192 research outputs found
Signal Processing for MIMO-NOMA: Present and Future Challenges
Non-orthogonal multiple access (NOMA), as the newest member of the multiple
access family, is envisioned to be an essential component of 5G mobile
networks. The combination of NOMA and multi-antenna multi-input multi-output
(MIMO) technologies exhibits a significant potential in improving spectral
efficiency and providing better wireless services to more users. In this
article, we introduce the basic concepts of MIMO-NOMA and summarize the key
technical problems in MIMO-NOMA systems. Then, we explore the problem
formulation, beamforming, user clustering, and power allocation of
single/multi-cluster MIMO-NOMA in the literature along with their limitations.
Furthermore, we point out an important issue of the stability of successive
interference cancellation (SIC) that arises using achievable rates as
performance metrics in practical NOMA/MIMO-NOMA systems. Finally, we discuss
incorporating NOMA with massive/millimeter wave MIMO, and identify the main
challenges and possible future research directions in this area.Comment: 14 pages (single column), 4 figures. This work has been accepted by
the IEEE Wireless Communications, the special issue of non-orthogonal
multiple access for 5
A Tutorial on Nonorthogonal Multiple Access for 5G and Beyond
Today's wireless networks allocate radio resources to users based on the
orthogonal multiple access (OMA) principle. However, as the number of users
increases, OMA based approaches may not meet the stringent emerging
requirements including very high spectral efficiency, very low latency, and
massive device connectivity. Nonorthogonal multiple access (NOMA) principle
emerges as a solution to improve the spectral efficiency while allowing some
degree of multiple access interference at receivers. In this tutorial style
paper, we target providing a unified model for NOMA, including uplink and
downlink transmissions, along with the extensions tomultiple inputmultiple
output and cooperative communication scenarios. Through numerical examples, we
compare the performances of OMA and NOMA networks. Implementation aspects and
open issues are also detailed.Comment: 25 pages, 10 figure
Exploiting Multiple-Antenna Techniques for Non-Orthogonal Multiple Access
This paper aims to provide a comprehensive solution for the design, analysis,
and optimization of a multiple-antenna non-orthogonal multiple access (NOMA)
system for multiuser downlink communication with both time duplex division
(TDD) and frequency duplex division (FDD) modes. First, we design a new
framework for multiple-antenna NOMA, including user clustering, channel state
information (CSI) acquisition, superposition coding, transmit beamforming, and
successive interference cancellation (SIC). Then, we analyze the performance of
the considered system, and derive exact closed-form expressions for average
transmission rates in terms of transmit power, CSI accuracy, transmission mode,
and channel conditions. For further enhancing the system performance, we
optimize three key parameters, i.e., transmit power, feedback bits, and
transmission mode. Especially, we propose a low-complexity joint optimization
scheme, so as to fully exploit the potential of multiple-antenna techniques in
NOMA. Moreover, through asymptotic analysis, we reveal the impact of system
parameters on average transmission rates, and hence present some guidelines on
the design of multiple-antenna NOMA. Finally, simulation results validate our
theoretical analysis, and show that a substantial performance gain can be
obtained over traditional orthogonal multiple access (OMA) technology under
practical conditions
Full-Duplex Non-Orthogonal Multiple Access for Modern Wireless Networks
Non-orthogonal multiple access (NOMA) is an interesting concept to provide
higher capacity for future wireless communications. In this article, we
consider the feasibility and benefits of combining full-duplex operation with
NOMA for modern communication systems. Specifically, we provide a comprehensive
overview on application of full-duplex NOMA in cellular networks, cooperative
and cognitive radio networks, and characterize gains possible due to
full-duplex operation. Accordingly, we discuss challenges, particularly the
self-interference and inter-user interference and provide potential solutions
to interference mitigation and quality-of-service provision based on
beamforming, power control, and link scheduling. We further discuss future
research challenges and interesting directions to pursue to bring full-duplex
NOMA into maturity and use in practice.Comment: Revised, IEEE Wireless Communication Magazin
Multiple Antenna Aided NOMA in UAV Networks: A Stochastic Geometry Approach
This article investigates the multiple-input multiple-output (MIMO)
non-orthogonal multiple access (NOMA) assisted unmanned aerial vehicles (UAVs)
networks. By utilizing a stochastic geometry model, a new 3-Dimension UAV
framework for providing wireless service to randomly roaming NOMA users has
been proposed. In an effort to evaluate the performance of the proposed
framework, we derive analytical expressions for the outage probability and the
ergodic rate of MIMO-NOMA enhanced UAV networks. We examine tractable upper
bounds for the whole proposed framework, with deriving asymptotic results for
scenarios that transmit power of interference sources being proportional or
being fixed to the UAV. For obtaining more insights for the proposed framework,
we investigate the diversity order and high signal-to-noise (SNR) slope of
MIMO-NOMA assisted UAV networks. Our results confirm that: i) The outage
probability of NOMA enhanced UAV networks is affected to a large extent by the
targeted transmission rates and power allocation factors of NOMA users; and ii)
For the case that the interference power is proportional to the UAV power,
there are error floors for the outage probabilities
Resource Allocation for Downlink NOMA Systems: Key Techniques and Open Issues
This article presents advances in resource allocation (RA) for downlink
non-orthogonal multiple access (NOMA) systems, focusing on user pairing (UP)
and power allocation (PA) algorithms. The former pairs the users to obtain the
high capacity gain by exploiting the channel gain difference between the users,
while the later allocates power to users in each cluster to balance system
throughput and user fairness. Additionally, the article introduces the concept
of cluster fairness and proposes the divideand- next largest difference-based
