According to recent reports from foreign media, the Huawei laboratory in Ottawa, Canada has begun to develop 6G technology. In June this year, Nokia, Ericsson and SK Telecom announced a strategic partnership to jointly work on 6G development.       

       In July 2018, the International Telecommunication Union established the 2030 Network Technology Research Group. Currently, countries such as the United States, China, Japan, South Korea, and Finland have all embarked on the research for 6G. In February this year, US President Trump repeatedly tweeted: "I want 5G and even 6G to enter the United States as soon as possible." Less than a month after that, the United States began to deploy research on 6G.       

       As early as March 2018, Minister of Industry and Information Technology Miao Wei stated in an interview that China had already begun to study 6G at the beginning of 2018. In December 2018, enterprises such as China Mobile, China Unicom, China Telecom, China Academy of Information and Communications Technology, Huawei, ZTE, Datang, OPPO and Vivo jointly initiated the "Vision and Requirements for Post-5G Systems" project.       

       It is clear that 6G has become the next goal for the global communication industry.

       What is 6G?
       6G refers to the sixth generation of mobile communication technology. 6G will be a fully connected world integrating terrestrial wireless and satellite communication. By integrating satellite communication into 6G mobile communication, global seamless coverage can be achieved. Network signals can reach any remote village, enabling patients in mountainous areas to receive remote medical care and children to receive remote education. Moreover, with the joint support of global satellite positioning systems, telecommunications satellite systems, earth imagery satellite systems and 6G ground networks, the ground-air全覆盖 network can also help humans predict weather and quickly respond to natural disasters. This is the future of 6G. 6G communication technology is not merely a breakthrough in network capacity and transmission rate； it is also aimed at narrowing the digital divide and achieving the "ultimate goal" of "everything connected"， which is the significance of 6G.       

       At the 2018 North American event of the Mobile World Congress held in the United States, Ms. Jessica Rosenworcel, an official of the Federal Communications Commission of the United States, stated: "6G will utilize the terahertz (THz) frequency band, and the 'densification' level of the 6G network will reach an unprecedented level. At that time, there will be numerous small base stations all around us."

       What is terahertz?
       The terahertz frequency band ranges from 100 GHz to 10 THz, which is a much higher frequency band than 5G. From the frequency of the first generation of communication (0.9 GHz) to the current 4G frequency (above 1.8 GHz), the wireless frequencies we use have been continuously increasing. Because the higher the frequency, the larger the bandwidth range that can be allocated, and the greater the amount of data that can be transmitted per unit time, which is what we usually call "the network speed has improved".       

       However, another major reason for the upward development of frequency bands is that the resources in the lower frequency bands are limited. Just like a road, no matter how wide it is, it can only accommodate a certain amount of traffic. When there is not enough space, vehicles will get stuck and unable to move freely. At this point, it is necessary to consider developing another road.       

       The same is true for spectrum resources. As the number of users and smart devices increases, the limited spectrum bandwidth needs to serve more terminals, which will result in a significant decline in the quality of service for each terminal. A feasible solution to this problem is to develop new communication frequency bands and expand the communication bandwidth.

       Why will 6G networks be "densified"?
       This involves the issue of the coverage range of the base station, which is also the problem of the transmission distance of the base station signal. Generally speaking, there are many factors that affect the coverage range of the base station, such as the frequency of the signal, the transmission power of the base station, and the height of the base station, etc. Regarding the frequency of the signal, the higher the frequency, the shorter the wavelength, so the ability of the signal to diffract (also known as diffraction, which refers to the phenomenon where electromagnetic waves encounter obstacles and when the size of the obstacle is close to the wavelength of the electromagnetic wave, the electromagnetic wave can be able to diffract around the edge of the object, diffraction can help cover shadow areas) becomes worse, and the loss becomes greater. This loss increases with the increase in transmission distance, and the coverage range that the base station can cover will also decrease. The frequency of 6G signals is already at the terahertz level, and this frequency has entered the spectral range of molecular rotational energy levels. It is easily absorbed by water molecules in the air, so the distance that 6G signals can propagate in space is not as far as that of 5G signals. 6G requires more base stations for "relay".       

       The frequency bands used by 5G are higher than those of 4G. Without considering other factors, the coverage area of 5G base stations is naturally smaller than that of 4G. Even at the higher frequency band of 6G, the coverage area of base stations will be even smaller. Therefore, the density of 5G base stations is much higher than that of 4G, and in the 6G era, the density of base stations will be incomparably higher.

       Can one purchase goods online using their thoughts in 6G?
       This bold imagination sounds very much like a scene from a science fiction movie. Professor Zhang Ping, a communication expert from Beijing University of Posts and Telecommunications, in his paper "Prospects of 6G Mobile Communication Technology", believes that 6G technology will be closely linked with AI. In the 6G era, the further development of AI will enable the extension from the real world to the virtual world. That is to say, through various technologies such as AR/VR, AI, and 6G, the real world can be sampled, transmitted, and reconstituted into a virtual world. People can use their thoughts to drive the network.       

