6G Wireless Communications and Mobile Networking E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
6G Wireless Communications and Mobile Networking introduces the key technologies behind 6G wireless communication and mobile networking to the reader. The book starts with a general vision of 6G technology, which includes the motivation that drives 6G research, the international organizations working on 6G standardization and recent progress in 6G research. Separate chapters on millimeter-wave and terahertz-wave technologies in 6G, the development of latest 6G antenna technology as well as related wireless communication applications are included in the contents. The book also provides details about the 6G network layer, such as self-organizing network driven by network slicing, software-defined networking and network function virtualization. Finally, it covers some popular research topics, including the challenges and solutions to massive 6G IoT networks, 6G cloud/edge computing and big data systems that may appear in the foreseeable future.
Key Features:
- Provides a complete introduction to 6G vision and technology
- Consists of both basic theories and frontier technologies
- Separate chapters on key topics such as 6G physical layers, millimeter wave and terahertz technology and advanced antenna arrays
- Covers future trends and applications such as intelligent management systems, 6G IoT networks, cloud/edge computing and big data applications
This focused reference will significantly enhance the knowledge of engineering students and apprentices involved in the field of telecommunications. Readers interested in cutting edge wireless networking technologies will also benefit from the information provided.
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Seitenzahl: 397
Veröffentlichungsjahr: 2021
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PREFACE
Although 5G mobile networks have been standardized and deployed worldwide since 2020, the requirements of wireless communication services are not completely met, taking into account industrial and other challenging applications. Thus, the 6G wireless technologies kicked off the initial research and are expected to be applied around 2030. Different from 5G, the next generation networks highlight new features like high time and phase synchronization accuracy, near 100% geographical coverage, and high cost-efficiency. Compared with Gbps-level transmission data rate in 5G, a number of useful applications in 6G, such as high-quality 3D video, virtual reality (VR), and a mix of VR and augmented reality (AR), need Tbps-level transmission data rate that could be achieved with terahertz (THz) and optical technologies. Due to the big datasets generated by heterogeneous networks, the technology of artificial intelligence (AI) is regarded as a promising aid to wireless systems in a bid to improve the quality of service (QoS), quality of experience (QoE), security, fault management, and energy efficiency.
With the booming of higher frequency and more energy-saving equipment, THz and photonic communications become economically feasible. The advanced integrated circuit (IC) technology nowadays makes radio frequency (RF) devices and antennas more flexibly designed and highly integrated. Flexible RF components could work with artificial intelligence (AI) algorithms to make wireless networks more adaptive to user demand and RF environment. The 6G will undoubtedly expand the frequency from below 95 GHz to the high-frequency millimeter-wave and terahertz range. Those new frequency bands will focus on short-range communications by enabling the design of much tinier RF devices to support technologies like ultra-massive antenna arrays.
6G networks are envisioned to be full dimensional and would address every potential demand of services. A smart city is such a typical scenario where various Internet of things (IoT) applications proliferate to help citizen services. Other than conventional scenarios, smart city IoT services will rely on 6G networks for broad coverage, ultra-low latency, and reliable connection. As different IoT applications may have different service holders, it becomes necessary to employ network slicing (NS) to gain distinct virtual networks and differentiated quality of service guarantees. In the meantime, computing technologies such as cloud computing, fog computing, and edge computing are critical to network resilience, lower latency, and time synchronization. Cloud computing provides the ability to use flexible and telescopic services through various hosted services provided by the Internet. Edge computing, on the other hand, prefers the open platform using the network, computing, storage resources close to the site of the object in order to avoid the relatively long delay of accessing the cloud data center. Finally, big data technology can work with 6G to get hidden patterns, unknown relevance, potential trends, and other information.
The content of this book is summarized as follows.
1. In Chapter 1, we provide readers with a general vision of 6G, including the inevitability of 6G research, the international organizations for 6G standardization, and also the 6G research progress.
