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Quadrature Amplitude Modulation

(QAM)

QAM

Date

Transmitting Information … (Analog or

Digital)

Modify a Signal

“Modulate”

Detect the Modifications

“Demodulate”

Any reliably detectable change in signal

characteristics can carry information

QAM

Date

Signal Properties

QAM

Date

Analog Modulation

Amplitude

(AM)Frequency -

FM

θV= A(t) sin[

(t)]

V= A(t) sin[2 f(t) +

(t)]

π φ

Phase - M

QAM

Date

Analog-to-Digital …

Amplitude

Frequency

Phase

Both Amplitude and

Phase

1 0 1 0

ASK

FSK

PSK

Discrete States

QAM

Date

Concept of I-Q …

PhaseMag

0 deg

(ref.)

“Q” - Quadrature

“I” – In PhaseI value

Q value

Polar Notation:

Magnitude &

Phase

I-Q Notation:

In Phase &

Quadrature

QAM

Date

Signal Changes or Modifications

Phase

Mag

0 deg

Magnitude Change

Phase0 deg

Phase Change

Frequency Change

Both Change0 deg

0 deg

QAM

Date

I-Q Modulation

Transmitter

Receiver

QAM

Date

I-Q Modulation Examples

I

Q

01I

Q0001

1011

BPSK

1bit/sym

Symbol rate = bit rate

QPSK

2bits/sym

Symbol rate = ½ bit rate

I

Q00000011

1100111

1 16-QAM

4bits/sym

Symbol rate =1/4 bit rate

Symbol

Constellation Diagrams

QAM

Date

Bit Rate and Symbol Rate

BIT RATE is the frequency of the system bit stream

SYMBOL RATE is the bit rate divided by the number of

bits that can be transmitted with each symbol (also

known as the BAUD RATE)

� Signal Bandwidth depends on the Symbol Rate

� More complicated modulation formats use less

(narrower) spectrum to provide the same

transmission capacity

QAM

Date

Spectral Efficiency

Trade-Off: as the complexity of the modulation increases, the radio becomes more spectrally efficient. However, it also becomes more susceptible to errors caused by noise and distortion.

QAM

Date

256-QAM

QAM

Date

Error Vector Magnitude

{

I

Q Magnitude Error (IQ error mag)

Error Vector

Ideal (Reference) Signal

Phase Error (IQ error phase)

TestSignal

φφφφ

(

)

average error magnitude

peak symbol magnitudeEVM (%) = x 100

EVM is a measure of the quality of modulation.

QAM

Date

Some QAM Applications

WLAN – 16/64 qam

Point-to-Point – 16/32/64/128/256 qam

Digital Video – 16/32/64 qam

Cable TV – 16/64/256 qam

QAM

Date

Lets look at some pre-recorded QAM

signals …

QAM

Date

256-QAM Recorded Signal