Quadrature Amplitude Modulation
Roshni A Chaudhari
ME EC
What is QAM?
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Quadrature amplitude modulation is both an analog
and a digital modulation scheme.
It conveys two analog message signals or two digital
bit streams, by changing amplitude of two carrier
waves, using the amplitude shift keying (ASK) digital
modulation scheme, or amplitude modulation scheme
for analog modulation.
Analog QAM
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The QAM is similar to DSB-SC but sends two message
signals over the same spectrum.
One of this message signal say m1(t) is sent in
phase, i.e by multiplying it with
and the other
message signal, m2(t) is sent in quadrature by
multiplying it with
and finally adding these two
signals.
A QAM signal represented as
QAM Transmitter
QAM Receiver
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The Demodulation uses coherent detection i.e
multiplication by carrier and then baseband low pass
filtering.
If
is multiplied with
we get
back and if we multiplied with
we get
back.
There is serious problem with QAM demodulation if
local oscillator has some phase error, this gives cochannel interference.
QAM Constellation
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In M-ary PSK modulation, amplitude of transmitted
signal was constrained to remain constant, there by
yielding a circular constellation.
By allowing the amplitude to also vary with the phase
we obtain QAM, so constellation consist of a squre
lattice of a signal points.
Constellation diagram of 16-ary QAM
QAM bits per symbol
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The advantage of using QAM is that it is a higher
order form of modulation and as a result it is able to
carry more bits of information per symbol.
By selecting a higher order format of QAM, the data
rate of a link can be increased.
Modulation
Bits per symbol
Symbol rate
BPSK
1
1*bit rate
QPSK
2
½*bit rate
8PSK
3
1/3*bit rate
16QAM
4
¼*bit rate
32QAM
5
1/5*bit rate
64QAM
6
1/6*bit rate
QAM noise margin
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While higher order modulation rates are able to offer
much faster data rates and higher levels of spectral
efficiency for the radio communications system, this
comes at a price. The higher order modulation
schemes are considerably less resilient to noise and
interference.
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As a result of this, many radio communications
systems now use dynamic adaptive modulation
techniques. They sense the channel conditions and
adapt the modulation scheme to obtain the highest
data rate for the given conditions.
Bandwidth and Power efficiency
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The power spectrum and bandwidth efficiency of
QAM modulation is identical to M-ary PSK
modulation
In terms of power efficiency QAM is superior to Mary PSK.
Bandwidth and Power
efficiency(cont…)
QAM advantages and disadvantages
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QAM increase the efficiency of transmission for radio
communications systems by utilising both amplitude
and phase variations.
It has a number of drawbacks. The first is that it is
more susceptible to noise. Receivers for use with
phase or frequency modulation are both able to use
limiting amplifiers that are able to remove any
amplitude noise and thereby improve the noise
reliance. This is not the case with QAM.
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The second limitation is also associated with the
amplitude component of the signal. When a phase or
frequency modulated signal is amplified in a radio
transmitter, there is no need to use linear amplifiers,
whereas when using QAM that contains an amplitude
component, linearity must be maintained.
Unfortunately linear amplifiers are less efficient and
consume more power, and this makes them less
attractive for mobile applications.
Almost always requires a highly stable local oscillator
In the optical domain this is very expensive
Applications of QAM
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Used in bandwidth-limited applications
„ Modems: telephones have 3kHz bandwidth,
excellent SNR (20dB) => M-ary QAM
„ Cellular Telephones: Bandwidth is at a premium,
very expensive (However, POWER
QAM is used in analog color television to multiplex
the chrominance signals, which carry the information
about the color.
Digital satellite television transmission also applies
QAM.
References
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John G. Proakis, Masoud Salehi, Communications
Systems Engineering, Prentice Hall 1994
Principles of communication systems by taub and
schilling
Modern digital and analog communication system by
B.P.Lathi