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Publications

Introduction

With traditional orthogonal multiple access (OMA) techniques, only one user can be served in each time/frequency resource block (RB). This drawback limits the applicability of these access techniques in the 5G and beyond era, which requires massive connectivity, high spectrum/energy efficiency, large throughput, and low latency. As an alternative, non-orthogonal multiple access (NOMA) has become a promising solution to overcome the limitations of OMA since it provides additional degrees of freedom from non-orthogonal resource utilization in either the power domain or the code domain. These domains extend the dimensionality of RBs so that multiple users can be served in the same RB. However, this unique arrangement results in several challenges for the design of NOMA systems. The first difficulty concerns the basic theoretical foundations of NOMA. Specifically, the principles for codebook design in code-domain NOMA and interference cancellation in power-domain NOMA have to be carefully established. The second challenge is mainly from the joint-design of NOMA with other communication technologies, such as multiple-input multiple-output (MIMO), cooperative communications, cognitive radio (CR) networks, etc. Depending on the type of network, NOMA needs to be adapted according to the various system requirements. The third challenge relates to resource allocation in NOMA systems. While it introduces novel domains, NOMA requires new resource management schemes addressing new problems including user clustering, and power allocation. The last challenge is the implementation of NOMA. In practice, imperfect channel state information (CSI) and channel estimation errors are unavoidable, which limits the performance of NOMA systems. Mitigating these effects in NOMA system design and implementation is still an open issue.

In this Best Readings, we first present textbooks that provide comprehensive guides to current and emerging NOMA techniques. Then, we introduce several review articles to highlight the corresponding NOMA opportunities, solutions, and future research directions. Subsequently, we list the special issues related to NOMA and the latest standardization activities. Lastly, we offer many selected technical papers addressing the aforementioned challenges. For clarity, we classify these papers into seven main technical areas, namely: Basic Principles of NOMA, Multiple Antenna Aided NOMA, Interplay Between NOMA and Cooperative Communications, Resource Management in NOMA Networks, Coexistence of NOMA and Other Emerging Technologies, Practical Forms of NOMA, and Implementation Challenges of NOMA.

Issued April 2020

Contributors

Yuanwei Liu, Queen Mary University of London, UK
Zhiguo Ding, The University of Manchester, UK
Octavia A. Dobre, Memorial University, Canada
Naofal Aldhahir, The University of Texas at Dallas, USA

Robert Schober, University of Erlangen-Nuremberg, Germany
George K. Karagiannidis, Aristotle University of Thessaloniki, Greece
Pingzhi Fan, Southwest Jiaotong University, China
Zhijin Qin, Queen Mary University of London, UK

Editorial Staff

Matthew C. Valenti
Editor-in-Chief, ComSoc Best Readings
West Virginia University
Morgantown, WV, USA

Xianbin Wang
Associate Editor-in-Chief, ComSoc Best Readings
Western University
London, ON, Canada

Textbooks

M. Vaezi, Z. Ding, and H. V. Poor, Multiple Access Techniques for 5G Wireless Networks and Beyond, Springer, 2019.
This book is the first comprehensive guide for current and emerging multiple access techniques. It highlights recent research from academia, industry, and standardization bodies. Based on a practical system model, this book introduces the key designs of NOMA in the power, code, and other domains. For power domain NOMA, it discusses the main features such as fundamentals, power allocation, clustering, and experiments. It reviews sparse code multiple access (SCMA), interleave-division multiple access (IDMA), low density spreading code division multiple access (LDS-CDMA), pattern division multiple access (PDMA), interleave-grid multiple access (IGMA), rate-splitting multiple access (RSMA), and repetition division multiple access (RDMA). In addition, this book proposes numerous NOMA applications, and provides an overview of challenges and future directions for 5G and beyond.

Y. Liu, Z. Qin, and Z. Ding, Non-Orthogonal Multiple Access for Massive Connectivity, Springer, 2019.
This book discusses various challenges (capability, sustainability, security, etc.) that arise in NOMA applications.  Various applications are covered, including heterogeneous networks (HetNets), MIMO enabled networks, and CR networks. Additionally, the most recent developments in artificial intelligence (AI) enabled NOMA networks are also presented, and the research challenges of NOMA to support massive number of devices are identified. 

