1
Non-Orthogonal Multiple Access for 5G and
Beyond
Yuanwei Liu, Member, IEEE, Zhijin Qin, Member, IEEE, Maged Elkashlan, Member, IEEE, Zhiguo Ding, Senior
Member, IEEE, Arumugam Nallanathan, Fellow, IEEE, and Lajos Hanzo, Fellow, IEEE,
Abstract—Driven by the rapid escalation of the wireless
capacity requirements imposed by advanced multimedia ap-
plications (e.g., ultra-high-definition video, virtual reality etc.),
as well as the dramatically increasing demand for user access
required for the Internet of Things (IoT), the fifth generation
(5G) networks face challenges in terms of supporting large-
scale heterogeneous data traffic. Non-orthogonal multiple access
(NOMA), which has been recently proposed for the 3rd genera-
tion partnership projects long-term evolution advanced (3GPP-
LTE-A), constitutes a promising technology of addressing the
above-mentioned challenges in 5G networks by accommodating
several users within the same orthogonal resource block. By doing
so, significant bandwidth efficiency enhancement can be attained
over conventional orthogonal multiple access (OMA) techniques.
This motivated numerous researchers to dedicate substantial
research contributions to this field. In this context, we provide a
comprehensive overview of the state-of-the-art in power-domain
multiplexing aided NOMA, with a focus on the theoretical
NOMA principles, multiple antenna aided NOMA design, on the
interplay between NOMA and cooperative transmission, on the
resource control of NOMA, on the co-existence of NOMA with
other emerging potential 5G techniques and on the comparison
with other NOMA variants. We highlight the main advantages of
power-domain multiplexing NOMA compared to other existing
NOMA techniques. We summarize the challenges of existing
research contributions of NOMA and provide potential solutions.
Finally, we offer some design guidelines for NOMA systems and
identify promising research opportunities for the future.
Index Terms—5G, cooperative communication, MIMO,
NOMA, resource allocation, power multiplexing
I. INTRODUCTION
A. Brief History of Wireless Standardization
Following the pioneering contributions of Maxwell and
Hertz, Marconi demonstrated the feasibility of wireless com-
munications across the Atlantic at the end of the 19th century.
By 1928 this technology became sufficiently mature for the
police, the gangsters as well as for the rich and famous to
enjoy tetherless communications.
An early European development was the Swedish Mobile
Telephone System introduced in 1957, which supported 125
users until 1967. In 1966 the Norwegian system was launched,
which operated until 1990. Following these, the 1980s led
This research was supported in part by the U.K. Engineering and Physical
Sciences Research Council (EPSRC) under grant number EP/N029720/1.
Y. Liu, M. Elkashlan and A. Nallanathan are with Queen Mary Uni-
versity of London, London, UK (email: {yuanwei.liu, maged.elkashlan,
a.nallanathan}@qmul.ac.uk).
Z. Qin and Z. Ding are with Lancaster University, Lancaster, UK (e-mail:
{zhijin.qin,z.ding}@lancaster.ac.uk).
L. Hanzo is with University of Southampton, Southampton, UK
(email:lh@ecs.soton.ac.uk).
to the roll-out of numerous national mobile phone systems,
most of which relied on analogue frequency modulation and
hence were unable to employ digital error correction codes.
Consequently, their ability to exploit the radical advances
in Digital Signal Processing remained limited. Hence the
achievable speech quality was typically poor, especially when
the Mobile Station (MS) roamed farther away from the Base
Station (BS).
Hence during the 1980s the member states of the European
Union launched a large-scale cooperative research programme,
which led to the standardization of the second generation (2G)
system known as the global system of mobile (GSM) com-
munications. GSM was the first digital international mobile
system, which rapidly spread across the globe. The success
of GSM shows the sheer power and attraction of global
standardization, motivating competitors to line up behind a
common worldwide solution.
Shortly after the ratification of GSM, a number of other
digital standards emerged, such as the pan-American digital
advanced mobile phone system (D-AMPS) and the direct
sequence code division multiple access (DS-CDMA) based
Pan-American system known as IS-95. IS-95 also had an
evolved counterpart, namely the Pan-American cdma2000
system, which had three parallel CDMA carriers, leading
to the first standardized multi-carrier CDMA (MC-CDMA)
system [1].
However, given the consumers’ thirst for higher bit rates,
during the early 1990s the research community turned its
attention to developing the third generation (3G) system,
which was also based on various CDMA solutions. The
detailed discussion and the performance characterization of
3G networks may be found in [2].
