  Cell-free massive MIMO and symbiotic radio communication have been recently
proposed as the promising beyond fifth-generation (B5G) networking architecture
and transmission technology, respectively. To reap the benefits of both, this
paper studies cell-free symbiotic radio communication systems, where a number
of cell-free access points (APs) cooperatively send primary information to a
receiver, and simultaneously support the passive backscattering communication
of the secondary backscatter device (BD). We first derive the achievable
communication rates of the active primary user and passive secondary user under
the assumption of perfect channel state information (CSI), based on which the
transmit beamforming of the cellfree APs is optimized to characterize the
achievable rate-region of cell-free symbiotic communication systems.
Furthermore, to practically acquire the CSI of the active and passive channels,
we propose an efficient channel estimation method based on two-phase
uplink-training, and the achievable rate-region taking into account CSI
estimation errors are further characterized. Simulation results are provided to
show the effectiveness of our proposed beamforming and channel estimation
methods.

Systems and Control;Systems and Control;Signal Processing