714 research outputs found
Optimal User Scheduling and Power Allocation for Millimeter Wave NOMA Systems
This paper investigates the application of non-orthogonal multiple access
(NOMA) in millimeter wave (mmWave) communications by exploiting beamforming,
user scheduling and power allocation. Random beamforming is invoked for
reducing the feedback overhead of considered systems. A nonconvex optimization
problem for maximizing the sum rate is formulated, which is proved to be
NP-hard. The branch and bound (BB) approach is invoked to obtain the optimal
power allocation policy, which is proved to converge to a global optimal
solution. To elaborate further, low complexity suboptimal approach is developed
for striking a good computational complexity-optimality tradeoff, where
matching theory and successive convex approximation (SCA) techniques are
invoked for tackling the user scheduling and power allocation problems,
respectively. Simulation results reveal that: i) the proposed low complexity
solution achieves a near-optimal performance; and ii) the proposed mmWave NOMA
systems is capable of outperforming conventional mmWave orthogonal multiple
access (OMA) systems in terms of sum rate and the number of served users.Comment: Submitted for possible publicatio
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
Joint Tx-Rx Beamforming and Power Allocation for 5G Millimeter-Wave Non-Orthogonal Multiple Access (MmWave-NOMA) Networks
In this paper, we investigate the combination of non-orthogonal multiple
access and millimeter-Wave communications (mmWave-NOMA). A downlink cellular
system is considered, where an analog phased array is equipped at both the base
station and users. A joint Tx-Rx beamforming and power allocation problem is
formulated to maximize the achievable sum rate (ASR) subject to a minimum rate
constraint for each user. As the problem is non-convex, we propose a
sub-optimal solution with three stages. In the first stage, the optimal power
allocation with a closed form is obtained for an arbitrary fixed Tx-Rx
beamforming. In the second stage, the optimal Rx beamforming with a closed form
is designed for an arbitrary fixed Tx beamforming. In the third stage, the
original problem is reduced to a Tx beamforming problem by using the previous
results, and a boundary-compressed particle swarm optimization (BC-PSO)
algorithm is proposed to obtain a sub-optimal solution. Extensive performance
evaluations are conducted to verify the rational of the proposed solution, and
the results show that the proposed sub-optimal solution can achieve a
near-upper-bound performance in terms of ASR, which is significantly improved
compared with those of the state-of-the-art schemes and the conventional mmWave
orthogonal multiple access (mmWave-OMA) system.Comment: 11Pages, 10 figure
A Survey on Non-Orthogonal Multiple Access for 5G Networks: Research Challenges and Future Trends
Non-orthogonal multiple access (NOMA) is an essential enabling technology for
the fifth generation (5G) wireless networks to meet the heterogeneous demands
on low latency, high reliability, massive connectivity, improved fairness, and
high throughput. The key idea behind NOMA is to serve multiple users in the
same resource block, such as a time slot, subcarrier, or spreading code. The
NOMA principle is a general framework, and several recently proposed 5G
multiple access schemes can be viewed as special cases. This survey provides an
overview of the latest NOMA research and innovations as well as their
applications. Thereby, the papers published in this special issue are put into
the content of the existing literature. Future research challenges regarding
NOMA in 5G and beyond are also discussed.Comment: to appear in IEEE JSAC, 201
A Two-Stage Beam Alignment Framework for Hybrid MmWave Distributed Antenna Systems
In this paper, we investigate the beam alignment problem in millimeter-wave
(mmWave) distributed antenna systems where a home base station communicates
with multiple users through a number of distributed remote radio units (RRUs).
Specifically, a two-stage schedule-and-align (TSSA) scheme is proposed to
facilitate efficient communications. In the first stage, a coarse beam scanning
over the entire angular space is performed while beam indices and the
corresponding peak-to-background ratios of the received power-angle-spectrum
are obtained from users' feedback. Then, by exploiting the user feedback, an
efficient user scheduling algorithm is developed to improve the system spectral
efficiency and to reduce the system misalignment probability. Next, the second
stage of beam search is performed by each RRU with reconfigured search angles,
search steps, and power levels to obtain a refined beam alignment. Simulation
results show that the proposed TSSA scheme can significantly outperform the
conventional one-stage method in both centralized and distributed mmWave
systems in terms of beam alignment accuracy and spectral efficiency.Comment: 5 pages, 5 figures, accepted by IEEE SPAWC 201
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
Fundamental Green Tradeoffs: Progresses, Challenges, and Impacts on 5G Networks
With years of tremendous traffic and energy consumption growth, green radio
has been valued not only for theoretical research interests but also for the
operational expenditure reduction and the sustainable development of wireless
communications. Fundamental green tradeoffs, served as an important framework
for analysis, include four basic relationships: spectrum efficiency (SE) versus
energy efficiency (EE), deployment efficiency (DE) versus energy efficiency
(EE), delay (DL) versus power (PW), and bandwidth (BW) versus power (PW). In
this paper, we first provide a comprehensive overview on the extensive on-going
research efforts and categorize them based on the fundamental green tradeoffs.
