What is 256-QAM Modulation?

256-QAM is a type of Quadrature Amplitude Modulation (QAM) in which a carrier wave of constant frequency can exist in one of 256 different discrete and measurable states in the constellation plot. The constellation diagram consists of two axes namely the in-phase (X-axis) and Quadrature (Y-axis). Both the in-phase and quadrature axes of the modulated signal are orthogonal to each other i.e. 90˚ out of phase with respect to each other.

Each symbol in 256-QAM is a constellation state that contains eight bits and each symbol is one possible combination out of 256 different states ranging from 0000 0000 to 1111 1111. Since this modulation scheme uses binary data for operation, the total number of possible combinations for 8 bits is 28 i.e. 256. The number of bits can be computed in terms of the logarithmic value as (1/6 of bit rate).

Using 256-QAM, it is possible to modulate both the amplitude as well as phase of the carrier wave and transmit a relatively larger number of bits, thereby resulting in a higher bit rate compared to other lower order QAMs such as 64-QAM and 16-QAM, QPSK, and BPSK schemes. The maximum data rate that can be achieved using 256-QAM scheme is up to 38 Mbps (downstream).

The wireless networking standard IEEE 802.11ac employs 256-QAM which can pack 33% additional information relative to 64-QAM modulation scheme.

The Modulation Coding Scheme (MCS) is a standard metric that is based on several parameters of a Wi-Fi connection between any two communicating devices. The significance of this 256-QAM scheme is the availability of two new data rates that are MCS 8 and MCS 9. They have the capability to achieve a relatively higher throughput compared to the predecessor MCS 7, given the air time.

256-QAM vs. Lower Order QAM Schemes

Since 256-QAM modulation packs more data into the carrier waveform signal due to a larger number of closely spaced constellation points, more sensitive and robust equipment is required to differentiate between points with different amplitudes and phases. Therefore, this modulation scheme requires a larger amount of transmit power or a cleaner signal to be transmitted between the client and the Wi-Fi AP (Access Point) when compared to 64-QAM, 16-QAM and lower QAM modualtions.

256-QAM is also more sensitive to channel noise and interference due to neighboring users in the environment. To ensure the detectability of signals with a higher Signal-to-Noise Ratio (SNR) and low Bit Error Rate (BER), a line of sight access between the Access Point and client can help. If for whatever reason the SNR is low, most wireless systems are designed to switch to a lower order QAM modulation scheme like 64-QAM (MCS 7) and 16-QAM, QPSK, or BPSK.

The use of beamforming can also improve the directivity in a particular direction, thereby enhancing the SNR. But, other factors such as the total cost of a beamforming system, algorithm computational complexity, and hardware complexity need to be taken into account while utilizing beamforming techniques in an IC.

The image below shows the required receiver sensitivity when using 256-QAM and other types of modulation schemes. 

The above figure shows the performance analysis of bandwidth bonding vs. receiver sensitivity when a 256-QAM is employed. For 256-QAM, the threshold of receiver sensitivity is lowered as a function of bandwidth bonding. Therefore, it means that with more and more bandwidth, a corresponding larger transmit power is required at the AP side to reap the benefits of 256-QAM.

Applications

This modulation technique is ideal for use in Wi-Fi, digital cable television, 370 Mbps of data download (for channel size of 56 MHz), cable modems, and Freeview-HD applications.