NOMA for 5G.pdfDriven by the rapid escalation of t

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详细说明：Driven by the rapid escalation of the wireless capacity requirements imposed by advanced multimedia applications (e.g., ultra-high-definition video, virtual reality etc.), as well as the dramatically increasing demand for user access required for the Internet of Things (IoT), the fifth generation (5G) networks face challenges in terms of supporting large-scale heterogeneous data traffic. Non-orthogonal multiple access (NOMA), which has been recently proposed for the 3rd generation partnership projects long-term evolution advanced (3GPP-LTE-A), constitutes a promising technology of addressing the above-mentioned challenges in 5G networks by accommodating several users within the same orthogonal resource block. By doing so, significant bandwidth efficiency enhancement can be attained over conventional orthogonal multiple access (OMA) techniques. This motivated numerous researchers to dedicate substantial research contributions to this field. In this context, we provide a comprehensive overview of the state-of-the-art in power-domain multiplexing aided NOMA, with a focus on the theoretical NOMA principles, multiple antenna aided NOMA design, on the interplay between NOMA and cooperative transmission, on the resource control of NOMA, on the co-existence of NOMA with other emerging potential 5G techniques and on the comparison with other NOMA variants. We highlight the main advantages of power-domain multiplexing NOMA compared to other existing NOMA techniques. We summarize the challenges of existing research contributions of NOMA and provide potential solutions. Finally, we offer some design guidelines for NOMA systems and identify promising research opportunities for the future.Table I: LIST OF ACRONYMS Amplify-and-Forward BC Base station CDMA Code Division Multiple Access Channel Impuls COMP Coordinated multipoint C-RAN Claud-based Radio Access Networks Channel state Informa CSIT Channel state information at the transmitter Decode-and-Forward Full-Duplex FDMA Lenev Division Multiple access eave Division Multiple Access Iterative Multi-user Detecton LDS Low-Density Signature DPC LPMA Lattice Partition Multiple Access LTE Long term evolu LMMSE Linear Minimum Mean Square Error Multiple access MAO Wireless Network visualization IUSA Multi-User Shared Access MUST Multi-User Superposition Transmissio Orthogonal Multiple Access OFDE Multiple acces OMA onal Multiple Access cation Pattern Division Multiple A PLS Physical Layer Secerity PT Primary Transmi QoS ervice Resource block Rela Signal alignment Su Space Division Multiple Access Software defined network SDR Software defined radio SD-NOMA Software Defined NOMA SR econdary receiver different-throughput modem and channel coding modes, depending on the near-instantaneous channel What is so beautiful about multi-carrier solutions is their impressive flexibility, since they have a host of different parameters which allow us to appropriately configure them and programme them, whatever the circumstances are -regardless of the propagation environment and regardless of the quality of service (QoS) requirements, as facilitated by the employment of adaptive modulation and coding(amc) Our hope is dear Colleague that would allow us now to briefly review the evolution of signal processing and communications techniques over the past three decades in an anecdotal style with reference to Fig. l At the time of writing we are gradually approaching the 5G Place'on our road map of Fig. l We are indeed also approaching the bit-rate limits upper-bounded by the channel capacity of both the classic single-input/single-output systems as well as of the MiMo systems. Observe at the top left hand corner of Fig. I how the various MIMO solutions, such as bell lab's layered space-time(BLAST)Drive pace-time coding (STC)'Street, Beam-Forming Closeand linear dispersion coding (LDc)"Street merge into mimo Square After decades of evolution, the classic orthogonal multiple access(OMA)schemes, such as time divi- sion multiple access (TDMa)'Street, frequency division multiple access(FDMA), orthogonal variable spreading factor based code division multiple access(OVSF-CDMA), interleave division multiple access (DMA)and orthogonal frequency division multiple access(OFDMA)'Street converged to OMA/non- orthogonal multiple access(NOMA)'Square'of Fig. I They have also evolved further along spatial division multiple access (sda)and multi-functional antenna array Street'-these solutions have found their way into the 4G OFDMA systems. As seen at the bottom left corner of Fig. I the various advance channel coding schemes have competed