On determining the optimum modulation index for reverse path lasers in hybrid fiber/coax networks

Previous work in modeling laser clipping phenomena in hybrid/fiber coax uses a signal model consisting of equal amplitude subcarriers. This model is inappropriate for the case of the reverse path where there may be only a small number of subcarriers, or a small number of subcarriers may take on an amplitude that is significantly greater than the other subcarriers. A model is presented that produces a probability density function (pdf) for laser drive current that accurately reflects the various mixtures of subcarriers typically present in the reverse path. This pdf is used to determine the relationship between laser rms modulation index, μ, and clipping distortion power. A method is shown to combine this relationship with laser RIN, shot noise and receiver noise current performance in order to determine the value of μ that produces optimum overall link performance.