       Over the course of these 10 years, a great many technologies will undergo updates and iterations, and new technologies will keep emerging. At that time, it might really be possible to use big data to understand human thoughts.       

       There are quite a few difficulties in the specific implementation process.       

       At present, 5G has not yet been widely adopted. As for 6G, there are no clear goals or formal visions yet. Therefore, all the research and development efforts of enterprises and countries are still in the early stage of theoretical research.       

       However, in the field of communication, there is a widely circulated saying of "using one generation, researching one generation, and preparing one generation". Now that the standards for 5G have been initially formulated, the entire standard will be completed under the 3GPP framework around 2021. It is now the right time to start the research and development of 6G. Let's take a look at what shortcomings 5G still has and what 6G needs to address.

       Meanwhile, although the prospects of 6G are promising, at present, there are still many technical and implementation challenges that need to be addressed. These mainly include the following points -       

       1. The route plan still needs to be verified.
       At present, various technical routes for achieving 6G have been proposed by international communication technology research institutions. However, all these proposals are still at the conceptual stage and their feasibility needs to be verified. Experts stated that the Wireless Communication Center of the University of Oulu in Finland is the first institution worldwide to start 6G research. Currently, it is conducting research in four areas: wireless connection, distributed computing, device hardware, and service applications.

       Wireless connection utilizes terahertz or even higher frequency radio waves for communication; distributed computing addresses the latency issues caused by large amounts of data through algorithms such as artificial intelligence and edge computing; the hardware of the devices mainly focuses on terahertz communication, and corresponding antennas, chips and other hardware are developed; service applications are researching the possible application areas of 6G, such as autonomous driving, etc. "At present, there are only these four directions, and the specific details have not been clearly defined yet."

       It is known that the Information and Communication Technology Center of SK Group in South Korea proposed a 6G technology solution in 2018 called "Terahertz - Deco-Cellularized Structure High-altitude Wireless Platform (such as satellites, etc.)". This solution not only applies terahertz communication technology but also aims to completely transform the existing cellular communication architecture and establish an integrated communication network across air, land and space.

       2. The technology of space multiplexing is very difficult.
       Technically, 6G will employ "spatial multiplexing technology". 6G base stations will be capable of simultaneously connecting hundreds or even thousands of wireless connections, and their capacity will be 1,000 times that of 5G base stations. As mentioned earlier, 6G will utilize the terahertz frequency band. Although this high-frequency band has abundant frequency resources and large system capacity, mobile communication systems using high-frequency carriers face severe challenges in improving coverage and reducing interference.

       When the frequency of the signal exceeds 10GHz, its main mode of propagation is no longer diffraction. For non-line-of-sight transmission links, reflection and scattering are the main ways for signal propagation. At the same time, the higher the frequency, the greater the propagation loss, the shorter the coverage distance, and the weaker the diffraction capability. These factors will significantly increase the difficulty of signal coverage.

       3. The promotion and application process is very costly.
       "Further development of wireless communication requires significant investment." Experts stated that there are two approaches to increasing communication speed: One is to make the base stations more densely distributed and increase the deployment volume. Although the power of the base stations can be reduced, the increase in the number of base stations will still lead to an increase in costs. The second approach is to use higher-frequency communication, such as terahertz or millimeter waves. However, higher frequencies have higher requirements for hardware equipment such as base stations and antennas. Currently, the cost of conducting terahertz communication hardware experiments is extremely high and exceeds the affordability of most research institutions. Additionally, from the perspective of the number of base station antennas, the number of antennas for 4G base stations is only 8, while 5G can achieve 64, 128, or even 256 antennas. The antennas for 6G may be even more, and the replacement of base stations will also increase the application costs.

       "Without changing the existing communication frequency bands, it is difficult to achieve the envisioned 6G vision merely by relying on measures such as algorithm optimization. Completely replacing all base stations is also impractical." Experts believe that in the future, it is highly likely to adopt a non-standalone networking approach, that is, to deploy 6G equipment on the basis of the existing base stations and other facilities. 6G will coexist with 5G, 4G, and 4.5G networks. 6G will mainly be used in densely populated areas or for high-end applications in vertical industries such as autonomous driving, remote medical care, and intelligent factories.

       Although there are still various difficulties with 6G at present, in the field of communication, technological progress is an eternal topic. Especially, after going through the stages of "1G blank, 2G imitation, 3G breakthrough, 4G parallel, and 5G leading", Chinese communication professionals have deeply understood a principle: whoever enjoys more intellectual property rights and patents in the field of mobile communication can have more say, and achieving greater success in the 6G field is obviously one of the goals of China's communication industry.