for adoption in the 4G standard, which relies of a variety of coding arrangements, including automatic repeat request(ARQ) At the time of writing the community turned towards the standardization of the 5G systems, with a special emphasis on the Noma techniques detailed in this treatise, as indicated by the broad NOMA Parkway,, which symbolizes 15 different NOMa proposals. The family of MFAAs also entails the recent spatial modulation (SM)and large-Scale (Ls) MIMO systems. Since the road along millimeter wave(mm Wave) Street' is rather unexplored and the attenuation is high, the employment of bF is rather crucial, if we want to exploit these rich spectral reserves In the bottom right corner of Fig. a number of novel technological advances converge at HetNet Square, where cognitive radio(CR) and software defined networks meet device-to-device(D2D)and Internet-of-Things(loT) networks. A range of sophisticated ideas are also under intensive investigation to resolve the network-centric versus user-centric design options. There is a strong evidence that the latter is MFAA MIMO LS-MIMO MFAA St BE Close Sq Terrace DC 4G ve tuI Sq IA Park 5G FD MA A MA OMA/ NOMA OVSF-CD MA St Sq HetNets CR SDN urbo st FEC LDPC St Figure 1: The roadmap for illustrating the brief history of wireless standardization more promising, because it is also capable of simultaneous load-balancing. There are also strong proposals on decoupling the uplink and downlink tele-traffic, with the motivation that mobile-initiated uplink traffic can reach a small-cell Bs at a lower transmit power than that of the bss downlink transmission Optical wireless based on visible-light communications is also developing quite rapidly, with Giga-bit copper backhaul networks making promising progress. Whilst no doubt the classic re systems will continue to evolve towards the next generation, an idea, whose time has come is Quantum communications, as demonstrated by the Science article "Satellite-based entanglement distribution over 1200 km"by Yin et l.4. As the lte system is reaching maturity and the 4G systems have been commercially deployed researchers have turned their attention to the 5G cellular network. The latest visual network indeX (vnD reports pointed out that by 2020s, the data traffic of mobile devices will become an order of magnitude higher compared to that in 2014 [5]. Apart from meeting the escalating data demands of mobile devices other challenges of dealing with the deluge of data as well as with the high- rate connectivity required b bandwidth-thirsty applications such as virtual reality (vR), online health care and the IoT further aggravate the situation. Driven by this, the 5G networks are anticipated with high expectations in terms of making a substantial breakthrough beyond the previous four generations. The often-quoted albeit potentially unrealistic expectations include 1,000 times higher system capacity, 10 times higher system throughput and 10 times higher energy efficiency per service than those of the fourth generation(4G)networks [6]. Several key directions such as ultra-densification, mm Wave communicationS, massive MIMO arrangements, D2D and machine-to-machine(M2M) communication, full-duplex(FD)solutions, energy harvesting(EH) cloud-based radio access networks(C-ran, wireless network virtualization (WNv), and software defined networks(SDN) have been identified by researchers [71-19 Fig. 2 illustrates the whole 5G network structure, including most of the existing/promising techniques Ultra Wideband (cmWave, mmWave NOMA ower 自之向 Figure 2: Illustration of the future 5G network architecture B. State-of-the-art of Multiple Access Techniques As mentioned before, sophisticated multiple access (Ma) techniques have also been regarded as one of the most fundamental enablers, which have significantly evolved over the consecutive generations in wireless networks [101, [11]. Let us have a deeper looker at the development of ma techniques below As illustrated in Fig. I the past three to four decades have witnessed historic developments in wirele communications and standardization in terms of ma techniques. Looking back to the development of the Ma formats as we briefly discussed above, in the first generation(IG), FDMa was combined with an analog frequency modulation based technology, although digital control channel signaling was used In the 2G GSM communications TDMA was used [12]. Then CDMA, which was originally proposed by Qualcomm [131, became the dominant MA in the 3G networks. In an effort to overcome the inherent limitation of CDMA- namely that the chip rate has to be much higher than the information data rate OFDMA was adopted for the 4G networks [14 Based on whether