Hybrid Beamforming Design for Reconfigurable Holographic Surface-Assisted Multi-User Multi-Device Symbiotic Radio Network

Symbiotic radio (SR) is a promising technology to achieve ultra-low-power transmission for the next generation wireless network. However, its enabling technique, ambient backscatter transmission, suffers from double fading and path loss that severely restricts its service range. In this work, the reconfigurable holographic surface (RHS) is introduced at the transmitter to tackle this problem. Unlike the widely studied reconfigurable intelligent surface (RIS), the RHS that uses amplitude control can offer higher integration and lower power consumption. Seeing that the treatment of the direct-link interference in the received signal is the key problem for the receiver of the backscatter devices to achieve successful signal detection, we propose two strategies. InStrategy I, the ambient data symbol is detected for canceling the direct-link interference, while inStrategy II, we design the data stream sent by the ambient transmitter and its transmission frame in collaboration with zero-forcing beamforming to alleviate the interference among all ambient users and backscatter devices. Based on the two strategies, respectively, problems are formulated to jointly optimize the transmit power, the digital beamforming, and the holographic beamforming of the RHS such that the energy efficiency of the SR network (SRN) is maximized. For each of the two problems, we develop specific hybrid beamforming algorithm that decomposes the original problem into three sub-problems. These sub-problems respectively optimize the transmit power, the digital beamforming, and the holographic beamforming that are solved in an alternative manner. Simulation results verify that the proposed strategies can well exploit the potential of the RHS in enhancing the transmission performance of the SRN.