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Emerging cooperative MIMO-NOMA networks combining TAS and SWIPT protocols assisted by an AF-VG relaying protocol with instantaneous amplifying factor maximization

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Abstract

In this study, we examine an emerging Multiple-Input-Multiple-Output (MIMO) Non-Orthogonal Multiple Access (NOMA) network which combines Transmit Antenna Selection (TAS) and Simultaneous Wireless Information and Power Transfer (SWIPT) protocols. A pre-coding channels matrix was designed to aid the TAS protocol. We deployed a strong user as a relay to assist a weak user. Three cooperative MIMO-NOMA network scenarios were investigated. In the first scenario, we assumed the relay was deployed with (i) Decode-and-Forward (DF), (ii) Amplify-and-Forward (AF) with Fixed Gain (FG) and (iii) AF with a Variable Gain (VG) protocols to improve system performance over the first scenario. In scenario (iii), we investigated an amplify coefficient based on the maximum of instantaneous CSI and obtained the Outage Probability (OP) in novel closed form. The analytical results showed that although the Base Station (BS) sent information and energy at the same instant, the OP performance at the relay was sufficient due to assistance from multi-antennas and the TAS protocol. However, the OP performance at the user under cooperation with a relay using the AF-VG protocol achieved better system performance than the DF and AF-FG protocols. We proved and verified the analytical results with Monte Carlo simulations.

Introduction

Non-Orthogonal Multiple-Access (NOMA) improves spectral sharing and the capability for a greater number of connections in the same time/frequency slot [1], [2]. Therefore, NOMA in fifth-generation (5G) mobile wireless networks and beyond could serve large numbers of User Equipment (UE). NOMA features technology based on sending a superimposed signal to all UE in a network via a multiplexing channel in the same power domain and time slot, although with different Power Allocation (PA) factors [3]. A larger PA factor is allocated at a terminal UE, i.e. the end device, which has poorer Channel State Information (CSI), than at other UE. NOMA’s featured concept is the superimposition of UE messages at the BS with different PA factors. At the receiver, the application of Successive Interference Cancellation (SIC) removes multi-user interference before detecting its own signal. The UE performs SIC by treating user information which has a stronger PA factor as interference before detecting its own information [4]. The UE with the weakest PA factor, which has the strongest CSI, must decode its own information after decoding and removing other poorer UE messages with stronger PA factors. Several studies have significantly contributed to NOMA through the application of new techniques in 5G networks [5], [6], [7], [8]. Dai et al. [5] fully described the advantages of NOMA. However, NOMA has many research challenges which require careful investigation, such as extension of the network under cooperation and combination with MIMO and TAS. In [3], the authors stated that NOMA network system performance is a result of PA. The authors in [5] verified that PA in NOMA could be determined by the user equipment CSI and bit rate thresholds, total power domain and system objective. As a result, unsuitable PA factors not only result in unfair QoS for users but also cause outage transmission because SIC is unsuccessful [6]. The authors mathematically formulated, characterized and analyzed PA problems under a range of constraints and utility functions [7]. The authors in [8] proposed fairness in NOMA networks through the incorporation of instantaneous and average CSI.

Recently, cooperative technology has drawn much attention in research as an emerging solution to combat channel fading. In a cooperative NOMA network, UE with a stronger CSI is designated as a relaying device to receive and forward superimposed signals to other UE with poorer CSI. The scope/distance of a NOMA network is thereby expanded and its reliability is enhanced by improving QoS for users [9], [10], [11], [12]. In [13], the authors examined the Outage Performance (OP) of a cooperative NOMA which uses Amplify-and-Forward (AF) and Decode-and-Forward (DF) scenarios. A relay with Full-Duplex (FD) instead of Half-Duplex (HD) was used to avoid wasting time slots [14]. Although a cooperative NOMA network enhances QoS for the UE far from the BS, it increases the bandwidth costs. This issue can be solved with the application of an FD relay technique. A relay with an FD protocol can simultaneously receive and forward a signal in the same frequency band [15]. The authors investigated a cooperative NOMA network over Nakagami-m fading channels and examined DF and AF relaying protocols with FG based on instantaneous/mean CSI [16] and imperfect CSI [17].

However, a disadvantage of the FD protocol is the effect of the Loop Interference (LI) channel from its own transmitter antenna, modeled as a fading channel. LI channels are a major problem in the deployment of FD relays [18]. The authors proposed various interference cancellation techniques, such as passive cancellation, active analog cancellation, and active digital cancellation [19]. The studies [20], [21] issued two main types of relaying FD technique with DF and AF. The authors also investigated a Cognitive Radio (CR) NOMA in an FD/HD relay [22]. The solution of switching between HD and FD is based on transmission power adaptation [23]. Another full study of HD/FD relays applied in combination with the DF protocol is investigated in [24]. Based on previous research results, the principal question is which relaying protocol offers more suitable forwarding channels. This question is the motivation of our study, which examines several protocols to determine the best relaying protocol.

A number of studies have made significant contributions to cooperative NOMA. Research results have shown that system performance can be improved with the selection of an appropriate relay. Ding et al. [25] proposed a two-stage relay selection strategy which outperforms max-min relay selection. Another potential technology for future 5G networks is radio frequency Energy Harvesting (EH) [26]. However, the initial studies on wireless high-power transmission show that high-power devices are potentially dangerous to health, thus inhibiting further development of wireless EH. The work in [27] offers a deep survey of the advantages of Simultaneous Wireless Information and Power Transfer (SWIPT) over other Wireless Power Transfer (WPT) techniques. The authors surveyed several SWIPT technologies, including SWIPT enabled multi-carrier systems, full-duplex SWIPT systems, etc. As a result of the explosion in the number of networked devices, such as Internet of Things (IoT) devices, energy use in these devices has become an especially important issue. Time Splitting (TS) and Power Splitting (PS) represents a solution for simultaneous data and energy transmission, as proposed in [28]. The authors designed the “precoding” matrices for maximizing system throughput in a multi-cell MIMO-NOMA network. Although a wireless EH solution has not yet achieved practical effectiveness, the EH technique provides a viable solution for recharging mobile or low-energy IoT devices which are not powered from the power grid.

