2. Related Works and Contributions

There are a few related works of literature that proposed the vision, requirements, enabling technologies, and design of 6G wireless networks ^{[33][37][38][39][40]}[33,39,40,41,42]. In particular, David et al. [33] opined that 6G would enable wireless charging and high data rates. The authors also identified the need for socio-ethics in the 6G design. Nayak and Patgiri ^[37][39] proposed 6G to change the perceptual experience in lifestyle, business, and society. The study also presents some technology-driven challenges in 6G wireless networks and the probable solutions. In ^[38][40], the authors examined critical features such as security, secrecy, and privacy to make 6G truly human-centric. Tariq et al. ^[39][41] present an extension to the existing vision of 5G and show speculatively how the 5G vision and technologies can be enhanced to drive the anticipated 6G. Yang et al. ^[40][42] proposed integrating machine learning and big data to facilitate intelligent transmission. The application of Big data and AI has also been considered in the scholarly works of literature ^[24][41][42][24,43,44]. Other enabling technologies such as Intelligent Reflecting Surfaces [17], Blockchain [28], Terahertz communication [11], and more, proposed for 6G communication, have also been surveyed. Additionally, the future projections of wireless communication presented in existing reports are highlighted.

This paper extensively considers existing research, clearly outlining the enabling technologies and the associated challenges, applications, and new applications such as the IoBNT and Digital Replica, which are not given adequate treatment in many related papers. Use cases in agriculture, education, media and entertainment, and more are discussed extensively. Apart from the technical challenges associated with the enabling technologies, this paper examines the social, psychological, and health concerns that could pose a challenge to 6G adoption. This paper also explores the recent research breakthroughs in 5G and the limitations of 5G, which make 6G a highly prospective candidate. A comparative review of some of the proposed enabling technologies, open research issues and lessons learned, and proposed future research directions are also discussed extensively.

In summary, Table 1 examines the limitations of some of the existing surveys and our contributions in this paper to fill the knowledge gap. We hope this paper gives the reader a panoramic view of what 6G will be, clearly outlining the possible challenges associated with the 6G-enabling technologies, applications, use cases, and more. Finally, this paper provides a future outlook of what needs to be done to facilitate this desirable generation of wireless communication towards achieving the United Nations Sustainable Development Goals.

Table 3.

Table 2. Comparing the different generations of wireless communication from 1G to 6G.

3. The Evolution from 1G to 6G

The analog wireless cellular network, which formed the first generation of wireless communication, was in use in the 1980s. This facilitated voice calls between mobile users. The Advanced Mobile Phone System (AMPS), International Mobile Telecommunications Standard (IMTS), and Point to Call formed the basis for the 1G. Additionally, some European countries adopted the Nordic Mobile Radio System (NMR). This resulted from incompatibility challenges. The first-generation network was monopolized and was not affordable for many. 2G brought about high-quality, secure mobile voice and basic data services such as fax and text messaging. GSM was at the core of the 2G. It was regarded as the “Groupe Speciale Mobile”—a group of technical personnel set up by the Postal and Telecommunication Administration (CEPT) Conference to develop digital mobile communication technology ^[48][50]. This was developed as the wireless counterpart of the land-lined Integrated Services Digital Network (ISDN) system. The acronym was later changed to refer to “Global Systems for Mobile Communication.” The GSM standard was deployed in 1991, using the 900 MHz bands ^[49][51].

The GSM architecture comprises the Mobile Station, Network and Switching Subsystem, and the Base Station Subsystem (BSS), also known as the radio network. Additionally, included is an intelligent network subsystem that enables intelligent functionality such as prepaid services and short message services (SMS). GSM utilized Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) for simultaneous communication between the subscriber and the base station ^[50][52]. The former allowed communication using multiple frequencies, while the latter enabled communication through multiplexing by time slots. General Packet Radio Service (GPRS) was developed to facilitate features such as always-on, higher capacity, internet-based content, packet-based data services, enabling services such as color internet service, email on the move, and visual communications, multimedia messages, and location-based services ^[51][53].

In the early twenty-first century, 3G was developed as an upgrade to the features in 2G. It permitted faster data rates in the range of 300 kbps–30 Mbps and services such as video conferencing, remote supervision systems, and enabled information services. Technologies such as Wideband Code Division Multiple Access (WCDMA), Universal Mobile Telephone Service (UMTS) were key to achieving the 3G. The 3rd Generation Partnership Programme (3GPP) was formed in 1998 to oversee UMTS implementation and other enabling technology for 3G. 3GPP2 was also formed in the United States to develop global specifications for 3G systems. Critical concepts for evolution toward beyond 3G networks are presented in ^[52][54].

