Why is amplitude measurement of a quadrature amplitude modulation (QAM) channel taken from the average power across the channel bandwidth?

The measurement of amplitude in a quadrature amplitude modulation (QAM) channel is based on the average power across the channel bandwidth because of the nature of how QAM works. In QAM, both amplitude and phase of the signal carry information, and the effective signal can be represented as a constellation where each point corresponds to a unique amplitude and phase combination.

In a QAM channel, the carrier frequency is typically situated at the center of the bandwidth allocation. Therefore, when measuring amplitude, it’s essential to evaluate the average power of the transmitted symbols over the entire bandwidth to accurately capture the variations in amplitude resulting from the modulation process. This average power measurement allows for a clearer understanding of signal integrity across the bandwidth, which is crucial for maintaining data quality and minimizing error rates.

The significance of the carrier being above the edge of the bandwidth relates to how the QAM signal is constructed and transmitted, ensuring that it remains effective and detectable across the entire frequency range allocated for that channel. This positioning contributes to measuring amplitude through the average power effectively, as it ensures that the carrier contributes meaningfully to the overall signal, enhancing reliability in performance assessments.