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To meet with both the energy and spectral efficiency, cooperative non-orthogonal multiple access (NOMA) (termed as CNOMA) and simultaneous wireless information and power transfer (SWIPT) are integrated in this paper. Two different protocols such as CNOMA-SWIPT-PS and CNOMA-SWIPT-TS are proposed, by employing power-splitting (PS) and time-switching (TS) energy harvesting techniques, respectively. In the proposed protocols, a base station directly communicates with a NOMA near user, whereas the communication with a NOMA far user is performed with the assistance of an energy constrained relay node. The relay node harvests energy from the transmitted signal by the base station, employing PS or TS energy harvesting technique. We investigate ergodic sum capacity and outage probability of the proposed protocols along with analytical derivations over Nakagami$-m$ fading channels. Finally, the effectiveness of the proposed schemes over existing strategy and conventional multiple access is demonstrated through analysis and simulation.
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Received: 1 July 2018 Revised: 23 September 2018 Accepted: 20 November 2018
DOI: 10.1002/ett.3571
Cooperative non-orthogonal multiple access with SWIPT
over Nakagami-mfading channels
Md. Fazlul Kader1Mohammed Belal Uddin2Anik Islam2Soo Young Shin2
1Department of Electrical and Electronic
Engineering, University of Chittagong,
Chittagong, Bangladesh
2Department of IT Convergence
Engineering, Kumoh National Institute of
Technology, Gumi, South Korea
Md. Fazlul Kader, Department of
Electrical and Electronic Engineering,
University of Chittagong,
Chittagong 4331, Bangladesh.
Funding information
University of Chittagong, Bangladesh;
MSIT (Ministry of Science, ICT), Korea,
Grant/Award Number:
To meet with both the energy and spectral efficiency, cooperative
non-orthogonal multiple access (NOMA) (termed as CNOMA) and simul-
taneous wireless information and power transfer (SWIPT) are integrated
in this paper. Two different protocols such as CNOMA-SWIPT-PS and
CNOMA-SWIPT-TS are proposed, by employing power-splitting (PS) and
time-switching (TS) energy harvesting (EH) techniques, respectively. In the pro-
posed protocols, a base station (BS) directly communicates with a NOMA near
user, whereas the communication with a NOMA far user is performed with the
assistance of an energy-constrained relay node. The relay node harvests energy
from the transmitted signal by the BS, employing PS or TS EH technique. We
investigate ergodic sum capacity and outage probability of the proposed pro-
tocols along with analytical derivations over Nakagami-mfading channels.
Finally, the effectiveness of the proposed schemes over existing strategy and
conventional multiple access is demonstrated through analysis and simulation.
To cope with the anticipated challenges of the future wireless networks, fifth-generation wireless systems, abbreviated
5G, are expected to be emerged by 2020.1,2Among the various challenging requirements of 5G, spectral efficiency and
energy efficiency are the two key requirements. To deal with both of these requirements, non-orthogonal multiple access
(NOMA) and simultaneous wireless information and power transfer (SWIPT) can be integrated, which is the main focus
of this paper.
NOMA is an exciting and emerging multiple access technique, which is viewed as one of the key players in 5G, because
of its manifold spectral gains.3-5In a NOMA-based system, multiple signals are superimposed in power domain at the
transmitter side, by the same code at the same frequency, which is unlike the conventional multiple access (orthogonal
multiple access [OMA]) schemes (eg, frequency/time/code division multiple access). At the receiver side, signal decoding
technique such as successive interference cancelation (SIC) can be carried out to decode each signal.6NOMA can be
applied in a wide range of application scenarios. Among them, cooperative NOMA is one of the most active areas of
research. Cooperative NOMA can be broadly categorized into two different types.7In the first type, NOMA-strong users
act as relays to the NOMA-weak users.8On the contrary, dedicated relays are used to assist the NOMA users,9-13 in the
second type. In general, users residing close to the base station (BS) are called strong or near users, whereas users residing
close to the cell edge are called weak or far users.
Because of the vast amount of coexisting wireless technologies, radio frequency (RF) signals are abundant in nature.
These RF signals can be exploited to harvest energy for powering up the energy-constrained nodes.14,15 To prolong the
Trans Emerging Tel Tech. 2019;30:e3571. © 2019 John Wiley & Sons, Ltd. 1of16
... Corollary 2. The AOP of SU 1 as I th → ∞ can be obtained by setting lim (11) and is given by (13) P FD,asy ...
