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The paper describes an optical duobinary modulation scheme that reduces the waveform distortion caused by the effect of group velocity dispersion (GVD) and the combined self-phase modulation (SPM)-GVD effect as generated in non-dispersion shifted fibre (NDSF) with a length of over 300 km at 5 Gbit/s. Measured and simulated eye-opening penalties are used to compare the optical duobinary modulation scheme to the conventional non-return-to-zero (NRZ) intensity modulation scheme. A comparison of duobinary modulation with a duobinary filter against NRZ intensity modulation with a full-cosine roll-off filter shows that the former yields a compact spectrum even in the case of a single pulse, which needs the broadest bandwidth. NDSF repeaterless transmission at 5 Gbit/s and 363 km without dispersion compensation is established to confirm the effectiveness of the optical duobinary modulation scheme.