FPLA: A Flexible Physical Layer Authentication Mechanism for Distributing Quantum Keys Securely via Wireless 5G Channels  
[PERSON]  
1, [PERSON]  
1, [PERSON]  
1, [PERSON] and [PERSON]  
1,2,3,4  
Response (CIR), Carrier Frequency Offset (CFO), or Channel State Information (CSI) to authenticate devices [6], but they depend on stable and distinguishable channel features.  
MU-MIMO greatly improves spectrum efficiency, and various techniques such as spatial multiplexing and beamforming are implemented in MU-MIMO. Spatial multiplexing allows multiple data streams, called layers, to be transmitted over the same time-frequency resource. In multi-user scenarios, different UE can transmit over different layers, and Base Stations (BS) reuse the data streams and assign layers to ports according to pre-coding matrix [7]. This changes the CSI at the BS's receiving antennas, which affects the traditional authentication schemes. However, it also enriches the features for the proposed FPLA mechanism. FPLA is a low-complexity and low-overhead solution that can easily handle different CSI dimensions, and achieve high authentication accuracy in MU-MIMO systems.  
In this paper, a Flexible Physical Layer Authentication (FPLA) mechanism for wireless access of QKD networks is proposed against spoofing attacks, enabling authentication under different antenna diversity conditions. The mechanism can flexibly adapt to different antenna diversity and is also robust to antenna layer heterogeneity caused by changes in the number of UEs. It employs CSI fed back in the Up-Link (UL) of Time Division Duplex (TDD) systems to train authentication classifier. The motivation is that the reported CSI information from UEs exhibits unique characteristics due to factors such as multipath effects and delay distortions, which vary with the UE's location. As CSI is a physical-layer attribute, it is inherently difficult to tamper with or forge, thereby offering robust security properties. In 5G TDD systems, the same frequency band is used for both UL and Down-Link (DL) transmissions. The wireless radio channel exhibits reciprocity, allowing the determination of the DL beam based on the SRS transmitted by UEs. Consequently, investigating device authentication based on SRS holds significant engineering practicality. The contributions of this paper are as follows:  
1. A Dimensional Transformation Residual Network (DTRN) model and a dimensional transformation block (D-block) are proposed to prevent loss of detection accuracy caused by the variation in CSI dimensions;
2. A DTRN-based FPLA mechanism is proposed, which dynamically adapts to the antenna diversity in 5G MU-MIMO systems and conforms to QKD network wireless access;
3. Authentication performance was evaluated under a time-varying CDL channel model in the 5G FR1 n78 band (3.5 GHz), which aligns with the frequency allocation of China Telecom's 5G QKD network. The impact of antenna diversity, multiple user access, and various SNR on the authentication performance is evaluated.  
The rest of this paper is organized as follows: Section 2 provides an overview of the related work; Section 3 introduces the system model; Section 4 presents the framework and workflow of FPLA; Section 5 describes the structure and implementation method of DTRN. Section 6 presents the simulation results, and Section 7 concludes this paper.