Content uploaded by Hirofumi Suganuma
Author content
All content in this area was uploaded by Hirofumi Suganuma on Feb 18, 2020
Content may be subject to copyright.
Hybrid Multiple Access Using Simultaneously
NOMA and OMA
Hirofumi SUGANUMA, Hiroaki SUENAGA, and Fumiaki MAEHARA
Graduate School of Fundamental Science and Engineering, Waseda University
3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
E-mail: fumiaki m@waseda.jp
Abstract—This paper proposes a hybrid multiple access scheme
using both non-orthogonal multiple access (NOMA) and orthog-
onal multiple access (OMA) to overcome the inherent problems
of NOMA. In NOMA, as multiple access is performed within the
power domain a small difference in the channel gain between
users or an increase in the number of simultaneous users reduces
the effectiveness. To cope with these problems, the proposed
scheme applies a novel combination of NOMA and OMA as
well as NOMA for multiple access patterns, and then determines
the best pattern based on the system capacity. The effectiveness
of the proposed scheme is demonstrated in comparison with
conventional hybrid multiple access using NOMA and OMA
through computer simulations.
I. Introduction
The fifth-generation mobile communication systems (5G),
to be realized after 2020, require higher speed, greater ca-
pacity, and lower latency. One promising approach being
considered is non-orthogonal multiple access (NOMA) [1]–
[3], employing successive interference cancellation (SIC). This
is because NOMA is a multiple access technique to allocate
radio resources within the power domain, thus making it
possible to offer more bandwidth per user and to achieve better
spectral efficiency than orthogonal multiple access (OMA).
However, NOMA suffers from the inherent problem that
if the difference in the channel gain between users is small,
then resource allocation becomes insufficient owing to inter-
user interference. This results in a lower system capacity than
that of OMA [1]–[3]. Moreover, it has also been reported that
an increase in the number of simultaneously connected users
reduces the benefits of NOMA [2]. To cope with this problem,
hybrid multiple access [4], [5], which adaptively switches
between NOMA and OMA, in the time domain has been
proposed, and its effectiveness has been confirmed. However,
as this approach adopts either NOMA or OMA using the entire
bandwidth at any time, the improvement of the system capacity
is limited.
With this background, we propose a hybrid multiple access
scheme using both NOMA and OMA, to further improve the
system capacity of NOMA. The proposed approach involves
applying a combination of NOMA and OMA as well as
NOMA to multiple access patterns, and determining the best
multiple access pattern based on the system capacity. In the
proposed scheme, as a novel combination of NOMA and OMA
is introduced in the same bandwidth, the problems of NOMA
under small differences in the channel gain and increases in the
number of users can be effectively resolved. The effectiveness
of the proposed scheme is demonstrated in comparison with
conventional hybrid multiple access in terms of the system
capacity through computer simulations.
II. Proposed Scheme
NOMA shares the entire bandwidth with all simultaneous
users, allowing inter-user interference to occur, while OMA
orthogonally allocates resources for all users. To cope with
such inter-user interference, SIC is applied on the receiving
side to extract transmitted signals with superposition coding.
Figure 1 depicts the system configuration of NOMA. As
shown in Fig. 1, NOMA allocates more transmit power for
users with poor channel gains, while less transmit power is
allocated for users with satisfactory channel gains. Moreover,
considering that the users with less transmit power face severe
interference from other simultaneous users, such inter-user
interference is first decoded and then canceled out using SIC.
However, this approach suffers from the inherent problem that
when the difference in the channel gain is small SIC fails
to decode inter-user interference, which leads to a system
performance degradation. In such cases, NOMA provides a
lower system performance than OMA [1]–[3], and therefore
countermeasures to this problem are expected to be consid-
ered. With this background, we propose a hybrid multiple
access scheme, simultaneously using NOMA and OMA. In
the proposed scheme, as the combination of NOMA and OMA
is introduced in the same bandwidth, the resource allocation
flexibility can be increased, thereby alleviating the problems
occurring for small differences. Moreover, the coexistence
of OMA and NOMA in the entire bandwidth can reduce
the actual number of simultaneous users in NOMA, which
enhances the effectiveness of NOMA.
Figure 2 illustrates the concept of the proposed scheme,
where the number of users is set to three. As shown in
Fig. 2, the three different multiple access patterns of NOMA,
OMA, and a combination of NOMA and OMA are adaptively
employed. Considering the possible combinations of users,
there are five types of resource pattern in total. In the proposed
scheme, the instantaneous system capacities of possible multi-
ple access patterns are first calculated using the channel state
information (CSI), and then the access pattern achieving the
maximum system capacity is determined as the best multiple
access pattern. Using the channel gain |hk|2obtained from
6PDOOGLIIHUHQFH
_K_ §_K_
%6
3RRU
FDSDFLW\
/DUJHGLIIHUHQFH
_K_ !!_K_
%6
+LJK
FDSDFLW\
\
K[
8VHU
GHFRGLQJ
8VHU
GHFRGLQJ
[
[
K[Q
\ K[[Q
8VHU
%DVH
VWDWLRQ
8VHU
&KDQQHOJDLQ _KN_
+LJK /RZ
\
8VHU
GHFRGLQJ [
\ K[[Q
K
K
3
3
3RZHU
)UHTXHQF\
8VHU
: : :
8VHU
3
Fig. 1. System configuration of NOMA.