2. In Chapter 2, we introduce millimeter-wave technologies in 6G, including large-scale MIMO systems, precoding technology, and different kinds of beamforming structures. It also systematically summarizes the requirements on 6G millimeter-wave devices.
3. In Chapter 3, we focus on the development of the latest 6G antenna technology. In particular, it highlights the technical trends of a large-scale antenna from antenna design and synthesis to feed network and antenna selection.
4. In Chapter 4, we highlight the characteristics and application fields of the terahertz wave, especially the application in wireless communication. Two mainstream terahertz wireless communication systems are explained in detail under the context of 6G.
5. In Chapter 5, we propose a self-organizing network (SON) driven network slicing architecture, where software-defined networking (SDN) and network function virtualization (NFV) act as the key enablers. Some preliminary simulation results are given to validate the efficiency of the design.
6. In Chapter 6, we present an overview of the developing trend of IoT applications and discuss its relation to 6G. This chapter also sheds light on the challenges and solutions to future cellular massive IoT.
7. In Chapter 7, we give a systematic introduction to the cloud/edge computing and big data system in 6G. New applications, such as sensing, positioning, and slice-specific function, can significantly benefit from the new network computing architecture and AI-powered big data analysis.
List of Contributors
Explaining 6G Spectrum THz, mmWave, Sub 6, and Low-Band
Xianzhong Xie1,*,Bo Rong2,Michel Kadoch3Abstract
This chapter aims to provide readers with a general vision of 6G. Firstly, we give a simple overview of various aspects related to 6G, including inevitability of 6G research, international organizations for standardization, and also 6G research progress of some countries/regions. Then, 6G spectrum compositions are discussed in detail with emphasis on SUB-6, mmWAVE, and Terahertz (THz).
THE SIXTH GENERATION MOBILE COMMUNICATION (6G)
The development of mobile/wireless communication has gone through the process of 1G/2G/3G/4G, and it has entered a critical stage of 5G commercial development. From the historical perspective of industrial development, the mobile communication system has been updated every ten years. The increasing demand for user communication and the innovation of communication technology is the driving force for the development of mobile communication [1]. However, 5G will not meet all requirements of the future of 2030 and beyond [2]. Researchers now start to focus on the sixth-generation mobile communication (6G) networks. Some countries and organizations have already initiated the exploration of 6G technology with the launch of 5G commercial deployment in major countries around the world.
The Inevitability Of 6g Research
The 10-year Cycled Rule
Since the introduction of the first generation (1G) mobile communication system in 1982, a new generation of wireless mobile communication systems has been updated approximately every 10 years, as shown in Fig. (1). It will take about 10 years from conceptual research to commercial applications [3]. In other words, when the previous generation enters the commercial period, the next generation begins conceptual and technical research. 5G research started 10 years ago, and now 6G research is in line with the development law of mobile communication systems. It may take about ten years for 6G to arrive, but research on 6G cannot be delayed. Mobile communications will stride towards the 6G era.
Fig. (1)) The evolution of mobile communication systems.“Catfish Effect”
The “catfish effect” means that it also activates the survival ability of the small fish when the catfish disturbs the living environment of the small fish. It is to adopt a means or measures to stimulate some enterprises to become active and invest in the market to actively participate in the competition, which will activate enterprises in the same industry in the market. 5G is different from previous generations of mobile communication systems mainly aimed at IoT/vertical industry application scenarios. Many vertical industry members will definitely participate in the 5G ecosystem with the large-scale deployment of 5G networks. The in-depth participation of emerging companies (especially internet companies born with innovative thinking) in the future will have a huge impact on the traditional communications industry and even a revolutionary impact compared with the status quo dominated by traditional operators, which is called “catfish effect”.