Overviews and Tutorials

Y. Liu, Z. Qin, M. Elkashlan, Z. Ding, A. Nallanathan, and L. Hanzo, “Nonorthogonal Multiple Access for 5G and Beyond,” Proceedings of the IEEE, vol. 105, no. 12, pp. 2347-2381, December 2017.
This paper provides a comprehensive overview of the state of the art in power-domain NOMA. It includes the theoretical foundations of NOMA, MIMO-NOMA designs, NOMA aided cooperative communications, resource control methods, NOMA in emerging 5G techniques, and comparisons with other forms of NOMA.

Z. Ding, Y. Liu, J. Choi, Q. Sun, M. Elkashlan, C.-L. I, and H. V. Poor, “Application of Non-orthogonal Multiple Access in LTE and 5G Networks,” IEEE Communication Magazine, vol. 55, no. 2, pp. 185-191, February 2017.
Compared with other surveys, this paper highlights the interplay between NOMA and cognitive radio and the current standards for LTE and 5G networks.

S. M. R. Islam, N. Avazov, O.A. Dobre, and K. Kwak, “Power-Domain Non-Orthogonal Multiple Access (NOMA) in 5G Systems: Potentials and Challenges,” IEEE Communications Surveys & Tutorials, vol. 19, no. 2, pp. 721-742, Second Quarter 2017.
This paper comprehensively surveys the recent progress in power-domain NOMA in terms of capacity, beamforming strategies, power allocation methods, and user-pairing schemes.

L. Dai, B. Wang, Y. Yuan, S. Han, C.-L. I, and Z. Wang, “Non-Orthogonal Multiple Access for 5G: Solutions, Challenges, Opportunities, and Future Research Trends,” IEEE Communications Magazine, vol. 53, no. 9, pp. 74-81, September 2015.
In addition to power-domain NOMA, this paper discusses code-domain NOMA including sparse code multiple access, multi-user shared access, and pattern-division multiple access. Moreover, this paper proposes a software-defined multiple access (SoDeMA) technique to provide an adaptive configuration for massive connectivity.

Z. Ding, X. Lei, G. K. Karagiannidis, R. Schober, J. Yuan, and V. K. Bhargava, “A Survey on Non-Orthogonal Multiple Access for 5G Networks: Research Challenges and Future Trends,” IEEE Journal on Selected Areas in Communications, vol. 35, no. 10, pp. 2181-2195, October 2017.
This is the first survey to discuss single-carrier/multi-carrier NOMA and NOMA-enabled millimeter-wave (mmWave) communications.

Y. Cai, Z. Qin, F. Cui, G.Y. Li, and J.A. McCann, “Modulation and Multiple Access for 5G Networks,” IEEE Communications Surveys & Tutorials, vol. 20, no. 1, pp. 629-646, First Quarter 2018.
This paper focuses on the modulation schemes for NOMA techniques both in the power domain, code domain, and multiple other domains.

S.M.R. Islam, M. Zeng, O.A. Dobre, and K. Kwak, Resource Allocation for Downlink NOMA Systems: Key Techniques and Open Issues,” IEEE Wireless Communications, vol. 25, no. 2, pp. 40-47, April 2018.
This paper focuses on resource allocation strategies (user pairing, power allocation algorithms, etc.) for downlink NOMA systems. It introduces a divide-and-next-largest-difference-based user pairing algorithm to guarantee cluster fairness.

W. Shin, M. Vaezi, B. Lee, D.J. Love, J. Lee, and H. V. Poor, Non-Orthogonal Multiple Access in Multi-Cell Networks: Theory, Performance, and Practical Challenges,” IEEE Communications Magazine, vol. 55, no. 10, pp. 176-183, October 2017.
This paper surveys the current research on multi-cell NOMA, especially for advanced interference cancellation schemes.

Special Issues

Special Issue on Non-Orthogonal Multiple Access for 5G Systems,” IEEE Journal on Selected Areas in Communications, vol 35, no. 10, October 2017.

