On the performance of block transmission schemes in optical channels with a Gaussian profile

OFDM and its real valued version discrete multitone transmission (DMT) are popular schemes to compensate dispersion in direct detection optical systems. They employ an inverse fast Fourier transform (IFFT) at the transmitter and a fast Fourier transform (FFT) at the receiver, whereas the data symbols are processed block-wise. Block transmission combined with frequency domain equalization (FDE) has been recognized as a possible alternative to the DMT schemes, where the IFFT is moved from the transmitter to the receiver. This paper compares various bit loading enhanced DMT schemes such as asymmetrically clipped DMT and DC-biased DMT to PAM block transmission with FDE and to various single subcarrier FDE schemes. Moreover, a new approach termed asymmetrically clipped orthogonal PAM, is proposed in this work. Simulation results are presented for theoretical channels with a Gaussian low-pass profile. It will be shown that PAM-FDE performs best for this kind of channel, and neither bit loading enhanced DMT nor single subcarrier block transmission with FDE can offer a higher data rate at a given average optical power.