What is 64-QAM Modulation?

64-QAM is a type of Quadrature Modulation (QAM) in which a carrier wave of a fixed frequency can exist in one of sixty-four different discrete and measurable states in the constellation plot. The constellation plot consists of two component axis namely the in-phase (X-axis) and quadrature (Y-axis). The two components are orthogonal to each other or 90˚ out of phase with respect to each other.

Each symbol in 64-QAM is a constellation state that contains six bits and each symbol is one possible combination out of 64 different states ranging from 000 000 to 111 111. Since, this modulation scheme uses binary data, the total number of possible combinations using 6 bits is 26 i.e. 64. The number of bits can be computed in terms of the logarithmic value as .

Using the 64-QAM scheme, it is possible to modulate both amplitude as well as phase of the carrier wave and transmit a relatively larger number of bits, thereby achieving a higher bit rate compared to other lower order QAMs such as 16-QAM and 8-QAM, BPSK, and QPSK schemes. The maximum data rate that can be achieved using 64-QAM scheme is up to 26.9 Mbps (downlink).

When evaluating 64-QAM Modulation, the following parameters need to be evaluated.

Error Vector Magnitude (EVM): This parameter measures how accurately the Device Under Test (DUT) can transmit symbols within the 64-QAM constellation response.

Adjacent Channel Leakage Ratio (ACLR): It measures the average power compared to the transmit power that leaks into the adjacent channels thereby resulting in interference with other neighboring channels.

Spurious emissions: These are emissions resulting from unwanted transmitter effects such as harmonics emission and intermodulation products.

64-QAM has a relatively higher bit rate (6 bits per symbol) can be achieved as the number of bits per symbol is increased compared to lower order M-QAM schemes and other techniques such as BPSK and QPSK. As compared to BPSK (1 bit per symbol), the 64-QAM modulation achieves 6 times better bandiwdth efficiency and 3 times better bandiwidth efficiency as compared to QPSK (2 bits per symbol).

Applications of 64-QAM Modulation

64-QAM modulation technique is widely used for applications such as digital CATV systems mandated by the Society of Cable Telecommunications Engineers (SCTE), cable modem, and in digital digital terrestrial television (freeview).

This modulation is also used in LTE for uplink (UL) and downlink (DL) where the UL based 64-QAM transmission provides benefit in small cell environments. When higher data rates are required, more advanced QAM constellation schemes like 256-QAM, 1024 QAM and 4096 QAM can be used.