Special Issue on Signal Processing Advances for Non-Orthogonal Multiple Access in Next Generation,” IEEE Journal of Selected Topics in Signal Processing, vol 13, no. 3, June 2019.

Special Issue on Emerging Non-Orthogonal Multiple Access (NOMA) Techniques for 5G and Beyond,” Physical Communication, 2019

Advances in Signal Processing for Non-Orthogonal Multiple Access,” IEEE Access, 2019.

Special Issue on Massive Access for 5G and Beyond,” IEEE Journal on Selected Areas in Communications, 2020.

Special Issue on Non-Orthogonal Multiple Access for 5G and Beyond,” IEEE Open Journal of the Communications Society,  2020.

Standards-Related Articles

Y. Chen et al., Toward the Standardization of Non-Orthogonal Multiple Access for Next Generation Wireless Networks,” IEEE Communications Magazine, vol. 56, no. 3, pp. 19-27, March 2018.
This paper reviews state-of-the-art designs of NOMA systems and the related standardization activities.

A. Benjebbour, A. Li, K. Saito, Y. Saito, Y. Kishiyama, and T. Nakamura, NOMA: From Concept to Standardization,” in Proc. IEEE Conference on Standards for Communications and Networking (CSCN), Tokyo, October 2015, pp. 18-23.
This paper summarizes the current standardization progress and use cases of downlink NOMA in 3GPP.

Topics

Specific Technical Areas

This section provides a list of papers spanning several areas:

  • The Basic Principles of NOMA part lists papers introducing the fundamentals and unique advantages of NOMA systems.
  • The Multiple Antenna Aided NOMA part lists papers on MIMO-NOMA scenarios, which focus on redesigning the decoding order to utilize the multi-antenna resource. 
  • The Interplay Between NOMA and Cooperative Communications part lists papers exploring the benefits of applying cooperative communications in NOMA systems, especially for the diversity gain.
  • The Resource Management in NOMA Networks part lists papers on the trade-off between bandwidth efficiency and energy efficiency.
  • The Coexistence of NOMA and Other Emerging Technologies part lists papers evaluating the performance of NOMA in combination with other promising communication techniques.
  • The Practical Forms of NOMA part lists papers on NOMA systems with different domains and carrier patterns.
  • The Implementation Challenges of NOMA part lists papers discussing the implementation challenges and possible solutions for NOMA networks.
Topic: Basic Principles of NOMA

Y. Saito, A. Benjebbour, Y. Kishiyama, and T. Nakamura, “System-Level Performance Evaluation of Downlink Non-Orthogonal Multiple Access (NOMA),” in Proc. IEEE Annu. Symp. Personal, Indoor and Mobile Radio Communications (PIMRC), London, U.K., September 2013.
This paper uses simulations to evaluate the system-level performance of downlink NOMA via adaptive modulation and coding, time or frequency-domain scheduling, hybrid automatic repeat request, and outer loop link adaptation. The results of this paper confirm that NOMA outperforms OMA.

Z. Ding, Z. Yang, P. Fan, and H. V. Poor, “On the Performance of Non-Orthogonal Multiple Access in 5G Systems with Randomly Deployed Users,” IEEE Signal Processing Letters, vol. 21, no. 12, pp. 1501-1505, December 2014.
This pioneering and award-winning paper popularizes NOMA techniques. It evaluates the ergodic sum rate and outage probability of NOMA systems based on stochastic geometry. This paper is the foundation for the following design of various NOMA schemes.

Z. Ding, P. Fan, and H. V. Poor, “Impact of User Pairing on 5G Nonorthogonal Multiple-Access Downlink Transmissions,” IEEE Transactions on Vehicular Technology, vol. 65, no. 8, pp. 6010-6023, August 2016.
With the aid of both analytical and numerical results, this paper investigates the effect of user pairing on two NOMA systems, namely NOMA with fixed power allocation and cognitive-radio-inspired NOMA.

Y. Saito, Y. Kishiyama, A. Benjebbour, T. Nakamura, A. Li, and K. Higuchi, “Non-Orthogonal Multiple Access (NOMA) for Cellular Future Radio Access,” in Proc. IEEE 77th Vehicular Technology Conference (VTC Spring), Dresden, June 2013.
By using SIC and interference rejection combining techniques, this paper investigates possible extensions of NOMA to multi-antenna or multi-site systems to enhance the network capacity.

