This page covers
basics of QAM i.e. Quadrature Amplitude Modulation technique. It compares
16-QAM vs. 64-QAM vs. 256-QAM and mentions difference between 16-QAM,64-QAM and
256-QAM.
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QAM stands
for Quadrature Amplitude Modulation. It is digital
modulation technique. This modulation technique is a combination of both
Amplitude and phase modulation techniques. QAM is better than QPSK in terms of
data carrying capacity. QAM takes benefit from the concept that two signal
frequencies; one shifted by 90 degree with respect to the other can be
transmitted on the same carrier. For QAM, each carrier is ASK/PSK modulated.
Hence data symbols have different amplitudes and phases.
S(t)= d1(t) cos(2*pi*fc*t)+ d2(t) sin(2*pi*fc*t)
S(t)= d1(t) cos(2*pi*fc*t)+ d2(t) sin(2*pi*fc*t)
Figure mention the
constellation points and encoding rule,which is taken from IEEE standard
802.16-2004 to demonstrate the 16-QAM concept.
As mentioned for each symbol both phase and amplitudes are varied to represent different bits. There are two levels of amplitudes for each phase i.e. d1 level and d2 level . There are many variants to this technique. Most popular are 16-QAM, 64-QAM and 256-QAM. The example below explains 16-QAM. In 16-QAM each symbol represents 4 bits as mentioned in the constellation diagram above.
For example if the input is 1010 then the output is (-3-j*3)*KMOD.
Typically KMOD is 1/root (10) for 16-QAM.
As mentioned for each symbol both phase and amplitudes are varied to represent different bits. There are two levels of amplitudes for each phase i.e. d1 level and d2 level . There are many variants to this technique. Most popular are 16-QAM, 64-QAM and 256-QAM. The example below explains 16-QAM. In 16-QAM each symbol represents 4 bits as mentioned in the constellation diagram above.
For example if the input is 1010 then the output is (-3-j*3)*KMOD.
Typically KMOD is 1/root (10) for 16-QAM.
In 64-QAM, each symbol
is represented by 6 bits and in 256-QAM, each symbol is represented by 8 bits.
As the level increases, QAM technique becomes more bandwidth efficient but it
requires very robust algorithms in order to decode complex symbols to bits at
receiver.
For example 256-QAM
is complex than 16-QAM. QAM is more bandwidth efficient compare to BPSK but it
is less robust. Hence for better CINR in the system QAM is employed which leads
better data rate. For poor CINR, BPSK is employed. CINR stands for Carrier to
Interference and Noise Ratio.
Difference between 16-QAM, 64-QAM and 256-QAM
Following table
mentions difference between 16-QAM, 64-QAM and 256-QAM modulation techniques.
The purpose of KMOD here is to achieve the same average power for all the
mapped symbols (i.e. average power of 1).
Specifications
|
16-QAM
|
64-QAM
|
256-QAM
|
Number of bits per symbol
|
4
|
6
|
8
|
Symbol rate
|
(1/4) of bit rate
|
(1/6) of bit rate
|
(1/8) of bit rate
|
KMOD
|
1/SQRT(10)
|
1/SQRT(42)
|
1/SQRT(170)
|
Applications
• CDMA
• WiMAX-16d, 16e
• WLAN-11a OFDM
• Satellite
• DVB
• Cable modem
• WiMAX-16d, 16e
• WLAN-11a OFDM
• Satellite
• DVB
• Cable modem
What is QAM: quadrature amplitude
modulation?
QAM: Quadrature
Amplitude Modulation combines amplitude & phase changes to give additional
capacity & is widely used for data communications.
Quadrature Amplitude Modulation, QAM
utilizes both amplitude and phase components to provide a form of modulation
that is able to provide high levels of spectrum usage efficiency.
QAM, quadrature amplitude modulation
has been used for some analogue transmissions including AM stereo
transmissions, but it is for data applications where it has come into its own.
It is able to provide a highly effective form of modulation for data and as
such it is used in everything from cellular phones to Wi-Fi and almost every
other form of high speed data communications system.
Quadrature amplitude
modulation concept
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What is QAM,
quadrature amplitude modulation?
Quadrature Amplitude Modulation, QAM
is a signal in which two carriers shifted in phase by 90 degrees (i.e. sine and
cosine) are modulated and combined. As a result of their 90° phase difference
they are in quadrature and this gives rise to the name. Often one signal is
called the In-phase or “I” signal, and the other is the quadrature or “Q”
signal.
The resultant overall signal
consisting of the combination of both I and Q carriers contains 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.
A motivation for the use of
quadrature amplitude modulation comes from the fact that a straight amplitude
modulated signal, i.e. double sideband even with a suppressed carrier occupies
twice the bandwidth of the modulating signal. This is very wasteful of the
available frequency spectrum. QAM restores the balance by placing two
independent double sideband suppressed carrier signals in the same spectrum as
one ordinary double sideband supressed carrier signal.
