Outage Probability Analysis and Optimization in
Downlink NOMA Systems with Cooperative
Full-duplex Relaying
(Invited Paper)
Lin Zhang
†
, Ming Xiao
†∗
, Jiaqi Liu
†
, Gang Wu
†
, Dengsheng Lin
†
, and Shaoqian Li
†
†
National Key Lab of Science and Technology on Communications,
University of Electronic Science and Technology of China (UESTC)
∗
School of Electrical Engineering, Royal Institute of Technology, KTH
Email: linzhang1913@gmail.com
Abstract—We study a downlink non-orthogonal multiple access
(NOMA) system with cooperative full-duplex relaying, where the
near user in terms of the base station (BS) is enabled to act
as a full-duplex relay for the far user. In particular, we first
derive the outage probability with closed-form expressions when
the power allocations at the BS and relay (or the near user) are
fixed. Then, we analytically obtain the optimal power allocations
with closed-form expressions at the BS and relay to minimize the
outage probability. Numerical results validate the correctness of
the theoretical analysis and demonstrate the advantages of the
proposed algorithms over the state of arts.
Index Terms—Cooperative relaying, full-duplex, NOMA, out-
age probability, power allocation.
I. INTRODUCTION
It is foreseen that the increasing smart devices will lead
to amounts of wireless connections and tremendous data
traffic in the next generation of wireless communications
[1]. To accommodate these data traffic, spectrum-efficient
transmission strategies are required. Non-orthogonal multiple
access (NOMA) is one of the promising candidates to en-
hance the spectrum efficiency [2], [3]. Compared with the
conventional orthogonal multiple access (OMA), e.g., time
division multiple access (TDMA), frequency division multi-
ple access (FDMA), code division multiple access (CDMA),
which exploit the time/frequency/code domain for multiple
access, the key idea of NOMA is exploiting the power domain.
In particular, different transmissions are allowed to share
the same time/frequecy/code but with different power levels.
Although they may cause co-channel interference to each other
at receivers, these receivers are able to avoid the co-channel
interference with the successive interference cancelation (SIC)
technique. Therefore, NOMA can offer a higher spectrum
efficiency than OMA.
The basic feature of the original NOMA is equipping a
receiver with the capability to have the prior information of
the messages of other receivers for SIC. In fact, this prior
information can be utilized to further improve the system per-
formance. Then, the cooperative relaying is introduced into the
original NOMA (referred to CR-NOMA hereafter) to obtain a
spatial diversity gain [4]. For instance, in a two-user downlink
CR-NOMA system, the base station (BS) superimposes two
messages into a downlink signal and transmit it to both the
near user U1 in terms of the BS and the far user U2. According
to the NOMA protocol, U1 is allowed to decode the message
of U2. Then, U1 may act as a relay for U2 and forward this
message to U2. In this way, U2 may receive two copies of
desired signals, one is from the BS and the other one is from
the U2. By combining the two copies of signals, U2 may
improve the reliability of the received signals. This leverages
the spatial diversity gain.
Recently, the CR-NOMA has attracted much attention due
to the advantage of the spatial diversity gain. [5], [6], [7]
studied the CR-NOMA system with the half-duplex relaying
(referred to CHR-NOMA hereafter), where the transmitter
broadcasts the superimposed signal to multiple receivers in
the first phase and near receivers decode and forward the
messages of far receivers in the second phase. To further
improve the system performance in a CR-NOMA system, [8],
[9] studied the CR-NOMA system with the (in-band) full-
duplex relaying (referred to CFR-NOMA hereafter), where
the transmitter broadcasts the superimposed signal to multiple
receivers meanwhile near receivers decode and forward the
messages of far receivers. In particular, [8] assumed perfect
self-interference cancelation, and theoretically obtained the
achievable rate region, which can be regarded as a performance
upper bound in a CFR-NOMA system. [9] studied imperfect
self-interference cancelation of the full-duplex relaying and
analyzed the outage probability performance in the CFR-
NOMA system.
In this paper, we investigate a downlink CFR-NOMA sys-
tem. Different from [9], we consider a scenario, where there
does not exist a direct link from the BS to the far user due
to physical obstacles or heavy shadowing, and theoretically
analyze the outage probability performance with a closed-
form expression. Besides, we characterize the optimal power
allocations at both the BS and relay (or near user) with closed-
form expressions and minimize the outage probability in the
CFR-NOMA system.
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