S. Timotheou and I. Krikidis, “Fairness for Non-Orthogonal Multiple Access in 5G Systems,” IEEE Signal Processing Letters, vol. 22, no. 10, pp. 1647-1651, October 2015.
This article studies the power allocation problem for NOMA based on a fairness principle. Both systems with instantaneous CSI and with average CSI are discussed.

N. Zhang, J. Wang, G. Kang, and Y. Liu, “Uplink Nonorthogonal Multiple Access in 5G Systems,” IEEE Communications Letters, vol. 20, no. 3, pp. 458-461, March 2016.
This paper investigates power control strategies for uplink NOMA. Based on a typical power back-off scheme, it demonstrates that the received power of NOMA users obeys a geometric progression with a constant scale factor. The result shows that the outage performance depends on the target data rate and this scale factor.

Topic: Multiple Antenna Aided NOMA

S. Ali, E. Hossain, and D. I. Kim, “Non-Orthogonal Multiple Access (NOMA) for Downlink Multiuser MIMO Systems: User Clustering, Beamforming, and Power Allocation,” IEEE Access, vol. 5, pp. 565-577, 2017.
This paper investigates a typical downlink MIMO-NOMA system in which the total number of receiving antennas is larger than the number of transmitting antennas. After grouping the receiving antennas into several clusters, this work uses a linear beamforming method to cancel inter-cluster interference and enhance the network capacity.

Z. Ding, R. Schober, and H. V. Poor, “A General MIMO Framework for NOMA Downlink and Uplink Transmission Based on Signal Alignment,” IEEE Transactions on Wireless Communications, vol. 15, no. 6, pp. 4438-4454, June 2016.
By exploiting signal alignment, this article creates a novel MIMO-NOMA framework for both uplink and downlink communications and evaluates the outage performance.

M. F. Hanif, Z. Ding, T. Ratnarajah, and G. K. Karagiannidis, “A Minorization-Maximization Method for Optimizing Sum Rate in the Downlink of Non-Orthogonal Multiple Access Systems,” IEEE Transactions on Signal Processing, vol. 64, no. 1, pp. 76-88, January 2016.
This paper utilizes a minorization-maximization algorithm to optimize the downlink sum-rate in a NOMA system with linearly precoded antennas.

Y. Liu, M. Elkashlan, Z. Ding, and G. K. Karagiannidis, “Fairness of User Clustering in MIMO Non-Orthogonal Multiple Access Systems,” IEEE Communications Letters, vol. 20, no. 7, pp. 1465-1468, July 2016.
Based on fairness considerations, this article optimizes user clustering methods in a downlink MIMO-NOMA system for maximizing the throughput. It provides three sub-optimal algorithms to realize different tradeoffs between the final throughput and computational complexity.

M. Zeng, A. Yadav, O. A. Dobre, G. I. Tsiropoulos, and H. V. Poor, “Capacity Comparison Between MIMO-NOMA and MIMO-OMA with Multiple Users in a Cluster,” IEEE Journal on Selected Areas in Communications, vol. 35, no. 10, pp. 2413-2424, October 2017.
This article analytically compares the sum channel capacity and ergodic rate in MIMO-NOMA and MIMO-OMA systems. It also demonstrates that there is a tradeoff between the number of served users and the system capacity in MIMO-NOMA.

M. Zeng, A. Yadav, O. A. Dobre, G. I. Tsiropoulos, and H. V. Poor, “On the Sum Rate of MIMO-NOMA and MIMO-OMA Systems,” IEEE Wireless Communications Letters, vol. 6, no. 4, pp. 534-537, August 2017.
This letter proves that MIMO-NOMA dominates MIMO-OMA in terms of sum rate and ergodic sum rate for both two-user scenario and multiple-user scenario with two users paired into a cluster and sharing a common transmit beamforming vector.

