Transmit Beamforming Design for Multiuser Multi-IoT-Device Symbiotic Radios

Symbiotic radio (SR) is envisioned to support both Internet-of-Things (IoT) and cellular networks by forming a mutualistic relationship between them. In this paper, we propose a multiuser multi-IoT-device SR system where the base station (BS) transmits information to multiple users, and a number of IoT devices simultaneously backscatter their information to the users via the BS signal. Since the IoT information changes rather quickly compared to the channel variation, the IoT transmission introduces channel uncertainty when the users decoding the cellular information. To leverage the channel uncertainty, we apply the robust design method to design the beamforming vector at the BS. To be specific, the transmit power at the BS is minimized under the cellular transmission outage probability constraints and IoT transmission rate constraints. S-lemma is then utilized to transform the challenging probability constraints into a convex semi-definite programming (SDP) form. To deal with the non-convex rank-one constraint in SDP, we exploit successive convex approximations to transform the SDP problem into a difference-of-convex (DC) problem and solve it iteratively with DC programming. Simulation results have substantiated the multiuser multi-IoT-device symbiotic radio system and the effectiveness of the proposed algorithm.