782 IEEE WIRELESS COMMUNICATIONS LETTERS, VOL. 6, NO. 6, DECEMBER 2017
Energy-Efficient Power Allocation in Millimeter Wave Massive
MIMO With Non-Orthogonal Multiple Access
Wanming Hao, Ming Zeng, Zheng Chu, Member, IEEE, and Shouyi Yang
Abstract—In this letter, we investigate the energy effi-
ciency (EE) problem in a millimeter wave massive MIMO system
with non-orthogonal multiple access (NOMA). Multiple two-user
clusters are formulated according to their channel correlation
and gain difference. Following this, we propose a hybrid ana-
log/digital precoding scheme for the low radio frequency chains
structure at the base station. On this basis, we formulate a power
allocation problem aiming to maximize the EE under users’
quality of service requirements and per-cluster power constraint.
An iterative algorithm is proposed to obtain the optimal power
allocation. Simulation results show that the proposed NOMA
scheme achieves superior EE performance than conventional
OMA scheme.
Index Terms—Massive MIMO, millimeter wave, hybrid
precoding, energy efficiency, NOMA.
I. INTRODUCTION
P
OWER domain non-orthogonal multiple access (NOMA)
has been recognized as a promising candidate for next
generation wireless communication systems [1]. By apply-
ing superposition coding and successive interference cancel-
lation (SIC), NOMA allows multiple users to access the
same time-frequency resource, leading to further increase
in the spectral efficiency (SE) compared with orthogonal
multiple access (OMA) [2]. Recently, MIMO has been applied
to NOMA (MIMO-NOMA) systems to further increase
SE [3]–[7]. In a MIMO-NOMA system, users are paired into
clusters, with users in each cluster sharing the same beam-
forming [3]–[6]. Reference [3] proposes a user clustering and
power allocation algorithm to maximize the sum capacity.
In [4], a low-feedback NOMA scheme is proposed for a mas-
sive MIMO (mMIMO) system, in which the performance of
two scenarios, namely perfect user ordering and one-bit feed-
back, is evaluated under the proposed scheme. Reference [6]
jointly investigates user clustering, beamforming design and
power allocation problem to maximize the capacity.
Manuscript received June 17, 2017; revised July 23, 2017; accepted
August 13, 2017. Date of publication August 18, 2017; date of current version
December 15, 2017. This work was supported in part by the Special Project
for Inter-Government Collaboration of State Key Research and Development
Program under Grant 2016YFE0118400, and in part by the National Natural
Science Foundation of China under Grant U1604159. The associate editor
coordinating the review of this paper and approving it for publication was
K. W. Choi. (Corresponding author: Wanming Hao.)
W. Hao is with the School of Information Engineering, Zhengzhou
University, Zhengzhou 450001, China, and also with Kyushu University,
Fukuoka 819-0395, Japan (e-mail: wmhao@hotmail.com).
M. Zeng is with the Faculty of Engineering and Applied Science, Memorial
University, St. Johns, NL A1B 3X9, Canada (e-mail: mzeng@mun.ca).
Z. Chu is with the Faculty of Science and Technology, Middlesex
University, London NW4 4BT, U.K. (e-mail: z.chu@mdx.ac.uk).
S. Yang is with the School of Information Engineering, Zhengzhou
University, Zhengzhou 450001, China (e-mail: iesyyang@zzu.edu.cn).
Digital Object Identifier 10.1109/LWC.2017.2741493
Fig. 1. Cluster-based downlink mmWave mMIMO-NOMA system model.
However, the above works mainly focus on the SE of the
system. With energy efficiency (EE) becoming one of the
major concerns for 5G, more efforts need to be paid to its
study. So far, only a few works have studied NOMA with
the perspective of EE [7], [8]. Sun et al. [7] investigate the
EE maximization problem for two users and propose a near-
optimal power allocation scheme, whereas [8] considers the
EE maximization problem under SISO channel.
Different from previous works, in this letter, we investi-
gate the EE optimization problem in a downlink millimeter
wave (mmWave) mMIMO-NOMA system. Indeed, the use of
NOMA in mmWave is preferable because users’ channels can
be highly correlated due to the highly directional feature of
mmWave transmission [9]. To reduce the hardware complexity,
we apply low radio frequency (RF) chains structure at the base
station (BS), where the hybrid analog/digitial precoding is con-
sidered. Specifically, we first pair users into clusters according
to their channel correlation and gain difference. Following
this, we design an analog beamforming vector for each cluster
based on the codebook. For digital precoding, to coordinate
inter-cluster interference, we apply the conventional zero forc-
ing (ZF) precoding based on the channel of the strong user.
To this end, we formulate the power allocation problem aim-
ing to maximize the EE under the users’ QoS requirements.
To ensure the cluster’ fairness, per-cluster power constraint
is considered. We transform the fractional EE problem into
a subtractive-form one, while it is still nonconvex. Then, we
divide the problem into multiple independent convex optimiza-
tion problems by setting initial feasible power and propose an
iterative algorithm to obtain the optimal solution. Simulation
results verify that the EE under NOMA outperforms that under
conventional OMA.
Notation: We use the following notations throughout this
letter: (·)
T
and (·)
H
denote the transpose and Hermitian trans-
pose, respectively, · denotes the Frobenius norm, C
x×y
denotes the space of x × y complex matrix.
II. S
YSTEM AND CHANNEL MODEL
As shown in Fig. 1, we consider a downlink mmWave
mMIMO-NOMA transmission scenario, in which one BS
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