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Symbiotic radio (SR) is a promising technique to support cellularInternet-of-Things (IoT) by forming a mutualistic relationship between IoT andcellular transmissions. In this paper, we propose a novel multi-usermulti-IoT-device SR system to enable massive access in cellular IoT. In theconsidered system, the base station (BS) transmits information to multiplecellular users, and a number of IoT devices simultaneously backscatter theirinformation to these users via the cellular signal. The cellular users jointlydecode the information from the BS and IoT devices. Noting that the reflectivelinks from the IoT devices can be regarded as the channel uncertainty of thedirect links, we apply the robust design method to design the beamformingvectors at the BS. Specifically, the transmit power is minimized under thecellular transmission outage probability constraints and IoT transmission sumrate constraints. The algorithm based on semi-definite programming anddifference-of-convex programming is proposed to solve the power minimizationproblem. Moreover, we consider a special case where each cellular user isassociated with several adjacent IoT devices and propose a direction of arrival(DoA)-based transmit beamforming design approach. The DoA-based approachrequires only the DoA and angular spread (AS) of the direct links instead ofthe instantaneous channel state information (CSI) of the reflective linkchannels, leading to a significant reduction in the channel feedback overhead.Simulation results have substantiated the multi-user multi-IoT-device SR systemand the effectiveness of the proposed beamforming approaches. It is shown thatthe DoA-based beamforming approach achieves comparable performance as theCSI-based approach in the special case when the ASs are small.

Multi-User Multi-IoT-Device Symbiotic Radio: A Novel Massive Access Scheme for Cellular IoT