... Instead, P S and P R are directly related to P max , as can be observed in (2). Accordingly, the AOP (as I th → ∞ ) given by (13) is independent of I th . In this case, the OP becomes lower, as P max is increased since it allows both the SBS and the relay to choose larger values for P S and P R . ...
... log(I th ) . Substituting (13) in this equation and simplifying further, the diversity order can be observed to be zero. Furthermore, setting either I th → 0 or P p → ∞ in (12) makes the AOP → 1. ...
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This article considers non‐orthogonal multiple access (NOMA) enabled full‐duplex (FD) underlay cognitive relay networks (ie, NOMA‐FDCRNs) with partial relay selection scheme. The secondary network, where NOMA is used, consists of a secondary base station (SBS) sending messages to two secondary users (SUs), that is, a near SU (SU1) and a far SU (SU2), by utilizing a dedicated relay selected from a set of FD decode‐and‐forward nodes. We obtain analytical expressions for the outage probabilities (OPs) of the SUs and then deduce the asymptotic OP expressions as well. Further, expressions are obtained for the optimal power allocation (OPA) coefficients at the SBS and at the relay that separately maximizes the throughput of the secondary network in NOMA‐FDCRN. Furthermore, the jointly optimal power allocation (JOPA) coefficients that maximize the throughput are also determined. The analyses consider (i) imperfect successive interference cancelation conditions, (ii) the tolerable interference limit of the primary receiver, (iii) the secondary nodes' maximum transmit power values, (iv) interference generated by the primary transmitter, and (v) the residual self‐interference (RSI) at the FD relay. It is shown that the proposed OPA coefficients at the SBS and the JOPA can mitigate the impact of RSI and significantly improve the OP and throughput performance. The numerical results show that the proposed JOPA approach provides 32% and 106% improvement of throughput compared to random power allocation (RPA) and equal power allocation (EPA) strategies, respectively. The OP of SU1 and SU2 reduce by 68% and 73%, respectively, under the proposed JOPA compared to RPA, while compared to EPA, the OP of SU1 and SU2 reduce by 96% and 97%, respectively.
... In Fig. 2 the two phases are represented, where power-switching and energy-harvesting methods, exploiting the whole time slot for both energy and data transmission are considered [42]. This makes the proposed scenario distinct from the cooperative NOMA algorithms previously proposed in the literature, which are based on time switching for the energy harvesting [45][46][47]. Although time switching has convenient hardware implementation, it fully dedicates one time slot for energy transmission which is a disadvantage from the throughput efficiency point of view [48]. ...
... 1 signal is the strongest, i.e., |ℎ 10 | 2 2 ′ + |ℎ 20 | 2 2 < |ℎ 10 | 2 1 : Similar to the previous case, (41) is formulated as (46). Then, for → ∞ and 2 , 1 = 1 , the diversity order of 1 is obtained as follow: ...
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In 5G wireless networks, cooperative non-orthogonal multiple access (NOMA) and wireless power transfer (WPT) are efficient ways to improve the spectral efficiency (SE) and energy efficiency (EE). In this paper, a new cooperative NOMA scheme with WPT is proposed, where EE optimization with a constrained maximum transmission power and minimum required SE is considered for the user grouping and transmission power allocation of users. We obtain a sub-optimal solution by decoupling the original problem in two sub-problems: an iterative algorithm is considered for the user grouping, while, in addition, we utilize the Bat Algorithm (BA) for solving the power allocation problem, where BA was proved to be able to achieve a higher accuracy and efficiency with respect to other meta-heuristic algorithms. Furthermore, to validate the performance of the proposed system, analytical expressions for the energy outage probability and outage probability of users are derived, confirming the effectiveness of the simulation results. It is demonstrated that the proposed cooperative NOMA with WPT offers a considerable improvement in terms of SE and EE of the network compared to other methods. Finally, the effectiveness of BA in solving the EE optimization problem is demonstrated through a high convergence speed by comparing it with other methods.
... The performance investigation of NOMA system has recently been the subject of a lot of research work. [11][12][13][14][15][16][17][18][19][20] In Reference 11, effect of user pairing on the downlink NOMA network performance is discussed and compared with that of OMA system. The strategy of pairing and admission with NOMA users is discussed in References 12,13, which significantly enhances the performance of MIMO-NOMA system. ...
