RIS-NOMA Networks Under Near-Field: Limited Feedback Design

As a core candidate technology of next-generation communication systems, reconfigurable intelligent surface (RIS) can effectively improve the channel quality, realizing the smart radio environment. It is recognized that the optimal deployment of RIS should be closer to the transmitter/receiver, naturally clarifying necessity of near-field channel for RIS. Due to the complexity of near-field channel as spherical waves, the channel state information (CSI) for RIS downlink is quite difficult to be estimated. To solve above issues, this work focuses on the RIS assisted nonorthogonal multiple access (RIS-NOMA) system based on the frequency division duplex (FDD), and proposes the bit-allocation schemes to solve that of the feedback bit limitations. The optimal deployment location of RIS is investigated for two-users case, which ensures the minimum channel fading effect and reduces the complexity of RIS beamforming. Specifically, we also derive the quantized channel gain as a function of feedback bits, when the feedback overhead is limited. Meanwhile, we explore the applicable CSI range for RIS-NOMA downlink designed in this article. According to the characteristics of base station-RIS cascade subchannel, three bit-allocation schemes are provided, which can be easily extended to multiuser scenarios. By applying the RIS deployment and bit-allocation schemes, the limited feedback issue of RIS-NOMA downlink can be solved easily. Numerical results show that the proposed RIS deployment and feedback schemes effectively improve the quantization sum-rate for overall system, mitigating the performance loss caused by limited feedback.