Technology

6G Wireless Systems: Requirements, Characteristics, And Challenges

Over a decade ago, the keyword “Beyond 4G (B4G)” was coined to refer to the need to advance the evolution of 4G beyond the LTE standard. At the time, it was unclear what 5G might involve, and only pre-standard R&D level prototypes were in the works. The phrase B4G lasted for some time, referring to what could be possible and potentially useful beyond 4G. Ironically, the LTE standard is still evolving, and some aspects will be used in 5G. Like B4G, Beyond 5G (B5G) is seen as a path to 6G technologies that will replace 5G capabilities and applications. Several private wireless implementations of fifth-generation networks including LTE, 5G, and edge computing for enterprise and industrial customers have helped lay the foundation for 6G.

Although it is still early days for 5G wireless technology, the top players in the industry are still busy working on it. Only a few of them have started working on its successor. Since 5G is still in the evolving phase and has enough capability to support the Internet of Everything (IoE) ecosystem, it is too early to precisely define the features of any technology beyond 5G (B5G).

However, to become the successor to the existing powerful 5G ecosystem, a more disruptive contender will be needed – 6G, whose design is naturally tailored to the performance requirements of Internet of Everything (IoE) applications and the technological trends that come with them. We believe that 6G technology will be driven by the confluence of past trends such as densification, higher rates and emerging trends that include new services such as: ● AI integrated communications ● Tactile Internet ● Higher energy efficiency ● Reduced backhaul and access network congestion ● Enhanced data security

6G communication systems are expected to be represented by the following types of KPI associated services: ● Ubiquitous mobile ultra-broadband (uMUB) ● Ultra-high-speed with low-latency communications (uHSLLC) ● Massive machine-type communications (mMTC) ● Ultra-high data density (uHDD)

Compared to eMBB in 5G, it is expected that 6G will include ubiquitous services, i.e. uMUB. Ultra-reliable low-latency communication, which is a key 5G feature, will again be an essential driver in 6G communication, providing UHSLC by adding features such as end-to-end delay of less than 1 ms, reliability greater than 99.99999% and 1 Tbps peak data rate. Formally defining 6G is still an early stage, and any such discussions are more or less speculation. Nevertheless, there is no doubt that 6G will take shape while building on the 5G vision.

6G Network Characteristics

Connected Intelligence: Compared to the previous generation of wireless communication systems, 6G will be innovatively more transformational, and will update the wireless progress from “connected devices/things” to “connected intelligence”. AI will be introduced in every stage of communication activity as well as radio resource management. The ubiquitous introduction of AI will create a new paradigm of 6G communication systems. Therefore, unlike 5G, the ultra-dense complex network scenarios of 6G should require a full AI system, allowing intelligent communication devices to receive and execute the resource allocation process.

Satellite Integrated N/W: To provide always-on broadband global mobile connectivity, terrestrial and satellite systems are expected to be integrated to achieve the goal of 6G. Integrating all networks (terrestrial, satellite and aerial N/W) into a single wireless system will be crucial for 6G. Only this integration can meet the demand for ubiquitous mobile ultra-broadband (UMUB) services.

Small-cell N/W: The small-cell network idea is introduced to improve the quality of the received signal as a result of throughput, energy efficiency and spectral efficiency enhancement in cellular systems. As a result, small cell networks are an indispensable feature of 5G and beyond (5G) communication technologies. Therefore, 6G communication systems will also adopt this network feature.

Ultra-dense heterogeneous N/W: Ultra-dense heterogeneous networks will be another important feature of 6G. Multi-tier networks consisting of heterogeneous N/W will improve overall QoS and reduce costs.

High-capacity backbone: Backbone connectivity in 6G must be characterized by high-capacity backhaul networks to support a significant volume of 6G data traffic. High-speed optical fiber and free-space optics (FSO) systems are potential solutions to this problem.

Softwarization and Virtualization: Softwarization and virtualization will remain two important features that form the basis of the design process in 5G networks to ensure flexibility, reconfigurability and programmability.

