  Optical wireless communications (OWC) which utilizes 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. In this paper, 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 a 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.