Despite the 40-year research history of OFDM [3], multi-
carrier cellular solutions only emerged during the 2000s as
the dominant modulation technique in the context of the
3G partnership project’s (3GPP) long-term evolution (LTE)
initiative. Clearly, during the 2000s multi-carrier solutions
have found their way into all the 802.11 wireless standards
designed for wireless local area networks (WLANs), while
using different-throughput modem and channel coding modes,
depending on the near-instantaneous channel quality.
What is so beautiful about multi-carrier solutions is their
impressive flexibility, since they have a host of different
parameters which allow us to appropriately configure them and
programme them, whatever the circumstances are - regardless
of the propagation environment and regardless of the quality of
service (QoS) requirements, as facilitated by the employment
2
2000s 2010s 2020s
MIMO
Sq.
BF Close
OVSF-CDMA St.
OMA/
NOMA
Sq.
Turbo St.
FEC
Sq.
LDPC St.
BICM-ID St.
MFAA St.
4G
Sq.
HetNets
CR
SDN
Sq.
UL/DL decoupling St.
MFAA
LS-MIMO
Terrace
5G
Place
Telepr. Ave.
MPEG St.
1990s
Figure 1: The roadmap for illustrating the brief history of wireless standardization.
of adaptive modulation and coding (AMC).
Our hope is dear Colleague that would allow us now to
briefly review the evolution of signal processing and communi-
cations techniques over the past three decades in an anecdotal
style with reference to Fig. 1. At the time of writing we are
gradually approaching the ‘5G Place’ on our road map of
Fig. 1. We are indeed also approaching the bit-rate limits
upper-bounded by the channel capacity of both the classic
single-input/single-output systems as well as of the MIMO
systems. Observe at the top left hand corner of Fig. 1, how the
various MIMO solutions, such as bell lab’s layered space-time
(BLAST) ‘Drive’, space-time coding (STC) ‘Street’, Beam-
Forming ‘Close’ and linear dispersion coding (LDC) ‘Street’
merge into MIMO ‘Square’.
After decades of evolution, the classic orthogonal multi-
ple access (OMA) schemes, such as time division multiple
access (TDMA) ‘Street’, frequency division multiple access
(FDMA), orthogonal variable spreading factor based code
division multiple access (OVSF-CDMA), interleave division
multiple access (IDMA) and orthogonal frequency division
multiple access (OFDMA) ‘Street’ converged to OMA/non-
orthogonal multiple access (NOMA) ‘Square’ of Fig. 1. They
have also evolved further along spatial division multiple access
(SDMA) and multi-functional antenna array ‘Street’ - these
solutions have found their way into the 4G OFDMA systems.
As seen at the bottom left corner of Fig. 1, the various advance
channel coding schemes have competed for adoption in the 4G
standard, which relies of a variety of coding arrangements,
including automatic repeat request (ARQ).
At the time of writing the community turned towards the
standardization of the 5G systems, with a special emphasis on
the NOMA techniques detailed in this treatise, as indicated by
the broad NOMA ‘Parkway’, which symbolizes 15 different
NOMA proposals. The family of MFAAs also entails the
recent spatial modulation (SM) and large-scale (LS) MIMO
systems. Since the ‘road along millimeter wave (mmWave)
Street’ is rather unexplored and the attenuation is high, the
employment of BF is rather crucial, if we want to exploit
these rich spectral reserves.
In the bottom right corner of Fig. 1 a number of novel tech-
nological advances converge at HetNet ‘Square’, where cogni-
tive radio (CR) and software defined networks meet device-to-
device (D2D) and Internet-of-Things (IoT) networks. A range
of sophisticated ideas are also under intensive investigation to
resolve the network-centric versus user-centric design options.