Key Challenges

6G seeks several orders of scale upgrades compared to 5G in all aspects. But certain obstacles exist, where major efforts will be required. Some of them are already being considered and partially addressed in 5G systems.

Access networks for backhaul traffic: The recently formed ITU-T Focus Group Technologies for Network 2030 – FG NET-2030 (established by ITU-T Study Group 13 at its meeting in Geneva on 16-27 July 2018) raised concerns that fixed access networks are already lagging behind the capabilities of emerging 5G systems. It is anticipated that access networks for backhaul traffic will struggle to cope with the unique data growth and other quality requirements unless necessary steps are taken to boost the research effort. With technologies such as free space optical communication and quantum communication, we are hopeful for 6G backhaul to meet the requirements. However, these technologies are still in the early stages of development and how these technologies, even if they exist, will integrate with the 6G equipment ecosystem needs to be looked into.

Sub-millimeter wave and THz spectrum: Preliminary research indicates that frequencies in the THz and above range will be considered for 6G as those bands have plenty of free spectrum bands suitable to meet this requirement. A study indicates that 5G spectrum may not exceed 140 GHz due to several challenges including lack of understanding of channel and propagation modelling, inability of devices to operate at such high frequencies, etc. In contrast, 6G will use the spectrum above 140 GHz. With applications in very short-range communications or ‘whisper radio’. However, the sensitivity of the THz band to blockage, molecular absorption, sampling and circuits for A/D and D/A conversion and communication range are among the major challenges that need to be addressed in the coming years. An additional issue is that at higher frequencies, the antenna size and associated circuitry becomes smaller and is difficult to fabricate on-chip while ensuring noise and inter-component interference suppression. On the other hand, the exact propagation characteristics of the THz band are not well understood.

6G Standardization: Various study groups are drafting an initial set of standards based on unique use-cases and KPIs. 3GPP is hoping to make the 6G requirements public after 2023. Thus, full-fledged 6G systems will not mature before 2030. With 6G, we cannot expect to reach a time when testing will be complete, which is also true with 5G. The industry will need to continuously validate standards compliance, performance, security, and interoperability throughout the lifecycle, from the lab to preproduction, as we start to scale services, and as we reach larger numbers of users. And we will have to repeat that effort whenever there is a change in the environment. As with 6G, and for that matter with 5G, we are no longer measuring bytes coming in and out of the network equipment interface. We need to test the interaction of multiple virtualized nodal functions, at scale, in multiple ways. This includes testing standalone functions, their handoffs to adjacent functions, and their behavior as a holistic system compared to the monolithic boxes they replace.

We need to rethink our testing approaches from the ground up. More than ever, 5G and 6G systems will demand independent, vendor-agnostic testing and validation. And this will require a level of automation that legacy and in-house testing approaches cannot meet. If you cannot use prebuilt test-cases and run them automatically at scale, you will never be able to make it to market. Hence, there is a need to create a complete testing and standardization ecosystem, where we are still lagging behind. Such an ecosystem will be a prerequisite for developing any wireless communication system, be it 5G or 6G.

Fog Networking and Mobile Edge Computing: Fog networking and edge computing have been introduced in 5G to reduce the distance of UEs with serving base stations and service/application content servers respectively. However, edge caching breaks the network into a distributed cloud structure where training data resides at the network edges, which hinders AI technologies from being fully functional. In 6G networks are inevitably going to move to even smaller cells for more capacity and lower latency, and this situation will get worse.

Conclusion

As 5G is in its final testing phase and getting ready for launch in the rest of the world, discussions on what 6G could be have already begun. 6G will require many years to take shape, as we have seen with previous generations in the past. However, there are already high-profile initiatives underway around the world aimed at developing technology for 6G, such as the Next G Alliance in the US and Canada; Hexa-X, RISE-6G, and NEW-6G in Europe; TOWS for 6G Li-Fi in the UK. While it is still too early to define 6G and any such discussion inevitably contains omissions, with this piece we have tried to identify potential enabling technologies for 6G and beyond the potential of 5G.

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