Which modulation technique aids in reliably receiving STB upstream carriers below 20 MHz?

QPSK, or Quadrature Phase Shift Keying, is a modulation technique particularly well-suited for environments with limited bandwidth and lower frequency operations, such as upstream carriers below 20 MHz. It effectively transmits information by varying the phase of the carrier signal, allowing it to represent two bits of data per symbol, which enhances the efficiency of data transmission while maintaining robustness against signal degradation.

In scenarios where upstream communication is needed, such as in set-top box (STB) applications, the characteristics of QPSK allow for reliable reception even in the presence of noise. This is essential for maintaining a stable connection over the constraints imposed by lower frequencies. QPSK's lower complexity also enables easier implementation in various devices and systems.

While other modulation techniques such as 16-QAM, 64-QAM, and 8-VSB provide higher data rates and are effective in certain applications, they are generally more suited for higher frequency ranges or scenarios where bandwidth efficiency is prioritized, which is not as critical in the context of upstream carriers below 20 MHz. Thus, QPSK stands out as the appropriate choice for this specific range and application.