The work in the present study was inspired by the major studies [29], [30], [31], [32], [33], which applied PS and TS protocols to cooperative networks with AF relaying. In [29], the authors examined OP and system throughput. However, the system model in the study [29] served a single user. The authors in [30] extended the works in [29] by adopting a cooperative NOMA network to serve multiple users. The authors in [29] examined a SISO network. MIMO was applied to improve networking capacity. Although the system performance may potentially be enhanced by equipping multiple antennas, it would increase hardware costs. In studies [31], [32], [33], the authors deployed a TAS protocol to avoid the high hardware costs while preserving the benefits of multiple antennas. The authors in [33] proposed various TAS strategies, i.e. Optimal TAS (OAS) and Sub-optimal TAS (SAS).

In studies [34], [35], the authors investigated cooperative NOMA-SWIPT networks seeking to maximize the sum achievable user rate in the same cluster, based on PA and EH strategies. Specifically, the authors in [35] deployed maximum-ratio combining (MRC) to combine the received signals from the BS and relay. The authors in [36], [37] improved the performance of MISO-NOMA networks with SWIPT and TAS techniques. To the best of our knowledge, MRC and Selection Combining (SC) are two methods which trade off between performance and complexity. MRC maximizes linear combining but is difficult to implement since it requires multiple signals over multiple channel estimations and complex hardware. SC is simpler to deploy since MRC requires the best signal selected from multiple signals. In [38], the authors examined the cooperative MIMO-NOMA network and offered TAS/SC to optimize OP performance adopted AF-FG protocol. Notice that the works in our study also select the best antenna pairs at the BS and user U1 in the first time slot, and user U1 and user U2 in the second time slot where user U1 adopted AF-VG protocol.

The major contributions of the present study are:

  • A proposed emerging cooperative MIMO-NOMA network model. The network model deploys a combination of TAS/SC and SWIPT protocols. Hence, the study provides deep insight how MIMO and TAS improve system performance;

  • Stronger user functions as relays to cooperate with the weaker user. Various relaying technologies are examined, such as DF, AF-FG and AF-VG protocols.

  • Maximization of the instantaneous amplify coefficient based on maximization of the instantaneous CSI, which is deployed in the AF-VG scenario. To the best of our knowledge, this has never been previously proposed.

  • Three scenarios are investigated: cooperative MIMO-NOMA is considered in combination with the SWIPT, TAS and (i) DF protocols, (ii) AF-FG protocols and (iii) AF-VG protocols. The OPs are obtained in novel closed-forms. The analytic theoretical results are proved and verified by Monte Carlo simulation results.

The paper is organized as follows: Section 2 introduces system modeling. Section 3 presents an analysis of the system model. Section 4 presents and discusses the analysis and simulation results. Section 5 presents a summary of this paper.

Section snippets

System model

The present study examines a cooperative MIMO-NOMA network for emerging 5G wireless networks. Fig. 1 depicts the system model with a BS, a near user U1 as a relay, and a user U2 which is far from the BS. We denote A0, where A0>1,A1, where A1>1, and A2, where A2>1, as the number of antennas at the BS, U1 and U2, respectively. We apply the following notation:

  • .r×c or .r×c×l, representing the two dimensions (2D) of r×c or three dimensions (3D) of the r×c×l matrix.

  • max.r×c or max.r×c×l, representing

System performance analysis

This chapter describes the system performance of the network model depicted in Fig. 1 in terms of OP and throughput at U1 and U2:

  • 1.

    At U1, we analyzed the OP performance of the NOMA network with a combination of MIMO, TAS and SWIPT;

  • 2.

    At U2, we investigated the OP performance of the NOMA network with the deployment of U1 as a cooperative relay operating with the DF, AF-FG or AF-VG protocols.

Numerical results

In this chapter, we present the numerical results and discussion to prove the analytical expressions derived in the previous sections. The analytical results and Monte Carlo simulation results possess the same parameters as the fixed PA factors for users U1 and U2, where α1=0.25 and α2=0.75, respectively, and the user bit rate thresholds R1=R2=0.2bps/Hz The BS is equipped with antennas A0=4, and U1 and U2 with antennas A1=A2=2. In addition, each h1(a0,a1)H1 or each h2(a1,a2)H2 randomly

Conclusion

We examined a NOMA networking concept which deployed several emerging technologies: cooperative NOMA, MIMO, TAS, SWIPT, DF, AF-FG and AF-VG. To assist in the reduction of hardware costs, we designed a pre-coding channel matrix which can be applied to support the TAS protocol. We also investigated three different scenarios and obtained novel closed forms. To improve system performance, we proposed maximization of the instantaneous amplifying coefficient. This was based on maximization of the

Funding

The research leading to these results was supported by Czech Ministry of Education, Youth and Sports under project reg. No. SP2021/25 and also partially from the Large Infrastructures for Research, Experimental Development and Innovations project “e-Infrastructure CZ” reg. No. LM2018140.

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.

Acknowledgments

We would like to thank the editors and reviewers for their useful comments and suggestions to help improve the quality of this paper.

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