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Flexible-Duplex Cell-Free Architecture for Secure Uplink Communications in Low-Altitude Wireless Networks

Wei Shi, Wei Xu, Yongming Huang, Jiacheng Yao, Wenhao Hu, Dongming Wang

TL;DR

The paper tackles secure uplink UAV communications in low-altitude wireless networks by introducing a flexible-duplex cell-free architecture where each AP can act as a receive or jamming node. It develops a max-min secrecy-rate framework and solves it with a closed-form receive combiner and a penalty dual decomposition (PDD) algorithm, complemented by a scalable sequential AP-mode selection scheme. Key results show substantial secrecy-rate gains over fixed-role CF schemes and good fairness across UAVs, with the low-complexity approach delivering over 90% of the optimal performance at much lower complexity. The proposed framework enables practical, robust physical-layer security for LAWNs, making it a strong candidate for secure uplink support in future 6G networks.

Abstract

Low-altitude wireless networks (LAWNs) are expected to play a central role in future 6G infrastructures, yet uplink transmissions of uncrewed aerial vehicles (UAVs) remain vulnerable to eavesdropping due to their limited transmit power, constrained antenna resources, and highly exposed air-ground propagation conditions. To address this fundamental bottleneck, we propose a flexible-duplex cell-free (CF) architecture in which each distributed access point (AP) can dynamically operate either as a receive AP for UAV uplink collection or as a transmit AP that generates cooperative artificial noise (AN) for secrecy enhancement. Such AP-level duplex flexibility introduces an additional spatial degree of freedom that enables distributed and adaptive protection against wiretapping in LAWNs. Building upon this architecture, we formulate a max-min secrecy-rate problem that jointly optimizes AP mode selection, receive combining, and AN covariance design. This tightly coupled and nonconvex optimization is tackled by first deriving the optimal receive combiners in closed form, followed by developing a penalty dual decomposition (PDD) algorithm with guaranteed convergence to a stationary solution. To further reduce computational burden, we propose a low-complexity sequential scheme that determines AP modes via a heuristic metric and then updates the AN covariance matrices through closed-form iterations embedded in the PDD framework. Simulation results show that the proposed flexible-duplex architecture yields substantial secrecy-rate gains over CF systems with fixed AP roles. The joint optimization method attains the highest secrecy performance, while the low-complexity approach achieves over 90% of the optimal performance with an order-of-magnitude lower computational complexity, offering a practical solution for secure uplink communications in LAWNs.

Flexible-Duplex Cell-Free Architecture for Secure Uplink Communications in Low-Altitude Wireless Networks

TL;DR

The paper tackles secure uplink UAV communications in low-altitude wireless networks by introducing a flexible-duplex cell-free architecture where each AP can act as a receive or jamming node. It develops a max-min secrecy-rate framework and solves it with a closed-form receive combiner and a penalty dual decomposition (PDD) algorithm, complemented by a scalable sequential AP-mode selection scheme. Key results show substantial secrecy-rate gains over fixed-role CF schemes and good fairness across UAVs, with the low-complexity approach delivering over 90% of the optimal performance at much lower complexity. The proposed framework enables practical, robust physical-layer security for LAWNs, making it a strong candidate for secure uplink support in future 6G networks.

Abstract

Low-altitude wireless networks (LAWNs) are expected to play a central role in future 6G infrastructures, yet uplink transmissions of uncrewed aerial vehicles (UAVs) remain vulnerable to eavesdropping due to their limited transmit power, constrained antenna resources, and highly exposed air-ground propagation conditions. To address this fundamental bottleneck, we propose a flexible-duplex cell-free (CF) architecture in which each distributed access point (AP) can dynamically operate either as a receive AP for UAV uplink collection or as a transmit AP that generates cooperative artificial noise (AN) for secrecy enhancement. Such AP-level duplex flexibility introduces an additional spatial degree of freedom that enables distributed and adaptive protection against wiretapping in LAWNs. Building upon this architecture, we formulate a max-min secrecy-rate problem that jointly optimizes AP mode selection, receive combining, and AN covariance design. This tightly coupled and nonconvex optimization is tackled by first deriving the optimal receive combiners in closed form, followed by developing a penalty dual decomposition (PDD) algorithm with guaranteed convergence to a stationary solution. To further reduce computational burden, we propose a low-complexity sequential scheme that determines AP modes via a heuristic metric and then updates the AN covariance matrices through closed-form iterations embedded in the PDD framework. Simulation results show that the proposed flexible-duplex architecture yields substantial secrecy-rate gains over CF systems with fixed AP roles. The joint optimization method attains the highest secrecy performance, while the low-complexity approach achieves over 90% of the optimal performance with an order-of-magnitude lower computational complexity, offering a practical solution for secure uplink communications in LAWNs.
Paper Structure (32 sections, 65 equations, 7 figures, 2 algorithms)

This paper contains 32 sections, 65 equations, 7 figures, 2 algorithms.

Table of Contents

  1. Introduction
  2. System Model
  3. Uplink Signal and SINR Model
  4. Eavesdropper Signal and SINR Model
  5. Achievable Rate and Secrecy Rate
  6. Optimization Problem Formulation
  7. Proposed Joint Optimization Algorithm
  8. Closed-Form Solutions to Receive Combining
  9. PDD Framework for AP Mode and AN Optimization
  10. BCD Method for Solving Problem (\ref{['aLP']})
  11. Updating $\{y_m^C\}$, $\left\{\overline{\mathbf{V}}_m\succeq\mathbf{0}\right\}$, $r$ and $\left\{\chi_k\right\}$
  12. Updating $\mathbf{B}_k$
  13. Updating $\left\{x_m^C\right\}$, $r$, $\left\{\tau_k\right\}$, and $\overline{\mathbf{B}}_k$
  14. Low-Complexity Algorithm
  15. Heuristic AP Mode Selection
  16. ...and 17 more sections

Figures (7)

  • Figure 1: Illustration of a flexible-duplex CF architecture for secure uplink communications in a LAWN.
  • Figure 2: Three-dimensional topology of the considered LAWN.
  • Figure 3: Secrecy rate versus AN power budget, $P_m$.
  • Figure 4: Secrecy rate versus the number of APs, $M$.
  • Figure 5: Secrecy rate versus UAV transmit power, $p_k$.
  • ...and 2 more figures