6G: Envisioning the Key Technologies, Applications and Challenges

In 2030, 6G is going to bring remarkable revolution in communication technologies as it will enable Internet of Everything. Still many countries are working over 5G and B5G has yet to be developed, while some research groups have already initiated projects on 6G. 6G will provide high and sophisticated QoS e.g. virtual reality and holographic communication. At this stage, it is impossible to speculate every detail of 6G and which key technologies will mark 6G. The wide applications of ICT, such as IoT, AI, blockchain technology, XR (Extended Reality) and VR (Virtual Reality), has created the emergence of 6G technology. On the basis of 5G technique, 6G will put profound impact over ubiquitous connectivity, holographic connectivity, deep connectivity and intelligent connectivity. Notably, research fraternity should focus on challenges and issues of 6G. They need to explore various alternatives to meet desired parameters of 6G. Thus, there are many potential challenges to be envisioned. This review study outlines some future challenges and issues which can hamper deployment of 6G. We subsequently define key potential features of 6G to provide the state of the art of 6G technology for future research. We have provided a review of extant research on 6G. In this review, technology prospects, challenges, key areas and related issues are briefly discussed. In addition, we have provided technologies breakdown and framework of 6G. We have shed light over future directions, applications and practical considerations of 6G to help researchers for possible breakthroughs. Our aim is to aggregate the efforts and eliminate the technical uncertainties towards breakthrough innovations for 6G. Keywords—IoT; AI; communication technologies; holographic communication; blockchain


I. INTRODUCTION
Although the era of 5G is not fully developed, the limitations of 5G have created the demand for 6G networks. In 2019, communication synergy around the globe drafted first 6G white paper in world's first 6G summit in Finland. After that, many government organizations and research group from prestigious institutes started introducing their 6G projects. UK government has decided to invest in 6G technology [1], while Academy of Finland has launched "6 Genesis" project.
What is 6G technology? Some people expect more than just a faster version of 5G. For example, there should be no limitation of coverage to ground level. Instead, it must provide undersea and space coverage. It must enable higher Artificial Intelligence (AI) characteristics. In fact, some researchers consider it as an "AI-empowered" network [2]. It should not merely involve AI but it must integrate AI networking functions and tool. In addition, secrecy, privacy and risk mitigation must be a core component of its architecture [3]. In this review, we have investigated privacy and security challenges along with potential applications of 6G network. An overview of different dimensions of 6G networks is shown in Fig. 1.
After commercialization of 5G network, academia and industrial experts have started thinking about next 6G network, services and requirements behind it. If we look at standardization methods of 5G technology, three aspects were investigated as, ultra-reliable and low latency communications (URLLC), massive machine type communications (mMTC) and enhanced mobile broadband (eMBB). Although such scenarios are not fully investigated for 6G networks, however some pioneering works [4][5] forecast the idea to link everything via unlimited, reliable and instantaneous wireless resources. We have shown an overview of 6G coverage in Fig. 2.
To bring this revolution to connect everything worldwide, 6G will require extreme communication techniques such as smart living based wireless brain-computer interactions [6], smart working based on seamless holographic projection [7] and smart design considering real-time digital twins [8]. The evolution from 5G to 6G is summarized in Table I. We have provided some performance metrics for 6G networks below and compared with conventional 5G requirements.
 Mobility: The highest speed to be achieved will be increased from 500 km/h to 1000 km/h.
 Reliability: 99.99% reliability will be achieved to support unmanned vehicles including AUVs and collaborative robotics.
 Throughput: A maximum throughput of 1 Tb/s will be needed for 6G which is 1000 times speedy than 5G. 100 times advancement is expected.
 Energy and Spectrum Efficiency: 100 times energy efficiency and 10 times spectrum efficiency will be achieved.
The above described metrics involve disruptive features in 6G networks to use more flexible frame structure, more frequency bands and more spatial dimensions. Many industrial experts and technologies have discussed to meet these requirements. Such as, Space-Air-Ground integrated network [9] have suggested to enhance the spatial degrees of freedom by incorporating airborne, terrestrial and satellite networks, which extend 2D into 3D space for reliable and efficient connectivity [10]. Under-utilized high frequency bands can be explored through Terahertz (THz). Visible light communication (VLC) is a promising candidate for tens of GHz bandwidth [11] and 1 Tb/s throughput. Meanwhile, AI driven communication [12] with intelligent control will be possible.

