Spherical RIS-Assisted mmWave MIMO Wireless Communications With Concentric UCAs

Transmission at millimeter-wave (mmWave) frequencies is promising in the sixth generation (6G) and beyond systems due to abundant spectrum resources. Reconfigurable intelligent surface (RIS) assisted mmWave communications has been envisioned as an effective technique to effectively compensate for propagation loss and increase the capacity of line-of-sight (LOS) multiple-input-multiple-output (MIMO) wireless communication. To decrease the complexity of transceivers, the uniform circular array (UCA) is an effective antenna structure for RIS-assisted mmWave MIMO wireless communications when the transmit and receive UCAs are aligned. However, the complexity increases when the transceivers are not perfectly aligned and it is difficult to ensure the strict alignment in practice. To achieve low-complexity transceivers and increase the spectrum efficiency for RIS-assisted mmWave MIMO wireless communications, in this paper we investigate the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications. The channel model is derived for the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications. Then, a beamforming scheme at the transmit UCA, a phase compensation scheme at the spherical RIS, and a predetection scheme at the receive UCA, which can decrease the complexity of transceivers, are developed in the misaligned scenario. Our proposed fast symbol-wise maximum likelihood (ML) detection scheme can recover the signals with low complexity. Furthermore, we propose an algorithm to achieve the optimal spherical RIS design for increasing the spectrum efficiency of the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications. Simulation results are presented to illustrate the theory for the concentric UCAs based spherical RIS-assisted mmWave MIMO wireless communications.