On the Design of Secure Non-Orthogonal Multiple Access Systems (1612.06961v2)

Abstract: This paper proposes a new design of non-orthogonal multiple access (NOMA) under secrecy considerations. We focus on a NOMA system where a transmitter sends confidential messages to multiple users in the presence of an external eavesdropper. The optimal designs of decoding order, transmission rates, and power allocated to each user are investigated. Considering the practical passive eavesdropping scenario where the instantaneous channel state of the eavesdropper is unknown, we adopt the secrecy outage probability as the secrecy metric. We first consider the problem of minimizing the transmit power subject to the secrecy outage and quality of service constraints, and derive the closed-form solution to this problem. We then explore the problem of maximizing the minimum confidential information rate among users subject to the secrecy outage and transmit power constraints, and provide an iterative algorithm to solve this problem. We find that the secrecy outage constraint in the studied problems does not change the optimal decoding order for NOMA, and one should increase the power allocated to the user whose channel is relatively bad when the secrecy constraint becomes more stringent. Finally, we show the advantage of NOMA over orthogonal multiple access in the studied problems both analytically and numerically.

• The paper derives an optimal decoding order that remains effective under secrecy constraints, ensuring reliable NOMA performance.
• The paper presents a closed-form solution for minimizing transmit power while satisfying secrecy outage and QoS constraints.
• The paper introduces an iterative algorithm to maximize the minimum confidential rate, highlighting NOMA’s advantage over OMA in dense networks.

An Analysis of Secure Non-Orthogonal Multiple Access Systems

The presented paper introduces a comprehensive paper on enhancing the security of non-orthogonal multiple access (NOMA) systems, which are considered a vital component in meeting the massive connectivity and high spectral efficiency requirements of fifth-generation (5G) networks. The research specifically addresses the design of NOMA systems that accommodate security constraints in the presence of external eavesdroppers without known instantaneous channel state information (CSI). The paper investigates optimal strategies for decoding order, transmission rates, and power allocation to optimize the system's performance within these constraints.

Key Contributions

1. Decoding Order: The authors reveal that the optimal decoding order under secrecy outage constraints remains equivalent to that of traditional NOMA systems, which follows the descending order of channel gains normalized by noise.
2. Power Allocation and Secrecy Constraints: The paper derives a closed-form solution aimed at minimizing the transmit power subject to both secrecy outage and quality of service (QoS) constraints. A non-convex optimization problem is presented, introducing complexity beyond conventional NOMA designs due to the incorporation of secrecy constraints.
3. Maximization of Confidential Information Rate: The authors propose an iterative algorithm to maximize the minimum confidential information rate among NOMA users. Solutions presented include a closed-form solution for the two-user scenario, indicating the necessity of increasing power allocation to users with weaker channels as secrecy constraints become more stringent.
4. NOMA vs. OMA: The paper provides an analytical and numerical comparison between the performance of NOMA and orthogonal multiple access (OMA) systems, suggesting that NOMA consistently outperforms OMA, particularly as the number of users increases. Notably, the performance advantage of NOMA over OMA grows approximately linearly with the increase in user numbers.

Implications and Future Directions

This research contributes significantly to the integration of physical layer security in NOMA systems, ensuring robust data confidentiality despite the open nature of wireless channels susceptible to eavesdropping. The results have practical implications, especially in dense network scenarios typical of future wireless networks. The combination of optimal resource allocation and maintained QoS under secrecy constraints establishes a strong foundation for improving secure wireless communications.

Future research directions suggested include exploring secure NOMA systems with multiple-input and multiple-output (MIMO) technologies and addressing the inherent secrecy challenges among users in NOMA systems, a function of their ability to decode signals during transmission. Additionally, the complexity of extending secrecy provision to code-domain NOMA remains unexplored and poses a significant opportunity for ongoing research initiatives.

In conclusion, the paper provides a detailed model for integrating secrecy into NOMA systems, enhancing secure wireless communication frameworks vital for future network standards. The outlined methodologies and their implications contribute to advancing wireless technology towards more secure, efficient, and reliable communication systems.