II. HISTORICAL OVERVIEW
A. 1G and 2G -10 Times Reduction 1G and 2G networks provide the basic service of voice calling. Significant contribution has been made from 1G to 2G realization, such as China Mobile's annual report revealed 10 times price depletion from 0.1 to 0.01 US dollar/minute [13]. In addition, world's population using these services also increased from 10% (1G) to above 50% (2G) within 20 years [14].

B. 3G and 4G -1000 Times Reduction
3G and 4G networks provide the key service of data transmission. Technical development from 3G to 4G include orthogonal frequency division multiplexing (OFDM) and multiple-input multiple-output (MIMO) and user-sensitive goal of1000 times price reduction. Initial 3G users are limited to business community to access company resources and emails, while further enhancement occurs only after the deployment of 4G networks.

C. 5G and 6G -1000 Times Reduction
An explosive growth of 5g and beyond is found to facilitate human-to-machine and machine-to-machine communications. Although the existing 5G is still based on eMBB with the similar price strategy of 4G networks. However, it will be more reasonable to charge on the basis of connection rather than data traffic. According to FTTH systems, China is charging 100-200 US dollars for each terminal [15]. However, 100 trillion sensors are expected to be manufactured and connect to internet by the end of 2030 to revolutionize 6G. Hence, 1000 times price reduction will be required to develop a sustainable smart society.   [18][19][20]. There is a growing inclination in research publications in this domain. Recently, Yang Lu et al. [21] filtered extant articles about 6G as various institutes have been conducting research on several approaches towards 6G. Publishing trend between 2016 and 2020 is depicted in Fig. 3. X-axis shows the number of publications while Y-axis shows specific year. It can be seen that maximum papers were published in IEEE conferences and journals. E. Basar et al. [22] have discussed MIMO paradigm for 6G. They focused on research activities related to device manufacturing capabilities. S.M. Bohloul et al. [23] have made a good discussion about trends, opportunities and developments in 6G. They have outlined communication technologies e.g. tactile internet, flying networks and holographic calls for future networks in 2030. In [24] and [25], future trends and applications enabling 6G technology have been summarized. Blockchain technology, human centric services and key performance indicators of 6g are investigated in these studies. 6G prospect, challenges and key performance indicators are defined. Authors have illustrated the role of OWC [26] in 6G technology. Some recent articles have provided detailed discussions about green 6G network architecture [27], 6G spectrum management [28], security challenges [29], potential solutions [33], machine learning technologies for 6G [30][31] and performance evolution of terahertz [32] communications. Some publications have discussed data center connectivity [34] and practical implementation of multiple access [35] for 6G networks. Network patterns for 6G are highlighted in some studies [36][37]. 6G based AI applications [38][39] which will unlock the full potential of radio signals are outlined in some studies. Hardware foundation of AI [40] is proposed in an article. Zhao et al. [41] have provided a survey on intelligent reflective surfaces for 6G networks. These promising materials can enhance the spectral efficiency [42] in 6G networks. In addition, several countries have started research projects to initiate, develop, define and reshape framework of 6G networks. Table IV summarizes country wise research initiatives in 6G networks.

2021-2026
South Korea Government of Korea will invest $169 million to secure 6G and planning to launch 6G pilot project in 2026. www.ijacsa.thesai.org IV. TECHNOLOGY BREAKDOWN We have discussed each generation in the aspects of frequency, spatial and time domains as given below. Technology breakdown from 1G to 6G is also displayed in Fig. 4.

A. Spatial -10 Times
The purview of the Space-Air-Ground integrated network enfolds an extensive range of terminals, satellite communications, flying drones, which proffers two times cost reduction with low number of base stations. Ultra-scale MIMO can improve 50% throughput without extra costs; thereby 1.5 times cost reduction can be achieved. Intelligent adaptation of beam eventually brings three to four times reduction, while 10 times reduction is possible through different network architectures.

B. Frequency -10 Times
In frequency domain, the cost reduction is dependent on utilization of low cost spectrum. Although mmWave, VLC and THz are capable to offer significant bandwidth for wireless transfer, the befitting scenario is indoor users with pedestrian mobility, which is 70% of the overall traffics. Thus, higher frequency bands can facilitate with 3 times reduction. Moreover, another 3-4 times reduction is possible by flexible usage of multiple frequency bands.

C. Time -10 Times
Another prominent alternative is to profoundly impact the resolution of time-frequency resource to feature flexible frame structure and integrate modulation scheme like index modulation. A fast mode adaptation can enhance the performance with a massive combination of duplex schemes, modulation techniques and frame structures. By incorporating several techniques, we expect 1000 times reduction can be achieved. The core element is AI-assisted intelligent communication which can reduce cost up to 20-50 times.

