Modulation Design and Optimization for Multiplicative Multiple Access Channel in Symbiotic Radios

In symbiotic radio (SR), the secondary transmitter (STx) modulates its information over the RF signal from the primary transmitter (PTx). This modulation technology, also called “modulation in the air”, leads to the multiplication of the primary and secondary signals. Thus, SR can be modeled as a multiplicative multiple access channel (M-MAC). In this paper, we propose a modulation scheme for such an M-MAC, which consists of two additive parts: the symbol-invariant component used to aid the primary transmission and the symbol-varying component used to deliver STx information. By optimizing these two components, we can strike a balance between the primary and secondary transmissions. Particularly, due to the coupling between these two transmissions in the M-MAC, we focus on the composite signal formed by the primary and secondary signals. Then, we optimize the above two components by maximizing the minimum Euclidean distance as well as minimizing the Hamming distance between the adjacent constellations of the composite signal. Furthermore, we derive the closed-form solution of the optimal modulation scheme, which is related to the ratio of the direct link to the backscatter link. Finally, simulation results are provided to verify the effectiveness of our proposed scheme.