W. Shin, M. Vaezi, B. Lee, D. J. Love, J. Lee, and H. V. Poor, “Coordinated Beamforming for Multi-Cell MIMO-NOMA,” IEEE Communications Letters, vol. 21, no. 1, pp. 84-87, January 2017.
This letter proposed two new coordinated beamforming strategies to enhance the performance of multi-cell MIMO-NOMA systems.

Q. Sun, S. Han, C. I, and Z. Pan, “On the Ergodic Capacity of MIMO NOMA Systems,” IEEE Wireless Communications Letters, vol. 4, no. 4, pp. 405-408, August 2015.
This is the first paper to study the ergodic rate maximization problem in MIMO-NOMA with Rayleigh fading and statistical channel state information at the transmitter (CSIT).

J. Men and J. Ge, “Non-Orthogonal Multiple Access for Multiple-Antenna Relaying Networks,” IEEE Communications Letters, vol. 19, no. 10, pp. 1686-1689, October 2015.
This letter first designs NOMA for multi-antenna enabled relaying networks. Then, it theoretically evaluates the outage performance of the proposed system with the aid of several closed-form expressions.

Topic: Interplay Between NOMA and Cooperative Communications

Z. Ding, M. Peng, and H. V. Poor, “Cooperative Non-Orthogonal Multiple Access in 5G Systems,” IEEE Communications Letters, vol. 19, no. 8, pp. 1462-1465, August 2015.
In this paper, a cooperative NOMA scheme is provided to fully utilize the prior information of the paired user. Additionally, the diversity order and outage probabilities are derived with the aid of user pairing methods.

Z. Ding, H. Dai, and H. V. Poor, “Relay Selection for Cooperative NOMA,” IEEE Wireless Communications Letters, vol. 5, no. 4, pp. 416-419, August 2016.
This paper investigates the effect of different relay selection strategies on a cooperative NOMA system.

Y. Liu, Z. Ding, M. Elkashlan, and H. V. Poor, “Cooperative Non-Orthogonal Multiple Access with Simultaneous Wireless Information and Power Transfer,” IEEE Journal on Selected Areas in Communications, vol. 34, no. 4, pp. 938-953 April 2016.
This paper proposes a novel cooperative NOMA protocol by introducing simultaneous wireless information and power transfer (SWIPT). Compared with the traditional NOMA, this new protocol has the same diversity gain but higher system throughput.

C. Zhong and Z. Zhang, “Non-Orthogonal Multiple Access with Cooperative Full-Duplex Relaying,” IEEE Communications Letters, vol. 20, no. 12, pp. 2478-2481, December 2016.
This paper studies a full-duplex cooperative NOMA system. It shows that the considered NOMA scheme performs better than that with half-duplex in the moderate signal-to-noise ratio (SNR) regions.

J. Kim and I. Lee, “Capacity Analysis of Cooperative Relaying Systems Using Non-Orthogonal Multiple Access,” IEEE Communications Letters, vol. 19, no. 11, pp. 1949-1952, November 2015.
This letter theoretically analyzes the capacity performance of cooperative NOMA networks via asymptotic expressions and a suboptimal power allocation method.

J. Kim and I. Lee, “Non-Orthogonal Multiple Access in Coordinated Direct and Relay Transmission,” IEEE Communications Letters, vol. 19, no. 11, pp. 2037-2040, November 2015.
This work investigates the outage performance of a NOMA system with coordinated direct and relay transmission. Exact and closed-form expressions are provided to evaluate the system capacity and diversity gain.

J. Choi, “Non-Orthogonal Multiple Access in Downlink Coordinated Two-Point Systems,” IEEE Communications Letters, vol. 18, no. 2, pp. 313-316, February 2014.
This paper provides a new NOMA system with a coordinated superposition coding to enhance the performance of cell-edge users. Two coordinated base stations (BSs) utilize the Alamouti code to support cell-edge users.

Topic: Resource Management in NOMA Networks

Y. Sun, D. W. K. Ng, Z. Ding, and R. Schober, “Optimal Joint Power and Subcarrier Allocation for Full-Duplex Multicarrier Non-Orthogonal Multiple Access Systems,” IEEE Transactions on Communications, vol. 65, no. 3, pp. 1077-1091, March 2017.
This paper applies monotonic optimization to jointly optimize the power and subcarrier allocation in full-duplex NOMA systems. It also provides a suboptimal solution with the aid of successive convex approximation to reduce the computational complexity.

