  Optical wireless communications (OWC) utilizing infrared or visible light as
the carrier attracts great attention in 6G research. Resonant beam
communications (RBCom) is an OWC technology which simultaneously satisfies the
needs of non-mechanical mobility and high signal-to-noise ratio~(SNR). It has
the self-alignment feature and therefore avoids positioning and pointing
operations. However, RBCom undergoes echo interference. Here we propose an
echo-interference-free RBCom system design based on second harmonic generation.
The transmitter and the receiver constitute a spatially separated laser
resonator, in which the retro-reflective resonant beam is formed and tracks the
receiver automatically. This structure provides the channel with adaptive
capability in beamforming and alignment, which is similar to the concept of
intelligent reflecting surface (IRS) enhanced communications, but without
hardware and software controllers. Besides, we establish an analytical model to
evaluate the beam radius, the beam power, and the channel capacity. The results
show that our system achieves longer distance and smaller beam diameter for the
transmission beyond 10 Gbit/s, compared with the existing OWC technologies.

Systems and Control;Systems and Control;Optics