This work amalgamates spatial modulation (SM) with ambient backscattering (ABSc) to address the spectral and energy efficiency demands of the power constrained device-to-device (D2D) communications in the Internet-of-things. Though incorporating reconfigurable intelligent surfaces (RISs) in the communication process can help in extending the coverage of such power constrained devices, rich scattering in the operation environment, or broken links between the nodes involved in the end-to-end communication, can adversely affect the system performance. To cope up with this challenge, a selection-based cooperative RIS protocol is proposed, and the performance of the D2D communication system, founded on SM and ABSc at the transmitter and cooperative RISs, is evaluated in terms of the bit error rate, outage probability, and energy efficiency. A link budget analysis is conducted to comprehend the effects of the RIS sizes in countering the path loss effects, and the imperfection of the channel estimation and timing synchronization of multiple RISs are also analyzed. The results reveal that the proposed communication model with cooperative RISs can overcome the path loss effects and enhance the received power levels, thereby outperforming the baseline system where a single RIS intervenes in the end-to-end communication, with a signal-to-noise ratio gain of around 10 dB for the bit error rate, outage probability, and energy efficiency. Considering different prominent SM techniques for the system operation and comparing the performance in different set-ups, it is shown that the system implementing generalized SM performs the best.
