Opportunistic Non-orthogonal Multiple Access Research Articles

Performance Analysis of Opportunistic Beam Splitting NOMA in Millimeter Wave Networks

Applying non-orthogonal multiple access (NOMA) in millimeter-wave (mmWave) networks has the potential to significantly enhance the spectral efficiency. In this paper, we propose an opportunistic beam-splitting NOMA scheme to fully exploit the antenna array gain in downlink mmWave networks, where single- and double-beam NOMA transmissions are dynamically enabled according to the spatial angle difference between the paired NOMA users, thereby enhancing the flexibility of applying NOMA in mmWave networks. The base station with a single radio frequency chain implements the double-beam NOMA transmission by adopting the beam-splitting technique. We develop a unified theoretical performance analysis framework for the proposed scheme, while taking into account the spatially random users, link blockage model, and general distance-based user pairing strategy. By using tools from stochastic geometry, we derive the exact expressions of the coverage probability and the sum rate of the proposed scheme under both the LoS exponential and ball models. We further derive the corresponding upper bound to simplify the analytical expressions. Simulations validate the theoretical analysis and demonstrate the performance gains of the proposed scheme in terms of the coverage probability and sum rate over the conventional single-beam NOMA and orthogonal multiple access (OMA) schemes.

Cache‐enabled reinforcement learning scheme for power allocation and user selection in opportunistic downlink NOMA transmissions

Non‐orthogonal multiple access (NOMA) allows multiple user equipment (UE) to simultaneously share the same resource blocks using varying levels of transmit power at the base station (BS) side. Proper allocation of transmission power and selection of candidate users for pairing over the same resource block are critical for an efficient utilization of the available resources. Optimal UE selection and power splitting among paired UEs can be made through an exhaustive search over the space of all possible solutions. However, the cost incurred by such approach can render it practically infeasible. Reinforcement learning (RL) deploying double deep‐Q networks (DDQN) is a promising framework that can be adopted for tackling the problem. In this article, an RL‐based DDQN scheme is proposed for user pairing in opportunistic access to downlink NOMA systems with capacity‐limited backhaul link connections. The proposed algorithm relies on proactive data caching to alleviate the throttling caused by backhaul bottlenecks, and optimized UE selection and power allocation are accomplished through the continuous interaction between an RL agent and the NOMA environment to increase the overall system throughput. Simulation results are presented to showcase the near‐optimal strategy achieved by the proposed scheme. © 2022 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

An Opportunistic NOMA Scheme for Multiuser Spatial Multiplexing VLC Systems

For indoor visible light communication (VLC) systems, multiuser spatial multiplexing (SMP) has become a crucial technology to enhance the network capacity by using multiple light-emitting diodes (LEDs). However, the performance of a VLC system based on multiuser SMP is dynamic-range-limited since the dynamic range of some LEDs is usually underutilized in this case. To solve this problem, an opportunistic non-orthogonal multiple access (NOMA) is put forward to utilize the unsaturated dynamic range by allowing more user connections and as a result the spectral-efficiency is improved. Therein, the effects of user location distribution and successive interference cancellation (SIC) error propagation are investigated. In order to further improve the spectral-efficiency, we have developed an adaptive dynamic-range utilization scheme. Simulation results demonstrate that the proposed scheme effectively improves the spectral-efficiency under the primary users' quality of service (QoS) requirement.

Performance analysis of opportunistic NOMA strategy in uplink coordinated multi-points systems

Non-orthogonal multiple access (NOMA) has attracted a great deal of interest due to its potential contribution to the 5th generation (5G) mobile networks. In conventional power-domain NOMA, one of the disadvantages is that the edge-users which are far from the receivers may suffer edge-interference issue, and they may not be served if the propagation channels are extremely poor. In this study, an amplify-and-forward (AF) user-relay based opportunistic NOMA (UR-ONOMA) strategy, which aims to adaptively improve the coverage of high-quality service to the far-end edge users with weak channel scenarios, is designed for the uplink cooperative networks. The transmission data rates of a two-layer UR-ONOMA strategy are theoretically analyzed, and the closed-form expressions of outage probability are derived and evaluated. The numerical results show that the proposed cooperative UR-ONOMA has superior performance in terms of outage probability and throughput than the NOMA without the aid of user-relays when the far-end users are with weak channels.

