per
symbol generally yields a higher data rate for a given
signal bandwidth.
• However, the larger the number of bits per symbol,
the greater the required received SNR for a given
target BER.

4. Digital Modulation
• The modulation scheme can be broken down in to
three main types.
• ASK or Amplitude Shift Key
• FSK or Frequency Shift Key
• PSK or Phase Shift Key

5. Digital Modulation (ASK or Amplitude Shift
Key)
• This form of modulation is when the carrier amplitude
is varied in proportion to message signal.

6. Digital Modulation (FSK or Frequency Shift
Key )
• This type of modulation transmits data by varying the frequency
of the carrier.
• In this modulation, the carrier has two predefined frequencies.

7. Digital Modulation (PSK or
Phase Shift Key)
• With PSK, the phase of the carrier is shifted to transmit data.

8. Digital Modulation (ASK &
FSK)

9. Digital Modulation (ASK & FSK)
• Figure shows a basic serial digital signal of binary zeros
and ones to be transmitted and the corresponding AM
and FM signals resulting from modulation.
• There are two types of AM signals: on-off keying (OOK)
and amplitude shift keying (ASK).
• In Figure (a), the carrier amplitude is shifted between two
amplitude levels to produce ASK.
• In Figure 1b, the binary signal turns the carrier off and on
to create OOK.

10. Digital Modulation (ASK & FSK)
• Frequency shift keying (FSK) shifts the carrier
between two different frequencies called the mark
and space frequencies, or fm and fs :
• FM produces multiple sideband frequencies above
and below the carrier frequency.

11. Digital Modulation (PSK)
• A very popular digital modulation scheme, phase shift
keying (PSK), shifts the carrier sine wave 180° for each
change in binary state also called Binary Phase Shift
Keying (BPSK)
• BPSK is coherent as the phase transitions occur at the
zero crossing points.
• The proper demodulation of BPSK requires the signal to
be compared to a sine carrier of the same phase.
• This involves carrier recovery and other complex circuitry.

12. Digital Modulation (PSK)
• A very popular digital modulation scheme, phase shift keying
(PSK), shifts the carrier sine wave 180° for each change in binary
state also called Binary Phase Shift Keying (BPSK)
• BPSK is coherent as the phase transitions occur at the zero
crossing points.

13. Digital Modulation (BPSK)
• A simpler version is differential BPSK or DPSK,
where the received bit phase is compared to the
phase of the previous bit signal.
• BPSK is very spectrally efficient in that you can
transmit at a data rate equal to the bandwidth or 1
bit/Hz.
• BPSK has more variations ; quadrature PSK (QPSK),

14. Digital Modulation (QPSK)
• The modulator produces two sine carriers 90° apart.
• The binary data modulates each phase, producing four
unique sine signals shifted by 45° from one another.
• The two phases are added together to produce the final
signal.
• Each unique pair of bits generates a carrier with a
different phase:
• shown in following table

15. Digital Modulation (QPSK)

16. Digital Modulation (QPSK)
Phasor diagram

17. Digital Modulation (QPSK)
• In ASK, a binary 0 is one amplitude and a binary 1 is
another amplitude.
• In FSK, a binary 0 is one carrier frequency and a
binary 1 is another frequency.
• BPSK uses a 0° shift for a binary 0 and a 180° shift
for a binary 1.
• In each of these cases there is one bit per symbol.