The Explosive Potential of IoT Business Models
IoT is the inevitability of the internet from top to bottom in the industry. It is an extension from the inside out, with the cloud platform as the center. Just as the emergence of smartphones stimulated 3G applications and triggered the demand for large-scale deployment of 4G, it is believed that certain IoT business models will also stimulate the 5G industry to burst at a certain point in the 5G era, which will stimulate the future needs of 6G networks. To accommodate the stringent requirements of their prospective applications, we need to have enough imagination. We must prepare in advance for the possible future network and lay a good technical foundation [4]. Based on the above analysis, we can draw the conclusion that now is the right time to start the research on the next generation wireless mobile communication system.
The 5G Performance Would Limit New IoT Applications
Despite the strong belief that 5G will support the basic MTC and URLLC related IoT applications, it is arguable whether the capabilities of 5G systems will succeed in keeping pace with the rapid proliferation of ultimately new IoT applications [5]. Meanwhile, following the revolutionary changes in the individual and societal trends, in addition to the noticeable advancement in human-machine interaction technologies, the market demands by 2030 are envisaged to witness the penetration of a new spectrum of IoT services. These services deliver ultrahigh reliability, extremely high data rates, and ultralow latency simultaneously over uplink and downlink [6]. The unprecedented requirements imposed by these services will push the performance of 5G systems to its limits within 10 years of its launch. Moreover, these services have urged that 6G should be capable of unleashing the full potentials of abundant autonomous services comprising past as well as emerging trends.
International Organization for Standardization
International Telecommunication Union (ITU)
According to the ITU work plan, the RA-19 meeting in 2019 will not establish a new IMT technical research resolution. It indicates that the research cycle from 2019 to 2023 is still mainly for 5G and B5G technology research, but the 6G vision and technology trend research will be carried out from 2020 to 2023. The mainstream companies in the industry generally believe that it is more appropriate to establish the next generation IMT technology research and naming resolution at the RA-23 meeting in 2023. ITU-T SG13 (International Telecommunication Union Telecommunication Study Group 13) established the ITU-T Focus Group Technologies for Network 2030 (FG NET-2030) at its meeting in July 2018. The FG NET-2030 intends to define the requirements of networks of the year 2030 and beyond [7]. 6G research work was carried out at ITU-R WP5D (the 34th International Telecommunication Union Radio Communication Sector 5D Working Group meeting) held in February 2020, which includes the formulation of a 6G research timetable, future technology trend research reports, and the writing of future technology vision proposals.
The Third Generation Partnership Project (3GPP)
3GPP is the main promoter and integrator of communication system technical specifications managing the standardization work such as the introduction of communication system requirements, system architecture design, security, and network management. 3GPP has completed the development of the first version of the 5G international standard Release 15 (R15) focusing on supporting enhanced mobile broadband scenarios and ultra-high reliability and low-latency scenarios in June 2018. The development of a complete 5G international standard Release 16 (R16) will be completed in Autumn 2020, which will fully support the three application scenarios determined by the ITU. Also, 3GPP is promoting 6G research and standardization activities. 3GPP Release 17 (R17) has started to investigate advanced features that would shape the evolution toward 6G [8], and substantive 6G international standardization is expected to start in 2025.
Institute of Electrical and Electronics Engineers (IEEE)
To better summarize and sort out the related technologies of next-generation networks, IEEE launched the IEEE 5G Initiative in December 2016 and renamed it IEEE Future Networks in August 2018 to enable 5G and the future network. IEEE is also developing corresponding 5G standards, which are expected to be submitted to ITU for approval in 2020. At present, IEEE has carried out some 6Gtechnical seminars. The 6G wireless summit will be held by IEEE in March every year, and the first 6G wireless summit was initiated by IEEE in the Netherlands on March 25, 2019. The industry and academia were invited to publish the latest insights on 6G. The theoretical and practical challenges that need to be addressed to realize the 6G vision are discussed. The global 6G research vision, requirements and potential approaches were published in 6G White Papers at the end of June 2020.
