Intelligent Reflecting Surface Enhanced Wireless Networks: Two-Timescale Beamforming Optimization

By Mingming Zhao, Qingqing Wu, Minjian Zhao, Rui Zhang. IEEE Transactions on Wireless Communications, 2019

319 citations recorded by the source corpus.

Intelligent reflecting surface (IRS) has drawn a lot of attention recently as a promising new solution to achieve high spectral and energy efficiency for future wireless networks. By utilizing massive low-cost passive reflecting elements, the wireless propagation environment becomes controllable and thus can be made favorable for improving the communication performance. Prior works on IRS mainly rely on the instantaneous channel state information (I-CSI), which, however, is practically difficult to obtain for IRS-associated links due to its passive operation and large number of reflecting elements. To overcome this difficulty, we propose in this paper a new two-timescale (TTS) transmission protocol to maximize the achievable average sum-rate for an IRS-aided multiuser system under the general correlated Rician channel model. Specifically, the passive IRS phase shifts are first optimized based on the statistical CSI (S-CSI) of all links, which varies much slowly as compared to their I-CSI; while the transmit beamforming/precoding vectors at the access point (AP) are then designed to cater to the I-CSI of the users’ effective fading channels with the optimized IRS phase shifts, thus significantly reducing the channel training overhead and passive beamforming design complexity over the existing schemes based on the I-CSI of all channels. Besides, for ease of practical implementation, we consider discrete phase shifts at each reflecting element of the IRS. For the single-user case, an efficient penalty dual decomposition (PDD)-based algorithm is proposed, where the IRS phase shifts are updated in parallel to reduce the computational time. For the multiuser case, we propose a general TTS stochastic successive convex approximation (SSCA) algorithm by constructing a quadratic surrogate of the objective function, which cannot be explicitly expressed in closed-form. Simulation results are presented to validate the effectiveness of our proposed algorithms and evaluate the impact of S-CSI and channel correlation on the system performance.

Subfields: Aerospace Engineering, Electrical and Electronic Engineering

View Paper: https://www.semanticscholar.org/paper/58a055eaf3f33406e9e336d262d01f6376e3bde9

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Citations in the corpus

Smart Radio Environments Empowered by Reconfigurable Intelligent Surfaces: How It Works, State of Research, and The Road Ahead (2020, background): “In [188], the authors consider an RIS-aided multi-user system and design a two-timescale joint active and passive beamforming optimization approach.”
Intelligent Reflecting Surface-Aided Wireless Communications: A Tutorial (2020, methodology): “has been widely adopted to obtain high-quality suboptimal solutions, by iteratively optimizing one of the transmit and reflect beamforming vectors/coefficients with the other being fixed, until the convergence is reached [9], [75], [76].”
Smart Radio Environments Empowered by Reconfigurable Intelligent Surfaces: How it Works, State of Research, and Road Ahead (2020, background): “In [187], the authors consider an RIS-aided multi-user system and design a two-timescale joint active and passive beamforming optimization approach.”
A Comprehensive Overview on 5G-and-Beyond Networks With UAVs: From Communications to Sensing and Intelligence (2020, background): “tradeoff when considering practical hardware imperfections such as discrete and/or coupled reflection amplitudes and phase shifts [139]–[142] remains an open problem.”
An Overview of Signal Processing Techniques for RIS/IRS-Aided Wireless Systems (2021, unknown): “a) Rounding method [89], [118], [141], [143], [144]: The main idea of this method is first to obtain a continuous solution that fulfills the unit modulus constraint.”
Reconfigurable Intelligent Surfaces: Principles and Opportunities (2020, background): “Scenarios Phase shifts CSI Main Objectives Techniques/Characteristics [73] SU/MU-DL-MISO Continuous Perfect Transmit power AO-SDR-based algorithm and two stage algorithm [74] SU/MU-DL-MISO Discrete Perfect Transmit power Near-optimal ZF-based successive refinement method [75] MU-DL-MISO Continuous P”