M. S. Ali, H. Tabassum, and E. Hossain, “Dynamic User Clustering and Power Allocation for Uplink and Downlink Non-Orthogonal Multiple Access (NOMA) Systems,” IEEE Access, vol. 4, pp. 6325-6343, 2016.
This article optimizes the user clustering and power allocation in both uplink and downlink NOMA networks for maximization of the sum throughput. By applying the Karush-Kuhn-Tucker optimality conditions, it derives closed-form solutions for optimal power allocations for any cluster size.

Z. Yang, Z. Ding, P. Fan, and N. Al-Dhahir, “A General Power Allocation Scheme to Guarantee Quality of Service in Downlink and Uplink NOMA Systems,” IEEE Transactions on Wireless Communications, vol. 15, no. 11, pp. 7244-7257, November 2016.
This paper provides a new dynamic power allocation strategy for uplink and downlink NOMA systems to guarantee various quality of service constraints. Compared with traditional NOMA, the proposed system has more flexibility and it is able to ensure the desired performance gain.

M. Zeng, A. Yadav, O. A. Dobre, and H. V. Poor, “Energy-Efficient Power Allocation for MIMO-NOMA with Multiple Users in a Cluster,” IEEE Access, vol. 6, pp. 5170-5181, February 2018.
In this paper, energy-efficient power allocation is investigated for a multi-cluster multi-user MIMO-NOMA system. A closed-form solution is first provided for the corresponding sum rate maximization problem, and on this basis, the EE maximization problem is solved by applying non-convex fractional programming.

F. Fang, H. Zhang, J. Cheng, and V. C. M. Leung, “Energy-Efficient Resource Allocation for Downlink Non-Orthogonal Multiple Access Network,” IEEE Transactions on Communications, vol. 64, no. 9, pp. 3722-3732, September 2016.
In contrast to other papers focusing on the throughput performance, this paper studies the problem of energy-efficient resource allocation. In order to maximize energy efficiency, it proposes a suboptimal algorithm with low complexity to find the optimal power and subchannel allocation.

L. Lei, D. Yuan, C. K. Ho, and S. Sun, “Power and Channel Allocation for Non-Orthogonal Multiple Access in 5G Systems: Tractability and Computation,” IEEE Transactions on Wireless Communications, vol. 15, no. 12, pp. 8580-8594, December 2016.
In this paper, algorithmic solutions for jointly optimizing the power and channel allocation in NOMA systems are provided. Polynomial-time solutions for global optimization are proposed for tractable special cases. For an intractable general case, it offers an algorithm with the aid of dynamic programming and Lagrangian duality to achieve suboptimal results.

B. Di, L. Song, and Y. Li, “Sub-Channel Assignment, Power Allocation, and User Scheduling for Non-Orthogonal Multiple Access Networks,” IEEE Transactions on Wireless Communications, vol. 15, no. 11, pp. 7686-7698, November 2016.
This paper optimizes the power and subchannel allocation to maximize the weighted sum-rate of NOMA networks via matching theory. In this work, the fairness of users is considered.

Y. Zhang, H. Wang, T. Zheng, and Q. Yang, “Energy-Efficient Transmission Design in Non-orthogonal Multiple Access,” IEEE Transactions on Vehicular Technology, vol. 66, no. 3, pp. 2852-2857, March 2017.
This paper provides an optimal power allocation policy to maximize the energy efficiency of a downlink NOMA system with multiple users.

M. Zeng, A. Yadav, O. A. Dobre, and V. Poor, “Energy-efficient Joint User-RB Association and Power Allocation for Uplink Hybrid NOMA-OMA,” IEEE Internet of Things, vol. 6, issue 3, pp. 5119-5131, February 2019.
In the context of a hybrid NOMA-OMA system, the paper provides a solution to user clustering, channel assignment and power allocation to maximize the energy efficiency while satisfying a minimum rate requirement for each user. The swap matching and bipartite graph are employed to develop the solution. The proposed scheme outperforms other hybrid multiple-access- and OMA-based schemes.

