# Outage Probability Analysis in Relaying Cooperative Systems with NOMA Considering Power Splitting
## 1 Introduction  
In order to meet the demand for high energy consumption in future wireless communications, energy harvesting (EH) from radio frequency (RF) has increasingly become a promising technology, in which energy received from RF signals is converted into electricity to power wireless devices. Thanks to this technology, both energy and information can be simultaneously transmitted to wireless users located near the source node [1; 2; 3; 4; 5; 6]. In fact, EH and information transmission (IT) cannot be carried out at the same time. Thus, the received signal has to be divided for EH and IT by either two foundational receiver architectures, i.e., or time switching (TS) and power splitting (PS) [7; 8]. In particular, the literature in [9] developed two optimal TS policies, namely Optimal Time for Transmit Power at Source and Optimal Time for Transmit Power at Relay when considering a half-duplex (HD) decode-and-forward (DF) small cell cognitive relay network (CRN). Besides, an EH relaying network was evaluated in terms of the outage probability (_OP_) in [10], in which closed-form expressions for _OP_ over Nakagami-m fading channels for both amplify-and-forward (AF) and DF relaying schemes were obtained. Interestingly, it is worth notingthat PS protocol is proved to be better than TS although PS is complicated and inefficient for practical implementations. Nevertheless, there have been several investigations into the use of PS [11; 12; 13]. In [11], the authors focused on PS scheme in an AF system, in which an optimization issue was solved to minimize the impact of instantaneous channel state information (CSI), in which the PS ratio is able to adjust itself according to the instantaneous CSI. Similarly, the PS ratio can be changed based on CSI in [12]. In addition, the work in [13] addressed the trade-off between the achievable information rate and the harvested energy for a PS receiver architecture. In [14], a dynamic PS scheme was studied in which trade-offs between the maximum ergodic capacity for IT and the maximum average harvested energy for power transfer. Furthermore, the study in [15] considered PS protocol, in which the authors tried to optimize their PS ratios and the transmit power to maximize the data rate over multiple coherent time slots. In [16], different two-way relay networks was considered, in which the performance of these considered systems were examined based on PS and TS protocols. In addition, the literature in [17] focused on a symmetric wireless communication network with SWIPT in a K-user multiple-input multiple-output (MIMO) CRN, where PS ratio and power allocation were carefully investigated to guarantee the ideal system performance. In recent years, non-orthogonal multiple access (NOMA) has increasingly become a potential multiple access technology in order to enhance the spectral efficiency (SE) of mobile systems [18]. For instance, multiuser superposition transmission was proposed for Third Generation Partnership Project LTE-Advanced (3 GPP-LTE-A) networks as a downlink version of NOMA. Besides that, NOMA has caught much research interest as a key technique in future 5G mobile networks. Unlike previous wireless networks which depend on the time, frequency, and code domain, NOMA takes advantage of the power domain for multiple access. For example, the primary problem with frequency-division multiple access deployed by 3 GPP-LTE is the low SE if some bandwidth resources like subcarrier channels are allocated to users with poor channel conditions [19]. Meanwhile, NOMA technique allows access to all the subcarrier channels, so users with strong channel conditions are able to access bandwidth resources allocated to users with poor channel conditions. This nature results in better SE [20]. In addition, NOMA balances the user fairness and the system throughput while conventional orthogonal multiple access (OMA) only serves users with strong channel conditions [21].