the same time or frequency resource can be occupied by more than one user, the existing Ma techniques may be categorized into OMA and NOMA techniques [15]. Amongst the above-mentioned MA techniques, FDMA, TDMA and OFDMA allow only a single user to be served within the same time/frequency resource block(RB),which belong to the OMa approach. By contrast, CDMa allows multiple users to be supported by the same rB with the aid of applying different unique, user-specific spreading sequences for distinguishing them Fuelled by the unprecedented proliferation of new Internet-enabled smart devices and innovative applications, the emerging sophisticated new services expedite the development of 5G networks requirin new Ma techniques. NOMA techniques can be primarily classified into a pair of categories, namely, code-domain NOMA and power-domain NoMA 161 The most prominant representatives code-domain Noma techniques include trellis coded multiple acceSs(TCMA)[181, IDMA [191, low-density signature(LDS)sequence based CDMA [20]. These solu- tions are complemented by the more recently proposed multi-user shared access(MUSA)technique [211 pattern division multiple access(PDMA)[22 ), and sparse code multiple access(SCMA) The power-domain NOMA, which has been recently proposed to 3GPP LTE [23 ] exhibits a superior capacity region compared to OMA. The key idea of power-domain noma is to ensure that multiple users can be served within a given time/frequnecy RB, with the aid of superposition coding (SC) techniques at the transmitter and successive interference Cancellation(SIC) at the receiver, which is fundamentally Note that apart from the code-and power-domain, the spatial-domain can also be regarded as another domain for supporting multiple users within the same rb, which is achieved by exploiting the specific"spatial signature"constituted by the channel impulse responses(CIRs)of the users for distinguishing them [31. l10. A representative ma technique is space division multiple access (SDMA)[17 different from the classic oma techniques of FDMA/TDMA/OFDMa as well as from the code-domain NOMA techniques. The motivation behind this approach lies in the fact that again, NOMA is capable of exploiting the available resources more efficiently by opportunistically capitalizing on the users'specific channel conditions [24 and it is capable of serving multiple users at different QoS requirements in the same RB. It has also been pointed out that noma has the potential to be integrated with existing Ma paradigms, since it exploits the new dimension of the power domain The milestones of power-domain NOMA are summarized in the timeline of Table Il C. Motivation and Contribution.s While the above literature review has laid the basic foundation for understanding the development of Ma schemes in each generation of cellular networks, the power domain multiplexing ng based NOMA philosophy is far from being fully understood. There are some short magazine papers [16],[23 ) [431 [44]and surveys[45,[46] in the literature that introduce NOMA, but their focus is different from our work. More particularly, Dai et al. introduced some concepts of the existing NOMA techniques and identified some challenges and future research opportunities [ 16] both for power-domain and code-domain NOMA. A magazine paper on power-domain NoMa was presented by Ding et al. [23 with particular attention devoted to investigating the application of NOMA in LTE and 5G networks. Shin et al. [431 discussed the research challenges and opportunities in terms of NOMa in multi-cell networks, aiming for identifying techniques to manage the multi-cell interference in NOMA. As a further advance, Ali et al.[44] outlined a general framework for multi-cell downlink NOMA by adopting a coordinated multi- point(CoMP)transmission scheme by considering distributed power allocation(PA) strategy in each cell Regarding surveys, in [45 Islam et al. have surveyed several recent research contributions on power- domain NOMa, while providing performance comparisons to OMA in different wireless communications scenarios. In 1461, Tabassum et al. investigated the uplink and downlink of noMa in single cell cellular networks, identifying the impact of distance of users on the performance attained Although the aforementioned research contributions present either general concepts or specific aspects of NOMA, some important NOMA models, the analytical foundations of NOMA, and some of its significant applications in wireless networks have not been covered. Besides, a clear illustration of the historic development of power-domain NOMA milestones is missing. Finally, the comparisons between power-domain NOMA and other practical forms of NoMa