Topic: Coexistence of NOMA and Other Emerging Technologies

Y. Liu, Z. Ding, M. Elkashlan, and J. Yuan, “Nonorthogonal Multiple Access in Large-Scale Underlay Cognitive Radio Networks,” IEEE Transactions on Vehicular Technology, vol. 65, no. 12, pp. 10152-10157, December 2016. 
This paper evaluates the performance of NOMA-enabled CR networks with the aid of stochastic geometry. Two scenarios with different power constraints are discussed and the results show that NOMA improves the performance of CR networks.

B. Wang, L. Dai, Z. Wang, N. Ge, and S. Zhou, “Spectrum and Energy-Efficient Beamspace MIMO-NOMA for Millimeter-Wave Communications Using Lens Antenna Array,” IEEE Journal on Selected Areas in Communications, vol. 35, no. 10, pp. 2370-2382, October 2017.
This paper introduces NOMA to beamspace MIMO networks operating in the millimeter-wave bands to serve more users than the number of radio frequency chains. The proposed system outperforms existing beamspace MIMO systems in terms of spectrum/energy efficiency.

M. Zeng, W. Hao, O. A. Dobre, and V. Poor, “Energy-efficient Power Allocation in Uplink mmWave Massive MIMO with NOMA,” IEEE Transactions on Vehicular Technology, vol. 64, issue 3, pp. 3000-3004, March 2019.
This paper introduces an enhanced NOMA scheme for uplink transmission, in the context of mmWave mMIMO systems. The proposed scheme removes the inter-cluster interference through a smart ordering of the clusters, leading to improved decoding performance. Additionally, power allocation is performed to increase the EE performance. The enhanced NOMA scheme outperforms the conventional NOMA and OMA schemes in terms of EE.

Y. Liu, Z. Qin, M. Elkashlan, A. Nallanathan, and J. A. McCann, “Non-Orthogonal Multiple Access in Large-Scale Heterogeneous Networks,” IEEE Journal on Selected Areas in Communications, vol. 35, no. 12, pp. 2667-2680, December 2017.
This article investigates the advantages of applying NOMA to multi-tier HetNets. By analyzing the coverage performance and energy efficiency, it demonstrates that NOMA-HetNets outperform traditional OMA-HetNets.

M. Zeng, N. Nguyen, O. A. Dobre, and H. V. Poor, “Securing Downlink Massive MIMO-NOMA Networks with Artificial Noise,” IEEE Journal of Selected Topics in Signal Processing, vol. 13, no. 3, pp. 685-699, June 2019.
This paper focuses on securing the confidential information of massive MIMO-NOMA networks by exploiting artificial noise. The ergodic secrecy rate is first derived for downlink transmission, and on this basis, joint power allocation of the uplink training phase and downlink transmission phase is proposed to maximize the sum secrecy rates and energy efficiency of the system.

H. Marshoud, V. M. Kapinas, G. K. Karagiannidis, and S. Muhaidat, “Non-Orthogonal Multiple Access for Visible Light Communications,” IEEE Photonics Technology Letters, vol. 28, no. 1, pp. 51-54, January 1, 2016.
This paper applies NOMA to visible light communications (VLC) to enhance the achievable data rate. A new gain ration power allocation policy is provided to guarantee efficient and fair power allocation. Moreover, new degrees of freedom in terms of the transmission angles and the field of views can be achieved to enhance NOMA systems.

Z. Zhang, Z. Ma, M. Xiao, Z. Ding, and P. Fan, “Full-Duplex Device-to-Device-Aided Cooperative Nonorthogonal Multiple Access,” IEEE Transactions on Vehicular Technology, vol. 66, no. 5, pp. 4467-4471, May 2017.
This article studies a full-duplex cooperative NOMA system with device-to-device communications. It proposes adaptive multiple access schemes to further improve outage performance.

M. Zeng, W. Hao, O. A. Dobre, and H. V. Poor, “Energy-Efficient Power Allocation in Uplink mmWave Massive MIMO with NOMA,” IEEE Transactions on Vehicular Technology, vol. 68, no. 3, pp. 3000-3004, March 2019.
This paper first designs a hybrid analog-digital beamforming scheme to lower the number of radio frequency chains at the base station. Then, an iterative power allocation algorithm is proposed to maximize the energy efficiency.

