In transoceanic optical amplifier repeater systems with transmission distances reaching 10,000 km, various factors causing transmission performance degradation accumulate over distance. As a result, the chromatic dispersion and fiber nonlinearity lead to significant degradation of the signal waveform, making it difficult to achieve higher bit rates. After the development of the 5 Gb/s single-channel Trans-Pacific optical submarine cable system, TPC-5, significant efforts have been made to investigate high-speed transmission technologies to expand bit rates by 4–8 times. However, it was difficult to increase the bit rate to over 10 Gb/s with conventional rectangular NRZ-OOK optical signals in long-distance transmission over Trans-Pacific distance. This chapter discusses single-channel long-distance transmission using RZ-OOK optical signals suitable for high-speed transmission, targeting bit rates of 20–40 Gb/s.