%DVH
VWDWLRQ 8VHU
8VHU
8VHU
3RZHU
)UHTXHQF\
8VHU
8VHU 3
:
8VHU
120$
3RZHU
)UHTXHQF\
8VHU
8VHU
3
:
8VHU
20$
3RZHU
)UHTXHQF\
8VHU 8VHU
3
:
8VHU
3RZHU
)UHTXHQF\
8VHU 8VHU
3
:
8VHU
3RZHU
)UHTXHQF\
8VHU 8VHU
3
:
8VHU
&RPELQDWLRQRI120$DQG20$
5HVRXUFHSDWWHUQVHOHFWLRQ
_K_!_K_!_K_
Fig. 2. Concept of the proposed scheme.
the CSI, the instantaneous channel capacity of each multiple
access is given by
Rk,NOMA =Wklog2(1 +Pk|hk|2/(Pj,|hk|2<|hj|2Pj|hk|2+N0)) (1)
Rk,OMA =Wklog2(1 +Pk|hk|2/N0) (2)
where Wkand Pkdenote the bandwidth and transmit power of
the k-th user, respectively. N0is the noise power.
III. Numerical Results
In this section, we verify the effectiveness of the proposed
scheme by comparison with NOMA and OMA only in terms
of the system capacity. Table I presents the simulation param-
eters. In our performance evaluation, the number of cells is
set to 37, with a frequency reuse factor of 1. Moreover, we
assumed two types of calls: calls from an entire cell and calls
made only from the cell edge, reflecting the worst case with
a low SINR and only small differences in the channel gain
between users. Fixed power allocation (FPA) is adopted for
the transmit power allocation in NOMA, and the fixed power
ratio αf pa (0 < α f pa <1) is set to 0.25, which defines the
difference in the transmit power between simultaneous users.
Figure 3 shows the cumulative distribution of the system
capacity for the proposed scheme, where the number of users
is set to three. For our comparison, the performances of
NOMA only, OMA only, and the conventional hybrid multiple
access scheme [4], [5] are presented for reference. From
Fig. 3(a), it can be seen that NOMA only achieves a high
system capacity and OMA only reduces the probability of a
low system capacity for the entire-cell scenario. In contrast,
it is found from Fig. 3(b) that NOMA dramatically reduces
the system capacity in the cell-edge scenario owing to a small
difference in the channel gain. These performances represent
the general properties of NOMA and OMA. Moreover, it
TABLE I
Simulation Parameters
Number of existing users per cell 3
Total bandwitdh /Transmit power 20 [MHz] /12 [W]
Cell radius /Path loss model 1000 [m] /L=d−3.5(din m)
Channel model Rayleigh fading channel
Thermal noise density /Noise figure −174 [dBm/Hz] /10 [dB]
&DSDFLW\ >ESV+]@
&
X
P
X
O
D
WL
Y
H
'
L
V
W
U
L
EX
W
L
RQ
(QWLUHFHOOVFHQDULR
)5) )3$DISD
/ Gí GLQP
5D\OHLJKIDGLQJFKDQQHO
3URSRVHG
&RQYHQWLRQDO5HI>@>@
120$
20$
2QHXVHURQO\
ZEHVWFKDQQHOJDLQ
(a) Entire-cell scenario
&DSDFLW\ >ESV+]@
&XPXODWLYH 'LVWULEXWLRQ
2QHXVHURQO\
ZEHVWFKDQQHOJDLQ
&HOOHGJHVFHQDULR
)5) )3$DISD
/ Gí GLQP
5D\OHLJKIDGLQJFKDQQHO
3URSRVHG
&RQYHQWLRQDO5HI>@ >@
120$
20$
(b) Cell-edge scenario
Fig. 3. Cumulative distribution of system capacity for the proposed scheme.
can be observed that the proposed scheme outperforms the
conventional scheme, and yields the best system capacity
performance regardless of the type of call. This is because
the proposed scheme can effectively utilize a combination of
NOMA and OMA in the same bandwidth.
IV. Conclusion
In this paper, we proposed a hybrid multiple access scheme
using NOMA and OMA to further improve the system capac-
ity of NOMA. The proposed scheme features the application
of a combination of NOMA and OMA, as well as NOMA
or OMA, for multiple access patterns. The numerical results
demonstrated that because the adoption of OMA can im-
prove the effectiveness of NOMA in the same bandwidth, the
proposed scheme outperforms conventional hybrid multiple
access approaches using NOMA and OMA in a typical multi-
cell scenario.
Acknowledgment
This work was supported by JSPS Grant-in-Aid for Scien-
tific Research (C) Grant Number 19K04381.
References
[1] A. Benjebbour, A. Li, Y. Saito, Y. Kishiyama, A. Harada, and T. Naka-
mura, “System-level performance of downlink NOMA for future LTE,”
Proc. 2013 IEEE Globecom Workshops, pp. 66–70, Dec. 2013.
[2] K. Higuchi and A. Benjebbour, “Non-orthogonal multiple access
(NOMA) with successive interference cancellation for future radio ac-
cess,” IEICE Trans. Commun., vol. E98-B, no. 3, pp. 403–414, Mar. 2015.
[3] 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 Commun. Mag., vol. 55, no. 10, pp. 176–
183, Oct. 2016.
[4] Y. Yuan et al., “Non-orthogonal transmission technology in LTE evolu-
tion,” IEEE Commun. Mag., vol. 54, no. 7, pp. 68–74, July 2016.
[5] A. S. Marcano and H. L. Christiansen, “A novel method for improving
the capacity in 5G mobile networks combining NOMA and OMA,” Proc.
IEEE 85th Veh. Technol. Conf. (VTC 2017-Spring), pp. 1-5, June 2017.