There is a strong evidence that the latter is more promising,
3
Table I: LIST OF ACRONYMS
AF Amplify-and-Forward
BB Beamforming Based
BC Broadcast Channel
BS Base Station
BF Beamforming
CB Cluster-Based
CDMA Code Division Multiple Access
CF Compress-and-Forward
CIRs Channel Impulse Response
CoMP Coordinated Multipoint
CR Cognitive Radio
C-RAN Cloud-based Radio Access Networks
CS Compressive Sensing
CSI Channel State Information
CSIT Channel State Information at the Transmitter
D2D Device-to-Device
DF Decode-and-Forward
DL Downlink
DPC Dirty-Paper Coding
FD Full-Duplex
FDMA Frequency Division Multiple Access
IDMA Leave Division Multiple Access
IMD Iterative Multi-user Detection
IoT Internet of Things
LDS Low-Density Signature
LDPC Low-Density Parity-Check
LPMA Lattice Partition Multiple Access
LTE Long Term Evolution
LMMSE Linear Minimum Mean Square Error
MA Multiple Access
MAC Medium Access Control
M2M Machine-to-Machine
MNV Wireless Network Visualization
MPA Message Passing Algorithms
MUSA Multi-User Shared Access
MUST Multi-User Superposition Transmission
NP Non-deterministic Polynomial-time
NOMA Non-Orthogonal Multiple Access
OFDM Orthogonal Frequency Division Multiplexing
OFDMA Orthogonal Frequency Division Multiple Access
OMA Orthogonal Multiple Access
PA Power Allocation
PDMA Pattern Division Multiple Access
PF Proportional Fairness
PLS Physical Layer Secerity
P2P Peer-to-Peer
PR Primary Receiver
PT Primary Transmitter
PU Primary User
QoS Quality of Service
RB Resource Block
RF Radio Frequency
RBC Relaying Broadcast Channel
SA Signal Alignment
SC Superposition Coding
SCMA Sparse Code Multiple Access
SDM Space Division Multiplexing
SDMA Space Division Multiple Access
SDN Software Defined Network
SDR Software Defined Radio
SD-NOMA Software Defined NOMA
SIC Successive Interference Cancelation
SISO Single-Input Single-Output
SNR Signal-Noise Ratio
SR Secondary Receiver
ST Secondary Transmitter
SU Secondary User
TCMA Trellis Coded Multiple Access
TDMA Time Division Multiple Access
UL Uplink
V-BLAST Vertical-Bell Laboratories Layered Space-Time
VLC Visible Light Communication
VNI Visual Network Index
VR Virtual Reality
WPT Wireless Power Transfer
ZF Zero-forcing
because it is also capable of simultaneous load-balancing.
There are also strong proposals on decoupling the uplink
and downlink tele-traffic, with the motivation that mobile-
initiated uplink traffic can reach a small-cell BS at a lower
transmit power than that of the BS’s downlink transmission.
Optical wireless based on visible-light communications is
also developing quite rapidly, with Giga-bit copper backhaul
networks making promising progress. Whilst no doubt the
classic RF systems will continue to evolve towards the next
generation, an idea, whose time has come is Quantum com-
munications, as demonstrated by the Science article “Satellite-
based entanglement distribution over 1200 km” by Yin et
al. [4].
As the LTE system is reaching maturity and the 4G systems
have been commercially deployed, researchers have turned
their attention to the 5G cellular network. The latest visual net-
work index (VNI) reports pointed out that by 2020s, the data
traffic of mobile devices will become an order of magnitude
higher compared to that in 2014 [5]. Apart from meeting the
escalating data demands of mobile devices, other challenges
of dealing with the deluge of data as well as with the high-
rate connectivity required by bandwidth-thirsty applications
such as virtual reality (VR), online health care and the IoT
further aggravate the situation. Driven by this, the 5G networks
are anticipated with high expectations in terms of making a
substantial breakthrough beyond the previous four generations.
The often-quoted albeit potentially unrealistic expectations
include 1,000 times higher system capacity, 10 times higher
system throughput and 10 times higher energy efficiency per
service than those of the fourth generation (4G) networks [6].
Several key directions such as ultra-densification, mmWave
communications, massive MIMO arrangements, D2D and
machine-to-machine (M2M) communication, full-duplex (FD)
solutions, energy harvesting (EH), cloud-based radio access
networks (C-RAN), wireless network virtualization (WNV),
and software defined networks (SDN) have been identified by
researchers [7]–[9]. Fig. 2 illustrates the whole 5G network
structure, including most of the existing/promising techniques.
B. State-of-the-art of Multiple Access Techniques
As mentioned before, sophisticated multiple access (MA)
techniques have also been regarded as one of the most fun-
damental enablers, which have significantly evolved over the
consecutive generations in wireless networks [10], [11]. Let
us have a deeper looker at the development of MA techniques
below. As illustrated in Fig. 1, the past three to four decades
have witnessed historic developments in wireless communica-
tions and standardization in terms of MA techniques. Looking
back to the development of the MA formats as we briefly
discussed above, in the first generation (1G), FDMA was
combined with an analog frequency modulation based tech-
nology, although digital control channel signaling was used.