... Therefore, to obtain overall optimum performance, the rate requirement of the f th UE needs to be ensured to get the optimum performance. Further, the rate requirements of the f th and the nth UEs are always satisfied at the optimum power allocation and hence O f = O n = 0 and O o f = O o n = 0, which can also be verified by substituting (32) and (34) into (7), (11) and (15), (18) respectively. ▪ ...
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In this study, user selection‐based downlink power domain‐non‐orthogonal multiple access (NOMA) system is investigated over generalized fading channels, that is, η‐μ and κ‐μ fading channels. In particular, this system considers multiple users and out of which two users are selected to serve with NOMA principle in perfect channel state information scenario. The outage probability expressions of the system are derived with respect to the fixed target rate and the orthogonal multiple access (OMA) rate. In addition, the average sum‐rate and rate of paired users of the system are derived and compared with that of OMA. Further, the optimum power allocation coefficients are derived for the system to ensure fixed and dynamic rate requirements. The impact of imperfect successive interference cancelation on the system performance is discussed. The paper also investigates the effect of various parameters on the performance of the system. The extensive simulations show the accuracy of the derived expressions. This paper evaluates the performance of multi‐user downlink NOMA system with considering of the effect of possible pairing of users over the generalized fading channels. The mathematical expressions for outage probability and sum‐rate are derived over η‐μ and κ‐μ fading channels. Further, the optimum power allocation coefficients are derived to ensure fixed and dynamic rate requirements. The extensive simulations show the accuracy of the derived expressions.
... Integrated performance of NOMA with other techniques such as cooperative communication, simultaneous wireless information and power transfer (SWIPT) and MIMO have been proposed recently. 41,42 Sum rate has been used as a performance metric and also as an objective to formulate various optimization problems in MIMO-NOMA systems. 43,44 A zero-forcing (ZF) receiver-based user pairing and beamforming were studied in downlink multiuser NOMA networks, assuming perfect channel state information (CSI) at the transmitter. ...
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In this article, we investigate the performances of conventional cooperative sensing (CCS)‐ and superior selective reporting (SSR)‐based cooperative sensing schemes in a multiple‐input, multiple‐output (MIMO)‐aided non‐orthogonal multiple access (NOMA)‐driven cognitive radio (CR) networks. First, we derive expressions for the probabilities of false‐alarm and signal detection for an optimal detector in the considered MIMO‐NOMA CR setup. Next, we study a constrained optimization problem that maximizes the sum rates of CCS and SSR schemes, with a constraint on the power allocation at SUs. This problem is shown to be a constrained convex optimization problem. Later, we analytically characterize and compare the sum rate performances of CCS and SSR schemes in MIMO‐NOMA CR and MIMO‐based orthogonal multiple access (MIMO‐OMA) CR networks. Extensive computer simulation results are presented which corroborate our analysis, which also quantifies the gain in performance of the proposed MIMO‐NOMA CR network employing SSR scheme when compared to conventional MIMO‐OMA CR systems. Furthermore, we present an experimental study based on RTL‐SDR setup, which supports our theory and also paves way towards a practical implementation of the proposed framework. image
... where, Q * is the amount of energy consumed by the receiver circuit and, R * 1 is the target rate of CEU 1 . Substituting for Q 1 and R 1 , Equation (20) can be rewritten as, ...
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We consider a reconfigurable intelligent surface (RIS) aided coordinated multipoint (CoMP) transmission to enhance the achievable rates of the cell edge users (CEUs) in a downlink non-orthogonal multiple access (NOMA) network enabled with simultaneous wireless information and power transfer (SWIPT). The RIS-enabled SWIPT-CoMP-NOMA network improves the spectral efficiency and counterbalances the rate trade-off introduced by SWIPT. The distribution model of the effective channel gain plays a vital role in ergodic capacity and outage probability analysis. In this article, we approximate the effective channel gains of the CEUs using the inverse Gaussian (IG) function and validate its accuracy using the well-known Kolmogorov–Smirnov distance (KSD) test. Using the proposed channel gain model, we derive closed-form expressions for the ergodic rates and outage probabilities of the CEUs. Using Monte-Carlo simulations, we confirm the accuracy of the derived analytical expressions and show that the proposed IG function is more accurate than the Gamma function in estimating the outage probability. We also found that the proposed system always offers a higher ergodic rate than the SWIPT-CoMP-NOMA system.
