Abstract
Digital transmission over dispersive optical channels suffers from intersymbol interference (ISI).We study themutual information of PAM transmission under the intensity modulation and direct detection (IM/DD) constraint, where we model the optical channel as a Gaussian low-pass filter, which is a valid attempt for polymer optical fibers (POF) and allows closed form expressions. In particular, we study the properties of the sample whitened matched filter, which enables the derivation of the mutual information for PAM with IM/DD. The information rate (lower bound) of PAM with equally probable input symbols is compared to an information rate approximation of DC biased multiple sub-carrier modulation (MSM), where we approximate the effect due to threshold clipping as Gaussian distributed noise, and to the information rate of asymmetrically clipped MSM. The required optical power for uncoded block transmission at a bit error rate of 10−3 is also presented. In the case of PAM, a unique word approach is used, and decision-feedback equalization is performed directly at the output of a sample whitened matched filter realized in the frequency domain. The results show that PAM with a rectangular pulse shape outperforms the other schemes.
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