[Paper Review] Intelligent Reflecting Surface Aided MIMO Broadcasting for Simultaneous Wireless Information and Power Transfer

Cunhua Pan, Hong Ren+5 more|arXiv (Cornell University)|Aug 13, 2019

Advanced Wireless Communication Technologies42 references280 citations

TL;DR

This paper studies joint active and passive beamforming in an IRS-aided SWIPT MIMO system to maximize weighted sum rate while ensuring energy harvesting for ERs, using a BCD-based algorithm with WMMSE reformulation.

ABSTRACT

An intelligent reflecting surface (IRS) is invoked for enhancing the energy harvesting performance of a simultaneous wireless information and power transfer (SWIPT) aided system. Specifically, an IRS-assisted SWIPT system is considered, where a multi-antenna aided base station (BS) communicates with several multi-antenna assisted information receivers (IRs), while guaranteeing the energy harvesting requirement of the energy receivers (ERs). To maximize the weighted sum rate (WSR) of IRs, the transmit precoding (TPC) matrices of the BS and passive phase shift matrix of the IRS should be jointly optimized. To tackle this challenging optimization problem, we first adopt the classic block coordinate descent (BCD) algorithm for decoupling the original optimization problem into several subproblems and alternatively optimize the TPC matrices and the phase shift matrix. For each subproblem, we provide a low-complexity iterative algorithm, which is guaranteed to converge to the Karush-Kuhn-Tucker (KKT) point of each subproblem. The BCD algorithm is rigorously proved to converge to the KKT point of the original problem. We also conceive a feasibility checking method to study its feasibility. Our extensive simulation results confirm that employing IRSs in SWIPT beneficially enhances the system performance and the proposed BCD algorithm converges rapidly, which is appealing for practical applications.

Motivation & Objective

• Motivate and enable energy-efficient SWIPT by deploying an intelligent reflecting surface (IRS) near energy receivers (ERs).
• Formulate and solve a weighted sum rate maximization problem for IRS-assisted SWIPT in a multiuser MIMO downlink with ER energy constraints.
• Develop low-complexity, convergent algorithms to jointly optimize BS transmit precoders and IRS phase shifts.
• Assess feasibility and demonstrate performance gains of IRS in extending ER operation range and enhancing IRs’ rates.
• Provide insights on the impact of channel path loss and IRS location on system performance.

Proposed method

• Reformulate the WSR maximization via the weighted MMSE (WMMSE) equivalence to enable alternating optimization of BS precoding and IRS phase shifts.
• Apply a block coordinate descent (BCD) approach to iteratively optimize the TPC matrices and the IRS phase shift matrix.
• For fixed phase shifts, solve the TPC optimization with a low-complexity iterative method based on successive convex approximation (SCA) and Lagrangian dual decomposition, including a bisection search for dual variables.
• For fixed TPC, solve the phase shift optimization as a non-convex QCQP using majorization-minimization (MM) and price-based methods to obtain a KKT-point solution.
• Prove convergence to KKT points for each subproblem and for the overall BCD algorithm, and introduce a feasibility checking strategy.

Experimental results

Research questions

• RQ1 How can IRS phase shifts be jointly designed with BS transmit precoding to maximize the weighted sum rate of information receivers in a SWIPT MIMO system?
• RQ2 How does including energy harvesting requirements at ERs affect the joint design and feasibility of IRS-assisted SWIPT transmissions?
• RQ3 What low-complexity algorithms can reliably converge to KKT points for the coupled TPC and phase-shift optimization problem?
• RQ4 What is the impact of IRS deployment near ERs on the feasible operating range and harvested energy performance?
• RQ5 How do channel conditions and path loss exponents influence the benefits of IRS in SWIPT environments?

Key findings

• Introducing IRSs in SWIPT-MIMO systems substantially enhances energy harvesting performance and can extend ER operation range.
• The proposed BCD-based algorithm converges rapidly to KKT points for the joint TPC and phase shift optimization.
• A feasible and efficient optimization framework is achieved by decoupling the problem via WMMSE reformulation and solving subproblems with SCA, MM, and dual methods.
• The feasibility study reveals the EH constraint can restrict solutions, and a dedicated feasibility checking method helps determine problem solvability.
• Simulation results confirm IRS-aided SWIPT yields performance gains and illustrate the importance of IRS placement and path loss on system performance.