Obtaining Diversity Gain for Symbiotic Radio by Using Space-Time Coding

In symbiotic radio (SR) systems, backscatter devices (BDs) are able to achieve ultra-low-power transmissions by backscattering the primary signal of a primary transmission, offering a promising solution for future passive Internet-ofThings (IoT). This paper explores the use of multiple antennas at the BD, which allows it to combat fading in the cascaded channel of the backscatter link. To leverage the potential, the BD generates the secondary signal using Alamouti space-time coding across the multiple antennas. This enables the BD to transmit information without requiring channel state information (CSI) of the cascaded channel. Considering the secondary symbol period is identical to the primary symbol period in the SR system, we propose a theoretical model to evaluate the diversity order for both the primary and the secondary transmissions using their pairwise error probability (PEP). Interestingly, based on the theoretical results, it is found that the secondary signal cannot obtain the diversity gain provided by the space-time coding with the conventional non-biased constellation but can obtain it with a direct current (DC)-biased constellation. Extensive numerical results are provided to demonstrate the accuracy of the theoretical analysis and to show how to obtain the diversity gain for SR with space-time coding.