Abstract
The channel capacity of an intensity-modulated optical transmission system with direct detection is compared with data rates of realizable modulation schemes. It is assumed that the additive Gaussian noise dominates in the receiver. Multilevel pulse amplitude modulation (PAM) and multilevel pulse position modulation (PPM) are investigated and their parameters are optimized. The numerical evaluation proves that only approx. 20 to 25 % of the channel capacity can be achieved with these modulation schemes. In contrast to results from literature it is shown in the present contribution that under realistic conditions, PAM yields higher data rates than PPM. As an alternative to the modulation schemes with impulsive carrier signals the multitone modulation is investigated. It is shown that the sum of the channel capacity of all sub-bands amounts to approx. 47% of the original channel capacity. Because of this result, with multitone modulation an increase of channel capacity is expected in comparison with modulation schemes with impulsive carrier signals.
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