  The rising demand for energy and spectrum resources in next-generation
Internet-of-things (IoT) systems accounts for innovative modes of information
and power transfer. One potential solution is to harness the active
transmission capability of devices to facilitate data transmission and wireless
energy harvesting (WEH) for backscatter communication so as to form a symbiotic
radio (SR) environment in a mutualistic manner. Additionally, incorporating
reconfigurable intelligent surfaces (RISs) into the SR environment can provide
an additional link and enhance the reliability of backscatter communication,
thereby reinforcing the symbiotic relationships between active and passive
devices. This paper proposes a novel SR system where a standalone RIS sustains
its functions through WEH based on a low-power RIS structure and establishes
mutualistic symbiosis by utilizing a signal conveyed by the primary transmitter
(PTx) to assist ongoing transmissions and convey information to the primary
receiver (PRx). The PTx employs time index modulation (TIM) to transmit
information to the PRx and power to the RIS and energy harvester (EH). A
log-likelihood ratio (LLR)-based detector is presented to address challenges in
the TIM scheme. Finally, the performance of the proposed scheme is investigated
in terms of harvested direct current (DC) power at the RIS and EH, as well as
the bit error rate (BER) at the PRx.