J. Choi, “Minimum Power Multicast Beamforming with Superposition Coding for Multiresolution Broadcast and Application to NOMA Systems,” IEEE Transactions on Communications, vol. 63, no. 3, pp. 791-800, March 2015.
This paper explores the potential of applying NOMA in multicast beamforming with superposition coding systems. A closed-form expression for the optimal power allocation is provided.

Topic: Practical Forms of NOMA

S. Chen, B. Ren, Q. Gao, S. Kang, S. Sun, and K. Niu, “Pattern Division Multiple Access—A Novel Nonorthogonal Multiple Access for Fifth-Generation Radio Networks,” IEEE Transactions on Vehicular Technology, vol. 66, no. 4, pp. 3185-3196, April 2017.
This paper studies PDMA techniques which maps transmitted data to a resource group. The resource group can be any combinations of space, frequency, and time. Compared with orthogonal frequency division multiple access (OFDMA), the proposed techniques provide six times improvement in terms of connectivity and 30% improvement in spectrum efficiency.

Li Ping, Lihai Liu, Keying Wu, and W. K. Leung, “Interleave Division Multiple-Access,” IEEE Transactions on Wireless Communications, vol. 5, no. 4, pp. 938-947, April 2006.
This article comprehensively studies IDMA techniques. The bit-error-rate (BER) is analyzed based on the density evolution method.

H. Nikopour and H. Baligh, “Sparse Code Multiple Access,” in Proc. IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), London, 2013, pp. 332-336.
This paper proposes an SCMA technique. By introducing a multi-dimensional constellation, this technique has higher performance than traditional code-division multiple access (CDMA) techniques. It also provides a tractable suboptimal design for the SCMA codebook to minimize the block error rate.

M. Taherzadeh, H. Nikopour, A. Bayesteh, and H. Baligh, “SCMA Codebook Design,” in Proc. IEEE 80th Vehicular Technology Conference (VTC), Vancouver, BC, September 2014.
This paper focuses on designing SCMA codebooks based on lattice constellation principles to enhance the achievable performance.

R. Hoshyar, F. P. Wathan, and R. Tafazolli, “Novel Low-Density Signature for Synchronous CDMA Systems Over AWGN Channel,” IEEE Transactions on Signal Processing, vol. 56, no. 4, pp. 1616-1626, April 2008.
In this paper, a new Low-Density Signature (LDS) method is created for symbol-synchronous communications. It shows that the performance of the proposed system is close to a single-user scenario.

Topic: Implementation Challenges of NOMA

Y. Liu, Z. Qin, M. Elkashlan, Y. Gao, and L. Hanzo, “Enhancing the Physical Layer Security of Non-Orthogonal Multiple Access in Large-Scale Networks,” IEEE Transactions on Wireless Communications, vol. 16, no. 3, pp. 1656-1672, March 2017.
This article studies the physical layer security of NOMA systems via stochastic geometry. Both single-antenna and multi-antenna scenarios are considered. By applying a protected zone and artificial noise at the BSs, the security performance is enhanced.

Z. Yang, Z. Ding, P. Fan, and G. K. Karagiannidis, “On the Performance of Non-orthogonal Multiple Access Systems with Partial Channel Information,” IEEE Transactions on Communications, vol. 64, no. 2, pp. 654-667, February 2016.
This paper investigates the performance of downlink NOMA under two types of partial CSI. The first type (named Imperfect CSI) has channel estimation errors. The second type (named Second-Order Statistics (SOS)) only has information about distances between BSs and users. It shows that the SOS-based NOMA has similar performance as the perfect CSI NOMA at low SNRs.

Y. Zhang, H. Wang, Q. Yang, and Z. Ding, “Secrecy Sum Rate Maximization in Non-orthogonal Multiple Access,” IEEE Communications Letters, vol. 20, no. 5, pp. 930-933, May 2016.
This letter maximizes the secrecy sum rate of NOMA systems by considering the users’ quality of service. It provides an optimal power allocation strategy to solve this problem.

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