A 300-GHz CMOS transmitter that covers the vast unallocated frequency band above 275 GHz with six channels has recently been reported. It employed a tripler-based subharmonic mixer called the cubic mixer. However, the maximum perchannel data rate was limited to 17.5 Gb/s with 32 QAM because of the interference between the desired signal and the unwanted image signals at approximately 300 GHz, originating from an image of the second intermediate frequency signal, IF2, at around 100 GHz. In this study, we show that the image signals at around 300 GHz can be reduced significantly by using a set of LO frequencies different from that used previously, without impacting the allocation of the six channels. This is accomplished because the image of IF2 is driven out-of-band of the IF amplifier for IF2. As a result, the maximum per-channel data rate reached 21 Gb/s with 64 QAM. The aggregate data rate over six channels is 123 Gb/s.