Opportunistic NOMA for Uplink Short-Message Delivery With a Delay Constraint

In this paper, we study the application of opportunistic non-orthogonal multiple access (NOMA) mode to short-message transmissions with user's power control under a finite power budget. It is shown that opportunistic NOMA mode, which can transmit multiple packets per slot, can dramatically lower the session error probability when $W$ packets are to be transmitted within a session consisting of $W_{\mathrm{ S}}$ slots, where $W_{\mathrm{ S}} \ge W$ and the slot length is equivalent to the packet length, compared to orthogonal multiple access (OMA) where at most one packet can be transmitted in each slot. From this, opportunistic NOMA mode can be seen as an attractive approach for uplink transmissions. We derive an upper-bound on the session error probability as a closed-form expression and also obtain a closed-form for the NOMA factor that shows the minimum possible ratio of the session error probability of opportunistic NOMA to that of OMA. Simulation results also confirm that opportunistic NOMA mode has a much lower session error probability than OMA.

Impact of Wireless Backhaul Unreliability and Imperfect Channel Estimation on Opportunistic NOMA

We propose a new opportunistic non-orthogonal multiple access (NOMA) scheme under wireless backhaul unreliability and fronthaul channel uncertainty, where fronthaul represents regular radio access link. In particular, we propose two opportunistic methods for the proposed NOMA, which allow the best transmitter selection approaches based on either a near or a far-away receiver, considering the wireless backhaul unreliability and the fronthaul fading impairment. For the performance analysis, new closed-form expressions of exact and approximated outage probabilities of the grouped receivers are derived. The theoretical analysis provides an insight into the impact of wireless backhaul unreliability and imperfect channel estimation on the behaviour of outage probabilities at NOMA receivers. Furthermore, we analytically investigate how the number of multiple transmitters in the proposed opportunistic NOMA can determine the outage floors. We show that under unreliable wireless backhauls, a dominant receiver in the proposed NOMA scheme can achieve more than 3 dB gain in outage performance, compared to the orthogonal multiple access. In addition, the outage probability at a dominant receiver is less influenced by imperfect channel information, while the outage probability at a non-dominant receiver is significantly sensitive. The analytical expressions and asymptotic results have been validated through Monte Carlo simulations, thus verifying the derived impact analysis of NOMA under wireless backhaul unreliability and imperfect channel estimation.

Stable Throughput Regions of Opportunistic NOMA and Cooperative NOMA With Full-Duplex Relaying

In this paper, we consider downlink non-orthogonal multiple access (NOMA) transmission with dynamic traffic arrival for spatially random users of different priorities. By exploiting limited channel state information, we propose an opportunistic NOMA scheme to enable NOMA for high- and low-priority users when high-priority users experience good channel conditions. Opportunistic NOMA improves the transmission opportunities of low-priority users while reducing the adverse effect of NOMA on high-priority users. Moreover, we propose a cooperative NOMA scheme with full-duplex relaying, where low-priority users act as full-duplex relays to assist the high-priority users. The high-priority user constructively combines the signal and its delayed version transmitted by the base station and a selected relay, respectively. The adopted relay selection scheme takes into account the users’ spatial distribution, queue status, and channel conditions. By using tools from queueing theory and stochastic geometry, we derive the stable throughput regions of both proposed schemes. Furthermore, we derive the conditions under which the proposed NOMA schemes achieve larger stable throughput regions than orthogonal multiple access (OMA). At the expense of a higher implementation complexity and with appropriate parameter setting, cooperative NOMA with full-duplex relaying achieves a larger stable throughput region than opportunistic NOMA, which in turn outperforms OMA.

Achievable Rate Analysis for Opportunistic Non-orthogonal Multiple Access-Based Cooperative Relaying Systems

In this paper, we propose the opportunistic non-orthogonal multiple access (NOMA)-based cooperative relaying system (CRS) with channel state information (CSI) available at the source, where CSI for the source-to- destination and source-to-relay links is used for opportunistic transmission. Using the CSI, for opportunistic transmission, the source instantaneously chooses between the direct transmission and the cooperative NOMA transmission. We provide an asymptotic expression for the average achievable rate of the opportunistic NOMA-based CRS under Rayleigh fading channels. We verify the asymptotic analysis through Monte Carlo simulations, and compare the average achievable rates of the opportunistic NOMA-based CRS and the conventional one for various channel powers and power allocation coefficients used for NOMA.