Long-Term Evolution (LTE) was deployed in 2009. With the proliferation of smartphones and tablets, online gaming, and other services, 4G has been a significant success, enabling these services with its data speed of 100 Mbps–1000 Mbps. Although the first release of LTE was in 2005 by 3GPP in release 6, the full development was only achieved in release 8 in 2008. Further details on the technical solutions for the 3G long-term evolution are reported in ^[53][55]. The LTE is often regarded as the 4G; however, the LTE-Advanced features such as increased peak data rate, spectral efficiency, simultaneous active subscribers, and improved cell-edge performance make it the true 4G. Key performance indicators for 4G LTE are given by ^[54][55][56,57], and radial basis function neural network pathloss prediction model in LTE network was reported in ^[56][58]. A higher data rate was achieved and lower latency in 20 ms–100 ms, which was lower than that obtained in 3G. 4G facilitated video streaming, online gaming, and more. The need for a higher data rate and lower latency gave room for interest in 5G.

5G commercialization started in 2019, and it opens up new use cases in the Internet of Things (IoT), immersive gaming, virtual reality, and more. 6G is expected to have a higher data rate in the range of 100 Gbps–1 Tbps and latency lower than 1ms. This opens up applications in holographic communication, tactile internet, extended reality, and more. Table 2 summarizes the technology, data rates, and supporting applications from 1G to 6G. The change in latency from 1G to 6G is also shown. Furthermore, a comparative analysis of 5G, Beyond 5G (B5G), and 6G is presented in

This table compares the different generations of wireless communication from 1G to 6G with respect to the supporting technologies, data features, and enabling applications. It shows that with higher data rates and lower latency, more sophisticated applications are enabled.

Table 3. A comparative analysis of 5G, B5G, and 6G.

, and more. The requirements are high reliability [7], the high data rate in the order of 1 Tb/s, ultra-low latency of less than 1ms, high energy and spectral efficiency ^[85][87], security and privacy ^[86][88], and ubiquitous connectivity that connects everyone, including people in rural areas ^[87][88][89,90]. The Ultra-Reliable Low Latency Communication (URLLC) required in 6G networks is also under more stringent conditions than that obtainable in 5G, with delay jitters, context awareness, and UAV/Satellite compatibility being considered. Sustainability is also desirable in future wireless networks, and there is a need for Green Networking architecture that will be environmentally friendly ^[89][91]. Figure 1 shows a brief overview that compares the Key Performance Indicators in 5G and those expected in 6G.

 

 

Figure 1. Comparative Analysis of 5G and 6G Key Performance Indicators.

5. Enabling Technologies and Challenges for 6G

Here, we present a robust discussion of the enabling technologies of the 6G communication system. We envision that Artificial Intelligence, which was introduced in 5G, will be further explored and central in achieving an intelligent 6G network. Reconfigurable Intelligent Surfaces will be deployed on doors, windows, buildings, and these reflect signals and help in places where maintaining Line of Sight (LoS) is tricky. The advantages of this over conventional relay systems are discussed. Cell-free Massive MIMO, TeraHertz, and Optical Wireless Technology, and more are also treated in this section. Quantum communication has been proposed, although research is in its inchoate state. This will improve the computing efficiency and security of future wireless networks. Unmanned Aerial Vehicles (UAV) and Cubesats are proposed to facilitate space communication. Some literature presents this as the Internet of Space Things ^[31][90][31,37]. This is important as it expands coverage and facilitates ubiquitous connectivity. The desirable wireless charging can be enabled by Ambient Backscatter Communication System (ABCS). Figure 2 gives a pictorial guide to the enabling technologies.

Figure 2. 6G Enabling Technologies.

6. 6G Applications

Every generation of wireless networks has facilitated new applications, and 6G is not an exception. The need for higher data rates, lower latency, high reliability, and more have given rise to the development and deployment of new wireless generation networks. In this section, we examine the applications to be facilitated by 6G. Although these or similar applications have been proposed to be enabled by 5G, 5G cannot meet the requirements to facilitate these applications seamlessly at the current development stage. Some limitations in 5G, such as scarce bandwidth and high-energy consumption, have been identified ^[91][231]. Although 5G tests have shown prospects with the actualization of a data rate of 8 Gbps ^[92][232] and 1 Gbps at 6.5 km distance ^[93][233], these are still below the peak data of 20 Gbps proposed in the literature. Therefore, existing 5G networks cannot meet high-speed intensive applications such as holographic communication, which require 1 Tbps for seamless communication ^[37][39]. Other limitations identified with 5G wireless networks are high interference due to massive interconnection, insufficient computing capacity, and lack of ubiquitous connectivity. Hence there is a need for 6G communication, which promises better features than 5G.

