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.
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