QAM Symbol Rate and Bandwidth Calculator

This tool calculates both the symbol rate and bandwidth for a Quadrature Amplitude Modulation (QAM) signal.

Inputs required:

  • Modulation order
  • Bit rate

Background

QAM is a popular modulation format that’s used in many modern waveforms such as 802.11 (Wi-Fi).

The following image animation shows a 16 QAM constellation diagram. There are 16 points, each representing a unique data symbol. In this case, each symbol is composed of 4 bits.

As the modulation order increases, the number of bits per symbol increases. Higher order modulations are more susceptible to the effects of noise and interference. In order to maintain a fixed bit error rate, it takes a higher signal-to-noise ratio for 64 QAM than for 8 QAM.

You can understand this visually looking at the constellation above. As the modulation order increases, the points get closer to each other and the probability of error for the same level of noise increases.

Examples

64 QAM Bandwidth Calculation

In the case of 64 QAM, there are 6 bits per symbol. Consider a bit rate of 100 Mbit/s. To transmit this information, 16.7 MHz of bandwidth is required.

256 QAM Bandwidth Calculation

When using 256 QAM, there are 8 bits per symbol. For the same bit rate of 100 Mbit/s, only 12.5 MHz of bandwidth is required.

Summary

Increasing the modulation order reduces the bandwidth requirements. However, as mentioned earlier, the SNR requirements increase as well.

In general, designing a communication system to maintain high SNR can be challenging. For instance, the receiver has to be designed to minimize the noise floor.

As well, there are other factors such as:

  • operating frequency
  • propagation considerations
  • antenna design
  • transmit power

that all need to be taken into account to maximize the signal received. Check out our antenna range calculator to understand how these factors influence the level of the received signal.