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What is 4096-QAM? What is its data rate? What technologies use this modulation scheme?

4096-QAM is a subset of the Quadrature Amplitude Modulation (QAM) scheme in which a carrier waveform of fixed frequency can exist in one of 4096 possible discrete and measurable states in the constellation plot. The constellation plot consists of two axes namely the in-phase (X-axis) and Quadrature (Y-axis). The two axes orthogonal to each other i.e. they are 90˚ out of phase with respect to each other.

Each symbol in a 4096-QAM scheme is a constellation state consisting of 12 bits that represents one possible combination out of 4096 different constellation points. Since this modulation scheme utilizes binary data, the total number of possible combinations for 12 bits is 2^{12} i.e. 4096. The number of bits can be computed in terms of the logarithmic value of (1/12 of a bit rate).

The maximum data rate that can be achieved with 4096-QAM is 575 Mbps over a channel bandwidth of 56 MHz. Many technologies like Wi-Fi 7 (802.11 be) use 4096QAM. These technologies use multiple channels, MIMO and other algorithms to further enhance the data rate that they can support.

Capacity Gain of 4096-QAM vs. Lower Order QAMs

In general, for a fixed channel size, increasing the order of QAM scheme increases the link capacity. However, as we go up in complexity over a point the incremental capacity gains start to decrease. For instance, when moving towards 4096-QAM from 1024-QAM, a capacity gain of roughly 11% can be obtained whereas when moving from 16-QAM to 64-QAM the gain is 25%.

As we reach 4096-QAM from the lower order QAMs, the constellation size becomes extremely large while the points get very close to each other. In the presence of noise and multipath interference, there is a higher likelihood of two points overlapping with each other, thereby reducing the RF signal strength.

In such cases, increasing the transmit power does not yield significant improvement in the link performance since the linearity of systems, for e.g. amplifiers, will get affected. Therefore, the system gain will be lower for a given transmit power. In order to maintain a stable carrier-to-interference ratio (CIR), the integrated power amplifiers (PAs) will also have to be operated in the linear region. This requires the transmit power to be reduced in schemes like 4096-QAM compared to other lower order QAM schemes such as 256-QAM, 64-QAM, 16-QAM, and 8-QAM.

Another way to ensure a higher Signal to Noise Ratio (SNR) and to maintain link performance is to move to a lower order QAM modulation.

However, alternative signal processing methods such as Adaptive Coding and Modulation (ACM) can be utilized to offset this system gain and accurately detect signals in dense constellation plots under such difficult propagation conditions while still using higher order QAM schemes.

Applications of 4096-QAM

Wi-Fi 7, the next generation of Wi-Fi is looking to adopt this modulation scheme. It will support throughputs of over 30 Gbps with very low latency. This technology will adopt the 4096-QAM technique that increases the modulation rate from its predecessors. Wi-Fi 7 will encode up to 12 bits per symbol per OFDM sub-carrier. Thus, by integrating this modulation technique, the data rate is increased by 20% over the 1024-QAM.

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