  The heterogenous wireless services and exponentially growing traffic call for
novel spectrum- and energy-efficient wireless communication technologies. In
this paper, a new technique, called symbiotic radio (SR), is proposed to
exploit the benefits and address the drawbacks of cognitive radio (CR) and
ambient backscattering communications(AmBC), leading to mutualism spectrum
sharing and highly reliable backscattering communications. In particular, the
secondary transmitter (STx) in SR transmits messages to the secondary receiver
(SRx) over the RF signals originating from the primary transmitter (PTx) based
on cognitive backscattering communications, thus the secondary system shares
not only the radio spectrum, but also the power, and infrastructure with the
primary system. In return, the secondary transmission provides beneficial
multipath diversity to the primary system, therefore the two systems form
mutualism spectrum sharing. More importantly, joint decoding is exploited at
SRx to achieve highly reliable backscattering communications. To exploit the
full potential of SR, in this paper, we address three fundamental tasks in SR:
(1) enhancing the backscattering link via active load; (2) achieving highly
reliable communications through joint decoding; and (3) capturing PTx's RF
signals using reconfigurable intelligent surfaces. Emerging applications,
design challenges and open research problems will also be discussed.