V. 6G REQUIRES A NEW PARADIGM
Next generation 6G network requires wide bandwidth for high resolution and high carrier frequencies for small antennas. A potential issue is to analyze and process radio systems over wide bandwidth without prior information of signal, modulation and carrier frequency. An idea option is photonics defined system as it can provide high spectrum capacity with extreme bandwidth. It is an extended version of microwave photonics through coherent optics, optical computing and photonics DSP. A paradigm shift and hyper-S curve [43] presenting a revolution of mobile of communication technologies is shown in Fig. 5.
Open loop control, reduced feedbacks, software defined systems and interference cancellation have developed this system. A radical innovation is expected in case of 6G which will result into a new S curve. The logical start of 6G is shown in Fig. 6.

VI. KEY AREAS IN 6G NETWORKS
We will discuss key areas in 6G networks and we have also investigated privacy and security issues in these areas.

A. Real-Time Intelligent Edge
It is not fully possible to implement Unmanned Aerial Vehicle (UAV) networks with existing technologies as it needs real time intelligence and extremely low latency to control the network. Although 5G technology has supported autonomous driving, however prediction, self-adaption and self-awareness for network entities is not supported [44]. Thus, a new technology is required to overcome these issues. It will be possible through 6G technology to enable AI-powered services. As AI will be incorporated in vehicle networks, it will support several security mechanisms. However, it will cause new privacy and security issues. Tang et al. [45] www.ijacsa.thesai.org investigated that both network and physical environments should be considered for a vehicle network as it can reduce malicious activities.

B. Distributed AI
6G networks will support Internet of Everything (IoE). It will make 6G network advance enough to take intelligent decisions [27]. In addition, IoT needs to support various requirements. 1) The edge device must compute and store data.
2) It should have the capability to clean and abstract data [46]. This approach can improve the privacy and security of the network. Machine learning algorithms can be integrated with 6G to ensure security [47] and data integrity.

C. 3D Intercoms
In 6G network, network optimization and designing will move from 2D to 3D [48]. 6G technologies will be capable of supporting 3D communication to enable undersea, UAVs and satellite communication. A 3D intercom can facilitate this feature with accurate time and location. In addition, resource management, routing and mobility characteristics also require network optimization in 3D intercom. Currently, THz bands are being experienced. With this band, some new technologies e.g. quantum and molecular communications can be applied for remote communication [49]. Wei et al. [50] highlighted some security risks for authentication process. In addition, performance of 6G networks in undersea environment is still unpredicted. Once 6G network operations in undersea environment are possible, more opportunities and challenges will emerge in near future. Fig. 7 illustrates some application scenarios supported by 6G technologies.

D. Intelligent Radio
The transceiver devices can be separated in 6G while they were designed together in earlier generations. Hence, it has the capability to update itself. Some operating systems are developed on the basis of hardware information and AI technology. Researchers have investigated signal jamming and suspicious activities in data transmission. Thus, 6G will enable intelligent and secure data transmission. VII. 6G TECHNOLOGIES In this section, we have discussed 6G technologies and associated privacy and security concerns. Table V presents an overview of 6G technologies and security issues. While Fig. 8 illustrates potential key technologies of 6G networks. A. AI AI plays the most important role in future network infrastructures as compared to other technologies. AI has gained a lot of attention from various research groups. With this growing research, various privacy and security problems are also emerging. Although AI is also integrated in 5G technology, however it is considered as the key component of future 6G. AI technologies are subdivided into physical layer consisting of network infrastructure, architecture layer, computing layer which contains software defined networks, edge/cloud computing and network function virtualization.

B. Quantum Communication
Another promising technology in 6G network is quantum communication. It can significantly increase reliability and security of data transmission. Quantum state is affected with any adverse eavesdrop. Quantum communication offers security with essential breakthroughs. It can provide solutions and elevate communication which is not possible to achieve www.ijacsa.thesai.org through traditional communication techniques [61]. However, it is not the only panacea for all security threats. Although research has been carried out to develop quantum cryptography, but fiber attenuation is a serve issue in long distance quantum communication. Zhang et al. [62] and Nawaz et al. [52] have presented quantum mechanism for secure communication through quantum key distribution models.

C. Blockchain
Another prominent technology is 6G network is blockchain. It has several used such as spectrum sharing, distributed ledger technology and network decentralization. S. Dang et al. [48] used network decentralization to enhance network performance. Strinati et al. [63] also increased authentication security through distributed ledger technology. Blockchain technology can also overcome spectrum monopoly and low spectrum utilization [64]. Blockchain privacy concerns are related to communication, authentication and access control. X. Ling et al. [65] have illustrated authentication and secure network access features through blockchain technology.

