Elsevier

Physical Communication

Volume 53, August 2022, 101711
Physical Communication

Full length article
Performance analysis of overlay cognitive NOMA network with imperfect SIC and imperfect CSI

https://doi.org/10.1016/j.phycom.2022.101711Get rights and content

Abstract

In this paper, we investigate a multiple users cooperative overlay cognitive radio non-orthogonal multiple access (CR-NOMA) network in the presence of imperfect successive interference cancellation (SIC) and imperfect channel state information (CSI). In the context of cellular network, cell-center cognitive secondary users act as relays to assist transmission from the primary user (PU) transmitter to the cell-edge PU receiver via NOMA. According to the received signals between the primary transmitter and multiple cognitive secondary center users, the best cell-center cognitive SU with the maximum signal to noise ratio (SNR) is selected to transmit the PU’s signals and its own signal to cell-edge users through NOMA principle. Then, the PU cell-edge user combine the signals received from direct transmission in the first phase and relay transmission from the best cell-center cognitive SU in the second phase by selection combining (SC). To measure the performance of the system quantitatively, we derive the end-to-end outage probability and capacity for the primary and secondary networks by taking the imperfect SIC and CSI into consideration. Finally, the performance analysis is validated by the simulations, and show that serious interference caused by imperfect SIC and (or) imperfect CSI reduce the system performance.

Introduction

The advent of the fifth generation mobile communication technology (5G) brings the realization of the Internet of everything a step forward. With the wide application of wireless communication and the increasing number of devices connected to the Internet, it is important for 5G to meet the needs of users for high bandwidth, low latency, and a large number of connected services. Non-orthogonal multiple access (NOMA) has been recognized as a promising technique to improve spectrum efficiency in the 5G networks [1]. Different from the traditional orthogonal multiple access (OMA), NOMA allows multiple users to use the same channels, the same frequency, and the same code resources to transmit the information in the same time [2]. At the destination, multiple users receive the superimposed signal, where the signal with the highest signal to interference plus noise ratio (SINR) is decoded first and other signals are considered as interference. Then the successive interference cancellation (SIC) is performed to detect the superimposed signal [3]. The use of multi-user diversity and cooperation diversity can also improve system performance, but usually in order to make effective use of system resources, only one user is allowed to transmit information at any given time in multi-users system. Therefore, the best user selection and (or) relay selection play a very significant role to optimize the performance of system and improve reliability, network coverage, and achievable rate of mobile networks [4]. However, the Internet of everything brings convenience to people as well as the pressure of data transmission. In order to alleviate the pressure, cooperative NOMA is applied to cellular systems to improve cell-edge users with poor channel conditions [5], [6], [7]. In order to improve the system performance, the outage sum rate and energy efficiency of the cooperative NOMA network is investigated in multi-cell network in [8], where multiple cooperative base stations send the superposed message to the corresponding users according to the NOMA principle and the near users act as relays to forward decoded messages to the far users. In [9], considering a cooperative NOMA network with single relay, the user with better channel conditions has prior information about the others’ messages, thus, the strong user decodes and transmits messages of the weak user. In [10], the authors investigate the ergodic capacity and outage probability of a multiple relays cooperative NOMA network, where the best relay is selected according to the channel state information of the first hops.

Cognitive radio (CR) is another promising technique to improve spectrum efficiency [11], [12], [13], [14]. The licensed spectrum allocated to the primary users (PUs) is prohibited for the secondary users (SUs) to access under the traditional static spectrum utilizing way. Therefore, the shortage of spectrum resources becomes increasingly prominent [15]. CR allows the SU to use the licensed spectrum to realize spectrum sharing so that spectrum utilization can be improved. Recently, cognitive radio NOMA (CR-NOMA) which allows more SUs or more PUs to transmit concurrently and meets the requirements of high spectrum efficiency, low power consumption, wide coverage, has received great attention from industry and academia [16], [17], [18], [19], [20]. An underlay CR-NOMA network was investigated in [16], where SUs were maintained a non-cooperative service through superimposed NOMA messages. In [17], the authors considered a cooperative underlay CR-NOMA network with single relay, which allowed SU transmit simultaneously under primary interference power constraints. In the study of [18], a cooperative underlay CR-NOMA network with multiple relays was proposed, the authors selected the best relay to maximize SINR of the second hop for the destination user with weak channel condition. The authors considered an overlay CR-NOMA network with single relay in [19], proving CR-NOMA can improve the secrecy performance by deriving outage probability and secrecy throughput. In [20], an overlay multi-user CR-NOMA network was analyzed, in which SUs acted as relays to help the transmission of PU, in order to obtain resources to send its own signal as a reward.

It is noted that perfect SIC is considered in the aforementioned works. However, there are several challenges to be tackled to make cognitive NOMA widely adoptable. The high self interference may result in unsuccessful SIC at the NOMA receivers in practical application so that the superimposed signals cannot be separated perfectly and the inter-user interference on the cognitive NOMA networks cannot be combated completely [21], [22]. With this regard, there have been some works on the investigation of the system performance undertaking imperfect SIC. The authors in [23] compared imperfect SIC with perfect condition in CR-NOMA network and proved that imperfect SIC had higher bit error rate (BER). In the study of [24], in the scenario of imperfect SIC, it was shown the outage probability and throughput of CR-NOMA network are superior to CR-OMA network. In addition, imperfect CSI caused by channel estimation errors is inevitable in practice communication system. In order to perform more exact deployment for practical system, imperfect CSI should be taken into account for CR-NOMA system [25]. In [26], the authors derived exact closed-form expressions for the outage probability of two NOMA secondary destination users in the case of imperfect CSI and showed the superiority of the CR-NOMA compared with OMA. In [27], NOMA was used in D2D communication cellular networks to support super connectivity and reduce the latency, where only imperfect CSI was considered. In [28], the authors studied the outage probability and the spectral efficiency of PU and SU in the single SU CR-NOMA networks under imperfect SIC and CSI.