... For example, in [12][13][14], different types of NOMA-SWIPT cooperative relay network models are proposed, and the resource allocation optimization methods is studied with the goal of outage performance of the networks. On this basis, other studies also comprehensively consider the impact of the throughput [15][16][17][18][19], diversity gain [20][21][22], and user transmission distance [23,24], verifying that the cooperative relay network of NOMA-SWIPT fusion has a good prospect in energy and resource optimization. Due to the adoption of the relay cooperation scheme, under the multi-relay model, the reasonable formulation of the relay selection methods can effectively improve the network performance. ...
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In order to improve the energy efficiency (EE) performance of cooperative networks, this study combines non-orthogonal multiple access (NOMA) with simultaneous wireless information and power transfer (SWIPT) technologies to construct a cooperative relay network composed of one base station (BS), multiple near users, and one far user. Based on the network characteristics, a time-division resource allocation rule is proposed, and EE formulas regarding direct-link mode and cooperative mode are derived. Considering user selection and decoding performance, to obtain the optimal EE, this study utilizes a DinkelBach iterative algorithm based on the golden section (GS-DinkelBach) to solve the EE optimization problem, which is affected by power transmitted from the BS, achievable rates under three communication links, and quality of service (QoS) constraints of users. The simulation results show that the GS-DinkelBach algorithm can obtain precise EE gains with low computational complexity. Compared with the traditional NOMA–SWIPT direct-link network model and the relay network model, the optimal EE of the established network model could be increased by 0.54 dB and 1.66 dB, respectively.
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The non-orthogonal multiple access (NOMA) technique is a prospective solution to support the massive connectivity of an ever-increasing number of wirelessly connected devices and address the spectrum scarcity issue. In this article, the outage probability and ergodic capacity of a two-hop cooperative NOMA network with an energy-limited relaying node are quantified over a generalized 𝛼 − 𝜅 − 𝜇 statistical model. The relay acts in an amplify-and-forward mode and performs energy harvesting (EH) using the time-switching and power splitting relaying protocols. Moreover, the impact of hardware impairments (HIs) is incorporated into the performance evaluation. The obtained results prove the importance of HIs and allow one to evaluate the outage probability and ergodic capacity over various statistical models and system parameters. Finally, the results suggest that the optimal performance at a specific scenario depends on the combination of multiple factors, such as channel conditions, HI level, transmit power, and EH protocol.
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This paper investigates a relay assisted simultaneous wireless information and power transfer (SWIPT) for downlink in cellular systems. Cooperative non-orthogonal multiple access (C-NOMA) is employed along with power splitting protocol to enable both energy harvesting (EH) and information processing (IP). A downlink model consists of a base station (BS) and two users is considered, in which the near user (NU) is selected as a relay to forward the received signal from the BS to the far user (FU). Maximum ratio combining is then employed at the FU to combine both the signals received from the BS and NU. Closed form expressions of outage propability, throughput, ergodic rate and energy efficiency (EE) are firstly derived for the SWIPT based C-NOMA considering both scenarios of with and without direct link between the BS and FU. The impacts of EH time, EH efficiency, power-splitting ratio, source data rate and distance between different nodes on the performance are then investigated. The simulation results show that the C-NOMA with direct link achieves an outperformed performance over C-NOMA without direct link. Moreover, the performance of C-NOMA with direct link is also higher than that for OMA. Specifically, (1) the outage probability for C-NOMA in both direct and relaying link cases is always lower than that for OMA. (2) the outage probability, throughput and ergodic rate vary according to $$\beta$$ β , (3) the EE of both users can obtain in SNR range of from $$-10$$ - 10 to 5 dB and it decreases linearly as SNR increases. Numerical results are provided to verify the findings.
Non-orthogonal multiple access (NOMA) and full duplex (FD) relaying are two promising techniques to enhance the spectral efficiency of the fifth generation (5G) wireless networks. Simultaneous wireless information and power transfer (SWIPT) technique has recently emerged as an effective solution to improve the energy efficiency of wireless networks. This paper investigates outage and throughput performance of SWIPT enabled FD cooperative NOMA network under power splitting relaying (PSR) protocol, i.e., SWIPT enabled FD-PSR-NOMA network. We assume a single cell network consisting of a base station (BS) and two pre-paired users, i.e., a near user and a far user, where the near user acts as FD relay to assist the BS for information delivery to the far user, and power splitting (PS) technique is used at the relay for energy harvesting (EH). We derive analytical expressions for the outage probabilities experienced by the downlink users, system outage probability and delay limited system throughput under imperfect successive interference cancellation (i-SIC). We also derive analytical expressions for outage probabilities and throughput of SWIPT enabled half duplex (HD) cooperative NOMA network under PSR, i.e., SWIPT enabled HD-PSR-NOMA network. The optimal power allocation (OPA) factor at the BS and optimal power splitting (OPS) factor at the relay that independently minimizes the asymptotic system outage probabilities of SWIPT enabled FD/HD-PSR-NOMA networks are determined. Further, we explore the jointly optimal power allocation and power splitting factor that minimize the system outage probability. Furthermore, we derive analytical expression for the OPS factor that maximizes the asymptotic system throughput. We provide extensive numerical and simulation results to establish that system outage and throughput improve significantly under the proposed schemes.
