Deep Learning Based Joint Detection and Decoding
of Non-Orthogonal Multiple Access Systems
Fuqiang Sun, Kai Niu, and Chao Dong
Key Laboratory of Universal Wireless Communications, Ministry of Education
Beijing University of Posts and Telecommunications, Beijing, China 100876
Email: {sunfuqiang, niukai, dongchao}@bupt.edu.cn
Abstract—In recent years, many studies have applied deep
learning techniques to the field of communication. Many of them
have achieved reduced complexity or improved performance. In
this paper, we apply deep learning techniques to joint multiuser
detection and decoding of non-orthogonal multiple access system.
We construct the neural network detector and decoder based on
message passing algorithm (MPA) and belief propagation (BP)
algorithm, respectively. And the decoder is cascaded with the
detector to form a larger neural network. To achieve this, we
assign weights to the edges of the respective factor graphs and
then use deep learning approaches to train these weights to get
better performance. Compared with the conventional method (i.e.
MPA for multiuser detection and BP for decoding), the proposed
method improves the joint detection and decoding performance,
while the computational complexity is not greatly increased.
I. INTRODUCTION
After decades of development, many excellent training
methods and neural network structures have been invented in
the field of deep learning to accomplish various tasks [1],
[2], [3]. In some areas, deep learning methods outperform
traditional methods, especially in computer vision and speech
processing [4]. In the last decade, there have been continuous
attempts to apply deep learning methods to the field of com-
munications. And many satisfactory results [5], [6], [7]have
demonstrated that this method is feasible.
The fifth-generation wireless communication requires high-
er spectral efficiency, lower latency, and massive connec-
tions. Current orthogonal multiple access technologies hardly
meet these requirements [8]. Non-orthogonal multiple ac-
cess(NOMA) is a feasible solution to meet these requirements
[8], [9]. Unlike orthogonal multiple access, an orthogonal
resource can be allocated to multiple users in NOMA system.
At the transmitting end, messages from different users are sent
non-orthogonally and interference information is actively in-
troduced. Compared to OMA system, the receiver complexity
is increased in NOMA system, but higher spectral efficiency
can be achieved. Like linear codes, such as BCH codes and
low-density parity-check (LDPC) codes, a NOMA system can
be represented by a factor graph in which there are two types
of nodes, variable nodes (VNs) corresponding to the users, and
function nodes (FNs) corresponding to the orthogonal resource
elements (REs). Message passing algorithm can be performed
in this factor graph to do multiuser detection.
This work is supported by the National Natural Science Foundation of
China (No. 61671080, No.61601047).
The message passing algorithm is essentially the same
as the belief propagation (BP) algorithm commonly used in
LDPC decoding. In the two algorithms, messages are both
transmitted over edges of the factor graph. In [5], the author
assign weights to the edges of the Tanner graph that represents
corresponding linear code. And then these weights are trained
by deep learning method. Inspired by this, We suspect that
the weights on the edges of the NOMA factor graph may
affect the performance of multiuser detection. So we design
a neural network multiuser detector which is an alternative
representation of the factor graph. The nodes in the neural
network multiuser detector is corresponding to messages trans-
mitted over edges. Due to the multi-ary modulation, multiple
messages are delivered over one edges of the factor graph
when the node is updated.
In this paper, we proposed a scheme of joint detection
and decoding of NOMA systems based on deep learning.
Unlike the fully connected neural network, the proposed neural
network is constructed based on the corresponding factor
graph and is a sparse neural network. And if only multi-user
detection is performed, the gain from channel coding cannot be
utilized. So we design the multi-user detector and decoder as
neural networks to form a large neural network. And then use
deep learning technologies to train the whole neural network
to get better performance.
The remainder of the paper is organized as follows. Section
II briefly describes the system model of NOMA. We present
the proposed deep neural network in Section III. The numerical
results and conclusion are presented in Section IV and Section
V.
II. SYSTEM MODEL
Consider a simplified uplink non-orthogonal multiple access
system where V users multiplex onto F orthogonal resource
elements. For vth user, each K-bit input binary data u
v
is
channel-coded into an N-bit coded block c
v
.
c
v
= u
v
G (1)
Where G is the generator matrix. Given the modulation order
J, each user’s N-bit codeword block will be transmitted in
N
J
time slots. For the tth time slot, t = 1, 2, · · ·
N
J
, every J
bits in c
v
of the vth user constitute a J-bits vector b
t
v
and are
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