The cellular network standard is gradually moving towards the 6th generation (6G). In 6G, the pioneering and exclusive features, such as creating connectivity in space and even under water, are attracting governments, organizations, and researchers to spend massive time, money, efforts in this field. Towards Intelligent Network Management and Distributed Secured Systems, Artificial Intelligence (AI) and Blockchain are going to form the backbone of 6G, respectively. However, there is a need for study of 6G architecture and technology, such that researchers can identify the scopes of improvement in 6G. Therefore, in this survey, we discuss the primary requirements of 6G along with its overall architecture and technical aspects. We also highlight the critical challenges and future research directions in 6G networks, which can lead to successful practical implementation of 6G, as per the objective of its introduction in the next generation cellular networks.
Introduction
Industry and academia are now working together towards 6th generation (6G) cellular networks. Unlike previous generations, 6G is expected to use a wide range of artificial intelligence (AI) services from the core network to terminal devices, leading to the concept of “connected intelligence” with the optimization of 6G networks. In the near future, we need connections underwater and in space. We need nanosecond response times, such that the network is fast enough to download 100 hours of Netflix in one second. The network must be able to support 10 times more devices that are 100 times more reliable. 6G networks are expected to use sub-millimeter or terahertz waves, leading to extremely high data rates. Samsung recently suggested the inclusion of artificial intelligence by merging AI into the 6G framework. This means that devices connected in the network can automatically talk to each other and manage data between them in a smart way. These modern technologies will help in building a stable communication source like 6G.
When the 5th generation (5G) cellular network was built, the idea was to upgrade its capacity so 6G could use the existing towers and frameworks as opposed to starting from scratch. Another important consideration should be the awareness of the damage we are causing to the planet and since we are trying to slow down the pollution levels, 6G can be allowed to use as much energy as 5G. The increase in data traffic and the growing number of connected devices are the main driving forces behind the development of the next generation cellular network i.e., 6G. Therefore, we need an informative survey on the basic architecture and technologies of 6G network systems. The key features of the 6G network systems are expected to be extremely low latency, ultra high data rates, high energy efficiency, enhanced security, and ubiquitous network coverage worldwide. Another major purpose of the 6G network system is to meet the needs and requirements of Internet of Things (IoT) applications and to meet the wider application and use of the latest technological trends. There has been a significant stress on the application of distributed ledger technology such as blockchain in the 6G network system, as it is able to provide privacy and security, which are the requirements of the digital world in the era of the Internet.
Purpose of this Survey: In this survey, we discuss the overall proposed architecture and technologies for the 6G network. We expect three tiers of network to replace the existing terrestrial network -air tier network, space tier network and underwater tier network. In the new architectural design of the 6Gnetwork, we will make extensive use of artificial intelligence and machine learning in designing unmanned aerial vehicles (UAVs), flying base stations, satellites and more advanced forms of technologies. We also present the challenges in the design and deployment of the 6G network. Based on the overview of the architecture and technologies of 6G, we also discuss the future research scopes in 6G, which can help in practically implementing the proposed 6G networks, leading to intelligent network management with higher security and data rates than the existing cellular networks. Conse quently, 6G will help merge the economically challenged population with the technologically advanced world, which leads to opening a wide range of benefits and possibilities to ease the way of living tomorrow.
6G Architecture
A. Vision of 6G Architecture
6G mobile network targeted ubiquitous intelligence, computing power, and high-speed wireless connectivity in air, space, and sea. The vision to achieve this objective is integrating underwater communication and satellite communication networks to provide worldwide network coverage. A super speedy service is required in 6G mobile network with data speed close to 1000 Mbps. Some of the requirements of 6G can be listed as holographic communication, ultra high broadband, multi-sense transmission, ultra-high throughput, reliability, low latency, etc.
Possible solutions can be using smaller cell size and higher frequency bands. However, smaller sizes of cells will lead to more power consumption and higher operating costs, and higher frequency bands may suffer path loss. Therefore, we have to put a limit to reduce the size of cells and increase the frequency band. Fully Decoupled Radio Access Network (FD-RAN) archi tecture has been proposed, where network functionality is fully decoupled and will be deployed by each independent network entity. The implementation of multi-point coordination and centralized resource management can achieve an elastic resource collaboration in the fully decoupled RAN. The control base station of FD-RAN coordinates with the decoupled uplink and decouples the downlink in a centralized manner, which has a similar architecture to Cloud-RAN. 6G will use a variety of computing, networking, communication, and multiple sensing technologies to offer smart applications. The key enablers are AI, edge-intelligence, blockchain, homomorphic encryption, network slicing, and integrated networks for space, sea, and land.
B. 4-Tier Ubiquitous Coverage 6g Network
The ubiquitous coverage of 6G network has 4-tiers, as follows.
1) Space Network Tier: Space Network Tier will support wireless space internet services using densely deployed Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geostationary
2) Air Network Tier: Air network can be broadly divided into two categories- Low Altitude Platform (LAP) and High Altitude Platform (HAP). LAP operates within the altitude range of few kilometers and HaP is generally in the op stratosphere. HAP has longer endurance and wider coverage capabilities, but HAP overlaps with LEO satellite network. On the other hand, LAP uses low frequency, millimeter-wave (MMWAVE) bands, and microwaves to provide flexible and reliable connectivity by dense employment of flying base stations (BSS) and UAVs. UAVSCAN acts as relay nodes for long distance communication, and thus promotes integration of space and terrestrial networks. But the most interesting and useful feature of UAVs is that it can facilitate mobile communication in areas where adequate infrastructure is absent. UAVs have high speed mobility, and hence, more advanced mobile network protocols are required for UAVs. Adaptive hybrid communication has potential and surpasses existing protocols. A fully integrated 6G network can be expected to have multilayer architecture including UAV based LAPs, LEO satellites, GEO satellites and heterogeneous terrestrial networks.
3) Terrestrial Network: Terrestrial network will be a highly dense heterogeneous network, requiring high capacity X-haul deployment as large number of small BSSs need to be deployed to prevent path loss. Terrestrial networks will support microwave, MMWAVE, low frequency and Tetra Hertz bands.
4) Under-sea Network: Under-sea network aims to provide internet facility under the seas and oceans. But it is still a very controversial topic whether it will be a part of 6G network or not. This network system involves optical and acoustic communication and radio frequency. But the difficulty lies due to the unpredictability of the underwater environment, challenges faced, and risk factors as water exhibits different propagation properties which are different from aerial or terrestrial terrains. Therefore, a lot of issues are yet to be resolved in under water 6G network.
C. New Network Protocol
The current Internet Protocol architecture is not applicable for 6G due to its limitations in terms of latency and throughput. Recently, some new protocols like Quick UDP Internet Connection (QUICK) have been created which have mitigated these issues to some extent. But, unfortunately these protocols make the Internet more complex and cannot serve as a complete solution.
D. Large Dimensional Network Architecture
Multi-connectivity techniques can help in connecting multiple access network tiers to achieve increased cover age. However, such connectivity arrangement between aerial/space, and terrestrial network tiers is non-ideal, and hence leads to a large latency which in turn will affect the information transfer. Moreover, the high mobility of the BSS makes it difficult to obtain channel state information for connectivity management. The BSS can also generate errors in channel estimation. Another problem that can arise is interference between different tiers. A possible solution to deal with the interference of tiers can be done by setting up a schedule system where information will be shared to upgrade user scheduling. The authors discuss the need for high performance algorithms for scheduling.
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