Chapter 4 part 2_a Digital Modulation Techniques.

Chapter 4 part 2_a

Digital Modulation Techniques

Chapter 4 (part 2a) Overview

Digital Modulation techniques (part 2) Bandpass data transmission

Amplitude Shift Keying (ASK) Phase Shift Keying (PSK) Frequency Shift Keying (FSK) Quadrature Amplitude Modulation (QAM)

Digital Modulation Techniques

Digital modulation The process by which digital symbols are

transformed into waveforms that are compatible with the characteristic of the channel.

Bandpass modulation Process whereby the amplitude, frequency, or

phase of an RF carrier, or a combination of them, is varied in accordance with the information to be transmitted.

Digital Bandpass Modulation A carrier signal has three parameters which can be

used for impressing:

)](cos[)()(

)()(

)(cos)()(

0

0

tttAts

ttt

ttAts

Amplitude Frequency Phase

Digital Bandpass Modulation

• If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK)

• If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)

Digital Bandpass Modulation

If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK)

If both the amplitude,V and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)

Amplitude Shift Keying (ASK)

ASK demonstrates poor performance, as it is heavily affected by noise and interference.

Used in radio telegraphy in the early 1900s

Amplitude Shift Keying (ASK)

M was chosen to be equal to 2, so it is corresponding to two waveform types. Also know as Binary ASK signaling (also called

on-off keying)

Frequency Shift Keying (FSK)

Bandwidth occupancy of FSK is dependant on the spacing of the two symbols. A frequency spacing of 0.5 times the symbol period is typically used.

FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states.

Frequency Shift Keying (FSK) M was chosen to be equal to 3, corresponding to the 3

waveform types (3-ary). Emphasize the mutually perpendicular axes.

The signal set is characterized by Cartesian coordinates, such that each of the mutually perpendicular axes represents a sinusoid with a different frequency. Such mutually perpendicular vectors are called orthogonal

signals.

Phase Shift Keying (PSK) Phase Shift Keying (PSK) demonstrates better performance

than ASK and FSK. PSK can be expanded to a M-ary scheme, employing multiple

phases and amplitudes as different states. Filtering can be employed to avoid spectral spreading. Widely used in both military and commercial communications

system.

Phase Shift Keying (PSK)

M was chosen as to be as 2, and it is called binary PSK (BPSK)

The modulating signal shifts the phase of the wave si(t) to one of two states, either zero or π (180º).

For the BPSK example, the vector picture illustrates the two 180º opposing vectors.

Signal sets that can be depicted with such opposing vectors are called antipodal signal sets.

Phase Shift Keying (PSK)

Constellation of two-level PSK

Phase Shift Keying (PSK)

Phase Shift Keying (PSK)

4-PSK has more efficient usage of bandwidth than 2-PSK, because each signal unit has two bits. For the same bandwidth, the data bit rate doubles.

Phase Shift Keying (PSK) Excellent performance of 2-PSK encourages us to go

with 4-PSK, also called quadrature PSK (Q-PSK)

Phase Shift Keying (PSK) The idea can be extended to 8-PSK, 16-PSK, 32-

PSK,…. The limitation is the ability of equipment to

distinguish small differences in signal’s phase.

8 PSK

BPSK Modulator

Binary PSK (BPSK) modulation can be accomplished by simply multiplying the original signal d(t) (which is a binary random sequence) by the carrier signal, which is an analog sinusoidal oscillation. After multiplication a bandpass filter is required

QPSK Modulator

Quadrature Amplitude Modulation (QAM)

Combination of ASK and PSK which helps making a contrast between signal units. The number of amplitude shifts should be lower than the number of phase shifts due to noise susceptibility of ASK.

Quadrature Amplitude Modulation (QAM)

Summarize