  Resonant beam communications (RBCom) is capable of providing wide bandwidth
when using light as the carrier. Besides, the RBCom system possesses the
characteristics of mobility, high signal-to-noise ratio (SNR), and
multiplexing. Nevertheless, the channel of the RBCom system is distinct from
other light communication technologies due to the echo interference issue. In
this paper, we reveal the mechanism of the echo interference and propose the
method to eliminate the interference. Moreover, we present an exemplary design
based on frequency shifting and optical filtering, along with its mathematic
model and performance analysis. The numerical evaluation shows that the channel
capacity is greater than 15 bit/s/Hz.
