Abstract: An optical line terminal (OLT) is configured to communicate with optical network units (ONUs) in a passive optical network. The downstream transmission from the OLT to the ONUs are organized in frames including segments, wherein a respective segment is modulated according to at least a first or second modulation format. The OLT is configured to: send, to an ONU, a request to estimate a channel quality based on at least one reference segment modulated according to the second modulation format, the ONU operating according to the first modulation format; receive, from the ONU, a response including an indication of the estimated channel quality; and decide to switch the segments in the downstream transmission assigned to the at least one ONU to the second modulation format based on the response.