Cooperative Modulation for Symbiotic Radios: Design Methodology, Challenges, and Solutions

Recently, symbiotic radio (SR) has gained increasing attention due to its mutualistic spectrumand energy-sharing features. In SR, the secondary system leverages backscattering radio technology to modulate its information bits over the primary signal by exploiting the spectrum and energy resources of the primary system. As a reward, the secondary system provides an additional backscattering link for the primary system. Particularly, the backscattering modulation of the secondary transmitter leads to a novel multiplicative multiple access channel for SR, where the primary and secondary signals are multiplied over the backscattering link. However, this multiplicative relationship poses significant challenges for modulating and decoding both the primary and secondary signals. In this article, we propose the concept of cooperative modulation for facilitating primary and secondary transmissions. Toward this end, we first introduce transmission technologies and the signal model of SR as the basis, followed by the design methodology of cooperative modulation. Then, we address the main technical challenges for implementing cooperative modulation, including channel state information acquisition, synchronization, easy-to-implement index modulation design, and signal detection. Finally, numerical results are presented to validate the effectiveness of the proposed cooperative modulation design.