Positioning Information Based High-Speed Communications with Multiple RISs: Doppler Mitigation and Hardware Impairments

Abstract

In this paper, we consider a multiple reconfigurable intelligent surface (RIS)-assisted system using positioning information (PI) to explore the potential of Doppler effect mitigation and spectral efficiency (SE) enhancement in high-speed communications (HSC) in the presence of hardware impairments (HWI). In particular, we first present a general multi-RIS-assisted system model for HSC with HWI. Then, based on PI, different phase shift optimization strategies are designed and compared for maximizing SE, eliminating Doppler spread, and maintaining a very low delay spread. Moreover, we compare the performance of different numbers of RISs with HWI in terms of SE and delay spread. Finally, we extend our channel model from line-of-sight to the Rician channel to demonstrate the effectiveness and robustness of our proposed scheme. Numerical results reveal that the HWI of RISs increases the delay spread, but has no impact on Doppler shift and spread. Additionally, the multiple RIS system not only suffers a more severe delay spread, but is limited in SE due to the HWI. When the number of RISs increases from 2 to 16, the range of average spectral efficiency and delay spread are from 4 to 4.6 Bit/s/Hz and from 0.7 µs to 2.5 µs, respectively. In contrast to conventional RIS-assistedsystems that require channel estimation, the proposed PI-based RIS system offers simplicity without compromising effectiveness and robustness in both SE enhancement and Doppler mitigation.

RIS-Assisted High-Speed Communications with Time-Varying Distance-Dependent Rician Channels

“…Considering the optimal phase shift φ opt mn (t) in ( 23), recall the DS f 0 (t) in (3) and f mn (t) in (12), and note that d(ζ mn (t))/dt = 0, we have…”

“…Interestingly, they showed that the HWI enlarges the delay spread of the HSC, but not the ability to mitigate Doppler effects. Moreover, the authors of [12] showed that due to the HWI, a multiple RIS system suffers from a wider delay spread as well as significant degradation of the SE. However, the channel models considered in [10][11][12] did not take into account the non-light-of-sight (NLoS), i.e., the Rayleigh component.…”

“…Besides, it is noteworthy that [7][8][9][10][11][12] did not consider distance-dependent Rician factors, which is not practical in HSC systems. This is because the distances of the direct and the cascaded links vary dramatically considering the mobility of the transceiver [6,13]; it is more practical to include the distance-dependent Rician factor in the channel model for HSC systems.…”

“…[10,11,14,15] did not consider the NLoS part, the DS term, or the distance-dependent Rician factor; Refs. [12,16,19] ignored the DS term and the distancedependent Rician factor. Besides, although Refs.…”