6G Research Progress In Some Countries/regions
6G communication is still in its infancy. The 6Gresearch race from academia can be said to have started in March 2019, when the first 6G Wireless Summit was held in Levi, Finland. Some researchers also defined 6G as B5G or 5G+. Preliminary research activities have already started in some countries/regions. The US president has requested the deployment of 6G in the country. China has already started the concept study for the development and standardization of 6G communications in 2019. Most European countries, Japan, and Korea are planning several 6G projects.
European Union
The European Union initiated consultation for 6G technology research and development projects aiming to study key technologies for next generation mobile communications in 2017. Finnish 6G research activity is coordinated by the University of Oulu in 2018, where a 6G initiative was launched. The EU's preliminary assumptions for 6G are that the peak rate should be greater than 100 Gbit/s, the single channel bandwidth can reach 1 GHz and the terahertz frequency band higher than 275 GHz should be used. The European Union launched a three-year 6G basic technology research project in 2019. The main task is to study the next generation error correction coding, advanced channel coding and modulation technologies used in 6G networks. Besides, the EU has also initiated a number of terahertz research and development projects. The EU has listed the development of terahertz communications as a 6G research program. A research group based on the EU’s Terranova project is now working toward the reliable 6G connection with 400 Gbit per second transmission capability in the terahertz spectrum.
United States
The FCC (The United States Federal Communications Commission) launched CBRSD (Public Wireless Broadband Service) in the 3.5GHz frequency band in 2015, which dynamically manages different types of wireless traffic through a centralized spectrum access database system to improve spectrum utilization efficiency. The experts from FCC proposed three key 6G technologies at the “Mobile World Congress 2018-North America” summit in September 2018, including new spectrum (terahertz frequency band), large-scale spatial multiplexing technology (supporting data hundreds of ultra-narrow beams) and blockchain based dynamic spectrum sharing technology. In addition, FCC announced that it would open the terahertz frequency band (95 GHz-3 THz) for using in 6G technology trials in March 2019, thereby setting the US as the pacesetter in the 6G research race.
Japan
Nihon Keizai Shimbun reported that Japan’s NTT group has successfully developed new technologies for B5G and 6G in July 2018. One is Orbital Angular Momentum (OAM) technology. It has realized the superimposed transmission of 11 radio waves that are several times of 5G. OAM technology uses a circular antenna to rotate radio waves into a spiral for transmission. Due to the physical characteristics, the high number of revolutions will make the transmission more difficult. NTT plans to realize the superposition of 40 radio waves in the future. And the other is terahertz communication technology. The development of terahertz technology is listed as the top of the “ten key strategic goals of national pillar technologies”. And a budget of more than 1 billion yen is proposed in the fiscal year 2019 to start research on 6G technology. The peak transmission rate reaches up to 100 Gbit/s. Japan still faces the problem of extremely short transmission distances, but the transmission speed can reach 5 times that of 5G. Japan readies US$2 billion to support industry research on6G technology. NTT and Intel have decided to form a partnership to work on 6Gmobile network technology. In addition, an EU–Japan project under Horizon2020 ICT-09-2017 funding called “Networking Research beyond 5G” also investigated the possibility of using the THz spectrum from 100 to 450 GHz.
South Korea
Experts from SK Telecom’s ICT R&D Center presented 3 technologies for future 6G networks at a cutting-edge technology seminar held at New York University in October 2018, including terahertz communications and de-cellular architecture (fully virtualized RAN+ large-scale antennas) and non-terrestrial wireless networks. Samsung Electronics and SK Telecom work together to develop technologies and business models related to 6G. LG Electronics established a 6G research center in collaboration with the Korea Advanced Institute of Science and Technology. In addition, SK telecom has reached an agreement with two equipment manufacturers, Ericsson and Nokia, to jointly develop 6G technologies. Korean operators achieved download speeds of 193~430 Mbps in the 3.5 GHz (sub-6GHz) frequency band in April 2019.