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Matlab code of the paper: N. T. Do, D. B. da Costa, T. Q. Duong, and B. An, “A BNBF User Selection Scheme for NOMA-Based Cooperative Relaying Systems With SWIPT,” IEEE Communications Letters, vol. 21, no. 3, pp. 664–667, Mar. 2017.
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In this paper, we study the performance of a cell-edge user in a two-user multiple-input single-output non-orthogonal multiple access (MISO-NOMA) system. Since the outage performance and fairness data rate of cell-edge users are essential issues in NOMA systems, we focus on how to resolve such problems. To this end, we propose three cooperative downlink transmission schemes utilizing hybrid simultaneous wireless information and power transfer (SWIPT) and transmit antenna selection (TAS) protocols. Particularly, in each scheme, a cell-center user acts as a relay to assist the cell-edge user and its relaying operation is powered by a hybrid time-switching/power-splitting (TS/PS) SWIPT protocol. Additionally, each scheme employs a different TAS criterion to exploit the spatial diversity gain of a multiple antennas base station. In particular, we derive tight closed-form approximate expressions for the outage probabilities (OPs) and the corresponding asymptotic OPs to provide significant insights into the impact of our proposed schemes on the system performance. Our numerical results demonstrate the achievable performance improvements of the proposed schemes in comparison to that of the orthogonal multiple access (OMA) and non-cooperative NOMA systems. In addition, the proposed schemes achieve various level of diversity gains accompanying with different complexity requirements.
Non-orthogonal multiple access (NOMA) is recognized as a promising radio access technology for next generation wireless networks. In this paper, a novel full-duplex (FD) cooperative relaying scheme for a NOMA based system (termed FD-NOMA-VP) is proposed, where K similar gain near users and relay-aided N similar gain far users are distributed into different NOMA cluster. In each cluster, one near user is virtually paired (VP) with multiple far users over non-overlapping frequency bands. In FD-NOMA-VP, the near user directly communicates with the base station, whereas far users communicate via a dedicated FD relay, in each cluster. The performance of FD-NOMA-VP is studied comprehensively in terms of the ergodic sum capacity, outage probability, and outage sum capacity along with analytical derivations under both perfect and imperfect interference cancellation (IC) scenarios. The results demonstrate that FD-NOMA-VP shows significant performance gains compared to the conventional multiple access scheme under perfect IC, while the residual interference has a substantial impact on the performance gain under imperfect IC. In addition, a strong agreement between analytical and simulation results affirms the correctness of our analysis.
In this paper, a full-duplex (FD) non-orthogonal multiple access (NOMA) scheme for a cooperative relay sharing network (termed as FD-NOMA-RS) is presented, in which two source-destination pairs share a dedicated FD relay FD-R. Following the principle of uplink NOMA, both sources transmit their symbols to FD-R, forming a NOMA pair, by depending on their channel conditions with respect to FD-R. The FD-R then decodes these symbols and simultaneously transmits a superimposed composite signal to the destinations with a processing delay $\tau$ according to the principle of downlink NOMA. The ergodic sum capacity, outage probability and outage sum capacity of FD-NOMA-RS are investigated comprehensively along with analytical derivations, considering both perfect and imperfect interference cancellation. A simulation is conducted to corroborate the correctness of the analysis presented here. Moreover, the effectiveness of FD-NOMA-RS is demonstrated through analysis and simulation and then compared with that of its counterpart, the half-duplex system.
