# Enhancing Physical Layer Security in Dual-Hop Multiuser Transmission

## TL;DR

This paper addresses enhancing physical layer security in a dual-hop OFDMA system by jointly optimizing resource allocation, including sub-carrier assignment, relay selection, and power loading, to maximize secrecy rates under practical constraints.

## Contribution

It introduces a joint optimization framework for resource allocation in dual-hop OFDMA systems to improve physical layer security, including a low-complexity sub-optimal scheme.

## Key findings

- The proposed joint optimization significantly increases secrecy rates.
- The sub-optimal scheme achieves near-optimal performance with reduced complexity.
- Simulation results validate the effectiveness of the proposed methods.

## Abstract

In this paper, we consider the Physical Layer Security(PLS) problem in orthogonal frequency division multiple access (OFDMA) based dual-hop system which consists of multiple users, multiple amplify and forward relays, and an eavesdropper. The aim is to enhance PLS of the entire system by maximizing sum secrecy rate of secret users through optimal resource allocation under various practical constraints. Specifically, the sub-carrier allocation to different users, the relay assignments, and the power loading over different sub-carriers at transmitting nodes are optimized. The joint optimization problem is modeled as a mixed binary integer programming problem subject to exclusive sub-carrier allocation and separate power budget constraints at each node. A joint optimization solution is obtained through Lagrangian dual decomposition where KKT conditions are exploited to find the optimal power allocation at base station. Further, to reduce the complexity, a sub-optimal scheme is presented where the optimal power allocation is derived under fixed sub-carrier-relay assignment. Simulation results are also provided to validate the performance of proposed schemes.