China
China began to study the 6G mobile communication system to meet the inconstant and rich demands of the IoT in the future at the end of 2017, such as medical imaging, augmented reality and sensing. In addition to solving the problems of wireless communication between people and wireless internet access, it is also necessary to solve the communication between things and things and between people and things with the expansion of the use of mobile communication in the future. 6G communication technology mainly promotes the development of the IoT. The Ministry of Science and Technology of China (MSTC) declared its goal of leading the wireless communication market in the 2030s by expanding research investment in 6G, and issued a notice on the annual project application guidelines for key special projects in 2018 such as “Broadband Communications and New Networks” for the national key research and development plan, 5 of which involve B5G/6G. In 2019 the MSTC also planned to set up two working groups to carry out the 6G research activities: the first is from government departments to promote 6G research and development, the second is made up of 37 universities, research institutes and companies, focusing on the technical side of 6G.
6G SPECTRUM COMPOSITION
Spectrum Requirements for 6G
Our society will become data-led due to almost no time-delay wireless connections by around 2030. Therefore, 6G will be expected to be used to promote the development of the wireless technology we are familiar with today. And it will be expected to achieve a quite good system performance. Fig. (2) presents a synopsis of the evolving wireless cellular communication generation. Specifically, frequency spectrum used by various generations of mobile communication systems is shown. In order to increase the data rate 100 to 1000 times faster than 5G in terms of frequency spectrum [9], 6G may use a higher frequency spectrum than previous generations as a vision for the future.
4G and the previous mobile communication systems all use the Sub-6 GHz frequency band, while the 5G mobile communication system uses both the Sub-6 GHz frequency band (FR1 band, 450 MHz-6 GHz) and the 24-100 GHz frequency band (FR2 band, 24.25 GHz-52.6 GHz) [10]. Researchers realize that although 5G expands the spectrum bandwidth, the current frequency band is still not enough to meet the increasing demand for communication services in the rapidly developing human society. Therefore, in the study of 6G networks, we will consider spectrum resources above 100 GHz such as millimeter wave (mmWave) and terahertz (THz) to increase the transmission bandwidth.
The 6G wireless communication system will use multi-band and high-spread spectrum to increase the transmission rate. The ultimate vision is to make the end-to-end transmission rate reach hundreds of gigabits. It is expected that in the future 6G, the ground mobile communication network, satellite system, and Internet will merge into a large space-air-ground-sea network. Thus, 6G spectrum needs to support space-air-ground-sea integrated communications. Most operating frequencies for space-air-ground-sea communications are assigned by the International Telecommunication Union (ITU) [11]. The mmWave bands can be used in both space-ground and air-ground channels as well as space-air transmission.
Fig. (2)) Spectrum compositions of mobile communication systems.SUB-6
Sub-6 GHz bands have been the primary working frequency in the third generation, 4G, and 5G due to wide coverage capabilities and low cost, which are also indispensable in 6G [12].
The Low Frequency Spectrum
Added Spectrum of 6 GHz
As the saying goes, “A single wire cannot form a thread, and a single tree cannot form a forest.” The development of 5G not only requires a large amount of spectrum resources, but also requires high, medium and low frequency collaborative work. The World Radio Communication Conference in 2019 (WRC-19) reached a global consensus on the 5G millimeter wave frequency band to meet the business needs of 5G systems for ultra-large capacity and high-speed transmission. At the same time, in order to solve the problem of large-scale and deep coverage for 5G systems and achieve a better balance between network capacity and coverage, many countries are focusing on continuous 5G spectrum in the middle and low frequency bands. The new IMT (5G or 6G) usage rules for the 6GHz (5925 MHz-7125MHz) frequency band was included in the agenda of WRC-23 with the vigorous promotion of the Chinese delegation at the WCR-19. 6425 MHz-7025 MHz becoming a new regional (Arab countries, Africa, Europe, CIS countries) IMT frequency band and 7025 MHz-7125 MHz becoming a new global IMT frequency band are being investigated.
