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Reconfigurable Intelligent Surface-Assisted Multiple-Antenna Coded Caching

Xiaofan Niu, Minquan Cheng, Kai Wan, Robert Caiming Qiu, Giuseppe Caire

TL;DR

A new RIS-assisted interference nulling algorithm to determine the phase-shift coefficients of the RIS and a new framework, referred to as RIS-assisted Multiple-Antenna Placement Delivery Array (RMAPDA), to construct the cache placement and delivery phases.

Abstract

Reconfigurable Intelligent Surface (RIS) has emerged as a promising technology to enhance the wireless propagation environment for next-generation wireless communication systems. This paper introduces a new RIS-assisted multiple-antenna coded caching problem. Unlike the existing multi-antenna coded caching models, our considered model incorporates a passive RIS with a limited number of elements aimed at enhancing the multicast gain (i.e., Degrees of Freedom (DoF)). The system consists of a server equipped with multiple antennas and several single-antenna users. The RIS, which functions as a passive and configurable relay, improves communication by selectively erasing certain transmission paths between transmit and receive antennas, thereby reducing interference. We first propose a new RIS-assisted interference nulling algorithm to determine the phase-shift coefficients of the RIS. This algorithm achieves faster convergence compared to the existing approach. By strategically nulling certain interference paths in each time slot, the transmission process is divided into multiple interference-free groups. Each group consists of a set of transmit antennas that serve a corresponding set of users without any interference from other groups. The optimal grouping strategy to maximize the DoF is formulated as a combinatorial optimization problem. To efficiently solve this, we design a low-complexity algorithm that identifies the optimal solution and develops a corresponding coded caching scheme to achieve the maximum DoF. Building on the optimal grouping strategy, we introduce a new framework, referred to as RIS-assisted Multiple-Antenna Placement Delivery Array (RMAPDA), to construct the cache placement and delivery phases. Then we propose a general RMAPDA design to achieve the maximum DoF under the optimal grouping strategy.

Reconfigurable Intelligent Surface-Assisted Multiple-Antenna Coded Caching

TL;DR

A new RIS-assisted interference nulling algorithm to determine the phase-shift coefficients of the RIS and a new framework, referred to as RIS-assisted Multiple-Antenna Placement Delivery Array (RMAPDA), to construct the cache placement and delivery phases.

Abstract

Reconfigurable Intelligent Surface (RIS) has emerged as a promising technology to enhance the wireless propagation environment for next-generation wireless communication systems. This paper introduces a new RIS-assisted multiple-antenna coded caching problem. Unlike the existing multi-antenna coded caching models, our considered model incorporates a passive RIS with a limited number of elements aimed at enhancing the multicast gain (i.e., Degrees of Freedom (DoF)). The system consists of a server equipped with multiple antennas and several single-antenna users. The RIS, which functions as a passive and configurable relay, improves communication by selectively erasing certain transmission paths between transmit and receive antennas, thereby reducing interference. We first propose a new RIS-assisted interference nulling algorithm to determine the phase-shift coefficients of the RIS. This algorithm achieves faster convergence compared to the existing approach. By strategically nulling certain interference paths in each time slot, the transmission process is divided into multiple interference-free groups. Each group consists of a set of transmit antennas that serve a corresponding set of users without any interference from other groups. The optimal grouping strategy to maximize the DoF is formulated as a combinatorial optimization problem. To efficiently solve this, we design a low-complexity algorithm that identifies the optimal solution and develops a corresponding coded caching scheme to achieve the maximum DoF. Building on the optimal grouping strategy, we introduce a new framework, referred to as RIS-assisted Multiple-Antenna Placement Delivery Array (RMAPDA), to construct the cache placement and delivery phases. Then we propose a general RMAPDA design to achieve the maximum DoF under the optimal grouping strategy.

Paper Structure

This paper contains 25 sections, 10 theorems, 50 equations, 4 figures, 2 tables, 4 algorithms.

Table of Contents

  1. Introduction
  2. Coded Caching
  3. Reconfigurable Intelligent Surface (RIS)
  4. Main Contribution
  5. System Model and Related Results
  6. System model
  7. MAPDA
  8. Tangent-Space Enhanced RIS-Assisted Interference Nulling algorithm
  9. Problem Formulation and Geometric Interpretation
  10. Tangent-Space Enhanced Alternating Projection algorithm
  11. Initialization Strategy
  12. Convergence Analysis
  13. Simulation Results and Analysis
  14. Optimal Grouping Approach
  15. RIS-Assisted Grouping Coded Caching Scheme
  16. ...and 10 more sections

Key Result

Lemma 1

Under the MAPDA structure, the maximum achievable sum-DoF is $\min\{K, L + K M/N \}$, when $KM/N$ is an integer.

Figures (4)

  • Figure 1: RIS-assisted multi-antenna coded caching system.
  • Figure 2: Comparison of two algorithms in interference power
  • Figure 3: Dividing Users into Interference-Free Groups via RIS.
  • Figure 4: Construction of RMAPDA for Example.

Theorems & Definitions (16)

  • Definition 1: MAPDA, yang2023multiplenamboodiri2023extended
  • Lemma 1: Maximum DoF yang2023multiple
  • Lemma 2: MN PDAmaddah2014fundamental
  • Lemma 3: SMK MAPDAshariatpanahi2016multi
  • Example 1
  • Example 2
  • Theorem 1: Local Convergence of the Tangent-Space Enhanced AP Algorithm
  • proof
  • Theorem 2
  • Proposition 1: Property of optimal grouping
  • ...and 6 more