This document discusses quadrature amplitude modulation (QAM) and orthogonal frequency-division multiplexing (OFDM). It provides background on digital modulation techniques before explaining the theories and mechanisms of QAM and OFDM. QAM uses two carriers shifted by 90 degrees that are modulated based on both amplitude and phase. OFDM is a multi-carrier modulation scheme where each carrier's amplitude is modulated and the subcarriers are separated by their orthogonal frequencies. The document outlines advantages such as high data rates and robustness to multipath fading, as well as disadvantages like sensitivity to frequency offsets. It concludes that OFDM performs better than single carriers for multipath channels when guard intervals are implemented properly.

2.  Background, history
 Review of digital modulation
 FDMA vs. Multi-carrier modulation
 Theory of OFDM an QAM
 Advantages and disadvantages
 Conclusion
2

3.  The first QAM-related so-called orthogonal
frequency division multiplexing (OFDM) schem
was proposed by Chang in 1966 for dispersive
fading channels, which has also undergonea
dramatic evolution due to the efforts of
Weinstein, Peled, Ruiz, Hirosaki, Kolb.
 OFDM was standardised as the European digital
audio broadcast (DAB) as well as digital video
broadcast (DVB) scheme.
 It constituted also a credible proposal for the
recent third-generation mobile radio standard
competition in Europe.

4.  Quadrature Amplitude Modulation, QAM is a
signal in which two carriers shifted in phase
by 90 degrees are modulated and the
resultant output consists of both amplitude
and phase variations.
 In view of the fact that both amplitude and
phase variations are present it may also be
considered as a mixture of amplitude and
phase modulation.
 Digital QAM modulates digital information
onto pulses that are modulated onto
Amplitudes of a sine and a cosine, or equivalently
Amplitude and phase of single sinusoidal.

5. 5
 Example: y(t) = f(t) cos(wc t)
Assume f(t) is an ideal lowpass signal with bandwidth w1
Assume w1 < wc
Y(w) is real-valued if F(w) is real-valued
 Demodulation: modulation then lowpass filtering
 Similar derivation for modulation with sin(w0 t)
w
0
1
w1
-
w1
F(w)
w
0
Y(w)
½
-wc -
w1
-wc +
w1
-wc
wc - w1 wc + w1
wc
½F(w - wc)½F(w + wc)

6. 6
 Example: y(t) = f(t) sin(wc t)
Assume f(t) is an ideal lowpass signal with
bandwidth w1
Assume w1 < wc
Y(w) is imaginary-valued if F(w) is real-valued
 Demodulation: modulation then lowpass
filtering
w
Y(w)
j ½
-wc -
w1
-wc +
w1
-wc
wc - w1 wc + w1
wc
-j ½F(w - wc)j ½F(w + wc)
-j ½
w
0
1
w1-w1
F(w)

7. 7
Serial/Parallel
Map to 2-D
constellation
Impulse modulator
Impulse modulator
Pulse shaping gT(t)
Local
Oscillator
+
90o
Pulse shaping gT(t)
d[n]
an
bn
a*(t)
b*(t)
s(t)
1 J
Matched
Delay
Matched delay matches delay through 90o phase shifter
bit
stream

8. 8
 90o phase shift performed by Hilbert
transformer
cosine => sine
sine => – cosine
 Frequency response of ideal
Hilbert transformer:
)(
2
1
)(
2
1
)2cos( 000 fffftf -++ 
)(
2
)(
2
)2sin( 000 ff
j
ff
j
tf --+ 
)sgn()( fjfH -







-



0if1
0if0
0if1
)sgn(
x
x
x
x

9. 9
 Magnitude
response
All pass except at origin
 For fc > 0
 Phase response
Piecewise constant
 For fc < 0
)2sin()
2
2cos( tftf cc 

 +
))(2sin()
2
)(2cos(
))
2
2(cos()
2
2cos(
tftf
tftf
cc
cc
-+-
+--







f
)( fH
-
90o
90o
f
|)(| fH
)sgn()( fjfH -

10.  Multi-carrier Amplitude modulation
scheme
◦ Each Carrier’s amplitude is modulated
 Well-suited for Wireless Communication

11.  OFDM carrier frequency is n・1/T
11
Symbol period T
cos( )2 1 0 1    +f t
T
f
1
0 
cos( )2 2 0 2    +f t
cos( )2 3 0 3    +f t
cos( )2 4 0 4    +f t
cos( )2 5 0 5    +f t
cos( )2 6 0 6    +f t

12.  OFDM is multi carrier modulation
 OFDM sub-carrier spectrum is overlapping
 In FDMA, band-pass filter separates each
transmission
 In OFDM, each sub-carrier is separated by
DFT because carriers are orthogonal
 Each sub-carrier is modulated by PSK,
QAM
12
Thousands of PSK/QAM symbol can be
simultaneously transmitted in one OFDM symbol

13. 13

16. 16

17. Advantages
 Very easy and efficient in dealing with multi-
path
 Ofdm/qam provide high data rate.
communication and effectively remove ISI.
 Disadvantages
 Sensitive to frequency offset and phase noise
 Peak-to-average problem reduces the power
efficiency of RF amplifier at the transmitter.

18.  Performs better than a single modulated
carrier in multipath fading
 With a properly implemented guard
interval:
– Time waveform appears periodic
– orthogonality of subcarriers is
ensured
– ISI and ICI are eliminated
18

19. THANK YOU