An, J., Xu, C., Ng, D. W. K., Yuen, C., & Hanzo, L. (2024). Adjustable-Delay RIS Is Capable of Improving OFDM Systems. IEEE Transactions on Vehicular Technology, 73(7), 9927–9942. https://doi.org/10.1109/tvt.2024.3362953
Abstract:
Reconfigurable intelligent surfaces (RIS) demonstrate
the potential to improve the spectrum and energy efficiency
of wireless networks. In this paper, we investigate multiple-RISassisted
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 themaximum
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.
License type:
Publisher Copyright
Funding Info:
This research / project is supported by the Australian Research Council’s Discovery Projects - NA
Grant Reference no. : DP230100603
This research / project is supported by the Engineering and Physical Sciences Research Council - NA
Grant Reference no. : EP/W016605/1, EP/X01228X/1, EP/Y026721/1
This research / project is supported by the European Research Council’s Advanced Fellow Grant QuantCom - NA
Grant Reference no. : 789028
This research / project is supported by the Ministry of Education, Singapore - Academic Research Fund Tier 2
Grant Reference no. : Award MOE-T2EP50220-0019
This research / project is supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research) Singapore - Manufacturing, Trade, and Connectivity Programmatic Fund
Grant Reference no. : M22L1b0110