868 research outputs found
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
Non-Orthogonal Multiple Access: Common Myths and Critical Questions
Non-orthogonal multiple access (NOMA) has received tremendous attention for
the design of radio access techniques for fifth generation (5G) wireless
networks and beyond. The basic concept behind NOMA is to serve more than one
user in the same resource block, e.g., a time slot, subcarrier, spreading code,
or space. With this, NOMA promotes massive connectivity, lowers latency,
improves user fairness and spectral efficiency, and increases reliability
compared to orthogonal multiple access (OMA) techniques. While NOMA has gained
significant attention from the communications community, it has also been
subject to several widespread misunderstandings, such as The above statements are actually false, and this paper aims at
identifying such common myths about NOMA and clarifying why they are not true.
We also pose critical questions that are important for the effective adoption
of NOMA in 5G and beyond and identify promising research directions for NOMA,
which will require intense investigation in the future.Comment: To appear in the IEEE Wireless Communication
Multi-Beam NOMA for Hybrid mmWave Systems
In this paper, we propose a multi-beam non-orthogonal multiple access (NOMA)
scheme for hybrid millimeter wave (mmWave) systems and study its resource
allocation. A beam splitting technique is designed to generate multiple analog
beams to serve multiple users for NOMA transmission. Compared to conventional
mmWave orthogonal multiple access (mmWave-OMA) schemes, the proposed scheme can
serve more than one user on each radio frequency (RF) chain. Besides, in
contrast to the recently proposed single-beam mmWave-NOMA scheme which can only
serve multiple NOMA users within the same beam, the proposed scheme can perform
NOMA transmission for the users with an arbitrary angle-of-departure (AOD)
distribution. This provides a higher flexibility for applying NOMA in mmWave
communications and thus can efficiently exploit the potential multi-user
diversity. Then, we design a suboptimal two-stage resource allocation for
maximizing the system sum-rate. In the first stage, assuming that only analog
beamforming is available, a user grouping and antenna allocation algorithm is
proposed to maximize the conditional system sum-rate based on the coalition
formation game theory. In the second stage, with the zero-forcing (ZF) digital
precoder, a suboptimal solution is devised to solve a non-convex power
allocation optimization problem for the maximization of the system sum-rate
which takes into account the quality of service (QoS) constraint. Simulation
results show that our designed resource allocation can achieve a
close-to-optimal performance in each stage. In addition, we demonstrate that
the proposed multi-beam mmWave-NOMA scheme offers a higher spectral efficiency
than that of the single-beam mmWave-NOMA and the mmWave-OMA schemes.Comment: Submitted for possible journal publicatio
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
Resource Optimization with Load Coupling in Multi-cell NOMA
Optimizing non-orthogonal multiple access (NOMA) in multi-cell scenarios is
much more challenging than the single-cell case because inter-cell interference
must be considered. Most papers addressing NOMA consider a single cell. We take
a significant step of analyzing NOMA in multi-cell scenarios. We explore the
potential of NOMA networks in achieving optimal resource utilization with
arbitrary topologies. Towards this goal, we investigate a broad class of
problems consisting in optimizing power allocation and user pairing for any
cost function that is monotonically increasing in time-frequency resource
consumption. We propose an algorithm that achieves global optimality for this
problem class. The basic idea is to prove that solving the joint optimization
problem of power allocation, user pair selection, and time-frequency resource
allocation amounts to solving a so-called iterated function without a closed
form. We prove that the algorithm approaches optimality with fast convergence.
Numerically, we evaluate and demonstrate the performance of NOMA for multi-cell
scenarios in terms of resource efficiency and load balancing
On User Pairing in NOMA Uplink
User pairing in Non-Orthogonal Multiple-Access (NOMA) uplink based on channel
state information is investigated considering some predefined power allocation
schemes. The base station divides the set of users into disjunct pairs and
assigns the available resources to these pairs. The combinatorial problem of
user pairing to achieve the maximum sum rate is analyzed in the large system
limit for various scenarios, and some optimum and sub-optimum algorithms with a
polynomial-time complexity are proposed. In the first scenario, users and
the base station have a single-antenna and communicate over subcarriers.
