Optical transmission system

A method of setting a light modulation index of an M-QAM signal in a CATV system that frequency division multiplexes an AM signal in a channel and an M-QAM signal in another channel, and that light intensity modulates the frequency division multiplexed signals under a condition where the total sum of the light modulation indices of each of the signals is 1 or more, comprising the steps of:

• measuring light modulation indices and corresponding error rates of signals transmitted over the CATV system, the signals having a total sum of modulation indices greater than 1;
• determining a curve which sets forth a relationship between a light modulation index and an error rate including an inflection point from which the error rate does not decrease any more even if the light modulation index of the M-QAM signal is increased;
• determining a light modulation index of the M-QAM signal by the use of the curve;
• and
• setting the determined light modulation index for the transmission of M-QAM signals across the optical transmission system.

An optical transmission system including a transmitting station that light intensity modulates a plurality of modulation signals, including at least one digital modulation signal, whose total sum of modulation indices is beyond 1, and a receiving station having a receiver comprising:

• modulation means for generating a plurality of modulation signals;
• frequency division multiplex means for frequency division multiplexing all of the modulation signals;
• light intensity modulation means for light intensity modulating the frequency division multiplexed signals;
• and
• modulation index adjustment means for adjusting the light modulation index of the digital modulation signal , prior to the light intensity modulation means, to a set value that marks an onset of diverging from an error rate given by a ratio between receiver thermal noise power and received signal power, while maintaining a light modulation index within a usable transmission margin.

A CATV system that frequency division multiplexes an AM signal in a channel and an M-QAM signal in another channel and light-intensity modulates the multiplexed signals for transmission to a receiver, comprising:

• AM modulation means for generating an AM signal at a predetermined channel;
• M-QAM modulation means for generating an M-QAM signal at another channel;
• frequency division multiplex means for frequency division multiplexing the signals of each of the channels;
• light intensity modulation means for modulating light intensity by the multiplexed signals;
• and
• light modulation index adjustment means for adjusting the light modulation index of the M-QAM signal to a set value that marks an onset of diverging from an error rate given by ratio between receiver thermal noise power and received signal power, while maintaining a light modulation index within a usable transmission margin.