# Utility Regions for DF Relay in OFDMA-based Secure Communication with Untrusted Users

## TL;DR

This paper analyzes the conditions under which a trusted relay enhances secure OFDMA communication with untrusted users, providing optimal subcarrier and mode allocation strategies and defining relay utility regions.

## Contribution

It introduces optimal subcarrier allocation policies, mode selection strategies, and defines relay utility regions for secure OFDMA systems with untrusted users.

## Key findings

- Optimal subcarrier allocation policies for direct and relayed modes.
- Identification of exclusive and mixed mode subcarriers.
- Asymptotically optimal mode selection strategy at various SNR levels.

## Abstract

This paper investigates the utility of a trusted decode-and-forward relay in OFDMA-based secure communication system with untrusted users. For deciding whether to use the relay or not, we first present optimal subcarrier allocation policies for direct communication (DC) and relayed communication (RC). Next we identify exclusive RC mode, exclusive DC mode, and mixed (RDC) mode subcarriers which can support both the modes. For RDC mode we present optimal mode selection policy and a suboptimal strategy independent of power allocation which is asymptotically optimal at both low and high SNRs. Finally, via numerical results we present insights on relay utility regions.