Undergrad Research

Dimensionality reduction for RIS-aided wireless communications

I spent 2.5 wonderful years in Broadband Wireless Communication and Signal Processing Laboratory led by Prof. Linglong Dai, where I finished my thesis on Dimensionality Reduction for RIS-aided Wireless Communications, and I was fortunate to be mentored by Zijian Zhang and to work with my friend Yuhao Chen.

Introduction

Reconfigurable intelligent surface (RIS) is a near-passive antenna array composed of a large number of elements. It can actively reconfigure the wireless channels by properly adjusting the phase-shifts of the incident electromagnetic waves. Benefiting from its characteristics of low cost and power consumption, RIS has been envisioned as a promising technique for future wireless communications. RIS is able to achieve high array gain that is proportional to the square of the number of RIS elements, which encourages RIS to be designed as large as possible. Although high-dimensional RIS enhances the performance of the system, it also brings about challenges considering size, power consumption, and pilot overhead. Thus, in my undergraduate thesis, we focus on the dimensionality-reduced design and validation of RIS-aided communications, preserving the high array gain of RIS while also addressing challenges in system design.

An RIS prototype with 2304 reflection units. [Source]

Challenge 1: mismatch in size

To tackle the problem of mismatch in size for high-dimensional RIS-aided system, we propose the concept of user-side RIS (US-RIS) that includes a multi-layer structure to save the space. We reveal the phenomenon that multi-layer RIS is able to adjust the magnitude of the signal, based on which we develop a phase-magnitude precoding design to maximize the achievable SNR. Simulation results verify the superiority of our proposed US-RIS that can enhance the uplink transmission of the user.

Challenge 2: high power consumption

To tackle the problem of high power consumption for high-dimensional RIS-aided system, we propose the concept of sub-connected active RIS, where multiple RIS elements share a same power amplifier to reduce the number of PAs for lower power consumption. We theoretically analyze the benefit of the sub-connected active RIS, and develop a hybrid precoding design to maximize the energy efficiency of the system. Simulation results verify that our proposed sub-connected active RIS as well as the precoding design can achieve higher EE in RIS-aided system.

Challenge 3: large number of pilot overhead

To tackle the problem of large number of pilot overhead for high-dimensional RIS-aided system, we propose the concept of sensing RIS, where a power detector is additionally integrated in each RIS element for detecting the magnitude of the signal. We conceive a signaling method to induce a interference random field (IRF), and propose a corresponding channel state information (CSI) acquisition method. Simulation results verify that our proposed solution can achieve dimension-independent CSI acquisition in RIS-aided system, which substantially reduces the pilot overhead. We further set up a hardware validation experiment that preliminarily demonstrates the practicability of our proposed solution.

Left: US-RIS. Middle: sub-connected active RIS. Right: sensing RIS.

This thesis was recognized as Excellent Bachelor Dissertation of Beijing and Excellent Bachelor Dissertation of Tsinghua University in 2022.

References

2023

Sensing RISs: Enabling Dimension-Independent CSI Acquisition for Beamforming

Jieao Zhu, Kunzan Liu, Zhongzhichao Wan, Linglong Dai, Tie Jun Cui, and H. Vincent Poor

IEEE Transactions on Information Theory, 2023

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Reconfigurable intelligent surfaces (RISs) are envisioned as a potentially transformative technology for future wireless communications. However, RISs’ inability to process signals and the attendant increased channel dimension have brought new challenges to RIS-assisted systems, including significantly increased pilot overhead required for channel estimation. To address these problems, several prior contributions that enhance the hardware architecture of RISs or develop algorithms to exploit the channels’ mathematical properties have been made, where the required pilot overhead is reduced to be proportional to the number of RIS elements. In this paper, we propose a dimension-independent channel state information (CSI) acquisition approach in which the required pilot overhead is independent of the number of RIS elements. Specifically, in contrast to traditional signal transmission methods, where signals from the base station (BS) and the users are transmitted in different time slots, we propose a novel method in which signals are transmitted from the BS and the user simultaneously during CSI acquisition. With this method, an electromagnetic interference random field (IRF) will be induced on the RIS, and we propose the structure of sensing RIS to capture its features. Moreover, we develop three algorithms for parameter estimation in this system, in which one of the proposed vM-EM algorithm is analyzed with the fixed-point perturbation method to obtain an asymptotic achievable bound. In addition, we also derive the Cramér-Rao lower bound (CRLB) and an asymptotic expression for characterizing the best possible performance of the proposed algorithms. Simulation results verify that our proposed signal transmission method and the corresponding algorithms can achieve dimension-independent CSI acquisition for beamforming.
@article{zhu2023sensing, title = {Sensing RISs: Enabling Dimension-Independent CSI Acquisition for Beamforming}, author = {Zhu, Jieao and Liu, Kunzan and Wan, Zhongzhichao and Dai, Linglong and Cui, Tie Jun and Poor, H. Vincent}, journal = {IEEE Transactions on Information Theory}, volume = {69}, number = {6}, pages = {3795--3813}, year = {2023}, publisher = {IEEE}, }

