Joint Precoding and Phase Shift Design in Reconfigurable Intelligent Surfaces-Assisted Secret Key Generation

TL;DR

This paper proposes a joint precoding and phase shift design in RIS-assisted systems to enhance secret key generation by maximizing channel randomness, leading to significant improvements in SKR, BDR, and randomness metrics.

Contribution

It introduces a novel channel probing protocol and an optimization algorithm for RIS-assisted secret key generation, improving key rates in challenging environments.

Findings

Enhanced SKR compared to existing protocols

Improved BDR and randomness metrics

Effective optimization of precoding and phase shifts

Abstract

Key generation is a promising technique to establish symmetric keys between resource-constrained legitimate users. However, key generation suffers from low secret key rate (SKR) in harsh environments where channel randomness is limited. To address the problem, reconfigurable intelligent surfaces (RISs) are introduced to reshape the channels by controlling massive reflecting elements, which can provide more channel diversity. In this paper, we design a channel probing protocol to fully extract the randomness from the cascaded channel, i.e., the channel through reflecting elements. We derive the analytical expressions of SKR and design a water-filling algorithm based on the Karush-Kuhn-Tucker (KKT) conditions to find the upper bound. To find the optimal precoding and phase shift matrices, we propose an algorithm based on the Grassmann manifold optimization methods. The system is evaluated in terms of SKR, bit disagreement rate (BDR) and randomness. Simulation results show that our protocols significantly improve the SKR as compared to existing protocol.