D. Visible Light Communication (VLC)
VLC is a promising technology to meet the rapidly growing needs of wireless connectivity [66]. VLC has been deployed in vehicular Ad Hoc networks and indoor positioning systems. J. Luo et al. [67] have presented an indoor positioning system based on VLC. It is noticed that VLC limits EM interference. Some research studies have demonstrated high speed data transmission by using LEDs. Some deficiencies exist which affect the performance of VLC communication. In particular, VLC technology mainly supports indoor scenario as it is severely affected by natural light. The security issues of VLC technology include communication problem and malicious activities. A SecVLC protocol [57] is developed for secure data transmission in a vehicular network. Fig. 9 presents an overview of OWC in 6G technologies. We have provided a detailed discussion of OWC and 6G our recent systematic study [68]. 6G is expected revolution in UWPT [69] and UWOC.

E. Terahertz Technology (THz)
Existing RF band cannot be utilized for future 6G technologies [70]. It has spurred the demand for THz technology. THz communication technology used 0.1-10 THz band. Moreover, it exploits optical signals and EM waves. Huang et al. [27] have highlighted several benefits of THz band including 100 Gbps data rate, high security and limited eavesdropping. THz can significantly minimize intercell impact [64]. Strianti et al. [63] have investigated energy consumption problem in THz communication. THz faces security risks of authentication and malicious attack. We have summarized a comparison between VLC and THz communication in Table VI.

VIII. POTENTIAL CHALLENGES
There are several critical challenges which can affect future 6G technology. In this section, we have discussed big data, power, latency and hardware design challenges.

A. Wireless Big Data
AI technology has proven its great stature in computer vision tasks. It has potential application in ImageNet big data sets. Such supervised learning method can solve complex optimization challenges in wireless communication. However, there exist many serious concerns for developing public wireless data sets for research purpose. As big data is processed and stored through cloud computing. The DIoE will cause new challenges to manage this data.

B. Portable and Low-Latency Algorithm
The current AI technologies are developed to meet certain requirements; however, it has limited migration capability. However, an important performance metric is to design portable and low latency algorithms. In addition, latency tradeoff and accuracy is highly required as compared to than traditional computer vision tasks.

C. Hardware Co-Design
High density parallel computing methods are required in AI-assisted technologies. Wireless network architecture requires certain parameters to support AI-assisted communication. Moreover, computer performance can face degradation in case of advance materials e.g. graphene transistors and high temperature superconductors. www.ijacsa.thesai.org

D. Power Supply
6G technology can make an efficient connection between mobile devices. Energy-efficient algorithms and strategies must be adopted in such cases. 6G will introduce new power control mechanism such as advance wireless power transfer (WPT) for smart devices. It will enable energy harvesting and optimization technique for efficient performance in harsh environment such as undersea environment.

E. Network Security Issue
Researchers need to focus on privacy concerns in future 6G technology. They must investigate new security approaches for secure data transmission. A significant extension in 5G security methods can also enable 6G security. Researchers can find new techniques to efficiently integrate THz with mmWaves. It can put profound impact on 6G privacy and security mechanism.

IX. 6G POTENTIAL APPLICATIONS
Every new epoch of network technology introduces new services. In this section, we have outlined some potential applications for future 6G technology.

A. Multi-Sensory XR Applications
The low latency and high bandwidth of 5G technology has extended the VR/AR experience for 5G users. Nonetheless, some existing challenges should be removed in 6G network. The VR/AR experience will be enhanced in 6G network. Multiple sensors can be allocated to gather sensory data. Hence, the XR in 6G network will be formulated from URLLC and eMBB. The security concerns of eMBB and URLLC include internal communication, access control and malicious attack. Chen et al. [71] have investigated security problems in URLLC applications. J.M. Hamamreh et al. [72] have suggested a technique to improve security again URLLC attacks. Similarly, Yamakami et al. [73] have proposed a 3D model for secrecy risks in XR applications.

B. Connected Robotics and Autonomous Systems
Another promising application of 6G technology is the connected robotics and autonomous systems. A comprehensive autonomous system is required in 6G network as compared to 5G. This system should be based on a multi-dimensional network. In addition, the system must be capable to embed AI across the network. This feature will support automatic controlling of internal components. Strianti et al. [63] have envisioned resource control, caching and automatic handling in network. They developed an automated factory which contains cloud services, database and UAV networks to make it a complete autonomous system. 6G will be helpful for underwater robotic tasks such as security, imaging and rescue. 6G will enable efficient surveillance, navigation and robotic communication. It will develop a reliable, secure and smooth communication channel for real-time applications. Low latency and high speed data transmission of 6G will be helpful to obtain video data.

