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.

翻译：共生无线电（SR）是一种通过构建物联网传输与蜂窝传输之间的互惠关系来支持蜂窝物联网（IoT）的有前景技术。本文提出了一种新颖的多用户多IoT设备共生无线电系统，以实现蜂窝物联网中的大规模接入。在所考虑系统中，基站（BS）向多个蜂窝用户传输信息，同时多个IoT设备利用蜂窝信号将自身信息反向散射至这些用户。蜂窝用户联合解码来自基站和IoT设备的信息。鉴于IoT设备的反射链路可视为直射链路的信道不确定性，我们应用稳健设计方法设计基站的波束赋形向量。具体而言，在蜂窝传输中断概率约束与IoT传输总速率约束下最小化发射功率。提出基于半正定规划和凸差规划的算法以求解该功率最小化问题。此外，我们考虑每个蜂窝用户与若干个邻近IoT设备相关联的特殊情形，并提出一种基于到达方向（DoA）的发射波束赋形设计方法。该DoA方法仅需直射链路的到达方向和角度扩展（AS），而无需反射链路的瞬时信道状态信息（CSI），从而显著降低信道反馈开销。仿真结果验证了多用户多IoT设备共生无线电系统及所提波束赋形方法的有效性。研究表明，在角度扩展较小的特殊情形下，基于DoA的波束赋形方法可获得与基于CSI方法相当的性能。