Why can the STB upstream carrier be positioned below 20 MHz?

The positioning of the STB upstream carrier below 20 MHz is primarily attributed to the use of narrow bandwidth and QPSK (Quadrature Phase Shift Keying) modulation. This type of modulation is very efficient and makes it possible to transmit data effectively over a limited frequency range.

By utilizing QPSK, data can be sent with fewer bandwidth requirements compared to other modulation techniques, which allows for a more compact use of the frequency spectrum. Narrow bandwidth is advantageous because it minimizes adjacent channel interference and ensures that the signal remains robust against potential environmental noise and other forms of interference. This positioning also aligns with the requirements for distortion and signal integrity, making it a strategic choice for upstream carrier positioning in a cable network.

The other options highlight different aspects of telecommunications but do not specifically address the technical capabilities that make positioning the carrier below 20 MHz effective in this context.