Different Types of Digital Modulation Schemes

Digital modulation techniques

Amplitude Shift Keying (ASK):

Change amplitude with each symbol

Frequency constant

Low bandwidth requirements

Very susceptible to interference

Frequency Shift Keying (FSK):

Change frequency with each symbol

Needs larger bandwidth

Phase Shift Keying (PSK):

Change phase with each symbol

More complex

Robust against interference

Signal Space Representation for ASK

Clearly, there is one basis function of unit energy

Then,

Average energy per bit

A binary ASK system is therefore characterized by having a signal space that is one-

dimensional

Probability of Error for ASK

Dependence of the error probability on the distance between two signal points. In

general, is

Thus, for ask Probability of Error is:

Concept of a constellation diagram

• A constellation diagram helps us to define the amplitude and phase of a signal when

we are using two carriers.

Constellation Diagram of ASK

s( t )={A cos(2 πf c t ) , binary 10 , binary 0

Use of ASK

• The simplest and most common form of ASK operates as a switch. (OOK)

• Very high speeds over optical fiber.

• Frequency Spectrum of ASK,

• Here, we can choose our fc so that the modulated signal occupies available

bandwidth.

Signal Space for FSK

• Unlike ASK, here two orthogonal basis functions are required to represent s1 (t) and s2

(t).

• Signal space representation,

Decision Regions of FSK

• Signal space diagram for binary FSK

• The receiver decides in favor of s1 (t) if the received signal point represented by the

observation vector r falls inside region R1. This occurs when r1 > r2

• When r1 < r2, r falls inside region R2 and the receiver decides in favor of s2 (t).

Probability of Error for FSK

Dependence of the error probability on the distance between two signal points. In

general, is

Thus,

Identical to that of ASK

Signal Space Representation for BPSK

• Clearly, there is one basis function of unit energy

• Then,

• A binary PSK system is therefore characterized by having a signal space that is in

one-dimension.

Decision Rule of BPSK

• Then the optimum decision boundary is the midpoint of the line joining these two

message points

Decision rule:

• Guess signal s1 (t) (or binary 1) was transmitted if the received signal point r falls in

region R1 (r > 0)

• Guess signal s2 (t) (or binary 0) was transmitted otherwise (r ≤ 0)

Probability of Error for BPSK

• Dependence of the error probability on the distance between two signal points. In

general, is

• Since the signals s1(t) and s2(t) are equally likely to be transmitted, the average

probability of error is

Constellation Diagram and Advantages of PSK

• Noise can change the amplitude easier than it can change the phase. In other words,

PSK is less susceptible to noise than ASK.

• PSK is superior to FSK because we do not need two carrier signals.

Probability of Error and the Distance Between Signals

Quadrature Phase Shift Keying (QPSK)

• In QPSK, we parallelize the bit stream so that every two incoming bits are split up

and PSK a carrier frequency.

• One carrier frequency is phase shifted 90 from the other - in quadrature.

• The two PSK signals are then added to produce one of 4 signal elements.

Quadrature Phase Shift Keying (QPSK):

• Also a type of linear modulation scheme

• The phase of the carrier takes on 1 of 4 equally spaced values, such as 0, p/2, p, and

3p/2 where each value of phase corresponds to a unique pair of message bits.

• The QPSK signal for this set of symbol states may be defined as:

s(t) = Ac cos( 2π fct + Φ (t)) Where, ϕ(t)= 0, 90, 180, 270

Simplifying Equation 1

This Gives the Idea about Transmitter design

QPSK Waveform and Transmitter design

QPSK Constellation Diagram

• Quadrature Phase Shift Keying has twice the bandwidth efficiency of BPSK

since 2 bits are transmitted in a single modulation symbol.