From 5G to 6G—Challenges, Technologies, and Applications
Abstract: As the deployment of 5G mobile radio networks gains momentum across the globe, the wireless research community is already planning the successor of 5G. In this paper, we highlight the shortcomings of 5G in meeting the needs of more data-intensive, low-latency, and ultra-high-reliability applications. We then discuss the salient characteristics of the 6G network following a hierarchical approach including the social, economic, and technological aspects. We also discuss some of the key technologies expected to support the move towards 6G. Finally, we quantify and
summarize the research work related to beyond 5G and 6G networks through an extensive search of publications and research groups and present a possible timeline for 6G activities.
Keywords: 3GPP; 6G; artificial intelligence; beyond 5G; edge computing; next-gen; THz
1. Introduction
The world’s global communication network has come a long way since the second-generation (2G) mobile radio network systems were deployed in the early 1990s. The second-generation network, undoubtedly, has been internationally recognized as the start
of a new era in digital communications. The aforesaid comes as no surprise based on the exploding rate of communication between users in the form of SMS texts and phone calls towards the end of the last century [1]. The world at that time experienced a paradigm shift on all levels, from individual users to large corporations, which created room for
new business models. Since then, the focus has been concentrated on offering faster communication speeds and supporting more users. To alleviate the connectivity issues that occur when many users try to access the network at the same time and to offer a better experience, third-generation (3G) systems were introduced in the early 2000s with new innovations, the most notable being the Universal Mobile Telecommunications System (UMTS), which has wideband code division multiple access at its essence [2]. However,3G was short-lived for a variety of reasons. Many analysts suggested that 3G faced regulatory and technical issues, leading to many operators phasing it out of their networks.
Conversely, the global, widespread media praise of 3G’s successor, i.e 4G, introduced around 2010, demonstrated that it was so far the most successful generation since 2G. The fourth-generation network is based on orthogonal frequency division multiplexing (OFDM) and multiple-input, multiple-output (MIMO) systems [3], offering theoretical speeds of
1 Gb/s and beyond, which until very recently was considered sufficient for almost all existing network services and applications. Figure 1 provides an overview of the timeline of the development of wireless networks.
Currently, many emerging services and network needs require speeds and network infrastructures well beyond the capabilities of 4G. The recently inaugurated 5G system is often presented as an integrated system that fills the gap between 4G and the current network demands, such as ultra-high communication speeds and very-low-link latency [4]. Nonetheless, a new research direction has recently commenced, investigating alternatives to 5G and looking beyond it. The drivers for this new direction are explored in depth in the next section. In essence, 5G is expected to be inadequate for the future network requirements. Furthermore, some challenges remain unresolved or overlooked in current 5G standards, such as dealing with signal propagation loss, which will inevitably increase with the use of higher frequencies (beyond 20 GHz), or maintaining efficient network management under increasingly complicated networks [5,6].Although research in the beyond 5G area has picked up momentum in recent With many surveys and discussions in the literature, we differentiate ourselves by consider-ing a hierarchical approach, providing a comprehensive review of the different aspects of 6G-enabling technologies and the current major research initiatives and publications related to 6G. Moreover, we discuss many of the deep learning methods to be used in 6G, provide an accurate count of papers discussing next-gen networks between 2015 and 2020, and show how the depth of the discussions on 6G have changed over the years. Additionally,We divided the published 6G surveys into seven categories: waveform, antennas, artificial intelligence (AI), security, blockchain, management, and architecture, and provide an up-to-date listing of 6G surveys at the time of writing of this paper. In this paper, we discuss the challenges facing 5G and how they are expected to stimulate the research towards 6G.In particular, the main contributions of this work are as follows:
• We highlight the main limitations of 5G and its key technologies;
• We present a holistic view of 6G that includes the social, technical, and
economic aspects;
• We provide a comprehensive review of recent research activities and projects related to 6G;
• We summarize the literature work on 6G’s vision and its potential technologies, as well as the timeline for 6 G's roll out on the market;
• We discuss 6G’s downsides, from the physical and mental health implications for
individuals, to the impact on the Earth’s ecosystems, and speculate about its existence in the future.The rest of the paper is organized as follows: Section 2 discusses the main shortcomings of 5G, while Section 3 discusses the aspects, requirements, and enabling technologies of 6G. Section 4 presents a summary of the current research related to 6G, and finally, Section 5 provides the conclusion. We show in Figure 2 the outline of our paper.
6G: Deep learning, TerraHertz, Human Chip Implants, Distributed Network Computations, Optical Wireless Communications, Intelligent Reflective Surfaces.
