Towards the Next Wireless Communications Standard - 6G

The prospect of offering large contiguous frequency bands to meet the demand for extremely high data transfer rates in the Terabit/s range will drive the next generation of wireless communications - 6G. Before 6G is proven, nationwide 5G networks are already opening the door for new application scenarios such as Industry 4.0 with enhanced mobile broadband, ultra-reliable low-latency communications, and massive machine-type communications.

The fundamentals of next-generation wireless communications are in progress and commercial deployment is expected around 2030. This will enable new possibilities including holographic applications, extended realities, and digital twins. These involve stringent requirements for data transmission rate and latency. This article provides a brief overview (albeit not comprehensive) of the main 6G research areas that have been identified to realize the vision and enable use cases for future wireless communications. While some of the technologies represent evolutionary changes already used in previous generations, other technologies are disruptive and could unlock potential beyond Shannon's limits. 

Use cases and KPI

It is difficult to predict which applications and use cases envisioned by the industry will eventually be the main driver for the next generation wireless standard and which in turn will shape the system requirements (KPIs) and technologies for 6G. The vision is that in the 6G era, the digital, physical and human worlds will seamlessly fuse (cyber-physical fusion), leading to a connected society in which communications connect people, machines, and virtual services with all of the necessary components of our daily life. 

Table 1 below summarizes the estimated KPI requirements. For 6G, the targeted values are more stringent by a factor of approx. 10 times to 100 times compared to 5G and will pose new challenges for the wireless communications and transport network.

In certain time-sensitive control applications for synchronization (e.g. in industrial application environments such as factory automation), instead of the net latency, it is important to have consistent and deterministic end-to-end latency with low fluctuations, i.e. low jitter. This is introduced as a new KPI. Space-time synchronization will allow time synchronization and mutual positioning by wireless technology for remote devices to work cooperatively.

Table 1: Expected enhancements of key performance requirements (KPIs) from 5G to 6G

Jitter is a new KPI defining a limit for latency fluctuations for time-sensitive operations.

Research areas of 6G

Figure 1 below summarizes some of the areas where development and research work is taking place to realize the vision of 6G wireless communications. We provide commentary on the most significant below.

Figure 1: Research areas of 6G: New waveforms and multiple access.

The efficient operation of a wireless system is highly dependent on the waveform used. While Orthogonal Frequency Division Multiplexing (OFDM) is still a strong candidate for 6G waveforms, alternative application-specific waveforms or unified scalable waveforms need to be explored. New waveforms under consideration include orthogonal time frequency space modulation (OTFS) for high mobility scenarios with large Doppler spreads. Furthermore, single carrier waveforms may play a more dominant role to fulfil the requirements for power efficiency. To allow more flexible usage of the wireless channel, it could be beneficial to consider non-orthogonal multiple access (NOMA).

New network topologies

The cellular layout used in current network architectures is designed to minimize interference at the cell borders between cells. However, achieving ultra-high speed, high capacity and high-reliability communications require communicating over short distances via a low-loss path. One option for such spatially distributed topologies involves cell-free networks where base stations – distributed over a large area – coordinate coherent joint transmission to provide service to each user. This supplies a higher signal-to-noise ratio and gain, and a more consistent quality of experience for users at different locations. However, implementation entails superior computing complexity and tight synchronization between base station locations, along with the requirement to exchange substantial amounts of data between sites. 

Extending the network coverage to three dimensions can deliver coverage to remote areas, sea and space. This could be achieved with non-terrestrial networks (NTN) using high-altitude platform stations and low earth orbit (LEO) satellite constellations. These operate as mobile base stations in the sky. 

Terahertz communication and sensing

Unlocking the potential of the sub-THz frequency region (100 GHz to 300 GHz) with extremely high bandwidths of several GHz is a technological way forward. As well as ultra-high data rates for wireless communications, this would also help sensing and imaging applications, and future medical diagnostics.

The concept of joint communications and sensing (JCAS) supports both applications natively, as part of the physical layer design: its waveforms and the network architecture. Not limited to THz frequencies it also encompasses the millimeter range. Wide bandwidths will also benefit high-precision sensing applications, from positioning, object detection and high-resolution RADAR to spectroscopy-type analysis. There is a particular interest in environmental sensing.

Photonic technologies and visible light communications (VLC)

Optical wireless communication (OWC) combines high speed and high fidelity with low deployment costs. Key advantages over radiofrequency access networks include the availability of 300 THz of license-free bandwidth available in visible and IR wavelengths, robustness against interference and secure communications: for example, in indoor environments where radiation cannot penetrate walls. Free-space optical communications (FSO) with wavelengths in the infrared region use modulated laser diodes for backhaul solutions or space-based communications. However, on Earth, it is affected by terrestrial weather conditions, atmospheric turbulence, and especially fog.

In visible light communications (VLC), data is transmitted via high-bandwidth intensity modulation of commercial LEDs. A photodiode serves as the receiver, and this cost-efficient approach allows easy integration into existing infrastructure primarily for line-of-sight indoor applications.

6G will also drive the evolution of future transportation networks in wireless networks. For example, the Innovative Optical and Wireless Networks Global Forum (IOWN) aims to develop technologies for computing and communications network architecture that achieves scalability, elasticity, energy efficiency, and latency manageability. 

Photonic technologies can help to cope with these challenges. The proposed open all-photonic network (APN) could help to streamline data transfer and processing and create large-capacity, low-latency, and low-energy consumption infrastructures. Integrated optical devices could offer routing and termination functions to implement end-to-end, all-optical connections. Furthermore, the trend for capacity demand increase in the 2030s will revolutionize long-haul transmission.

Another photonic technology that has recently attracted increasing interest is quantum communications. This could play a complementary role for 6G, for example ensuring trustworthiness for ultra-secure and reliable communications.

Reconfigurable intelligent surfaces (RIS) and metamaterials

The use of RIS on building facades or in indoor environments directs the energy of wireless signals towards a specific point, enabling better coverage in non-line-of-sight environments and reducing energy consumption. A reconfigurable intelligent surface is a planar structure that is designed to have properties enabling dynamic control of electromagnetic waves. Targeted scenarios with RIS emphasize ultra-dense network deployments predominantly in indoor environments.

Distributed computing

Although the future 6G application scenarios are currently undefined, we do know that future performance requirements will be even more demanding than 5G in terms of data rate, latency, spectral efficiency, security, reliability and energy consumption. This will also influence the processing architecture: information technologies and communications technologies will further merge. However, processing large amounts of data in distributed systems in networks leads to challenging data rates and latency requirements.

AI-native communications systems and machine learning

In the future, artificial intelligence (AI) will become an integral part of all areas of the wireless communications system. This could include a physical layer design that adapts to the specific propagation channel and environmental conditions with the possibility of end-to-end solutions. To cope with the increase in complexity of future 6G networks, AI and machine learning will play a key role in 6G deployment and operation, maximizing user experience and cost efficiency whilst minimizing energy consumption.

Conclusion

Terahertz systems and applications represent just one potential building block for future 6G wireless communications. Nevertheless, the technology promises to be indispensable – not only to achieve the target requirements including maximum throughput on the Tbit/s level as well as extremely low latencies - but also to implement intriguing novel applications. The 6G use cases envisioned in a recent White Paper span a plethora of applications ranging from communications, spectroscopy, imaging, and sensing. However, successful commercial implementation will require practical business models that have yet to be developed.

Click here to read the whitepaper for more information on THz waves and their properties for 6G-based communications.