Cooperative Beamforming and Flexible Index Modulation for Reconfigurable Intelligent Surface-Aided Symbiotic Radio Systems
Weidong Wan, Yi Liu, Hailin Zhang
Abstract
Symbiotic radio (SR) system is a cellular network (CN) and Internet of Things (IoT) integrated system that achieves mutual benefits. In the SR system, to make the spectral efficiency (SE) of IoT signal not limited by its modulation, and to provide greater freedom for the system design, this paper proposes a flexible modulation (FLEM) for reconfigurable intelligent surface (RIS) aided SR system based on the index modulation. The weighted sum capacity (WSC) is considered as the optimization goal for perfect channel state information (CSI) and imperfect cascaded CSI (CCSI) respectively. For each case, we propose a joint optimization algorithm, containing not only the cooperative beamforming, but also the optimization of the IoT modulation, to obtain the optimal solution for the SR system design. Simulations show that joint optimization improves the WSC of SR system and is robust to the channel estimation error. Furthermore, the optimal IoT SE is obtained by analyzing the trade-off between the IoT channel capacity and the SE provided by IoT modulation.