In the 2G GSM communications TDMA was used [12]. Then
CDMA, which was originally proposed by Qualcomm [13],
became the dominant MA in the 3G networks. In an effort to
overcome the inherent limitation of CDMA - namely that the
chip rate has to be much higher than the information data rate
4
Macro
cell
……
Massive
MIMO
Small cells
D2D
f
• Ultra Wideband
(cmWave, mmWave)
• NOMA
Power
f
V2V
M2M
IoT
Fronthaul
...
Cloud RAN
Forwarding
Virtualization
Software defined networking controller
Applications
...
IoT Health
Safety
Telco API
Radio access unit
VR
Figure 2: Illustration of the future 5G network architecture.
- OFDMA was adopted for the 4G networks [14]. Based on
whether the same time or frequency resource can be occupied
by more than one user, the existing MA techniques may be
categorized into OMA and NOMA techniques [15]. Amongst
the above-mentioned MA techniques, FDMA, TDMA and
OFDMA allow only a single user to be served within the
same time/frequency resource block (RB), which belong to the
OMA approach. By contrast, CDMA allows multiple users to
be supported by the same RB with the aid of applying different
unique, user-specific spreading sequences for distinguishing
them.
Fuelled by the unprecedented proliferation of new Internet-
enabled smart devices and innovative applications, the emerg-
ing sophisticated new services expedite the development of 5G
networks requiring new MA techniques. NOMA techniques
can be primarily classified into a pair of categories, namely,
code-domain NOMA and power-domain NOMA [16]
1
The most prominant representatives code-domain NOMA
techniques include trellis coded multiple access (TCMA) [18],
IDMA [19], low-density signature (LDS) sequence based
CDMA [20]. These solutions are complemented by the more
recently proposed multi-user shared access (MUSA) tech-
1
Note that apart from the code-and power-domain, the spatial-domain can
also be regarded as another domain for supporting multiple users within the
same RB, which is achieved by exploiting the specific “spatial signature”
constituted by the channel impulse responses (CIRs) of the users for dis-
tinguishing them [3], [10]. A representative MA technique is space division
multiple access (SDMA) [17].
nique [21], pattern division multiple access (PDMA) [22], and
sparse code multiple access (SCMA).
The power-domain NOMA, which has been recently pro-
posed to 3GPP LTE [23], exhibits a superior capacity region
compared to OMA. The key idea of power-domain NOMA
is to ensure that multiple users can be served within a given
time/frequnecy RB, with the aid of superposition coding (SC)
techniques at the transmitter and successive interference can-
cellation (SIC) at the receiver, which is fundamentally different
from the classic OMA techniques of FDMA/TDMA/OFDMA
as well as from the code-domain NOMA techniques. The mo-
tivation behind this approach lies in the fact that again, NOMA
is capable of exploiting the available resources more efficiently
by opportunistically capitalizing on the users’ specific channel
conditions [24] and it is capable of serving multiple users at
different QoS requirements in the same RB. It has also been
pointed out that NOMA has the potential to be integrated with
existing MA paradigms, since it exploits the new dimension of
the power domain. The milestones of power-domain NOMA
are summarized in the timeline of Table II.
C. Motivation and Contributions
While the above literature review has laid the basic foun-
dation for understanding the development of MA schemes
in each generation of cellular networks, the power domain
multiplexing based NOMA philosophy is far from being fully
understood. There are some short magazine papers [16], [23],
5
Table II: Timeline of power-domain NOMA milestones.
1972 · · · · · ·•
Cover first proposed SC and SIC
concepts [25].
1973 · · · · · ·•
Bergmans theoretically
demonstrated that SC is capable of
approaching the capacity of the
Gaussian broadcast channel
(BC) [26].
1986 · · · · · ·•
Verdu discovered the optimal
maximum-likelihood multi-user
receiver for CDMA systems [27].
1994 · · · · · ·•
Patel and Holtzman proposed to
apply SIC and PIC in CDMA
systems [28].
2001 · · · · · ·•
Li and Goldsmith studied the
capacity regions for fading BCs with
applying SC and SIC [29], [30].
2003 · · · · · ·•
Mostafa et al. demostrated that
SAIC can effectively suppress
downlink inter-cell interferences in
GSM networks [31].
2004 · · · · · ·•
Tse compared the capacity regions
of NOMA to OMA both in downlink
and uplink [32].
2005 · · · · · ·•
Andrews summarized the
development of interference
cancellation for cellular
systems [33].
2011 · · · · · ·•
Zhang and Hanzo offered a unified
treatment for SC aided
systems [34].
