Modulation formats in optical networks

1 . An optical line terminal configured to communicate with optical network units in a passive optical network, wherein downstream transmission from the optical line terminal to the optical network units is organized into frames including segments, wherein a respective segment is modulated according to at least a first modulation format or a second modulation format, and wherein the optical line terminal is configured to:

send, to at least one optical network unit, a request to estimate a channel quality based on at least one reference segment modulated according to the second modulation format, wherein the at least one optical network unit is configured to operate according to the first modulation format, wherein the at least one reference segment is located at one or more selected positions within downstream transmission frames, and wherein the downstream transmission frames further include segments modulated according to the first modulation format;

receive, from the at least one optical network unit, 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 optical network unit to the second modulation format based on the response.

2 . The optical line terminal according to claim 1 , wherein the second modulation format has a higher modulation order than the first modulation format.

3 . The optical line terminal according to claim 1 , wherein the optical line terminal is further configured to select a forward error correction code for the segments in the downstream transmission assigned to the at least one optical network unit based on the response.

4 . The optical line terminal according to claim 1 , wherein the optical line terminal is further configured to include the at least one reference segment within downstream transmission frames during an estimation period.

5 . The optical line terminal according to claim 1 , wherein the at least one reference segment includes data intended for at least one other optical network unit operating according to the second modulation format.

6 . The optical line terminal according to claim 1 , wherein the at least one reference segment includes a sequence.

7 . The optical line terminal according to claim 1 , wherein the request includes at least one position identifier to locate the at least one reference segment within downstream transmission frames.

8 . The optical line terminal according to claim 1 , wherein the request includes a modulation format identifier to identify the second modulation format.

9 . The optical line terminal according to claim 1 , wherein the request triggers configuration of receiver settings of the at least one optical network unit according to the second modulation format based on the at least one reference segment.

10 . The optical line terminal according to claim 1 , wherein the optical line terminal is further configured to omit data intended for the at least one optical network unit from downstream transmission frames until receiving the response.

11 . The optical line terminal according to claim 1 , wherein the indication of the estimated channel quality includes an indication of whether the estimated channel quality supports at least one of the second modulation format, a signal to noise ratio, a pre forward error correction bit error rate, a post forward error correction bit error rate, a symbol error rate, or a codeword error rate.

12 . An optical network unit configured to communicate with the optical line terminal according to claim 1 in the passive optical network, wherein downstream transmission from the optical line terminal to one or more optical network units is organized into frames including segments, wherein a respective segment is modulated according to at least a first modulation format or a second modulation format, and wherein the optical network unit, operating according to the first modulation format, is configured to:

receive, from the optical line terminal, a request to estimate a channel quality based on the at least one reference segment modulated according to the second modulation format; and

send, to the optical line terminal, a response including an indication of the estimated channel quality.

13 . A system comprising:

the optical line terminal according to claim 1 ; and

an optical network unit configured to

receive, from the optical line terminal, the request to estimate the channel quality based on the at least one reference segment modulated according to the second modulation format, and

send, to the optical line terminal, the response including the indication of the estimated channel quality.

14 . A method for communicating between an optical line terminal and optical network units in a passive optical network, wherein downstream transmission from the optical line terminal to the optical network units is organized into frames including segments, wherein a respective segment is modulated according to at least a first modulation format or a second modulation format, and wherein the method comprises:

sending, by the optical line terminal to at least one optical network unit, a request to estimate a channel quality based on at least one reference segment modulated according to the second modulation format, wherein the at least one optical network unit operates according to the first modulation format, wherein the at least one reference segment is located at one or more selected positions within downstream transmission frames, and wherein the downstream transmission frames further include segments modulated according to the first modulation format;

sending, by the at least one optical network unit to the optical line terminal, a response including an indication of the estimated channel quality to the optical line terminal; and

deciding, by the optical line terminal, to switch the segments in the downstream transmission assigned to the at least one optical network unit to the second modulation format based on the response.