5G: Enhanced Mobile Broadband, Massive Machine Type Communications, Ultra Reliable Low latency Communications, Cloud Computing, Software-defined Network
4G: MIMO Antennas, OFDM/OFDMA, Improved Modulation and Coding, Voice over IP
3G: High speed internet, IP technology, WCDMA, UMTS
2G: Digital voice communication, TDMA, CDM
2. Fifth-Generation Network’s
This section looks at how well 5G is expected to perform as it is being rolled out in more and more global markets recently. It is appropriate first to examine the key technologies of 5G that are quickly becoming outdated. Network densification is a key player in 5G
through the very wide deployment of small cells. However, the benefits of this deployment,i.e., enhanced coverage and higher data transfer rates, represent diminishing returns as more and more small cells are deployed due to the significant increase in infrastructure cost.
Another technology is carrier aggregation, which allows users to be served by more than a single-component carrier to offer a higher bandwidth [7]. However, this has implications for hardware on the end users’ side to support different frequency bands. It is worth looking at the cloud radio access network (C-RAN) as being a primary component of 5G to mitigate
the hardware limitations of end devices. However, as networks grow exponentially in size,it becomes evident that the cloud alone is not enough, and fog and edge node computations are needed. Moreover, security in the main 5G technologies is not advanced enough to be deployed on very large scales, such as in software-defined networks (SDNs), where it lacks the mechanisms to verify trust between the management apps and the controller.
Another example is network function virtualization (NFV), where attackers can target software-level components, such as the virtual infrastructure manager, and generate fake logs that hinder the operation of NFV [8]. Furthermore, 5G offers ultra-reliable and low-latency communication (URLLC) as one of its key drivers. However, it is limited to the edge of the network without real integration across the entire network (including the
core) [9]. Moreover, the concept of heterogeneous networks (HetNets) is at the core of 5G technologies, but currently, such network integration is limited to terrestrial networks. This has to be further expanded to be three-dimensional by including aerial and space mesh networks in the main network. It is also important to note that 5G is not immune to denial
of service (DoS) attacks or threats that compromise its availability [10]. It is crucial that this be improved in future networks to adjust for the size of ever-growing networks of billions
of nodes. Next, we present the global communication network requirements and demands that are expected to surpass 5G’s capabilities.2.1. A. Communication Speed and Scalability It is projected that by 2030, global mobile traffic will be 670-times what it was in 2010, mainly due to machine-to-machine (M2M) communications [11]. This is an unprece-
dented exponential growth that motivates researchers worldwide to achieve technological breakthroughs in many network aspects, especially in spectral and energy efficiency tech-niques. The fifth-generation network is portrayed to bring to the network enhanced mobile
broadband (eMBB), i.e., offer speeds up to 20 Gbps [12], and massive machine-type com-munication (mMTC) support, as shown in Figure 3. However, this will not be able to keep up with the near future demands, as it is expected by 2030 that 5G will reach its limits [13].
The demand-driven nature of communication speeds dictates that in less than 10 years from
now, the data transfer rates will have to experience substantial improvements to be well
beyond 1 Tbps (up to 10 Tbps) [14]. Thus, looking beyond 5G incorporates researching tech-niques that can offer such speeds. Moreover, 5G is designed to utilize the millimeter wave range of 20–100 GHz [15]. However, it is not possible in this range to achieve such high
speeds due to current transceiver designs and digital modulation techniques’ limitations,such as non-linear power amplifiers, phase noise, and poor analog-to-digital converter (ADC) resolution [16]. Consequently, the next leap in communication will consider look-ing at frequencies beyond 100 GHz, possibly up to a few THz [17], as this spectrum is available in abundance to achieve high data rates. It was shown in a comparison [18] that beyond 100 Gbps speeds can be achieved in the 300 GHz range compared to 4 Gbps in the
60 GHz range.The extremely high data rates are justified by the kind of services that are emerging or expected to be widely adopted in the near future. Services such as augmented reality
(AR), human nano-chip implants, connected robotics, autonomous systems, and tele-medicine [19] are currently under development and enhancement to be deployed on a wide scale in the near future. Additionally, with the envisioned growth in M2M communications, it is expected that there will be hundreds of billions of devices connected to the Internet [20]. However, 5G is expected to offer the best performance tradeoffs only up to the scale of a billion devices [21]. Therefore, the next major mobile network upgrade will be scaled to accommodate such a huge number of device connections and a more-than-every ondensed network.