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We theoretically investigate the performance of 10-Gb/s semiconductor optical amplifier (SOA)-boosted transmission systems using the nonreturn-to-zero (NRZ), conventional duobinary, and phase-modulated (PM)-duobinary modulation formats. NRZ and conventional duobinary modulation formats are based on the intensity modulation. When the NRZ and conventional duobinary modulation formats are used, the nonlinear characteristics of SOA severely distort the signals. On the other hand, the PM-duobinary modulation format is based on differential binary phase-shift-keying modulation. When the PM-duobinary modulation format is employed, the nonlinear characteristics of SOA scarcely affect the signals which maintain a constant intensity in SOAs. Using the PM-duobinary modulation format, we can significantly improve the power margin in SOA-boosted transmission systems.