HRIS-Assisted Communications: Trade-off between Combination and Separation

This correspondence considers a double-HRIS-assisted system to investigate the impact of separating an intact HRIS into two sub-RISs on system performance under a fixed total number of RIS elements. Specifically, an achievable rate maximization problem is formulated by jointly optimizing the allocation of active and passive elements, the transmit power at BS, and the amplifying power for two HRISs under the overall power constraint, and an alternating optimization algorithm is proposed to solve the problem. Based on the obtained solution, we further compare the proposed double-HRIS scheme with the single-HRIS scheme and the active RIS (ARIS) and passive RIS (PRIS) coexisting schemes under the same total power budget, and analyze the conditions under which the double-HRIS outperforms the other configurations. The analysis shows that the distance between the transmitter and the user plays a key role in the performance comparison. Finally, simulation results verify the correctness of the analysis and further reveal that the superiority of the HRIS over the coexisting ARIS-PRIS scheme becomes more pronounced as the communication distance increases, while the double-HRIS exhibits greater potential than its single-HRIS counterpart in wide-area communications.