Which of the following is NOT a primary method of transporting radio signals in an active DAS?

Analog modulation of the RF signal, typically implemented as intensity-modulated RF-over-fiber, is a well-established primary transport method in analog active DAS architectures.

Down conversion - shifting the RF carrier to a lower intermediate frequency at the headend - is a primary active DAS transport method that simplifies signal distribution across coaxial or fiber links.

Active DAS headends transport signals to remote antenna units using three primary methods - analog modulation such as RF-over-fiber, digital sampling via ADC-based digital transport, or down conversion of the RF signal to a lower intermediate frequency for easier cable or fiber distribution. Up conversion, which shifts a signal from an intermediate frequency back up to the original RF frequency, is the final processing step at the remote unit that prepares the signal for radiation by the antenna; it is the inverse of down conversion and occurs after transport is complete, not during it.

Digital sampling through analog-to-digital conversion is a primary transport method used in digital active DAS platforms to carry RF signals as packetized or streamed digital data to remote units.