Abstract
Owing to the superior performance in spectral efficiency, connectivity, and flexibility, nonorthogonal multiple access (NOMA) is recognized as the promising access protocol and is now undergoing the standardization process in 5G. Specifically, dozens of NOMA schemes have been proposed and discussed as the candidate multiple access technologies for the future radio access networks. This paper aims to make a comprehensive overview about the promising NOMA schemes. First of all, we analyze the state-of-the-art NOMA schemes by comparing the operations applied at the transmitter. Typical multiuser detection algorithms corresponding to these NOMA schemes are then introduced. Next, we focus on grant-free NOMA, which incorporates the NOMA techniques with uplink uncoordinated access and is expected to address the massive connectivity requirement of 5G. We present the motivation of applying grant-free NOMA, as well as the typical grant-free NOMA schemes and the detection techniques. In addition, this paper discusses the implementation issues of NOMA for practical deployment. Finally, we envision the future research challenges deduced from the recently proposed NOMA technologies.
Highlights
In the past several decades, the wireless communication system has evolved from the first generation, an analog communication network which only transfers voice messages, to LTE networks, which satisfies the great demands on mobile broadband data transmissions
superimposed radio resource sharing (SRRS) superimposes different uplink data streams on the same radio resources and applies successive interference cancelation (SIC) at the receiver, which can be regarded as a prototype of Nonorthogonal multiple access (NOMA), Despite all the related researches, NOMA is still not commercialized in the past decades due to the concern of high computational complexity of SIC-type receiver
Various multiuser receiving technologies facilitate the application of NOMA in different scenarios
Summary
In the past several decades, the wireless communication system has evolved from the first generation, an analog communication network which only transfers voice messages, to LTE networks, which satisfies the great demands on mobile broadband data transmissions. In uplink multiple access channel (MAC), the signals of different transmitters are overlapped and SIC receiver is applied to achieve the corner points of MAC capacity region. In 1990s, multiple access protocols, which exploited the differences between the power levels of the received packets, were proposed and studied by Shimamoto (1992) [3], Pedersen (1996) [4], and Mazzini (1998) [5], respectively. SRRS superimposes different uplink data streams on the same radio resources and applies SIC at the receiver, which can be regarded as a prototype of NOMA, Despite all the related researches, NOMA is still not commercialized in the past decades due to the concern of high computational complexity of SIC-type receiver.
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