Exploiting Outage Performance of Wireless Powered NOMA

Considering a dual-hop energy-harvesting (EH) non-orthogonal multiple access (NOMA) relaying system, the main problems are novel relaying protocol based on time power switching-based relaying (TPSR) and power switching-based relaying (PSR) scheme for two kinds of gain factors regarding amplify-and-forward (AF) mode. We introduce novel system model relaying network with impacts of energy harvesting fractions and derive analytical expressions for outage probability with respect to the information transmission link. It confirmed that right selection of power allocation for NOMA, and AF mode to obtain optimal performance as compared study in two considered schemes. The thoughput can be shown its optimal value achieved by numerical method. We also explore impacts of other key parameters of system to outage performance evaluation for different channel models. Simulation results are presented to corroborate the proposed methodology

power allocation coefficients, and better channel condition together with assigned successive interference cancellation (SIC) to eliminate the other users' signals before distinguishing its own signal [8]. Alternately, a special case of the information theoretic concept, namely superposition coding, is similar with the concept of NOMA. Furthermore, user fairness is considered as key feature of NOMA. As compared to conventional water-filling power distribution, worse channel condition is assigned for NOMA with higher power compared with those in case of better channel conditions, trade-off between system throughput and user fairness are improved. As a result, to an increase in spectral efficiency for all users in NOMA where shares the same time slot, frequency and spreading code.
The existing advanced schemes such as multiple-input multiple-output (MIMO) can be included in NOMA as an attractive property in research about NOMA [9]. In different system model, NOMA is studied in cooperative relaying networks (CRS) [10], heterogeneous system [11], and device-to-device (D2D) networks with full-duplex scheme [12]. In other trend, the cooperative NOMA (C-NOMA) together with CRS as investigation in [13] in which spatially multiplexed scheme to serve a single user, while the other papers [9], [11] concentrated on scheme related to a group of users. Outage performance of several relaying networks can be explored in [14][15][16][17].
Motivated by these discussions, in this paper we propose two schemes based on kind gain factor in AF mode and analyse a relaying system with WPT under collecting energy from an external energy resource.
The remainder of this paper is organized as follows: Section 2 presents the system model and energy harvesting protocols rea investigated. In Section 3, we derive the analytical expressions of outage probability and throughput in delay-limited transmission. Section 4 examines the simulation results. Finally, Section 5 completes with conclusion remarks for the paper and reviews the important results. between the source and the destination is unreliable or unavailable, so the transmission can only happen successfully with the aid of the wireless powered relay. In particular, the relay node deployed in this paper is characterized as energy-constrained. Furthermore, each node is furnished with a single antenna, and half-duplex mode using amplify-and-forward (AF) strategy is deployed in the relay. The relay acquires two independent data symbols during two time epochs, 1 x from S directly and 2 x through the relay, whereas the EH-NOMA delivers a single data symbol.

System Model
This paper uses power based relaying energy harvesting (PSR) protocol in Scheme II [2], [3]. We call  is power percentage to energy transfer in such PSR scheme. In particular, during the first phase S broadcasts a superposition-coded signal 1

. System model of EH-NOMA
During the first phase, the received signals at the relay is given by, stands for complex additive white Gaussian noise (AWGN) at relay. In this study, we consider two modes: (i) Varying gain based AF and (ii) Fixed gain based AF. Regrading case in SCHEME I, using variable gain in Amplify-and-Forward (AF) mode, the relay will be amplified with factor G as given by Then, D y can be expressed as: It is noted that the received signal at destination D y can be given by: Then, D1 perform SIC to detect 1 x signal with the received SNR given by: Similarly, after SIC operation at D2, the receiving SNR for detecting 2 x given by: In SCHEME II, we obtain the fixed gain as: In this SCHEME, we apply power splitting based energy harvesting as: The received signal at D1 can be computed as: Finallly, we have SNR at D1 before SIC and after SIC respectively as:

Numerical Results
Unless otherwise stated, we set the source transmission rate,  Figure 2 plot the outage probability for cooperative NOMA with different power allocation factors for AF relaying, where energy harvesting fractions contribute to change outage performance shown as different curves. Observing the Figure 2, one can conclude that compared among three cases of EH-NOMA, the proposed scheme with higher time aware energy harvesting allocation for energy harvesting can realize worse outage performance r. Furthermore, Figure 2 manifest that EH-NOMA can remarkably enhance the outage performance at high transmit power at source S P . More importantly, the analytical curves match very well with Monte-Carlo results.

 
In Scheme I, Figure 3 plots the outage probability for cooperative NOMA in case of fixed time aware energy harvesting factor is set and varying power aware energy harvesting factor in TPSR protocol depoying in Scheme I. As can be seen clearly that the proposed scheme with higher power aware energy harvesting allocation for energy harvesting can realize worse outage performance. Similarly, Figure 3 confirmed that that EH-NOMA can provide the best outage performance as reasonable selection of S P .
As can be seen in Figure 4, it shows the optimal throughput versus changing variance noise term in EH-NOMA in case of changing transmit power at source. It is noted that throughput can be shown as can realize better throughput performance due to more energy for signal processing. It is noted that noise term contributes to lower throughput, especially at high noise -20 to -10dB In Figure 5, we compare the outage performance for EH-NOMA in two schemes by using different gain factor in AF mode and different percentage of harvested energy. It can be observed from Figure 5 that Scheme I can achieve better outage performance than that in Scheme II. Furthermore, we can also see that the outage performance of NOMA is enhanced significantly with high transmit power at source. In addition, Figure 5 also demonstrates that energy harvesting remain operation of relay where has signal processing and outage performance at acceptable as reasonable selection of related parameters as in simulation result. Figure 6. Impact of bit rates on the throughput It is can be found optimal throughput at specific bit rate. As dervided expressions of outage behavior, bit rate is factor impacting outage performance as in Figure 6. In particular, the throughput in Scheme I is better than that in Scheme II at 3 cases of SNR value of the base

Conclusion
In this paper, we have proposed two kinds of AF modes as presented in two schemes. With regard to EH-NOMA relaying schemes, we have derived asymptotic analytical expressions for outage probability. These results provide guidline to design practical NOMA system. The proposed cooperative NOMA schemes not only achieve reasonable performance but also yield better outage performance at specific scenarios. In addition, compared to different power allocation fractions, the proposed NOMA schemes can further improve the outage probability at appropriate energy harvesting policy.