We will then focus on research progresses of 4G and 5G communications, such as
orthogonal frequency division multiplexing (OFDM) and non-orthogonal
aggregation (NOA), multiple input multiple output (MIMO), and heterogeneous
networks (HetNets). We will also discuss potential challenges and impacts of
fundamental green tradeoffs, to shed some light on the energy efficient
research and design for future wireless networks.Comment: revised from IEEE Communications Surveys & Tutorial
Energy Efficiency Optimization for NOMA UAV Network with Imperfect CSI
Unmanned aerial vehicles (UAVs) are developing rapidly owing to flexible
deployment and access services as air base stations. However, the channel
errors of low-altitude communication links formed by mobile deployment of UAVs
cannot be ignored. And the energy efficiency of the UAVs communication with
imperfect channel state information (CSI) hasnt been well studied yet.
Therefore, we focus on system performance optimization in non-orthogonal
multiple access (NOMA) UAV network considering imperfect CSI between the UAV
and users. A suboptimal resource allocation scheme including user scheduling
and power allocation is designed for maximizing energy efficiency. Because of
the nonconvexity of optimization function with an probability constraint for
imperfect CSI, the original problem is converted into a non-probability problem
and then decoupled into two convex subproblems. First, a user scheduling method
is applied in the two-side matching of users and subchannels by the difference
of convex programming. Then based on user scheduling, the energy efficiency in
UAV cells is optimized through a suboptimal power allocation algorithm by
successive convex approximation method. The simulation results prove that the
proposed algorithm is effective compared with existing resource allocation
schemes.Comment: to appear in IEEE Journal on Selected Areas in Communication
Spectrum and Energy Efficient Beamspace MIMO-NOMA for Millimeter-Wave Communications Using Lens Antenna Array
The recent concept of beamspace multiple input multiple output (MIMO) can
significantly reduce the number of required radio-frequency (RF) chains in
millimeter-wave (mmWave) massive MIMO systems without obvious performance loss.
However, the fundamental limit of existing beamspace MIMO is that, the number
of supported users cannot be larger than the number of RF chains at the same
time-frequency resources. To break this fundamental limit, in this paper we
propose a new spectrum and energy efficient mmWave transmission scheme that
integrates the concept of non-orthogonal multiple access (NOMA) with beamspace
MIMO, i.e., beamspace MIMO-NOMA. By using NOMA in beamspace MIMO systems, the
number of supported users can be larger than the number of RF chains at the
same time-frequency resources. Particularly, the achievable sum rate of the
proposed beamspace MIMO-NOMA in a typical mmWave channel model is analyzed,
which shows an obvious performance gain compared with the existing beamspace
MIMO. Then, a precoding scheme based on the principle of zero-forcing (ZF) is
designed to reduce the inter-beam interferences in the beamspace MIMO-NOMA
system. Furthermore, to maximize the achievable sum rate, a dynamic power
allocation is proposed by solving the joint power optimization problem, which
not only includes the intra-beam power optimization, but also considers the
inter-beam power optimization. Finally, an iterative optimization algorithm
with low complexity is developed to realize the dynamic power allocation.
Simulation results show that the proposed beamspace MIMO-NOMA can achieve
higher spectrum and energy efficiency compared with existing beamspace MIMO.Comment: To appear in IEEE Journal on Selected Areas in Communications.
Simulation codes are provided to reproduce the results presented in this
paper:
http://oa.ee.tsinghua.edu.cn/dailinglong/publications/publications.htm
Energy-Efficient Joint User-RB Association and Power Allocation for Uplink Hybrid NOMA-OMA
In this paper, energy efficient resource allocation is considered for an
uplink hybrid system, where non-orthogonal multiple access (NOMA) is integrated
into orthogonal multiple access (OMA). To ensure the quality of service for the
users, a minimum rate requirement is pre-defined for each user. We formulate an
energy efficiency (EE) maximization problem by jointly optimizing the user
clustering, channel assignment and power allocation. To address this hard
problem, a many-to-one bipartite graph is first constructed considering the
users and resource blocks (RBs) as the two sets of nodes. Based on swap
matching, a joint user-RB association and power allocation scheme is proposed,
which converges within a limited number of iterations. Moreover, for the power
allocation under a given user-RB association, we first derive the feasibility
condition. If feasible, a low-complexity algorithm is proposed, which obtains
optimal EE under any successive interference cancellation (SIC) order and an
arbitrary number of users. In addition, for the special case of two users per
cluster, analytical solutions are provided for the two SIC orders,
respectively. These solutions shed light on how the power is allocated for each
user to maximize the EE. Numerical results are presented, which show that the
proposed joint user-RB association and power allocation algorithm outperforms
other hybrid multiple access based and OMA-based schemes.Comment: Non-orthogonal multiple access (NOMA), energy efficiency (EE), power
allocation (PA), uplink transmissio
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