Spectrum resources are precious and scarce as the core resource for the development of mobile communication technology. Spectrum planning is the starting point of the industry and will also determine the development direction, rhythm and pattern of the industry. The successful establishment of the new allocation of IMT for the 6 GHz spectrum means that the 6 GHz frequency band will become a potential frequency band for IMT (5G or 6G).Countries around the world will give priority to this frequency band when they build 5G systems and future 6G systems, which promotes the research and development of IMT technology for 6 GHz band and the internationalization of the industrial chain and accelerates the process of 5G global commercial and 6G research and development.
Capacity and Coverage
5G new business applications have driven rapidly increasing in mobile data usage with the further acceleration of 5G commercial use. And enhanced mobile broadband services, fixed wireless broadband services and industrial applications such as smart cities and industrial manufacturing have accelerated the surge in mobile data usage. According to an industry analysis report, data usage per user per month in some leading markets will reach 150 GB in 2025. This requires a large amount of radio spectrum to support undoubtedly. The low and medium frequency bands can provide continuous bandwidth on the order of 100 MHz and good network coverage compared with the millimeter wave frequency band. Furthermore, the performance requirements of network capacity and coverage can be taken into consideration, and network construction costs can be greatly reduced. In addition, the propagation characterization and channel models in Sub-6 GHz bands have been extensively investigated in 5G. Therefore, they are important parts of the 5G or 6G spectrum. As one of the pioneers in the development and deployment of 5G systems, China’s radio management department has been committed to seeking more IMT frequency resources for 5G or future 6G technologies to support its future technologies and applications from the perspective of efficient frequency use and long-term planning.
According to the characteristics of each frequency band, the Sub-6 GHz spectrum will take into account the requirements of coverage and capacity, which is an ideal compromise between peak rate and coverage capability. The frequency
spectrum above 6GHz can provide ultra-large bandwidth, larger capacity and higher speed, but the continuous coverage capability is insufficiently shown in the Fig. (3).
5G wireless infrastructures for Sub-6 GHz will be widely deployed using a beamforming solution, which can greatly expand network coverage and penetration capabilities within buildings. Low frequency bands (such as 600 MHz, 700 MHz, 800 MHz, 900 MHz, 1.5 GHz, 2.1 GHz, 2.3 GHz and 2.6 GHz) have the characteristics of wide coverage and low cost, which are used the large-scale IoT in the future, the industrial automation and the key task IoT. Wireless throughput and capacity will show explosive growth with the mobile networks continuing to accelerate. The above 24GHz mmWave beamforming is also considered as a promising technology to provide ultra-high capacity in 6G coverage.
Fig. (3)) Coverage of frequency bands.The Spectrum Allocation of Sub-6
Spectrum for 5G NR
The spectrum resource from 1.7 GHz to 4.7 GHz is the Sub-6 GHz spectrum allocated to 5G NR by 3GPP, which is the FR1 band, and the maximum continuously allocated spectrum is 100 MHz [13]. The following is an introduction to several major FR1 bands. n77 (3300 MHz-4200 MHz) and n78 (3300 MHz-3800 MHz) are currently the most unified frequency bands for 5G NR in the world. n79 (4400 MHz-5000 MHz) is also used in 5G NR mainly promoted by China, Russia and Japan. n28 (700 MHz) is also highly valued because of its good coverage. This frequency band has been identified as a pioneer candidate frequency band for global mobile communications at WRC-15. If this frequency band cannot be fully utilized, it would be a pity. At present, the US operator T-Mobile has announced the use of n71 (600 MHz) to build 5G. The use of the 3.4 GHz-3.8 GHz band areas is the most complicated [14]. For example, in the United States, the 3.5 GHz frequency band is used for Citizen Broadcast Radio Service (CBRS), while 150 MHz of the frequency spectrum is used for radar communications. In addition, the frequency band can also be used for other commercial services using dynamic access [15]. This dynamic access method saves users from expensive spectrum licenses, only needs to pay the corresponding communication fees to the service provider [16].
Spectrum Selection of Systems
License-free frequency band technology simplifies the process and restrictions for users to connect to the network. Nevertheless, due to the existence of these unlicensed spectrum users in the network, the reliability and security of the wireless network will be disturbed.