Analogue and digital
QAM
Quadrature amplitude modulation, QAM
may exist in what may be termed either analogue or digital formats. The
analogue versions of QAM are typically used to allow multiple analogue signals
to be carried on a single carrier. For example it is used in PAL and NTSC
television systems, where the different channels provided by QAM enable it to
carry the components of chroma or color information. In radio applications a
system known as C-QUAM is used for AM stereo radio. Here the different channels
enable the two channels required for stereo to be carried on the single
carrier.
Digital formats of QAM are often
referred to as "Quantized QAM" and they are being increasingly used
for data communications often within radio communications systems. Radio
communications systems ranging from cellular technology as in the case of LTE
through wireless systems including WiMAX, and Wi-Fi 802.11 use a variety of
forms of QAM, and the use of QAM will only increase within the field of radio
communications.
Digital / Quantized
QAM basics
Quadrature amplitude modulation,
QAM, when used for digital transmission for radio communications applications
is able to carry higher data rates than ordinary amplitude modulated schemes
and phase modulated schemes.
Basic signals exhibit only two
positions which allow the transfer of either a 0 or 1. Using QAM there are many
different points that can be used, each having defined values of phase and amplitude.
This is known as a constellation diagram. The different positions are assigned
different values, and in this way a single signal is able to transfer data at a
much higher rate.
Constellation diagram
for a 16QAM signal showing the location of the different points
As shown above, the constellation
points are typically arranged in a square grid with equal horizontal and
vertical spacing. Although data is binary the most common forms of QAM,
although not all, are where there constellation can form a square with the
number of points equal to a power of 2 i.e. 4, 16, 64 . . . . , i.e. 16QAM,
64QAM, etc.
By using higher order modulation
formats, i.e. more points on the constellation, it is possible to transmit more
bits per symbol. However the points are closer together and they are therefore
more susceptible to noise and data errors.
The advantage of moving to the
higher order formats is that there are more points within the constellation and
therefore it is possible to transmit more bits per symbol. The downside is that
the constellation points are closer together and therefore the link is more
susceptible to noise. As a result, higher order versions of QAM are only used
when there is a sufficiently high signal to noise ratio.
To provide an example of how QAM
operates, the constellation diagram below shows the values associated with the
different states for a 16QAM signal. From this it can be seen that a continuous
bit stream may be grouped into fours and represented as a sequence.
Bit sequence mapping
for a 16QAM signal
Normally the lowest order QAM
encountered is 16QAM. The reason for this being the lowest order normally
encountered is that 2QAM is the same as binary phase-shift keying, BPSK, and
4QAM is the same as quadrature phase-shift keying, QPSK.
Additionally 8QAM is not widely
used. This is because error-rate performance of 8QAM is almost the same as that
of 16QAM - it is only about 0.5 dB better and the data rate is only
three-quarters that of 16QAM. This arises from the rectangular, rather than
square shape of the constellation.
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QAM advantages and
disadvantages
Although QAM appears to increase the
efficiency of transmission for radio communications systems by utilizing both
amplitude and phase variations, it has a number of drawbacks. The first is that
it is more susceptible to noise because the states are closer together so that
a lower level of noise is needed to move the signal to a different decision
point. 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.
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.
QAM vs. PSK &
other modes
When deciding on a form of
modulation it is worth comparing AM vs PSK and other modes looking at what they
each have to offer.
As there are advantages and
disadvantages of using QAM it is necessary to compare QAM with other modes
before making a decision about the optimum mode. Some radio communications
systems dynamically change the modulation scheme dependent upon the link conditions
and requirements - signal level, noise, data rate required, etc.
The table below compares various
forms of modulation:
SUMMARY OF TYPES OF MODULATION WITH DATA CAPACITIES
|
||||
MODULATION
|
BITS PER SYMBOL
|
-- ERROR MARGIN --
|
COMPLEXITY
|
|
OOK
|
1
|
1/2
|
0.5
|
Low
|
BPSK
|
1
|
1
|
1
|
Medium
|
QPSK
|
2
|
1 / √2
|
0.71
|
Medium
|
16 QAM
|
4
|
√2 / 6
|
0.23
|
High
|
64QAM
|
6
|
√2 / 14
|
0.1
|
High
|
Typically it is found that if data
rates above those that can be achieved using 8-PSK are required, it is more
usual to use quadrature amplitude modulation. This is because it has a greater
distance between adjacent points in the I - Q plane and this improves its noise
immunity. As a result it can achieve the same data rate at a lower signal
level.
However, the points no longer the
same amplitude. This means that the demodulator must detect both phase and
amplitude. Also the fact that the amplitude varies means that a linear
amplifier is required to amplify the signal.
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Source: rfwireless-world and electronics-notes
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