Non-orthogonal Multiple Access in Large-Scale Heterogeneous Networks (1705.03325v1)

Abstract: In this paper, the potential benefits of applying non-orthogonal multiple access (NOMA) technique in $K$-tier hybrid heterogeneous networks (HetNets) is explored. A promising new transmission framework is proposed, in which NOMA is adopted in small cells and massive multiple-input multiple-output (MIMO) is employed in macro cells. For maximizing the biased average received power for mobile users, a NOMA and massive MIMO based user association scheme is developed. To evaluate the performance of the proposed framework, we first derive the analytical expressions for the coverage probability of NOMA enhanced small cells. We then examine the spectrum efficiency of the whole network, by deriving exact analytical expressions for NOMA enhanced small cells and a tractable lower bound for massive MIMO enabled macro cells. Lastly, we investigate the energy efficiency of the hybrid HetNets. Our results demonstrate that: 1) The coverage probability of NOMA enhanced small cells is affected to a large extent by the targeted transmit rates and power sharing coefficients of two NOMA users; 2) Massive MIMO enabled macro cells are capable of significantly enhancing the spectrum efficiency by increasing the number of antennas; 3) The energy efficiency of the whole network can be greatly improved by densely deploying NOMA enhanced small cell base stations (BSs); and 4) The proposed NOMA enhanced HetNets transmission scheme has superior performance compared to the orthogonal multiple access~(OMA) based HetNets.

• The paper proposes a hybrid transmission framework combining NOMA in small cells and massive MIMO in macro cells to enhance coverage, spectrum, and energy efficiency in large-scale heterogeneous networks.
• It presents analytical expressions showing NOMA small cell coverage depends on power allocation, massive MIMO boosts spectrum efficiency with more antennas, and NOMA improves overall network energy efficiency.
• The findings highlight practical implications for optimizing user association policies and demonstrate NOMA's superior performance over OMA for spectrum efficiency and fairness in future 5G network designs.

Non-orthogonal Multiple Access in Large-Scale Heterogeneous Networks

This paper explores the integration of non-orthogonal multiple access (NOMA) in $K$-tier hybrid heterogeneous networks (HetNets), proposing a novel transmission framework combining NOMA in small cells and massive MIMO in macro cells. The authors aim to enhance coverage, spectrum efficiency, and energy efficiency in fifth-generation (5G) network scenarios by developing a unique user association scheme based on these technologies.

Key Contributions

The authors introduce several analytical expressions to evaluate the proposed network framework:

1. Coverage Probability: By deriving exact analytical expressions, the paper reveals that the coverage probability for NOMA-enhanced small cells is significantly influenced by power allocation and targeted transmit rates. They establish that inappropriate power sharing among users can lead to ineffective coverage.
2. Spectrum Efficiency: The research demonstrates that massive MIMO enabled macro cells greatly increase spectrum efficiency by scaling the number of antennas. This is quantified through exact analytical and lower bound expressions.
3. Energy Efficiency: It is established that the deployment of NOMA-based small cells substantially improves the energy efficiency of the entire network. The authors discuss a power consumption model that includes static power and power amplifier efficiency for both types of cells.

Implications

The research contributes valuable insights into the design of 5G networks by illustrating how combining NOMA and massive MIMO can optimize network performance. The findings suggest significant practical implications:

• Optimization of User Association: The flexibility in biased user association policies can potentially improve user experience and network balance, especially in scenarios where network resources are unevenly distributed.
• Enhanced Network Performance: NOMA demonstrates superior performance compared to traditional orthogonal multiple access (OMA) systems, supporting increased spectrum efficiency and fairness in user access.

Future Directions

Further research may focus on optimizing power sharing coefficients among NOMA users to fully leverage the advantages of the proposed HetNets framework. Other potential directions include:

• Complexity Reduction: Developing efficient algorithms to mitigate processing delays and computational complexity associated with the integration of NOMA in small cells.
• Expansion to Multi-Cell Networks: Extending these analytical models to more intricate cellular networks, potentially involving interactions with IoT devices and varying service requirements.

The utility of stochastic geometry in capturing network randomness and providing tractable results is affirmed, yet the challenges in large-scale deployments and multi-user interactions remain ripe for exploration.