UP algorithm to distribute the capacity gain among the NOMA clusters in a
controlled manner. Furthermore, performance comparison between multiple-input
multiple-output NOMA (MIMO-NOMA) and MIMO-OMA is conducted when users have
pre-defined quality of service. Simulation results are presented, which
validate the advantages of NOMA over OMA. Finally, the article provides avenues
for further research on RA for downlink NOMA.Comment: 5G, NOMA, Resource allocation, User pairing, Power allocatio
On Optimal Beamforming Design for Downlink MISO NOMA Systems
This work focuses on the beamforming design for downlink multiple-input single-output (MISO) nonorthogonal multiple access (NOMA) systems. The beamforming vectors are designed by solving a total transmission power minimization (TPM) problem with quality-of-service (QoS) constraints. In order to solve the proposed nonconvex optimization problem, we provide an efficient method using semidefinite relaxation. Moreover, for the first time, we characterize the optimal beam- forming in a closed form with quasi-degradation condition, which is proven to achieve the same performance as dirty- paper coding (DPC). For the special case with two users, we further show that the original nonconvex TPM problem can be equivalently transferred into a convex optimization problem and easily solved via standard optimization tools. In addition, the optimal beamforming is also characterized in a closed form and we show that it achieves the same performance as the DPC. In the simulation, we show that our proposed optimal NOMA beamforming outperforms OMA schemes and can even achieve the same performance as DPC. Our solutions dramatically simplifies the problem of beamforming design in the downlink MISO NOMA systems and improve the system performance
Power Efficient IRS-Assisted NOMA
In this paper, we propose a downlink multiple-input single-output (MISO)
transmission scheme, which is assisted by an intelligent reflecting surface
(IRS) consisting of a large number of passive reflecting elements. In the
literature, it has been proved that nonorthogonal multiple access (NOMA) can
achieve the capacity region when the channels are quasi-degraded. However, in a
conventional communication scenario, it is difficult to guarantee the
quasi-degradation, because the channels are determined by the propagation
environments and cannot be reconfigured. To overcome this difficulty, we focus
on an IRS-assisted MISO NOMA system, where the wireless channels can be
effectively tuned. We optimize the beamforming vectors and the IRS phase shift
matrix for minimizing transmission power. Furthermore, we propose an improved
quasi-degradation condition by using IRS, which can ensure that NOMA achieves
the capacity region with high possibility. For a comparison, we study
zero-forcing beamforming (ZFBF) as well, where the beamforming vectors and the
IRS phase shift matrix are also jointly optimized. Comparing NOMA with ZFBF, it
is shown that, with the same IRS phase shift matrix and the improved
quasi-degradation condition, NOMA always outperforms ZFBF. At the same time, we
identify the condition under which ZFBF outperforms NOMA, which motivates the
proposed hybrid NOMA transmission. Simulation results show that the proposed
IRS-assisted MISO system outperforms the MISO case without IRS, and the hybrid
NOMA transmission scheme always achieves better performance than orthogonal
multiple access
Performance Analysis of NOMA in Training Based Multiuser MIMO Systems
This paper considers the use of NOMA in multiuser MIMO systems in practical
scenarios where CSI is acquired through pilot signaling. A new NOMA scheme that
uses shared pilots is proposed. Achievable rate analysis is carried out for
different pilot signaling schemes including both uplink and downlink pilots.
The achievable rate performance of the proposed NOMA scheme with shared pilot
within each group is compared with the traditional orthogonal access scheme
with orthogonal pilots. Our proposed scheme is a generalization of the
orthogonal scheme, and can be reduced to the orthogonal scheme when appropriate
power allocation parameters are chosen. Numerical results show that when
downlink CSI is available at the users, our proposed NOMA scheme outperforms
orthogonal schemes. However with more groups of users present in the cell, it
is preferable to use multi-user beamforming in stead of NOMA.Comment: 13 pages, accepted in IEEE Transaction on Wireless Communication
A Survey of Rate-optimal Power Domain NOMA with Enabling Technologies of Future Wireless Networks
The ambitious high data-rate applications in the envisioned future B5G
networks require new solutions, including the advent of more advanced
architectures than the ones already used in 5G networks, and the coalition of
different communications schemes and technologies to enable these applications
requirements. Among the candidate schemes for future wireless networks are NOMA
schemes that allow serving more than one user in the same resource block by
multiplexing users in other domains than frequency or time. In this way, NOMA
schemes tend to offer several advantages over OMA schemes such as improved user
fairness and spectral efficiency, higher cell-edge throughput, massive
connectivity support, and low transmission latency. With these merits,
NOMA-enabled transmission schemes are being increasingly looked at as promising
multiple access schemes for future wireless networks. When the power domain is
used to multiplex the users, it is referred to as PD-NOMA. In this paper, we
survey the integration of PD-NOMA with the enabling communications schemes and
technologies that are expected to meet the various requirements of B5G
networks. In particular, this paper surveys the different rate optimization
scenarios studied in the literature when PD-NOMA is combined with one or more
of the candidate schemes and technologies for B5G networks including MISO,
MIMO, mMIMO, advanced antenna architectures, mmWave and THz, CoMP, cooperative
communications, cognitive radio, VLC, UAV and others. The considered system
models, the optimization methods utilized to maximize the achievable rates, and
the main lessons learnt on the optimization and the performance of these
NOMA-enabled schemes and technologies are discussed in detail along with the
future research directions for these combined schemes. Moreover, the role of
machine learning in optimizing these NOMA-enabled technologies is addressed.Comment: Accepted for publication in IEEE Surveys and Tutorials, July 202
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