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A comparison between 51.84-Gb/s vestigial sideband 4-level pulse amplitude modulation (PAM4) and ON-OFF keying (OOK) signals at a transmission distance of 80 km is experimentally demonstrated for the first time. An electro-absorption modulated laser-based intensity-modulation and direct-detection 80-km transmission is achieved using both modulation formats without dispersion compensating fiber. The relative frequency between single sideband filter and signal is adjusted to find an optimized condition for both carrier to signal power ratio and sideband suppression ratio. The experimental results show that with linear equalization and without bandwidth limitation, the OOK signal has a 4-dB better sensitivity compared with PAM4 after 80-km standard single-mode fiber transmission. [ABSTRACT FROM PUBLISHER]