Wireless communication using terahertz/sub-terahertz band can alleviate the spectrum scarcity in conventional RF ands and satisfy the drastically expanding demands for capacity. In this paper, a spatial-orthogonal ASK transmitter architecture is presented. The self-sustaining oscillator-based transmitter architecture has an ultra-compact size and excellent power efficiency. With the proposed high-speed constant-load switch, significantly reduced modulation loss is achieved. Using polarization diversity and multi-level modulation, the throughput is largely enhanced. Array configuration is also adopted to enhance the link budget for higher signal quality and longer communication range. Fabricated in a 130-nm SiGe BiCMOS technology, the transmitter achieves an EIRP of 21 dBm and dc-to-THz-radiation efficiency of 0.7% in each spatial channel. A 24.4-Gb/s total data rate over a 10-cm communication range is demonstrated. With an external Teflon lens system, the demonstrated communication range is further extended to 52 cm. Compared with prior art, the proposed transmitter shows much higher power efficiency.