? What is QAM
Quadrature amplitude modulation (QAM) including 16QAM, 32QAM, 64QAM,
128QAM, 256QAM, 512QAM, 1024QAM, 2048QAM and 4096QAM is both an analog
and a digital modulation scheme. It conveys two analog message signals,
or two digital bit streams, by changing (modulating) the amplitudes of
two carrier waves, using the amplitude-shift keying
(ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme
(ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme
? Why are higher QAM levels used
Modern wireless networks require higher capacities.
For a fixed channel size, increasing QAM modulation level increases the
link capacity. Note that incremental capacity gain at low-QAM levels
is significant; but at high QAM, the capacity gain is much smaller. For
example, increasing
From 1024QAM to 2048QAM gives a 10.83% capacity gain.
From 2048QAM to 4096QAM gives a 9.77% capacity gain.
From 1024QAM to 2048QAM gives a 10.83% capacity gain.
From 2048QAM to 4096QAM gives a 9.77% capacity gain.
? What are the penalties in higher QAM
The receiver sensitivity is greatly reduced. For every QAM increment
(e.g. 512 to 1024QAM) there is a -3dB degradation in receiver
sensitivity. This reduces the range. Due to increased linearity
requirements at the transmitter, there is a reduction in transmit power
also when QAM level is increased. This may be around 1dB per QAM
increment.
.
Advantages of QAM over other modulation types
Following are the advantages of QAM modulation:
• Helps achieve high data rate as more number of bits are carried by one carrier. Due to this it has become popular in modern wireless communication system such as LTE, LTE-Advanced etc. It is also used in latest WLAN technologies such as 802.11n 802.11 ac, 802.11 ad and others.
• Helps achieve high data rate as more number of bits are carried by one carrier. Due to this it has become popular in modern wireless communication system such as LTE, LTE-Advanced etc. It is also used in latest WLAN technologies such as 802.11n 802.11 ac, 802.11 ad and others.
Disadvantages of QAM over other modulation types
Following are the disadvantages of QAM modulation:
• Though data rate has been increased by mapping more than 1 bits on single carrier, it requires high SNR in order to decode the bits at the receiver.
• Needs high linearity PA (Power Amplifier) in the Transmitter.
• In addition to high SNR, higher modulation techniques need very robust front end algorithms (time, frequency and channel) to decode the symbols without errors
• Though data rate has been increased by mapping more than 1 bits on single carrier, it requires high SNR in order to decode the bits at the receiver.
• Needs high linearity PA (Power Amplifier) in the Transmitter.
• In addition to high SNR, higher modulation techniques need very robust front end algorithms (time, frequency and channel) to decode the symbols without errors
Understanding QAM Modulation
Starting with the QAM modulation process at the transmitter to
receiver in the wireless baseband (i.e. Physical Layer) chain. We will
use the example of 64-QAM to illustrate the process. Each symbol in the
QAM represents a unique amplitude and phase. Hence they
can be distinguished from the other points at the receiver.
- As shown in the figure-1, 64-QAM or any other modulation is applied on the input binary bits.
- The QAM modulation converts input bits into complex symbols which represent bits by variation in amplitude/phase of the time domain waveform. Using 64QAM converts 6 bits into one symbol at transmitter.
- The bits to symbols conversion take place at the transmitter while reverse (i.e. symbols to bits) take place at the receiver. At receiver, one symbol gives 6 bits as output of demapper.
- Figure depicts position of QAM mapper and QAM demapper in the baseband transmitter and receiver respectively. The demapping is done after front end synchronization i.e. after channel and other impairments are corrected from the received impaired baseband symbols.
- Data Mapping or modulation process is done before the RF upconversion (U/C) in the transmitter and PA. Due to this, higher order modulation necessitates use of highly linear PA (Power Amplifier) at the transmit end.
QAM Mapping Process
Fig:2, 64-QAM Mapping Process
In 64-QAM, the number 64 refers to 2^6.
Here 6 represents number of bits/symbol which is 6 in 64-QAM.
Similarly it can be applied to other modulation types such as 512-QAM, 1024-QAM, 2048-QAM and 4096-QAM as described below.
Here 6 represents number of bits/symbol which is 6 in 64-QAM.
Similarly it can be applied to other modulation types such as 512-QAM, 1024-QAM, 2048-QAM and 4096-QAM as described below.
Following table mentions 64-QAM encoding rule. Check the encoding
rule in the respective wireless standard. KMOD value for 64-QAM is
1/SQRT(42).
| Input bits (b5, b4, b3) | I-Out | Input bits (b2, b1, b0) | Q-Out |
|---|---|---|---|
| 011 | 7 | 011 | 7 |
| 010 | 5 | 010 | 5 |
| 000 | 3 | 000 | 3 |
| 001 | 1 | 001 | 1 |
| 101 | -1 | 101 | -1 |
| 100 | -3 | 100 | -3 |
| 110 | -5 | 110 | -5 |
| 111 | -7 | 111 | -7 |
QAM mapper Input parameters : Binary Bits
QAM mapper Output parameters : Complex data (I, Q)
QAM mapper Output parameters : Complex data (I, Q)
The 64-QAM mapper takes binary input and generates complex data
symbols as output. It uses above mentioned encoding table to do the
conversion process. Before the coversion process, data is grouped into 6
bits pair. Here, (b5, b4, b3) determines the I value and (b2, b1, b0)
determines the Q value.
Example: Binary Input: (b5,b4,b3,b2,b1,b0) = (011011)
Complex Output: (1/SQRT(42))* (7+j*7)
Complex Output: (1/SQRT(42))* (7+j*7)
512-QAM modulation
Fig:3, 512-QAM Constellation Diagram
The above figure shows 512-QAM constellation diagram. Note that 16
points do not exist in each of the four quadrants to make total 512
points with 128 points in each quadrant in this modulation type. It is
possible to have 9 bits per symbol in 512-QAM also. 512QAM increases
capacity by 50% compare to 64-QAM modulation type.
1024-QAM modulation
The figure shows a 1024-QAM constellation diagram.
Number of bits per seymbol: 10
Symbol rate: 1/10 of bit rate
Increase in capacity compare to 64-QAM: About 66.66%
Number of bits per seymbol: 10
Symbol rate: 1/10 of bit rate
Increase in capacity compare to 64-QAM: About 66.66%
2048-QAM modulation
Following are the characteristics of 2048-QAM modulation.
Number of bits per seymbol: 11
Symbol rate: 1/11 of bit rate
Increase in capacity from 64-QAM to 1024QAM: 83.33% gain
Increase in capacity from 1024QAM to 2048QAM: 10.83% gain
Total constellation points in one quadrant: 512
Number of bits per seymbol: 11
Symbol rate: 1/11 of bit rate
Increase in capacity from 64-QAM to 1024QAM: 83.33% gain
Increase in capacity from 1024QAM to 2048QAM: 10.83% gain
Total constellation points in one quadrant: 512
4096-QAM modulation
Following are the characteristics of 4096-QAM modulation.
Number of bits per symbol: 12
Symbol rate: 1/12 of bit rate
Increase in capacity from 64-QAM to 409QAM: 100% gain
Increase in capacity from 2048QAM to 4096QAM 9.77% gain
Total constellation points in one quadrant: 1024
Number of bits per symbol: 12
Symbol rate: 1/12 of bit rate
Increase in capacity from 64-QAM to 409QAM: 100% gain
Increase in capacity from 2048QAM to 4096QAM 9.77% gain
Total constellation points in one quadrant: 1024
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