We report the first single-channel 15.3 Tbit/s, 1.28 Tbaud, 64 QAM transmission using 670 fs coherent Nyquist pulses. We newly constructed an optical gate to improve the signal-to-noise ratio (SNR) of the homodyne detection signal, a coherent spectral expansion technique, and an optical phase-locked loop (OPLL) circuit with a 0.6 deg. phase noise. We also constructed an active 70 fs timing stabilization circuit between the OTDM signal and Nyquist LO pulse to realize precise homodyne detection. With these new techniques, we successfully achieved a record speed of 15.3 Tbit/s in a single channel transmission over 150 km with a spectral efficiency of 8.3 bit/s/Hz.