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optical true time delay pool-based beamforming and limited feedback for reconfigurable intelligent surface-empowered cloud radio access networks

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Science China(Information Sciences) 2021年第10期64卷 88-102页

作者： Huan HUANG Xiaowen WANG Chongfu ZHANG Jie PENG Muchuan YANG Songnian FU Deming LIU Kun QIU School of Information and Communication Engineering Zhongshan InstituteUniversity of Electronic Science and Technology of China School of Electronic Information University of Electronic Science and Technology of ChinaZhongshan Institute School of Information Engineering Guangdong University of Technology School of optical and Electronic Information Huazhong University of Science and Technology

Introducing reconfigurable intelligent surfaces(RISs) greatly improves the wireless propagation environment. Limited to feedback overheads and computing power, the low-complexity precoding and the cost-effective beamforming implementation are urgently demanded in RIS-empowered millimeter-wave cloud radio access networks(mmWave C-RANs). Herein, an optical true time delay pool-based hybrid beamforming(OTTDP-based HBF) scheme, enabling centralized analog beamforming control, is proposed for RIS-empowered mmWave C-RANs. By using codebook quantization techniques, we develop a non-iterative limited feedback(NI-LF) precoding algorithm for the RIS-empowered mmWave C-RAN using the proposed OTTDP-based HBF. In the NI-LF precoding algorithm, the complex joint optimization problem of hybrid precoding and phase-shifting of RIS is equivalently formulated as a maximum ratio transmission(MRT)problem and a sparse reconstruction problem. For an active antenna unit(AAU) equipped with eight antennas, a designed example of the proposed OTTDP is presented. Furthermore, for a RIS-empowered mmWave C-RAN at 28 GHz, the received signal-to-noise ratio(SNR) curves obtained via different precoding schemes are compared and discussed. Compared with the precoding without RIS, the proposed NI-LF precoding by using different codebooks to quantize the phase-shifting of the RIS respectively achieves 20.56, 19.55, and18.93 dB performance improvement for the user located near the 49-element RIS. Moreover, performance degradations due to limited feedback are analyzed. The computational complexity and overhead of the proposed NI-LF precoding algorithm are also given and discussed.

关键词： reconfigurable intelligent surface optical true time delay millimeter-wave cloud radio access network limited feedback

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optical true time delay pool based hybrid beamformer enabling centralized beamforming control in millimeter-wave C-RAN systems

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Science China(Information Sciences) 2021年第9期64卷 202-211页

作者： Huan HUANG Chongfu ZHANG Muchuan YANG Wei ZHENG Jie PENG Kun QIU School of Information and Communication Engineering Zhongshan InstituteUniversity of Electronic Science and Technology of China School of Electronic Information University of Electronic Science and Technology of ChinaZhongshan Institute

Effectively supporting millimeter-wave(mmWave) beamforming is still a major challenge in 5 G cloud radio access network(5 G C-RAN) systems with evolved common public radio interface-based(eCPRIbased)fronthaul. Herein, an optical true time delay pool based hybrid beamforming(OTTDP-HBF) scheme,enabling centralized beamforming control, is proposed for mmWave 5 G C-RAN systems. The weight control of the OTTDP-HBF is physically implemented by a pre-designed optical wavelength matrix which is mapped from a series of optical carriers. After introducing optical true time delay, this optical wavelength matrix then maps to the defined OTTDP. In this scheme, all physical implementation and the computational processing of analog beamforming can be centrally deployed into a centralized unit or distributed unit(CU/DU).Each active antenna unit(AAU) therefore becomes very simple. For single-user and multi-user scenarios,the OTTDP-based hybrid precoders are formulated respectively. In the developed OTTDP-based multiuser hybrid precoder, the spectral efficiency is improved by making use of all RF chains. For a 9-element uniform planar array deployed at an AAU, a designed example of the OTTDP-HBF is presented, where spectral efficiency curves obtained via different precoding schemes for single-user and multi-user scenarios are compared and discussed respectively.

关键词： optical true time delay MIMO system Millimeter wave millimeter wave cloud radio access network Multi-user hybrid precoding multi-user hybrid precoding

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