Adjustable-Delay RIS Is Capable of Improving OFDM Systems

Reconfigurable intelligent surfaces (RIS) demonstrate the potential to improve the spectrum and energy efficiency of wireless networks. In this paper, we investigate multiple-RIS-assisted orthogonal frequency division multiplexing (OFDM) communications. Specifically, we generalize the existing RIS concept conceived for frequency-flat channels to the adjustable-delay RIS by introducing varactor diodes. In contrast to conventional reflecting elements, each adjustable-delay RIS element is capable of storing and retrieving the impinging electromagnetic waves upon dynamically controlling its electromagnetically induced transparency (EIT), thus imposing an extra delay onto the incident signals. This allows for aligning multiple signal copies via multiple RISs. To this end, we formulate a rate-maximization problem by jointly optimizing the transmit power allocation and the RIS reflection coefficients as well as the RIS delays. To address the coupling issue between these optimization variables, we propose a computationally efficient algorithm to find a high-quality solution to the non-convex design problem by alternately optimizing the transmit power allocation and the RIS reflection pattern, including both the reflection coefficients and the delays. Furthermore, we conceive a low-complexity reflection optimization scheme upon aligning the strongest taps of all reflected channels, while ensuring that the maximum delay spread introduced by extra RIS delays does not exceed the length of the cyclic prefix. Our simulation results demonstrate that the proposed design significantly improves the achievable rate as well as the RIS's adaptability to wideband signals compared to various benchmark schemes operating without adjustable-delay RIS. Moreover, it is shown that there exists a fundamental trade-off between the adjustable delay margin to align different reflected channels and the practical component's power decay caused by delay.