C. Wireless Brain-Computer Interactions
The concept of wireless BCI is to develop a link between device and human brain. This device can be placed inside or outside the body. The key application of wireless BCI is to control auxiliary equipment for disabled people. It is expected that BCI will have more applications with involving 6G technology. In 2015, Chen et al. [74] developed a braincomputer interface to speed up spelling. The security risks of wireless BCI contain encryption and malicious behavior. Several research studies [75][76] have discussed security issues, protection techniques and hacking applications to mitigate security issues.

D. Accurate Indoor Positioning
With evolving GPS, outdoor positioning systems have been developed accurately. However, indoor position systems need research attention to cope up with complicated indoor EM propagation. New aspects of full-fledge applications are expected with reliable and accurate indoor positioning services. However, alone RF communication cannot achieve accurate indoor positioning. Such crucial application can only be realized with 6G technology.

E. Holographic Communications
6G will make it possible to realize virtual in-person meeting than traditional video conferencing. It can be achieved through a realistic projection of real time mobility in short time. It is not sufficient to transmit 3D image with voice to realize in-person presence. However, it requires a stereo audio incorporated in 3D video. We can state that user interacts with holographic data and can carry out possible modifications as needed. This scenario can be captured by reliable communication networks with extremely large bandwidth.

F. Tactile Communications
After realizing holographic communication for virtual inperson meeting, it is advantageous to carry out tactile communication to transfer the physical interaction remotely. Specifically, it includes interpersonal communication, cooperative automated driving and teleoperation. Stringent demands or these applications can be met through reliable cross-layer communication-system. Moreover, delay can be mitigated by carefully handling handover, scheduling, queuing and buffering.

G. Internet of Nano-Things
Nanotechnology is providing remarkable opportunities to design advance materials. It has developed nanodevices like nanosensors. It has the capability to perform simple task and enable internet connectivity. IoNT [77] is developed by integrating nanotechnology with IoT. It has the ability to sense and transmit information. IoNT can be deployed with allied technologies such as big data, cloud computing, WSN, UWSN [78][79] and IoT. However, IoNT faces limited memory space issue for real-time implementation as data storage depends on the size of nano memory. Another potential issue is high biological noise and congestion control in nanodevices. An overview of 6G applications is summarized in figure 10.

H. Intelligent Internet of Medical Things (IIoMT)
IIoMT will remove space and time hurdles to perform surgical operations. 6G will provide high speed communication for efficient performance of telesurgery beyond boundaries. IIoMT will make use of holographic communication, tactile communication and AR/VR to assist remote doctors. Thus, 6G www.ijacsa.thesai.org technology will ensure intelligent healthcare. It is expected to bring mobile hospital technology which can remove ambulance services. In future, it can offer medical devices to perform special medical tasks which can greatly reduce the possibility of medical staff in contacting with viruses. An overview of 6G applications is summarized in Fig. 10.

X. CONCLUSIONS
During the worldwide deployment of 5G, academia and industrial experts have started conceptualizing 6G. Unlike 5G networks, the next generation 6G will focus on communication among users, industries and multiple objects. Network transmission performance is no longer only important parameter; blockchain technology, IoT and AI have become important components. 6G network will keep penetrating into virtual society, human-perceived actions and ubiquitous spaces. It will provide a secure, reliable, intelligent, deep, seamless and holographic network infrastructure. 6G network will fulfil the growing demands of industries with continuous innovations of AI. We outlined research activities in different countries which aim to create a vision of 6G. 6G will enable many new technologies such as VLC, tactile and holographic communication. In conclusion, we expect that this review article will pave the way to identify 6G roadmap. This paper reviews the key technologies and areas of 6G networks and highlights a prospective on future research. We have presented a vision of 6G network as a research guide for readers. We have also addressed key features, security challenges and explained potential applications which will be supported in 6G. We have presented an overview of 1G to 6G. We then examine the key areas of 6G network. This review article started by highlighting the historical overview of communication technologies and their pivotal elements aiming at fostering future 6G in various dimensions. Then, we discussed technology breakdown, potential challenges associated with future 6G technology and possible solutions to foster 6G. In addition, we have profoundly examined research activities in different countries including industries and research institutes. Finally, this study concludes with potential applications of future 6G. The key contribution of our study is that it clarifies the promising solution for potential issues and challenges in 6G technology. Thus, this review will open new horizons for future research directions.