Some of the applications to be facilitated and fully enabled by 6G are holographic communication, teleoperated driving, tactile internet, Industry 4.0, and more. New applications that were not considered in the 5G context, such as IoBNT and Digital replica, are also introduced in this paper. Table 4 gives a summarized description of these applications. Furthermore, we outline the challenges in achieving some of these applications. Additionally, desirable features of these applications are considered. These applications, the 5G limitations, and how 6G wireless networks will enable them are highlighted in this section. In this literature, it has been established that the existing features of 5G are not efficient enough to enable the requirements of these technologies. We consider each application in this section, pointing out some of the requirements that make 6G an ideal candidate for these applications.

Table 4. Brief description of 6G applications.

	Applications			Brief Descriptions
	Holographic Communication			This enables human communication through holographs-3D images in thin air. To improve the experience of remote communication as we embrace a borderless workplace. Latency and high bandwidths are some of the challenges associated with Holographic Communication. 6G will solve these challenges.
	Tactile Internet			Enables human-to-machine interactions and machine-to-machine interactions.
	Industry 4.0 and beyond			Comprises cyber-physical systems, IoT, and cloud computing. Additionally, AI and ultra-fast wireless networks will drive the 4th industrial revolution. This enables smart cities, factories which are some of the vision for 6G.
	Teleoperated Driving			Allows cars to be controlled remotely. These cars are also referred to as semi-autonomous vehicles. Semi-autonomous cars require a fast and ubiquitous wireless network with ultra-low latency.
	Internet Bio-Nano Things			An interconnection of biological nano-sized objects(nanomachines). Takes application largely in healthcare. 6G is proposed to provide the perceptual requirements and ultra-low latency required by IoBNT.
	Multisensory XR Applications			AR/MR/VR that incorporates perceptual experience. Supported by URLLC and eMBB and perceptual factors to be supported by 6G. An excellent candidate to provide a better gaming experience.
	Blockchain and Distributed Ledger Technologies			Blockchain is postulated to provide security for 6G networks. They also require low latency, reliable connectivity, and scalability, which 6G networks will provide.
	Connected Robotics and Autonomous Systems (CRAS)			CRAS is required to improve industrialization through the use of robots and autonomous systems for industrial operations. They require a high rate and reliability and low latency.
	Wireless Brain-Computer Interface (BCI)			BCI enables communication between the brain and electronic devices. This requires ultra-low latency, high reliability, and high data rate.
	Digital Replica			These are also called digital twins, and they create a digital copy to replace people, places, systems, objects. This requires a very high data rate, which 6G will enable.

This table zooms in on the comparison of 5G, Beyond 5G, and 6G, presenting a specific latency, brief description of the architecture, and services such as MBRLLC, mURLLC that have been proposed. The device types are also shown to include BCI and smart implants in 6G.

4. Vision of 6G Wireless Communication

There have been different descriptions of what the 6G network should be by researchers ^{[38][43][77][78][79]}[40,45,79,80,81]. Furthermore, the authors in ^[80][82] defined it as a technology that will make human society a “Ubiquitous Intelligent Mobile Society.” In ^[39][41], the authors envisioned 6G to facilitate super smart cities with pervasive autonomous systems. It is expected that 6G will be supported by existing 5G infrastructure such as Software-Defined Networking ^[81][83], Network Function Virtualization (NFV) ^[82][84], and Network Slicing (NS) ^[83][85] together with new infrastructure. In order to give a future assessment of how well 6G has accomplished the required cases, this paper examines various visions and requirements projected by different researchers ^[40][41][42][42,43,44]. It presents a blend of what 6G will be. Just as we envisage, some researchers also believe there will be a pervasive application of AI to make 6G a reality ^[20][68][20,70]. There is also research on optical wireless communication to enable indoor and outdoor communication at high-data rates ^[84][86]. Simultaneous Wireless and Information Power Transfer (SWIPT) [35], which is an Energy Harvesting (EH) technique, has been proposed to improve the battery life of UEs ^[38][39][40,41].

6G is expected to support smart cities, the Internet of Everything (IoE) ^[37][39], tactile devices [9]