What allows the STB upstream carrier to be reliably received below 20 MHz?

The reliable reception of the STB upstream carrier below 20 MHz is primarily attributed to the narrow bandwidth and QPSK modulation. QPSK, or Quadrature Phase Shift Keying, is a modulation technique that encoding data in four different phase shifts of a carrier wave. This allows for a more efficient use of bandwidth as it transmits two bits of information per symbol, leading to better data integrity and reducing the likelihood of interference or degradation of the signal.

Narrow bandwidth is crucial in this context as it minimizes the chance of overlapping signals and mitigates the effects of noise, which can be particularly problematic in lower frequency ranges. By utilizing narrow bandwidth in conjunction with QPSK modulation, the STB upstream carrier can achieve a higher signal-to-noise ratio, thereby enhancing the reliability of the received signal even when operating close to the 20 MHz threshold.

The other options do not directly influence the ability to receive signals below 20 MHz with the same degree of reliability as the combination of narrow bandwidth and QPSK modulation does.