Reconfigurable intelligent surface (RIS)-aided terahertz (THz) communication has been considered as a promising technology for enabling future sixth-generation (6G) wireless systems. Due to the exploitation of extremely large bandwidth and large scale of RIS, RIS-aided THz communications would suffer from the beam split effect, where the generated beams cannot be aligned with the target physical direction in the whole bandwidth, so a severe array gain loss will be introduced. In this paper, the beam split effect is first analyzed in the existence of RIS. Then, a novel sub-connected RIS architecture is proposed to mitigate the beam split effect. The crux is to introduce additional time-delay (TD) modules and phase shifters into RIS elements so as to convert the classical phase-only precoding to the joint phase and delay precoding. Accordingly, a wideband precoding design is proposed to compensate for the severe array gain loss, and the performance analysis on the array gain is also provided. After that, we extend our discussions to the emerging scenarios with massive antennas equipped at the base station (BS), where the effect of “double beam split”, i.e., the coupling of beam split at the BS and the RIS, occurs. We prove the decomposability of the array gain, based on which the double beam split effect can be addressed by separately optimizing the wideband precoding at the BS and the RIS. Simulation results demonstrate that our proposed sub-connected RIS significantly alleviates the beam split effect with a small number of TD modules, and it is capable of achieving sub-optimal achievable rate performance with acceptable hardware cost and power consumption.

Reconfigurable intelligent surface (RIS) is an emerging technology to improve the spectral efficiency of wireless communication systems. However, the increase of RIS elements results in the non-negligible overhead of channel estimation and channel feedback, as well as the high complexity of beam design. Therefore, how to improve the system capacity with a limited number of RIS elements becomes a challenge. In this paper, we propose a brand new irregular RIS structure to enhance the capacity of RIS-aided wireless communications. The key idea is to irregularly configure a given number of RIS elements on an enlarged surface, which provides extra degrees of freedom and spatial diversity compared with the classical regular RIS. For the proposed irregular RIS-aided communication, we then formulate the joint topology and beamforming design problem to maximize the system capacity. Accordingly, we propose a joint optimization framework with low complexity to alternately optimize the RIS topology and the corresponding beamforming design. Finally, simulation results demonstrate that the proposed irregular RIS with a limited number of RIS elements can significantly enhance the system capacity compared with the traditional regular RIS.