  Symbiotic radio is a promising technology to achieve spectrum- and
energy-efficient wireless communications, where the secondary backscatter
device (BD) leverages not only the spectrum but also the power of the primary
signals for its own information transmission. In return, the primary
communication link can be enhanced by the additional multipaths created by the
BD. This is known as the mutualism relationship of symbiotic radio. However, as
the backscattering link is much weaker than the direct link due to double
attenuations, the improvement of the primary link brought by one single BD is
extremely limited. To address this issue and enable full mutualism of symbiotic
radio, in this paper, we study symbiotic radio with massive number of BDs. For
symbiotic radio multiple access channel (MAC) with successive interference
cancellation (SIC), we first derive the achievable rate of both the primary and
secondary communications, based on which a receive beamforming optimization
problem is formulated and solved. Furthermore, considering the asymptotic
regime of massive number of BDs, closed-form expressions are derived for the
primary and the secondary communication rates, both of which are shown to be
increasing functions of the number of BDs. This thus demonstrates that the
mutualism relationship of symbiotic radio can be fully exploited with massive
BD access.