Nonorthogonal multiple access (NOMA) has been regarded as an appropriate multiple-access technique for fifth generation due to its radio-frequency spectrum effective usage and significant capacity gains. In this paper, a cooperative network is considered, where 2 kinds of signal transmission links exist at the same time, ie, the direct link and the relaying link. The system performance is evaluated in a NOMA-based downlink multiple-input–multiple-output relaying network in terms of 2 measures, ie, the outage probability and the ergodic rate. Due to the uncertainty of the signal-to-interference-plus-noise ratio (SINR) at the users reached by the direct links, at the base station, the best antenna that maximizes the instantaneous system SINR at the relay is selected to transmit the signals. Furthermore, for the receiving antennas at the multiple users, maximal ratio combining is used to exploit efficiently the diversity offered by the multiple antennas. Closed-form expressions for the exact outage probability and the ergodic rate at high SINR are derived. Moreover, the diversity order is obtained by analyzing the asymptotic outage probability. Finally, the extensive simulations are performed to corroborate that NOMA, in combination with a successive interference canceller, represents a significant alternative to the orthogonal multiple access in catching up with the high demands for radio spectrum.
Integrating simultaneous wireless information and power transfer (SWIPT) into cooperative cognitive radio networks (CCRN) can increase both energy and spectral efficiencies. In contrast to most of existing works that only consider relay transmission, this paper studies opportunistic relaying along with dynamic energy harvesting in SWIPT-based CCRN, where a secondary transmitter (ST) can act as relay for primary transmission and a secondary receiver (SR) has the energy harvesting function. We propose a new framework that can unify the direct transmission, relay transmission and energy harvesting. Under the proposed framework, we first assume the transmit power of the ST is fixed and formulate the optimization problem of maximizing the ergodic capacity for SR while meeting the ergodic capacity for the primary receiver (PR) and average harvested energy for SR, by joint transmission mode selection and transmit power splitting ratio selection. Then, for the case when the transmit power of the ST can be adjusted, we investigate the joint optimization of transmit power control, transmit power splitting ratio and transmission mode selection. The two problems are non-convex and we propose the optimal policies by using the dual Lagrangian method. Simulations validate the effectiveness of the proposed schemes.
There is a wide consensus by the research community and the industry that it will not be possible to satisfy future mobile traffic demand and application requirements by simply evolving the current fourth-generation architecture. Instead, there is a need for a considerable revision of the mobile network system: such an effort is commonly referred to as the future fifth-generation (5G) architecture, and large-scale initiatives all around the globe have been launched worldwide to address this challenge. While these initiatives have not yet defined the future 5G architecture, the research community has already invested a very substantial effort on the definition of new individual technologies. The fact that all new proposals are tagged as 5G has created a lot of confusion on what 5G really is. The aim of this article is to shed some light on the current status of the 5G architecture definition and the trends on the required technologies. Our key contributions are the following: (1) we review the requirements for 5G identified by the different worldwide initiatives, highlighting similarities and differences; (2) we discuss current trends in technologies, showing that there is a wide consensus on the key enablers for 5G; and (3) we make an effort to understand the new concepts that need to be devised, building on the enablers, to satisfy the desired requirements.
The intensity in the requirements of Internet of Things and mobile internet makes the efficiency of fifth-generation (5G) wireless communications very challenging to achieve. Accomplishing the drastically increasing demand of massive connectivity and high spectral efficiency is a strenuous task. Because of the very large number of devices, 5G wireless communication systems are inevitable to satisfy the traffic requirements. Recently, nonorthogonal multiple-access (NOMA) schemes are immensely being explored to address the challenges in 5G, which include effective bandwidth utilization, support for a massive number of devices, and low latency. This paper provides the reader with a holistic view of multiple-access schemes, methods, and strategies for optimization in NOMA. First, we discuss the taxonomy of multiple-access schemes in the literature; then, we provide a detailed discussion of objectives, constraints, problem types, and solution approaches for NOMA. This paper also discusses the decoding methods and key performance indicators used in NOMA. Finally, we outline future research directions.
Non-orthogonal multiple access (NOMA) is an essential enabling technology for the fifth generation (5G) wireless networks to meet the heterogeneous demands on low latency, high reliability, massive connectivity, improved fairness, and high throughput. The key idea behind NOMA is to serve multiple users in the same resource block, such as a time slot, subcarrier, or spreading code. The NOMA principle is a general framework, and several recently proposed 5G multiple access schemes can be viewed as special cases. This survey provides an overview of the latest NOMA research and innovations as well as their applications. Thereby, the papers published in this special issue are put into the content of the existing literature. Future research challenges regarding NOMA in 5G and beyond are also discussed.