The performance of optimum pairing is derived for and
shown to be superior to random pairing and orthogonal multiple access
techniques. In the second setting, a novel NOMA scheme for a multi-antenna base
station and single carrier communication is proposed. In this case, the users
need not be aware of the pairing strategy. Furthermore, the proposed NOMA
scheme is generalized to multi-antenna users. It is shown that random and
optimum user pairing perform similarly in the large system limit, but optimum
pairing is significantly better in finite dimensions. It is shown that the
proposed NOMA scheme outperforms a previously proposed NOMA scheme with signal
alignment.Comment: Submitted to Transaction on Wireless Communication
Enhanced Energy-Efficient Downlink Resource Allocation in Green Non-Orthogonal Multiple Access Systems
Despite numerous advantages, non-orthogonal multiple access (NOMA) technique
can bring additional interference for the neighboring ultra-dense networks if
the power consumption of the system is not properly optimized. While targeting
on the green communication concept, in this paper, we propose an
energy-efficient downlink resource allocation scheme for a NOMA-equipped
cellular network. The objective of this work is to allocate subchannels and
power of the base station among the users so that the overall energy efficiency
is maximized. Since this problem is NP-hard, we attempt to find an elegant
solution with reasonable complexity that provides good performance for some
realistic applications. To this end, we decompose the problem into a subchannel
allocation subproblem followed by a power loading subproblem that allocates
power to each user's data stream on each of its allocated subchannels. We first
employ a many-to-many matching model under the assumption of uniform power
loading in order to obtain the solution of the first subproblem with reasonable
performance. Once the the subchannel-user mapping information is known from the
first solution, we propose a geometric programming (GP)-based power loading
scheme upon approximating the energy efficiency of the system by a ratio of two
posynomials. The techniques adopted for these subproblems better exploit the
available multi-user diversity compared to the techniques used in an earlier
work. Having observed the computational overhead of the GP-based power loading
scheme, we also propose a suboptimal computationally-efficient algorithm for
the power loading subproblem with a polynomial time complexity that provides
reasonably good performance. Extensive simulation has been conducted to verify
that our proposed solution schemes always outperform the existing work while
consuming much less power at the base station.Comment: 29 pages (Accepted
Robust Radio Resource Allocation in MISO-SCMA Assisted C-RAN in 5G Networks
In this paper, by considering multiple slices, a downlink transmission of a
sparse code multiple access (SCMA) based cloud-radio access network (C-RAN) is
investigated. In this regard, by supposing multiple input and single output
(MISO) transmission technology, a novel robust radio resource allocation is
proposed where considering uncertain channel state information (CSI), the worst
case approach is applied. The main goal of the proposed radio resource
allocation is to, maximize the system sum rate with maximum available power at
radio remote head (RRH), minimum rate requirement of each slice, maximum
frounthaul capacity of each RRH, user association, and SCMA constraints. To
solve the proposed optimization problem in an efficient manner, an iterative
method is deployed where in each iteration, beamforming and joint codebook
allocation and user association subproblem are solved separately. By
introducing some auxiliary variables, the joint codebook allocation and user
association subproblem is transformed into an integer linear programming, and
to solve the beamforming optimization problem, minorization-maximization
algorithm (MMA) is applied. Via numerical results, the performance of the
proposed system model versus different system parameters and for different
channel models are investigated.Comment: 11 pages, 8 figure
On the Performance Gain of NOMA over OMA in Uplink Communication Systems
In this paper, we investigate and reveal the ergodic sum-rate gain (ESG) of
non-orthogonal multiple access (NOMA) over orthogonal multiple access (OMA) in
uplink cellular communication systems. A base station equipped with a
single-antenna, with multiple antennas, and with massive antenna arrays is
considered both in single-cell and multi-cell deployments. In particular, in
single-antenna systems, we identify two types of gains brought about by NOMA:
1) a large-scale near-far gain arising from the distance discrepancy between
the base station and users; 2) a small-scale fading gain originating from the
multipath channel fading. Furthermore, we reveal that the large-scale near-far
gain increases with the normalized cell size, while the small-scale fading gain
is a constant, given by = 0.57721 nat/s/Hz, in Rayleigh fading
channels. When extending single-antenna NOMA to -antenna NOMA, we prove that
both the large-scale near-far gain and small-scale fading gain achieved by
single-antenna NOMA can be increased by a factor of for a large number of
users. Moreover, given a massive antenna array at the base station and
considering a fixed ratio between the number of antennas, , and the number
of users, , the ESG of NOMA over OMA increases linearly with both and
. We then further extend the analysis to a multi-cell scenario. Compared to
the single-cell case, the ESG in multi-cell systems degrades as NOMA faces more
severe inter-cell interference due to the non-orthogonal transmissions.
Besides, we unveil that a large cell size is always beneficial to the ergodic
sum-rate performance of NOMA in both single-cell and multi-cell systems.
Numerical results verify the accuracy of the analytical results derived and
confirm the insights revealed about the ESG of NOMA over OMA in different
scenarios.Comment: 51 pages, 7 figures, invited paper, submitted to IEEE Transactions on
Communication
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
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