To the best of our knowledge, NOMA-based multi-user cooperative overlay CR network with imperfect SIC and CSI has not been analyzed yet. However, imperfect SIC and CSI effects limit the performance of NOMA involved cognitive systems in practice. Therefore, in this paper, a cooperative overlay CR-NOMA system in the presence of imperfect SIC and CSI is considered to improve the performance of cell-edge users. In this network, cell-center cognitive SUs act as relays to assist transmitting information to cell-edge users with poor channel conditions. The outage probability expression of primary network and secondary network are deduced by co-existing of imperfect SIC and imperfect CSI over Rayleigh fading channels. Furthermore, we provide closed-form expressions for the capacity of system to research influence of residual interference level of imperfect SIC and error level of imperfect CSI on system performance. Finally, to gain a more in-depth insight, the numerical results validated by simulations show that: (1) The outage probability decreases as residual interference level of imperfect SIC decreasing, in other words, the better outage performance will be achieved with the more perfect SIC. (2) The capacity of system decreases as error level of imperfect CSI increasing, which means accurate channel estimation can improve the system performance effectively.

The rest of the article is organized as follows: Section 2 introduces the system model of the multi-user cooperative overlay NOMA system. In Section 3, the performance analysis is proposed, which including outage probability and capacity of primary network and secondary network. Section 4 details numerical simulations. Finally, Section 5 provides concluding remarks.

Section snippets

The system model

As shown in Fig. 1, we consider a wireless network based on CR-NOMA edge system, which is consisted of primary network and secondary network with overlay mode. In the primary network, the primary transmitter u send the primary signal xu towards primary cell-edge user v with transmission power Pu. In the secondary network, cell-center cognitive SUs sn 1nN act as relays to forward the primary signal xu to primary cell-edge user v in a half-duplex decode-and forward (DF) mode with transmission

Outage analysis

Outage probability is the probability that the end-to-end SINR in the network is less than the outage threshold, which is one of the most important performance indicators of wireless communication network.

Numerical results

In this section, we present the simulation results to validate the performance analysis. The configuration of simulation is given as follows: σuv2=σsbv2=σusb2=1 dB, α1=0.8,α2=0.2, N0=1 and γP=γS=10.

Fig. 2 shows the outage performance of PU versus γ for several numbers of cell-center cognitive SUs N=1,3,6 where the simulated and theoretical values are coincident which proves the accuracy of derivation. Obviously, we observe that the outage probability of PU decreases as γ increases, the reason

Conclusion

In this paper, we consider a wireless communication network based on cooperative overlay CR-NOMA edge system. Cell-center cognitive SUs act as relays to assist the primary transmitter send the signal to cell-edge users to improve the communication quality of edge users. At the same time, the influence of imperfect SIC and CSI in actual applications are considered. To obtain further insights, the closed-form expressions of the outage probability and the capacity of PU and SU are derived over.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

This work was supported by the National Natural Science Foundation of China under grant 61901201, 62001359, 61762058; the Science and Technology Department of Gansu Province, China Project under grant 20JR5RA387, 20JR5RA397; the Youth Science Foundation of Lanzhou Jiaotong University, China under grant 2017003; the Innovation Fund for Project of Gansu Provincial Higher Education, China under grant 2021B-107; the Foundation of A Hundred Youth Talents Training Program of Lanzhou Jiaotong

Haiyan Huang, born in 1988. She received the Ph.D. degree s from the Xidian University, Xi’an, China, in 2016. Since 2016, she has been with Lanzhou Jiaotong University, China, where she is currently an associate professor. Her main research interests include 5G/6G networks, wireless communications theory, non-orthogonal multiple access (NOMA), cooperative communications, intelligent reflecting surface (IRS), and simultaneous wireless Information and power transfer (SWIPT).

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    Haiyan Huang, born in 1988. She received the Ph.D. degree s from the Xidian University, Xi’an, China, in 2016. Since 2016, she has been with Lanzhou Jiaotong University, China, where she is currently an associate professor. Her main research interests include 5G/6G networks, wireless communications theory, non-orthogonal multiple access (NOMA), cooperative communications, intelligent reflecting surface (IRS), and simultaneous wireless Information and power transfer (SWIPT).

    Yujie Shi, born in 1998, She received the Bachelor’s degree in 2019. She is studying master’s degree in Lanzhou Jiaotong University. Her main research interests include cooperative transmission, cognitive radio, non-orthogonal multiple access (NOMA).

    Linlin Liang, born in 1986. He received the M.S. and Ph.D. degrees from the Xidian University, Xi’an, China, in 2014, and 2018, respectively. Since 2019, he has been with Xidian University, China, where he is currently an Lecturer. His current research interests include Covert Communications, Application & Security of IoT.

    Jianlin He, born in 1996, He received the Bachelor’s degree in 2019. He is currently pursuing a Master’s Degree at Lanzhou Jiaotong University. His research interests include cognitive radio, reconfigurable intelligent surface, and wireless communication.

    Xuejun Zhang, born in 1977. He received the Ph.D. degree s from the Xi’an Jiaotong University, in 2016. He is a professor in Lanzhou Jiaotong University and senior member of CCF and ACM currently. His main research interests include user privacy measurement and protection in location services, data privacy, machine learning and cloud computing.

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