have not been discussed. Motivated by all the aforementioned inspirations, we developed this treatise. More explicitly, the goal of this survey is to comprehensively survey the state-of-the-art research contributions that address the major issues challenges and opportunities of NOMA, with particular emphasis on both promising new techniques and Table Il: Timeline of power-domain NOMA milestones 1972 Cover first proposed SC and SIC concepts 25 1973,, apable of approaching the capacity of the Gaussian broadcast channel(BC:26 1986 m likelihood multi user receiver for CDMA systems PIC in CDMA Li and Goldsmith studied the capacity regions or fading BCs with applying SC and SIC 29 Mostafa et al. demostrated that SAIC can 2003 effectively suppress downlink inter-cell interferences in GSM networks 31 Tse compared the capacity regions of NoMA to 2004 OMA both in downlink and uplink 32 Andrews summarized the devolopment of 2005 interference cancellation for cellular systems 33 Zhanc and Han d a unified treatment for 2011 SC aided system Vanka et al. designed an experimental platform 2012 or investigating the implementing performance of SC 35 Saito et al. proposed the concept of two-user 2013 downl nk NoMA transmission for bandwidth efficiency enhancement 36 Ding et a/ developed a multi-user downlink 2014 NOMA transmission scheme with randomly Xiong ot al designed a practical cper source SDR based NOMA prototype for two Benjebbour et a/. measured the experimental 2015 Its on a noma test-bed for tw Choi et al. proposed a two-user MISO-NONA 2015 design for investigating the potential application nulti-antenna techniques in NOMA 40 2016 Ding et al. proposed a cluster-based multi-user MIMO-NOMA structure 41 beamforming for multi-cell MIMO-NOMA 2017 networks to enhance the cell-edge users h hput42 novel application scenarios. Table IImillustrates the comparison of this treatise with the existing magazine papers and surveys in the context of NOMa To highlight the significance of this contribution, we commence with a survey of NOMA starting with the basic principles, which provides the readers with the basic concepts of NOMA. We continue in the context of multiple antenna aided techniques combined with NOMA, followed by cooperative NOMa techniques. We then address another important issue of NOMA, namely its resource and pa problems Finally, we elaborate on invoking other 5g candidate techniques in the context of noMa networks The contributions of this survey are at least five fold, which are summarized as follows 1) We present a comprehensive survey on the recent advances and on the state-of-the-art in power- domain multiplexing aided NOMa techniques. The basic concepts of NOMa are introduced and key advantages are summarized. The research challenges, opportunities and potential solutions are also identified 2)We investigate the application of multiple-antenna aided techniques to NOMA. The pair of most dominant solutions namely cluster-based mIMo-Noma and beamformer-based MIMo-nOMa are reviewed and their benefits are examined. Furthermore, we highlight that specific massive- MIMO-NOMA Solutions are capable of improving the performance of NOMa networks to a large extent. A range of important challenges are elaborated on in the context of multiple-antenna assisted NOMA and the associated future opportunities are also underlined 3)By exploiting the specific characteristics of NOMA, we study the interplay between NOMA and cooperative communications. We demonstrate that cooperative noma constitutes a promising technique of improving the reliability of the users experiencing poor channel conditions 4)We identify the potential issues associated both with power-and user-allocation, which constitute the fundamental problems to be solved for ensuring fairness in the NOMA networks. We point out the significance of designing efficient algorithms for dynamically allocating the resources to the users. Furthermore, we propose the novel concept of a software-defined NOMA (SD-NOMA network architectures, where resource allocation -including the power -is performed on a generic hardware platform by taking into account the global view of the entire networ 5) We identify the major issues and challenges associated with the co-existence of NOMa and the other emerging 5G technologies. The potential solutions based on the current research contributions corresponding to these technologies are also surveyed We have also discussed the implementation issues and recent standardization progress for NOMA. Finally, power-domain Noma and other popular forms of NOMa are contrasted. We spotlight that the significance to provide a unified

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