What is the upper threshold of 256-QAM?

The upper threshold of 256-QAM (Quadrature Amplitude Modulation) is indeed around 20-45 dB. This modulation scheme is used to transmit data efficiently over a channel by encoding multiple bits into signal variations. The 256-QAM format allows for 8 bits per symbol, making it a high-density modulation technique suitable for high-data-rate communications.

In practical terms, the dB measurement refers to the signal-to-noise ratio (SNR) needed for reliable communication. A SNR of 20-45 dB indicates a sufficient signal clarity and minimal interference, which is necessary for the complex symbol constellation of 256-QAM. At this threshold, the system can achieve favorable error rates, enabling the effective transmission of data.

The other choices correspond to ranges that are either too low or too high for reliable 256-QAM operation. Values lower than 20 dB would likely result in increased error rates and signal degradation, while values exceeding 45 dB might indicate an over-engineered scenario where the system design could still function effectively at lower power levels. Thus, the correct range of 20-45 dB aligns with the operational requirements for achieving robust performance with 256-QAM modulation.