Underlay Cognitive NOMA-Based Coordinated Direct and Relay Transmission

854 IEEE WIRELESS COMMUNICATIONS LETTERS, VOL. 10, NO. 4, APRIL 2021

Underlay Cognitive NOMA-Based Coordinated Direct and Relay Transmission

Tien-Tung Nguyen , Thai-Hoc Vu , Toan-Van Nguyen , Daniel Benevides da Costa , Senior Member, IEEE,

and Chung Duc Ho

Abstract—In this letter, a cooperative underlay cognitive non￾orthogonal multiple access (NOMA)-based coordinated direct

and relay transmission is investigated. Specifically, exact closed￾form expressions for the outage probabilities (OPs) of the primary

and SNs are derived, based on which the respective asymptotic

analysis are carried out. Our results show the superiority of the

proposed system when compared with conventional cooperative

schemes. Also, it is shown that there exists an optimal point where

the transmit powers of the primary and STs can be set in order

to ensure a good performance for both networks.

Index Terms—Coordinated direct and relay transmission

(CDRT), non-orthogonal multiple access (NOMA), outage

performance, underlay cognitve radio network.

I. INTRODUCTION

RECENTLY, with ever-growing increase in the number of

mobiles and Internet-of-Things (IoT) devices, the demand

of spectrum usage has increased dramatically. In order to

address the spectrum shortage, non-orthogonal multiple access

(NOMA) technique has envisioned as a promising solution. In

principle, relying on the superposition coding and successive

interference cancellation (SIC), NOMA allows that the same

resource block can simultaneously be allocated for multiple

users [1].

In recent years, there has been an increasing interest in

cooperative NOMA strategies to enhance the spectral effi￾ciency and extend the transmission coverage. In particular,

a promising strategy is to integrate NOMA into a coor￾dinated direct and relay transmission (CDRT), where the

near user’s signal is directly transmitted by the source while

the far user receives the signal from the source with assis￾tance of a half-duplex relay [2]. Inspired by [2], the authors

in [3] evaluated the impact of full-duplex mode on outage

performance and ergodic sum capacity of a NOMA-based

Manuscript received November 27, 2020; accepted December 16, 2020.

Date of publication December 23, 2020; date of current version April 9, 2021.

This work was supported by Thu Dau Mot University under Grant DT.20.2-

016. The associate editor coordinating the review of this article and approving

it for publication was X. Li. (Corresponding author: Chung Duc Ho.)

Tien-Tung Nguyen is with the Telecommunication Division, Industrial

University of Ho Chi Minh City, Ho Chi Minh City 700000, Vietnam (e-mail:

[email protected]).

Thai-Hoc Vu is with the School of Electrical Engineering, University of

Ulsan, Ulsan 44610, South Korea (e-mail: [email protected]).

Toan-Van Nguyen is with the Faculty of Electronics, Posts and

Telecommunications Institute of Technology, Chi Minh City 70000, Vietnam

(e-mail: [email protected]).

Daniel Benevides da Costa is with the Department of Computer

Engineering, Federal University of Ceará, Sobral 62010-560, Brazil (e-mail:

[email protected]).

Chung Duc Ho is with the Institute of Electrical, Electronics Engineering

and Computer Sciences, Thu Dau Mot University, Thu Dau Mot 750000,

Vietnam, and also with the Electronics and Telecommunication Research

Group, Thu Dau Mot University, Thu Dau Mot 750000, Vietnam (e-mail:

[email protected]).

Digital Object Identifier 10.1109/LWC.2020.3046779

CDRT scheme. Regarding multi-relay scenario, an efficient

relay selection scheme was proposed in [4] to attain the max￾imum successful decoding at the near user. Very recently, the

authors in [5] derived closed-form expressions for the outage

probability (OP) of satellite network under both shadowed￾Rician and Nakagami-m fading. Finally, a device-to-device

communication NOMA-based CDRT system was examined

in [6].

Another crucial approach to alleviate the spectrum scarcity

is to employ cooperative NOMA in cognitive radio networks

(CRNs) [7]–[11]. Enabling cooperative NOMA in CRNs, a

user in secondary networks (SNs) was recruited as a cooper￾ative relay in [7] while dedicated relays in NOMA systems

were deployed in [8], [9]. A spectrum sharing strategy was

studied in overlay CR-NOMA networks [10]. The outage

performance of underlay CR-NOMA network with Detect￾and-Forward relaying was analyzed in [11]. However, no

previous works have studied NOMA-based CDRT in CRNs

subject to interference from primary transmitter. Motivated by

this fact, we propose a novel cognitive NOMA-based CDRT

model, where a NOMA-based CDRT operates with a primary

network (PN) to further improve spectrum utilization thanks to

the adopted underlay transmission mode. Taking into account

the maximum power of the ST as well as the interference

constraints imposed by the PNs, exact closed-form as well as

asymptotic expressions for the OPs of secondary and PNs are

derived. Our results indicate that the proposed system setup

outperforms its orthogonal multiple access (OMA) counter￾part. In addition, it is shown that there exists an optimal point

where the transmit powers of the primary and STs can be set

in order to ensure a good performance for both networks.

II. SYSTEM MODEL

Consider a PN that coexists with a SN, as shown in Fig. 1.

The PN consists of a transceiver pair (i.e., transmitter P and

receiver D), and the SN is a NOMA-based CDRT system com￾posed by one source, i.e., secondary transmitter (ST), one near

user U1, one relay R, and one far user U2. An underlay cogni￾tive radio strategy is adopted. It is assumed that the direct link

between ST and U2 does not exist due to obstacles [2]–[4].

Let hAB ∼ CN (0, ΩAB) be the channel coefficient of the

transmitter A and the receiver B, with A ∈ {ST, R, P} and B

∈ {U1, U2, R, D}. The whole communication process in the

SN is carried out in two consecutive time-slots.

A. Allowable Transmit Power at the ST and Relay

The ST and relay adjust their respective powers in order to

keep the interference at the PN below of a given threshold Ip.

Thus, the transmit powers of S and R in the first and second

time-slots, respectively, must be constrained as [8]

Pi ≤ min

Pm

i , Ip

|hiD|

2

, (1)

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