Channel capacity of intensity-modulated optical transmission systems with direct detection receivers

The channel capacity (in the sense of information theory) of an intensity-modulated optical transmission system is calculated with realistic boundary conditions. it is shown that a boundary condition for the transmitted signal in which the mean optical power is limited, does not lead to a limitation of the channel capacity. Because of this reason, a new and more realistic boundary condition is derived, in which a linear combination of the mean and the mean squared value of the optical power is limited. For the calculation, the model of the optical transmission system can be reduced to a single-sided hard limiter which prevents the optical power from becoming negative, and an additive coloured Gaussian noise source. For this transmission channel, no general solution for the channel capacity is known in literature. In the pre sent contribution, with the help of simple approximations, upper and lower bounds for the channel capacity are derived. A numerical example shows that these bounds are very close together.