2022

Compact User-Specific Reconfigurable Intelligent Surfaces for Uplink Transmission

Kunzan Liu, Zijian Zhang, Linglong Dai, and Lajos Hanzo

IEEE Transactions on Communications, 2022

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Large-scale antenna arrays employed by the base station (BS) constitute an essential next-generation communications technique. However, due to the constraints of size, cost, and power consumption, it is usually considered unrealistic to use a large-scale antenna array at the user side. Inspired by the emerging technique of reconfigurable intelligent surfaces (RIS), we firstly propose the concept of user-specific RIS (US-RIS) for facilitating the employment of a large-scale antenna array at the user side in a cost- and energy-efficient way. In contrast to the existing employments of RIS, which belong to the family of base-station-specific RISs (BSS-RISs), the US-RIS concept by definition facilitates the employment of RIS at the user side for the first time. This is achieved by conceiving a multi-layer structure to realize a compact form-factor. Furthermore, our theoretical results demonstrate that, in contrast to the existing single-layer structure, where only the phase of the signal reflected from RIS can be adjusted, the amplitude of the signal penetrating multi-layer US-RIS can also be partially controlled, which brings about a new degree of freedom (DoF) for beamformer design that can be beneficially exploited for performance enhancement. In addition, based on the proposed multi-layer US-RIS, we formulate the signal-to-noise ratio (SNR) maximization problem of US-RIS-aided communications. Due to the non-convexity of the problem introduced by this multi-layer structure, we propose a multi-layer transmit beamformer design relying on an iterative algorithm for finding the optimal solution by alternately updating each variable. Finally, our simulation results verify the superiority of the proposed multi-layer US-RIS as a compact realization of a large-scale antenna array at the user side for uplink transmission.
@article{liu2022compact, title = {Compact User-Specific Reconfigurable Intelligent Surfaces for Uplink Transmission}, author = {Liu, Kunzan and Zhang, Zijian and Dai, Linglong and Hanzo, Lajos}, journal = {IEEE Transactions on Communications}, volume = {70}, number = {1}, pages = {680--692}, year = {2022}, publisher = {IEEE}, }
Active Reconfigurable Intelligent Surface: Fully-Connected or Sub-Connected?

Kunzan Liu, Zijian Zhang, Linglong Dai, Shenheng Xu, and Fan Yang

IEEE Communications Letters, 2022

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To overcome the “multiplicative fading effect” introduced by passive reconfigurable intelligent surface (RIS), the concept of active RIS has been recently proposed to amplify the radiated signals. However, the existing fully-connected architecture of active RIS consumes high power due to the additionally integrated active components. To address this issue, we propose the sub-connected architecture of active RIS. Different from fully-connected architecture, where each element integrates a dedicated power amplifier, in the sub-connected architecture, multiple elements control their phase shifts independently but share a same power amplifier, which significantly reduces the number of power amplifiers for power saving at the cost of fewer degrees of freedom (DoFs) for beamforming design. Fortunately, our analysis reveals that performance loss introduced by the subconnected architecture is slight, indicating that it can achieve much higher energy efficiency (EE). Furthermore, we formulate the EE maximization problem in the active RIS-aided system for both architectures and develop a corresponding joint beamforming design. Simulation results verify the proposed sub-connected architecture as an energy-efficient realization of active RIS.
@article{liu2022active, title = {Active Reconfigurable Intelligent Surface: Fully-Connected or Sub-Connected?}, author = {Liu, Kunzan and Zhang, Zijian and Dai, Linglong and Xu, Shenheng and Yang, Fan}, journal = {IEEE Communications Letters}, volume = {26}, number = {1}, pages = {167--171}, year = {2022}, publisher = {IEEE}, }