In the Sub-6 GHz spectrum area, the most noteworthy unlicensed spectrums are the 2.4 GHz and 5 GHz bands, which are currently the two frequency bands with the highest utilization rate. Among them, bandwidth resources in the 2.4 GHz band are scarce and have been preempted by existing services, making it very crowded. Compared with 2.4 GHz, the available frequency of the 5 GHz band is wider. But with the development of wireless communication networks, it is expected that the 5 GHz band will also be taken up in the next few years. And the current 5 GHz band is divided into several parts by different wireless access methods, which cannot be used uniformly.
Because the spectrum resources used by wireless communication networks and radar systems overlap, the Dynamic Frequency Selection (DFS) mechanism is required by the FCC and ETSI to be used in the Sub-6 GHz band to avoid interference to the radar system. And most of the access methods for the available spectrum in the low-frequency band are the Listen Before Talk (LBT). LBT will cause a large amount of delay idle periods in the use of spectrum, resulting in low spectrum utilization, which is the major resistance to the low-latency vision of 5G networks.
Recently, FCC has promoted additional spectrum for unlicensed usage in the 5.925 GHz - 7.125 GHz range. This is commonly referred to the 6 GHz band, and its regulations are currently been defined. The current trend of extending the availability of unlicensed access, even in the below 10 GHz spectrum region, copes with the necessity of dealing with the spectrum crunch due to the exponential increase of wireless applications [17]. Unlicensed access also eliminated the obsolete licensing paradigm that is known to lead to inefficient spectrum utilization.
mmWAVE
It is known that the target data rate of Sub-6 GHz 5G mobile communications is Gbps level, and the target data rate of 5G mmWave is about 10 Gbps. There are two key ways to increase the wireless transmission data rate: one is by improving the spectral efficiency and the other is by using large frequency bandwidth or spectrum resource [18].
6G mmWave Communication
Advantages of mmWave
In the development of mobile communication systems in the past, low-frequency microwave communications represented by 2.4 GHz and 5 GHz bands have been fully studied. However, as a result of its limited physical frequency width and being occupied by more and more network traffic, low-frequency microwave communication has become very crowded and cannot support the capacity requirements of a new generation of mobile communication systems [19].
Therefore, millimeter wave (mmWave) communication will become one of the main supporting technologies of a new generation of wireless communication systems by virtue of its large bandwidth, low latency, high rate, and friendly positioning function. The mmWave band is usually defined as electromagnetic waves in the 30GHz-300GHz frequency domain. It is located in the overlapped wavelength range of microwave and far-infrared waves, so it has the characteristics of both spectrums. Since its wavelength is 1ms-10ms, it is called the millimeter wave.
Since the development of the Internet, the democratization of information has improved education, e-commerce has promoted economic growth, and business innovation has been accelerated by supporting broader cooperation. And now we are entering an era of the Internet of Everything (IoE) integrating billions or even trillions of connections. Everything will gain contextual awareness, enhanced processing capabilities, and better sensing capabilities. Therefore, The IoE will become an application scenario that must be considered in the new generation of mobile communication systems. The large bandwidth and high speed that mmWave can bring make it possible for businesses such as high-definition video, virtual reality, augmented reality, dense urban information services, factory automation control, and telemedicine. It can be said that the development and utilization of mmWave provides a broad space for 5G applications on the basis of Sub-6 services.
But there is a huge disadvantage that mmWave has serious attenuation during propagation compared with other frequency bands [20]. And because of its large available bandwidth, the original resolution of mmWave communication systems is smaller than that of traditional low-band communication systems [21, 22]. Therefore, mmWave has not appeared in the field of mobile communication for a long time. Qualcomm prescribes the right medicine. Firstly, it uses multi-beam technology to address mobility challenges and improve coverage, robustness and non-line-of-sight operation. Secondly, it uses path diversity to deal with blocking problems, and uses terminal antenna diversity to improve reliability. Finally, several indoor and outdoor OTA tests were carried out to verify mobility. Therefore, the application of mmWave in mobile phones is realized.
