Optimizing D2D Communication With Practical STAR-RIS and Irregular Configurations

Device-to-device (D2D) communication offers significant potential for future wireless networks but faces challenges such as limited range, signal blockage, and interference. Reconfigurable intelligent surfaces (RISs) can mitigate these issues by dynamically controlling signal reflections. However, existing RIS-assisted D2D systems often rely on impractical infinite-resolution phase shifters and achieve limited coverage. This paper addresses these limitations by proposing a novel D2D communication system using simultaneous transmitting and reflecting RISs (STAR-RISs) with coupled and low-resolution phase shifters for cost-effective and full-space coverage D2D communication. We further introduce irregular STAR-RIS configurations where a subset of elements is strategically activated for enhanced spatial diversity. To optimize this system, a unified cross-entropy optimization (CEO) framework is developed for joint optimization of element selection (for irregular configurations) and the continuous amplitudes for transmission and reflection, along with the discrete phase shifts. Simulation results reveal that the proposed CEO-based algorithm achieves significantly higher sum rates compared to the benchmark algorithms. Furthermore, irregular STAR-RIS configurations provide additional gains in both sum rate and energy efficiency.