  Symbiotic radio (SR) is a promising technique to support cellular
Internet-of-Things (IoT) by forming a mutualistic relationship between IoT and
cellular transmissions. In this paper, we propose a novel multi-user
multi-IoT-device SR system to enable massive access in cellular IoT. In the
considered system, the base station (BS) transmits information to multiple
cellular users, and a number of IoT devices simultaneously backscatter their
information to these users via the cellular signal. The cellular users jointly
decode the information from the BS and IoT devices. Noting that the reflective
links from the IoT devices can be regarded as the channel uncertainty of the
direct links, we apply the robust design method to design the beamforming
vectors at the BS. Specifically, the transmit power is minimized under the
cellular transmission outage probability constraints and IoT transmission sum
rate constraints. The algorithm based on semi-definite programming and
difference-of-convex programming is proposed to solve the power minimization
problem. Moreover, we consider a special case where each cellular user is
associated with several adjacent IoT devices and propose a direction of arrival
(DoA)-based transmit beamforming design approach. The DoA-based approach
requires only the DoA and angular spread (AS) of the direct links instead of
the instantaneous channel state information (CSI) of the reflective link
channels, leading to a significant reduction in the channel feedback overhead.
Simulation results have substantiated the multi-user multi-IoT-device SR system
and the effectiveness of the proposed beamforming approaches. It is shown that
the DoA-based beamforming approach achieves comparable performance as the
CSI-based approach in the special case when the ASs are small.
