Abstract
We compare the performance of several direct-detection
orthogonal frequency-division multiplexing (OFDM) schemes to that of
ON–OFF keying (OOK) in combating modal dispersion in multimode fiber
links. We review known OFDM techniques, including dc-clipped OFDM
(DC-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM) and
pulse-amplitude modulated discrete multitone (PAM-DMT). We describe
an iterative procedure to achieve optimal power allocation for
DC-OFDM and compare analytically the performance of ACO-OFDM and
PAM-DMT. We also consider unipolar <i>M</i>-ary pulse-amplitude
modulation (<i>M</i>-PAM) with minimum mean-square error
decision-feedback equalization (MMSE-DFE). For each technique, we
quantify the optical power required to transmit at a given bit rate
in a variety of multimode fibers. For a given symbol rate, we find
that unipolar <i>M</i>-PAM with MMSE-DFE has a better power performance
than all OFDM formats. Furthermore, we observe that the difference
in performance between <i>M</i>-PAM and OFDM increases as the spectral
efficiency increases. We also find that at a spectral efficiency of
1 bit/s/Hz, OOK performs better than ACO-OFDM using a symbol rate
twice that of OOK. At higher spectral efficiencies, <i>M</i>-PAM performs
only slightly better than ACO-OFDM using twice the symbol rate, but
requires less electrical bandwidth and can employ analog-to-digital
converters at a speed only 81% of that required for
ACO-OFDM.
© 2011 IEEE
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