  In this paper, a novel technique, called symbiotic radio (SR), is proposed
for passive Internet-of-Things (IoT), in which a backscatter device (BD) is
integrated with a primary transmission. The primary transmitter is designed to
assist the primary and BD transmissions, and the primary receiver decodes the
information from the primary transmitter as well as the BD. We consider a
multiple-input single-output (MISO) SR and the symbol period for BD
transmission is designed to be either the same as or much longer than that of
the primary system, resulting in parasitic or commensal relationship between
the primary and BD transmissions. We first derive the achievable rates for the
primary system and the BD transmission. Then, we formulate two transmit
beamforming optimization problems, i.e., the weighted sum-rate maximization
problem and the transmit power minimization problem, and solve these non-convex
problems by applying semi-definite relaxation technique. In addition, a novel
transmit beamforming structure is proposed to reduce the computational
complexity of the solutions. Simulation results show that when the BD
transmission rate is properly designed, the proposed SR not only enables the
opportunistic transmission for the BD via energy-efficient passive
backscattering, but also enhances the achievable rate of the primary system by
properly exploiting the additional signal path from the BD.
