Abstract
Due to the increasingly complicated communication scenarios and network architectures as well as growing traffic demands for high speed connectivity, dynamic spectrum allocation in fifth generation (5G) networks becomes insufficient to guarantee the satisfaction of main network requirements in terms of spectrum efficiency (SE), scalability, delay, and energy efficiency (EE). Enormous multiple access schemes and cognitive radio (CR) network scenarios come to fulfill these requirements and enhance network functionalities. With multiple access schemes, users are able to transmit their data streams simultaneously under maximum capacity constraints. On the other hand, vacant spectrum holes are exploited in an opportunistic manner via CR and software defined radio. In order to exploit these spectrum holes as well as meeting different network requirements, several multiple access techniques have been presented that have been initiated through the adoption of orthogonal multiple access (OMA) scheme. Additionally, non-orthogonal multiple access (NOMA) and space division multiple access (SDMA) are presented to achieve a promising multiplexing gain as well as to address the inefficient spectrum utilization incurred with OMA schemes. However, such multiplexing gain is limited as it depend on the channel conditions. Accordingly, a generalized multiple access scheme has been presented recently, namely rate splitting multiple access (RSMA), to further enhance the SE. In this paper, we provide a comprehensive study regarding the key multiple access schemes presented for CRNs to further enhance the use of spectral resources, and additionally highlights the key implementation challenges and the enabling techniques addressed to overcome it. We have given a special attention to the enhances provided by RSMA as compared with OMA, SDMA, and NOMA techniques. Finally, some open issues are spotted to shed lights on the need for further studies and future research efforts.
Highlights
A CCORDING to tremendous growth of the number of mobile devices and the rapid increase of wide-band and hungry data rate communication services, e.g. augmented reality (AR) and virtual reality (VR) [1], 5G networks require 1000x higher capacity, 10x higher spectral efficiency, and 100x higher connectivity density in comparison with the fourth generation (4G) [2]
Orthogonal allocation approaches suffer from in-efficient distribution of available resources and inability to support massive connectivity, which push towards new access schemes as well as capacity boosting technologies such as non-orthogonal multiple access (NOMA), rate splitting multiple access (RSMA) with massive Multi-Input Multi-Output, ultradense networks and millimeter wave deployments [9]
We have focused on multiple access techniques and its integration with CRNs to achieve one or more 5G requirement according to the desired application
Summary
A CCORDING to tremendous growth of the number of mobile devices and the rapid increase of wide-band and hungry data rate communication services, e.g. augmented reality (AR) and virtual reality (VR) [1], 5G networks require 1000x higher capacity, 10x higher spectral efficiency, and 100x higher connectivity density in comparison with the fourth generation (4G) [2]. Orthogonal allocation approaches suffer from in-efficient distribution of available resources and inability to support massive connectivity, which push towards new access schemes as well as capacity boosting technologies such as non-orthogonal multiple access (NOMA), rate splitting multiple access (RSMA) with massive Multi-Input Multi-Output (mMIMO), ultradense networks and millimeter wave deployments [9]. In this comprehensive survey, we provide a special focus on NOMA and RSMA with some enabling technologies that are highly expected to enhance system capacity, spectral/energy efficiency and accommodate massive connectivity. Flexible architecture of RSMA tolerates with two extremes of interference management: completely treat interference as noise (as in SDMA) and com-
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