WEI Zhiqiang YUAN Jinhong Derrick Wing Kwan Ng Maged Elkashlan and DING Zhiguo

Abstract

Non?orthogonal multiple access (NOMA) has been recognized as a promising multiple access technique for the next generation cellular communication networks. In this paper, we first discuss a simple NOMA model with two users served by a single?carrier simultaneously to illustrate its basic principles. Then, a more general model with multicarrier serving an arbitrary number of users on each subcarrier is also discussed. An overview of existing works on performance analysis, resource allocation, and multiple?input multiple?output NOMA are summarized and discussed. Furthermore, we discuss the key features of NOMA and its potential research challenges.

Keywords

non?orthogonal multiple access (NOMA); successive interference cancellation (SIC); resource allocation; multiple?input multiple?output (MIMO)

1 Introduction and Background

the fifth generation (5G) communication system is on its way. It is widely believed that 5G is not just an incremental version of the fourth generation (4G) communication systems [1] due to the increasing demand of data traffic and the expected new services and functionalities, such as internet?of?things (IoT) and cloud?based architectural applications [2]. These envisioned services pose challenging requirements for 5G wireless communication systems, such as much higher data rates (100-1000 times faster than current 4G technology), lower latency (1 ms for a

roundtrip latency), massive connectivity and support of diverse quality of service (QoS) (106 devices/km2 with diverse QoS requirements) [1]. From a technical perspective, to meet the aforementioned challenges, some potential technologies, such as massive multiple?input multiple?output (MIMO) [3], [4], millimeter wave [5], [6], and ultra densification and offloading [7]-[9], have been discussed extensively. Besides, it is expected to employ a future radio access technology for 5G, which is flexible, reliable [10], and efficient in terms of energy and spectrum [11], [12]. Radio access technologies for cellular communications are characterized by multiple access schemes, such as frequency?division multiple access (FDMA) for the first generation (1G), time?division multiple access (TDMA) for the second generation (2G), code?division multiple access (CDMA) used by both 2G and the third generation (3G), and orthogonal frequency division multiple access (OFDMA) for 4G. All these conventional multiple access schemes are categorized as orthogonal multiple access (OMA) technologies, where different users are allocated to orthogonal resources in either time, frequency, or code domain in order to mitigate multiple access interference (MAI). However, OMA schemes are not sufficient to support the massive connectivity with diverse QoS requirements. In fact, due to the limited degrees of freedom (DoF), some users with better channel quality have a higher priority to be served while other users with poor channel quality have to wait to access, which leads to high unfairness and large latency. Besides, it is inefficient when allocating DoF to users with poor channel quality. In this survey, we focus on one promising technology, non?orthogonal multiple access (NOMA), which in our opinion will contribute to disruptive design changes on radio access and address the aforementioned challenges of 5G.

In contrast to conventional OMA, NOMA transmission techniques intend to share DoF among users via superposition and consequently need to employ multiple user detection (MUD) to separate interfered users sharing the same DoF, as illustrated in Fig. 1. NOMA is beneficial to enlarge the number of connections by introducing controllable symbol collision in the same DoF. Therefore, NOMA can support high overloading transmission and further improve the system capacity given limited resource (spectrum or antennas). In addition, multiple users with different types of traffic request can be multiplexed to transmit concurrently on the same DoF to improve the latency and fairness. The comparison of OMA and NOMA is summarized in Table 1. As a result, NOMA has been recognized as a promising multiple access technique for the 5G wireless networks due to its high spectral efficiency, massive connectivity, low latency, and high user fairness [13]. For example, multiuser superposition transmission (MUST) has been proposed for the third generation partnership project long?term evolution advanced (3GPP?LTEA)

networks [14]. Three kinds of non?orthogonal transmission schemes have been proposed and studied in the MUST study item. Through the system?level performance evaluation, it has been shown that the MUST can increase system capacity as well as improve user experience.

Recently, several NOMA schemes have been proposed and received significant attention. According to the domain of multiplexing, the authors in [13] divided the existing NOMA techniques into two categories, i.e., code domain multiplexing (CDM) and power domain multiplexing (PDM). The CDM?NOMA techniques, including low?density spreading (LDS) [15]-[17], sparse code multiple access (SCMA) [18], pattern division multiple access (PDMA) [19], etc, introduce redundancy via coding/spreading to facilitate the users separation at the receiver.

For instance, LDS?CDMA [15] intentionally arranges each user to spread its data over a small number of chips and then interleave uniquely, which makes optimal MUD affordable at receiver and exploits the intrinsic interference diversity. LDS?OFDM [16], [17], as shown in Fig. 2, can be interpreted as a system which applies LDS for multiple access and OFDM for multicarrier modulation. Besides, SCMA is a generalization of LDS methods where the modulator and LDS spreader are merged. On the other hand, PDM?NOMA exploits the power domain to serve multiple users in the same DoF, and performs successive interference cancellation (SIC) at users with better channel conditions. In fact, the non?orthogonal feature can be introduced either in the power domain only or in the hybrid code and power domain. Although DM?NOMA has the potential code gain to improve spectral efficiency, PDM?NOMA has a simpler implement since there is almost no big change in the physical layer procedures at the transmitter side compared to current 4G technologies. In addition, PDM?NOMA paves the way for flexible resource allocation via relaxing the orthogonality requirement to improve the performance of NOMA, such as spectral efficiency [20], [21], energy efficiency [22], and user fairness [23]. Therefore, this paper will focus on the PDM?NOMA, including its basic concepts, key features, existing works, and future research challenges.