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The technical feasibility of a straightforward and cost-effective extension of the current 100 Gb/s Ethernet 40-km physical medium dependent (PMD) architecture for single-mode fiber to a higher speed of 400 Gb/s is demonstrated by means of simulations. A 16-wavelength configuration, each running at 25 Gb/s in non-return-to-zero modulation format and using plain direct detection is numerically analyzed and optimized. It is shown that error-free performance is achievable when a channel plan slightly shifted from the zero-dispersion wavelength of the transmission fiber and having a channel spacing of 400 GHz is utilized. However, to meet the power budget requirement in a fiber having an attenuation coefficient of 0.50 dB/km, a semiconductor optical pre-amplifier with a small-signal gain of 23 dB and transmitters having a minimum average output power of +2.9 dBm and an extinction ratio of 8 dB have to be employed. Based on the calculated design margins, the use of flexible active devices is suggested for span lengths shorter than 40 km.