Quadrature amplitude modulation can be used with a
variety of different formats: 8QAM, 16QAM, 64QAM, 128QAM, 256QAM, but there are
performance differences and trade-offs
QAM, quadrature amplitude modulation
provides some significant benefits for data transmission. As 16QAM transitions
to 64QAM, 64QAM to 256 QAM and so forth, higher data rates can be achieved, but
at the cost of the noise margin.
Many data transmission systems migrate
between the different orders of QAM, 16QAM, 32QAM, etc., dependent upon the
link conditions. If there is a good margin, higher orders of QAM can be used to
gain a faster data rate, but if the link deteriorates, lower orders are used to
preserve the noise margin and ensure that a low bit error rate is preserved.
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As the QAM order increases, so the distance between the
different points on the constellation diagram decreases and there is a higher
possibility of data errors being introduced. To utilise the high order QAM
formats, the link must have a very good Eb/No otherwise
data errors will be present.When the Eb/No deteriorates,
then other the power level must be increased, or the QAM order reduced if the
bit error rate is to be preserved.
Accordingly there is a balance to be
made between the data rate and QAM modulation order, power and the acceptable
bit error rate. Whilst further error correction can be introduced to mitigate
any deterioration in link quality, this will also decrease the data throughput.
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QAM applications
QAM is in many radio communications and
data delivery applications. However some specific variants of QAM are used in
some specific applications and standards.
For domestic broadcast applications for
example, 64 QAM and 256 QAM are often used in digital cable television and
cable modem applications. In the UK, 16 QAM and 64 QAM are currently used for
digital terrestrial television using DVB - Digital Video Broadcasting. In the
US, 64 QAM and 256 QAM are the mandated modulation schemes for digital cable as
standardised by the SCTE in the standard ANSI/SCTE 07 2000.
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In addition to this, variants of QAM are
also used for many wireless and cellular technology applications. Here the link
conditions can vary and accordingly the order of the QAM modulation used can
normally be altered dynamically with the level of error correction to achieved
the best throughput. This means balancing the QAM order with the level of error
correction against the prevailing link conditions. As data rates have risen and
the demands on spectrum efficiency have increased, so too has the complexity of
the link adaptation technology. Data channels are carried on the cellular radio
signal to enable fast adaptation of the link to meet the prevailing link
quality and ensure the optimu data throughput, balancing transmitter power, QAM
order, and forward error correction, etc.
Constellation
diagrams for QAM
The constellation diagrams show the
different positions for the states within different forms of QAM, quadrature
amplitude modulation. As the order of the modulation increases, so does the
number of points on the QAM constellation diagram.
The diagrams below show constellation
diagrams for a variety of formats of modulation:
16QAM constellation
32QAM constellation
64QAM constellation
It can be seen from these few QAM
constellation diagrams, that as the modulation order increases, so the distance
between the points on the constellation decreases. Accordingly small amounts of
noise can cause greater issues.
It is also found that the higher the
order of modulation for the QAM signal, the greater the amount of amplitude
variation. For transmitter RF amplifiers for everything from Wi-Fi to cellualr
and more, it means that linear amplifiers are required. As linear amplifiers
are less efficient than those that can be run in saturation, it means that
techniques like Doherty amplifers and envelope tracking may be needed.
QAM bits per
symbol
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.
The table below gives a summary of the
bit rates of different forms of QAM and PSK.
Bit mapping for a 16QAM signal
|
QAM FORMATS & BIT RATES COMPARISON
|
||
|
MODULATION
|
BITS PER SYMBOL
|
SYMBOL RATE
|
|
BPSK
|
1
|
1 x bit rate
|
|
QPSK
|
2
|
1/2 bit rate
|
|
8PSK
|
3
|
1/3 bit rate
|
|
16QAM
|
4
|
1/4 bit rate
|
|
32QAM
|
5
|
1/5 bit rate
|
|
64QAM
|
6
|
1/6 bit rate
|
The power spectrum and bandwidth
efficiency of QAM modulation is identical to M-ary PSK modulation, in other
words for the same order phase shift keying, the power spectrum and bandwidth
efficiency levels are exactly the same whether quadrature amplitude modulation
or phase shift keying is used.
QAM noise margin
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.
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. As signal to noise ratios decrease
errors will increase along with re-sends of the data, thereby slowing
throughput. By reverting to a lower order modulation scheme the link can be
made more reliable with fewer data errors and re-sends.
|
QAM FORMATS & NOISE PERFORMANCE
|
||
|
MODULATION
|
ηB
|
EB / NO FOR BER = 1 IN 106
|
|
16QAM
|
2
|
10.5
|
|
64QAM
|
3
|
18.5
|
|
256QAM
|
4
|
24
|
|
1024QAM
|
5
|
28
|
Selecting the right order of QAM
modulation for any given situation, and having he ability to dynamically adapt
it can enable the optimum throughput to be obtained for the link conditions for
that moment. Reducing the order of the QAM modulation enables lower bit error
rates to be achieved and this reduces the amount of error correction required.
In this way the throughput can be maximised for the prevailing link quality.
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Source: electronics-notes
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