RFUnderstanding QAM: 512-QAM vs 1024-QAM vs 2048-QAM vs 4096-QAM
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QAM, which stands for Quadrature Amplitude Modulation, is a digital modulation technique where each symbol in the QAM constellation represents a unique amplitude and phase. This allows the receiver to distinguish between different points in the constellation, enabling the transmission of multiple bits per symbol.
Let’s delve into the QAM modulation process at the transmitter and receiver within the wireless baseband (Physical layer) chain. We’ll use 64-QAM as an example to illustrate the concept.
64-QAM Mapping and Demapping
Fig:1, 64-QAM Mapping and Demapping
- As depicted in Figure 1, 64-QAM (or any other QAM variant) is applied to the input binary bits.
- The QAM modulation process converts these input bits into complex symbols. These symbols represent bits by varying the amplitude and phase of the time-domain waveform. In the case of 64-QAM, 6 bits are converted into one symbol at the transmitter.
- This bits-to-symbols conversion happens at the transmitter, while the reverse process (symbols-to-bits) occurs at the receiver. At the receiver, one symbol is demapped to yield 6 bits as output.
- The figure illustrates the position of the QAM mapper and demapper in the baseband transmitter and receiver, respectively.
- Demapping takes place after front-end synchronization, meaning after the channel and other impairments have been corrected from the received, impaired baseband symbols.
- Data mapping, or modulation, occurs before the RF upconversion (U/C) in the transmitter and the Power Amplifier (PA). Due to this, higher-order modulation schemes necessitate the use of highly linear PAs at the transmitting end.
64-QAM Mapping Process
Fig:2, 64-QAM Mapping Process
In 64-QAM, the number 64 refers to 26. The exponent, 6, represents the number of bits per symbol. This principle applies to other QAM modulation types such as 512-QAM, 1024-QAM, 2048-QAM, and 4096-QAM, as we’ll explore below.
The following table outlines a 64-QAM encoding rule. Note that the specific encoding rule can vary depending on the wireless standard in use. The 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
- Output parameters: Complex data
The 64-QAM mapper takes binary input and generates complex data symbols as output, using the encoding table above to perform the conversion. Before the conversion, the data is grouped into 6-bit pairs.
Here, (b5, b4, b3) determines the I (in-phase) value, and (b2, b1, b0) determines the Q (quadrature) value.
Example:
- Binary Input: (b5,b4,b3,b2,b1,b0) = (011011)
- Complex Output: (1/SQRT(42)) * (7 + j*7)
In digital modulation, the baseband signal is separated into in-phase (I) and quadrature-phase (Q) components. The combination of I and Q is known as the baseband modulating signal, often represented as an IQ diagram.
The constellation diagram represents all the possible modulated symbols used by the modulation technique to map the information bits. These different symbols are represented in the complex plane, with their amplitude and phase information.
Higher Order QAM Constellations
Let’s look at higher-order QAM constellations.
512-QAM Modulation
Fig-3: 512-QAM constellation diagram
The figure above depicts a 512-QAM constellation diagram. In this modulation type, approximately 16 points are intentionally omitted from each of the four quadrants, resulting in a total of 512 points with 128 points per quadrant. This allows for 9 bits per symbol in 512-QAM. 512-QAM offers a 50% increase in capacity compared to 64-QAM.
1024-QAM Modulation
Fig: 1024-QAM constellation diagram
The figure above depicts a 1024-QAM constellation diagram.
- Number of bits per symbol: 10
- Symbol rate: 1/10 of the bit rate
- Increase in capacity compared to 64-QAM: Approximately 66.66%
2048-QAM Modulation
Following are the characteristics of 2048-QAM modulation.
- Number of bits per symbol: 11
- Symbol rate: 1/11 of bit rate
- Increase in capacity compared to 64-QAM: Approximately 83.33%
- 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 the bit rate
- Increase in capacity compare to 64-QAM: Approximately 100%
- Total constellation points in one quadrant: 1024
Comparison of QAM Modulation Types
The following table compares 512 QAM, 1024 QAM, 2048 QAM, and 4096 QAM modulation types, highlighting the differences between these techniques:
| Specifications | 512 QAM | 1024 QAM | 2048 QAM | 4096 QAM |
|---|---|---|---|---|
| Number of bits per symbol | 9 | 10 | 11 | 12 |
| Symbol rate | 1/9 th of bit rate | 1/10 th of bit rate | 1/11 th of bit rate | 1/12 th of bit rate |
| Total points in constellation diagram | 512 | 1024 | 2048 | 4096 |
| Increase in capacity compare to 64-QAM | 50 % | 66.66 % | 83.33 % | 100 % |
Advantages and Disadvantages of QAM
Advantages of QAM
- High Data Rate: QAM helps achieve high data rates because it allows for a greater number of bits to be carried by a single carrier. This has made it popular in modern wireless communication systems like WiMAX, LTE, LTE-Advanced, and WLAN technologies such as 802.11n, 802.11ac, and 802.11ad.
Disadvantages of QAM
- High SNR Requirement: While QAM increases data rates by mapping more than 1 bit onto a single carrier, it also requires a high Signal-to-Noise Ratio (SNR) in order to accurately decode the bits at the receiver.
- Linear Power Amplifier: QAM necessitates the use of a highly linear Power Amplifier (PA) at the Transmitter.
- Robust Front-End Algorithms: In addition to high SNR, higher modulation techniques demand very robust front-end algorithms (time, frequency, and channel estimation) to decode the symbols without introducing errors.