2012 · · · · · ·•
Vanka et al. designed an
experimental platform for
investigating the implementing
performance of SC [35].
2013 · · · · · ·•
Saito et al. proposed the concept of
two-user downlink NOMA
transmission for bandwidth
efficiency enhancement [36].
2014 · · · · · ·•
Ding et al. developed a multi-user
downlink NOMA transmission
scheme with randomly deployed
users [37].
2015 · · · · · ·•
Xiong et al. designed a practical
open source SDR-based NOMA
prototype for two-user case [38].
2015 · · · · · ·•
Benjebbour et al. measured the
experimental results on a NOMA
test-bed for two-user case [39].
2015 · · · · · ·•
Choi et al. proposed a two-user
MISO-NONA design for
investigating the potential
application of multi-antenna
techniques in NOMA [40].
2016 · · · · · ·•
Ding et al. proposed a
cluster-based multi-user
MIMO-NOMA structure [41].
2017 · · · · · ·•
Shin et al. proposed to apply
coordinated beamforming for
multi-cell MIMO-NOMA networks to
enhance the cell-edge users’
throughput [42].
[43], [44] and surveys [45], [46] in the literature that introduce
NOMA, but their focus is different from our work. More
particularly, Dai et al. introduced some concepts of the existing
NOMA techniques and identified some challenges and future
research opportunities [16] both for power-domain and code-
domain NOMA. A magazine paper on power-domain NOMA
was presented by Ding et al. [23], with particular attention
devoted to investigating the application of NOMA in LTE and
5G networks. Shin et al. [43] discussed the research challenges
and opportunities in terms of NOMA in multi-cell networks,
aiming for identifying techniques to manage the multi-cell
interference in NOMA. As a further advance, Ali et al. [44]
outlined a general framework for multi-cell downlink NOMA
by adopting a coordinated multi-point (CoMP) transmission
scheme by considering distributed power allocation (PA) strat-
egy in each cell. Regarding surveys, in [45], Islam et al.
have surveyed several recent research contributions on power-
domain NOMA, while providing performance comparisons
to OMA in different wireless communications scenarios. In
[46], Tabassum et al. investigated the uplink and downlink of
NOMA in single cell cellular networks, identifying the impact
of distance of users on the performance attained.
Although the aforementioned research contributions present
either general concepts or specific aspects of NOMA, some
important NOMA models, the analytical foundations of
NOMA, and some of its significant applications in wireless
networks have not been covered. Besides, a clear illustration of
the historic development of power-domain NOMA milestones
is missing. Finally, the comparisons between power-domain
NOMA and other practical forms of NOMA have not been
discussed. Motivated by all the aforementioned inspirations,
we developed this treatise. More explicitly, the goal of this
survey is to comprehensively survey the state-of-the-art re-
search contributions that address the major issues, challenges
and opportunities of NOMA, with particular emphasis on both
promising new techniques and novel application scenarios.
Table III illustrates the comparison of this treatise with the
existing magazine papers and surveys in the context of NOMA.
To highlight the significance of this contribution, we com-
mence with a survey of NOMA starting with the basic prin-
ciples, which provides the readers with the basic concepts of
NOMA. We continue in the context of multiple antenna aided
techniques combined with NOMA, followed by cooperative
NOMA techniques. We then address another important issue
of NOMA, namely its resource and PA problems. Finally, we
elaborate on invoking other 5G candidate techniques in the
context of NOMA networks. The contributions of this survey
are at least five fold, which are summarized as follows:
1) We present a comprehensive survey on the recent ad-
vances and on the state-of-the-art in power-domain
multiplexing aided NOMA techniques. The basic con-
cepts of NOMA are introduced and key advantages are
summarized. The research challenges, opportunities and
potential solutions are also identified.
2) We investigate the application of multiple-antenna aided
techniques to NOMA. The pair of most dominant
solutions - namely cluster-based MIMO-NOMA and
beamformer-based MIMO-NOMA - are reviewed and
![Figure 4: Illustration of the sum-rate versus the minimum rate constraints with full CSIT knowledge for different user distances. The full parameter settings can be found in [88].](/figures/figure-4-illustration-of-the-sum-rate-versus-the-minimum-zaij8mo7.webp)
![Figure 13: Illustration of the superiority of NOMA-mmWave over OMA-mmWave networks both at 28 GHz and 60GHz, “OP” refers to optimized power allocation, “FP” refers to fixed power allocation. The detailed parameter settings are found in [192].](/figures/figure-13-illustration-of-the-superiority-of-noma-mmwave-42ql07x3.webp)