Unlicensed mmWave Bands
In the 60GHz mmWave frequency band, about 7 GHz bandwidth is available for unlicensed access, which is much larger than the unlicensed spectrum of sun-6GHz. In such a wide available spectrum, to achieve the peak transmission rate of 100 Gbps required by the 5G network, at least a spectrum utilization rate of 14 bit/Hz per unit time is required. However, the existing data modulation/demodulation technology and related original devices cannot meet such strict spectrum utilization and symbol fidelity. Consequently, one of the current research difficulties in the field of wireless communication systems is how to achieve a transmission rate of 100 Gbps in a high-frequency band above 100 GHz [23]. For example, the FCC decided to open up four mmWave frequency bands above 95 GHz in the United States in 2019. As shown in Table 1, a total of approximately 21 GHz bandwidth is provided for license-free access.
Spectrum Options for 6G
The 26 GHz-100 MHz frequency band is a big opportunity for mobile networks. However, it is the previously unpopular feature of mmWave that makes it very suitable for 5G. It relies on the use of micro-infrastructure, such as small units distributed in dense urban locations. 5G needs to use a wider spectrum than previous generations of mobile systems. According to this trend, it is expected that a higher frequency spectrum will be used in 6G wireless communication networks. Therefore, we initially set a 6G system to be deployed in the sub-terahertz band of 114 GHz-300 GHz as shown in Fig. (4). According to its ultra-high frequency physical characteristics, the sub-terahertz spectrum can complete ultra-narrow beam transmission, thereby improving spectrum efficiency and performing precise positioning [24]. This allows the sub-terahertz band to perform well in scenarios such as future backhaul networks and real-time short-distance communications.
On the other hand, It is predicted that the number of mobile devices worldwide will be more than 125 billion in the 2030s, including the mobile phone, tablet, wearable devices, integrated headsets, implantable sensors and other machine-type users. The Internet of Thing (IoT) system, connecting millions of people and billions of devices, will be one of the essential application scenarios for mmWave B5G and 6G.
Fig. (4)) Spectrum options for 6G.Terahertz (THz)
As the propagation medium of wireless mobile communication systems, spectrum resources will become increasingly scarce as the types and demands of communication services continue to develop. At the beginning of 5G research and development, there have been suggestions to seek scalable spectrum resources in the direction of terahertz and visible light.
Terahertz is a new radiation source with many unique advantages. It has a wide range of applications in many fields, such as semiconductor materials, tomography technology, label-free genetic examination, broadband communications, and microwave orientation. The study of radiation sources in this frequency band will not only promote the major development of theoretical research but also pose major challenges to solid-state electronics and circuit technology. The development and utilization of terahertz spectrum in fields such as communications have been highly valued by Europe, the United States, Japan and other countries and regions. And they have also received strong support from the ITU [25].
6G Terahertz Communication
Terahertz Spectrum
With the development of science and technology, the communication volume and connection volume of wireless services are increasing explosively. Especially since the 5G era, the rapid development of wireless communication technologies in space and sky fields has made 2-dimensional wireless channel performance analysis no longer applicable. In the 6G network, we need to perform three-dimensional modeling of the wireless channel and calculate the relevant performance parameters in the unit cubic space, which requires a wider RF spectrum bandwidth. The frequency spectrum is a physical quantity existing in nature, which cannot be increased or decreased, so it is extremely precious. Terahertz waves refer to electromagnetic waves with frequencies in the range of 0.1THz-10 THz, between millimeter waves and infrared light as shown in Fig. (5). It is the transition zone from macroscopic classical theory to microscopic quantum theory, as well as the transition zone from electronics to photonics, called the terahertz gap of the electromagnetic spectrum.
Fig. (5)) Spectrum of mmWave and terahertz.
