Exploiting Full/Half-Duplex User Relaying in NOMA Systems (1812.07389v1)

Abstract: In this paper, a novel cooperative non-orthogonal multiple access (NOMA) system is proposed, where one near user is employed as decode-and-forward (DF) relaying switching between full-duplex (FD) and half-duplex (HD) mode to help a far user. Two representative cooperative relaying scenarios are investigated insightfully. The \emph{first scenario} is that no direct link exists between the base station (BS) and far user. The \emph{second scenario} is that the direct link exists between the BS and far user. To characterize the performance of potential gains brought by FD NOMA in two considered scenarios, three performance metrics outage probability, ergodic rate and energy efficiency are discussed. More particularly, we derive new closed-form expressions for both exact and asymptotic outage probabilities as well as delay-limited throughput for two NOMA users. Based on the derived results, the diversity orders achieved by users are obtained. We confirm that the use of direct link overcomes zero diversity order of far NOMA user inherent to FD relaying. Additionally, we derive new closed-form expressions for asymptotic ergodic rates. Based on these, the high signal-to-noise radio (SNR) slopes of two users for FD NOMA are obtained. Simulation results demonstrate that: 1) FD NOMA is superior to HD NOMA in terms of outage probability and ergodic sum rate in the low SNR region; and 2) In delay-limited transmission mode, FD NOMA has higher energy efficiency than HD NOMA in the low SNR region; However, in delay-tolerant transmission mode, the system energy efficiency of HD NOMA exceeds FD NOMA in the high SNR region.

• The paper analyzes outage probability and ergodic rates for full/half-duplex user relaying in NOMA, finding FD NOMA outperforms HD NOMA and OMA at low SNR, especially with a direct link.
• Theoretical models reveal that while FD NOMA offers higher spectral efficiency at low SNR, residual interference limits its advantage over HD NOMA at high SNR.
• Energy efficiency analysis shows FD NOMA is superior in delay-limited modes due to higher throughput, whereas HD NOMA is better in delay-tolerant modes due to reduced residual interference effects.

Analysis of Exploiting Full/Half-Duplex User Relaying in NOMA Systems

This paper presents an advanced exploration of cooperative non-orthogonal multiple access (NOMA) systems complemented by decode-and-forward (DF) user relaying. Through the employment of full-duplex (FD) and half-duplex (HD) relaying modes, the authors investigate constraints and performance variations within two distinct cooperative relaying scenarios: the presence or absence of a direct link between the base station (BS) and the far user.

Key Contributions and Findings

1. Outage Probability Analysis:
   • The paper derives closed-form expressions for the outage probability of both near and far NOMA users, providing insights into maximizing system performance. In particular, it uncovers how the integration of a direct link for FD user relaying can enhance network reliability, effectively mitigating the zero diversity order issue typically associated with conventional FD relaying.
   • It is demonstrated that FD NOMA offers superior outage performance compared to HD NOMA and orthogonal multiple access (OMA) under low SNR conditions.
2. Ergodic Rate Calculations:
   • The paper establishes new theoretical models for understanding the asymptotic ergodic rates of two users. This ensures higher spectral efficiency, particularly in low SNR domains, where FD mode outstrips HD NOMA by a notable margin.
   • The analytic results suggest that, despite FD NOMA achieving higher ergodic sum rates where SNR is low, the dual-link structure does not extend this advantage to high-SNR scenarios due to residual interference (RI).
3. Energy Efficiency Metrics:
   • Detailed energy efficiency (EE) evaluations show that in delay-limited transmission modes, FD NOMA surpasses HD NOMA's EE, due to the higher throughput achievable when the system operates below maximal power levels.
   • Contrarily, in delay-tolerant models, the results indicate that HD NOMA surpasses FD NOMA. This is attributed to FD re-transmissions reaching their power ceilings due to the increasing effects of residual loop-back interference (LI).
4. Performance Enhancement via Direct Links:
   • By incorporating a direct communication link to the far user, the paper provides a systematic demonstration of relieving the diversity bottleneck innate to conventional FD systems. This leads to improved signal diversity orders and enhances the overall system reliability.

Implications and Future Directions

This comprehensive examination of NOMA systems enriched with FD/HD user relaying harnesses the potential of maximizing spectral efficiency within the evolving 5G networks. By realizing favorable conditions for energy-efficient transmissions, these findings underscore the importance of balancing duplexing modes and leveraging direct link capabilities for multi-user access scenarios.

Given the bold claims and robust numerical support, future work could deepen investigations into minimizing self-interference (SI) in practical systems to fully harness FD potential. Advanced antenna design and signal processing techniques may yet hold the key to unlocking a more pronounced efficiency gain—essentially bridging the gap between theoretical promise and practical application.

In summary, this exploration significantly advances the understanding of integrating FD user relaying with NOMA, offering potential pathways for enhanced communication efficiency in next-generation wireless networks.