Intelligent Reflecting Surface Enhanced Wireless Network via Joint Active and Passive Beamforming (1810.03961v4)

Abstract: Intelligent reflecting surface (IRS) is envisioned to be a new and revolutionizing technology for achieving spectrum and energy efficient wireless communication networks cost-effectively in the future. Specifically, an IRS consists of a large number of low-cost passive elements each reflecting the incident signal with a certain phase shift to collaboratively achieve beamforming and/or interference suppression at designated receivers. In this paper, we study an IRS-aided multiuser multiple-input single-output (MISO) wireless system where one IRS is deployed to assist in the communication from a multi-antenna access point (AP) to multiple single-antenna users. As such, each user receives the superposed signals from the AP as well as the IRS via its reflection. We aim to minimize the total transmit power at the AP by jointly optimizing the transmit beamforming by active antenna array at the AP and reflect beamforming by passive phase shifters at the IRS, subject to users' individual signal-to-interference-plus-noise ratio (SINR) constraints. However, the formulated problem is non-convex and difficult to be solved optimally.

• The paper demonstrates IRS-enhanced networks that jointly optimize AP active beamforming and IRS passive beamforming to minimize transmit power while meeting SINR constraints.
• It applies semidefinite relaxation and alternating optimization to address non-convex coupling between transmit and phase shift design.
• Simulation results reveal a quadratic (N²) transmit power reduction as IRS elements increase, outperforming traditional AF relay systems.

Intelligent Reflecting Surface Enhanced Wireless Network via Joint Active and Passive Beamforming

In the paper "Intelligent Reflecting Surface Enhanced Wireless Network via Joint Active and Passive Beamforming" by Qingqing Wu and Rui Zhang, the authors introduce the concept of Intelligent Reflecting Surface (IRS) as an innovative solution to enhance wireless communication systems. An IRS comprises numerous low-cost passive elements that can reflect incident signals independently with adjustable phase shifts. This enables three-dimensional passive beamforming without necessitating any transmit radio-frequency (RF) chains.

Problem Formulation

The paper investigates an IRS-aided single-cell wireless system where an IRS assists in communication between a multi-antenna access point (AP) and multiple single-antenna users. The primary objective is to minimize the total transmit power at the AP by jointly optimizing the active beamforming at the AP and passive beamforming through phase shifts at the IRS, ensuring that users' Signal-to-Interference-plus-Noise Ratio (SINR) constraints are met.

Methodology

The optimization problem posed is non-convex due to the coupling of active and passive beamforming alongside the unit-modulus constraints imposed by the phase shifters. To address this, the authors propose two main approaches:

1. Semidefinite Relaxation (SDR): This method relaxes the rank-one constraint that is inherently non-convex to solve the problem approximately. The SDR provides a high-quality approximate solution and a lower bound of the optimal value, facilitating the evaluation of other suboptimal solutions.
2. Alternating Optimization: This approach iteratively optimizes the transmit beamforming and phase shifts at the IRS. For given phase shifts, the optimal transmit beamforming is obtained using the minimum mean squared error (MMSE) criterion, and for given beamforming, the phase shifts are optimally updated.

Asymptotic Performance and Practical Insights

The asymptotic performance of IRS with an infinitely large number of reflecting elements is analyzed, and simulation results show significant performance gains over benchmark systems such as massive MIMO. Notably, the transmit power required scales down with the number of reflecting elements at a rate proportional to $N^2$. This theoretical insight underscores the potential of IRS in drastically reducing AP's transmit power.

Key Numerical Results

• Transmit Power vs. Number of Reflecting Elements: By doubling the number of reflecting elements, the transmit power can be significantly reduced, demonstrating the $N^2$ scaling law.
• Comparison with AF Relay Systems: The paper shows that an IRS-aided system achieves better or comparable performance to amplify-and-forward (AF) relay systems, especially as the number of reflecting elements increases.
• Deployment Insights: For effective deployment, it is advantageous to position the IRS in line-of-sight (LoS) with the AP and where it can best assist user communications.

Theoretical and Practical Implications

Theoretically, the use of IRS introduces a new degree of freedom in designing wireless networks by transforming the propagation environment. Practically, the reduced power consumption and hardware costs make IRS a compelling solution for future wireless systems, particularly in high-density indoor environments. The joint active and passive beamforming approach provides the necessary flexibility to dynamically adjust to varying user locations and channel conditions.

Future Directions

Future research should focus on practical aspects such as discrete phase shift design, robust beamforming in the presence of imperfect channel state information (CSI), and the integration of IRS in existing cellular and Wi-Fi networks. Additionally, experimental validations and the development of cost-effective IRS hardware will be key to transitioning this technology from theory to practice.

In conclusion, the paper by Wu and Zhang presents a comprehensive paper on the potential benefits of IRS in wireless communications, underpinned by rigorous theoretical analysis and supported by extensive simulation results. The insights derived pave the way for further exploration and eventual real-world implementation of IRS-enhanced wireless networks.