The 300-GHz band enables ultrahigh-speed wireless communication because of its vast frequency range. We present a wireless link with a 300-GHz-band CMOS transmitter that improves the system signal-to-noise ratio (SNR) by using a frequency-doubler-based subharmonic mixer called a 'square mixer' and an architecture with image and local oscillator (LO) suppression. It achieved wireless digital transmission at 56 Gbit/s over 5 cm with 16-QAM. In addition, we compare the performance of wireless links using a figure-of-merit (FoM). This wireless link has an approximately 7.5 times higher FoM than a recently reported wireless link based on a CMOS transmitter.