Making waves: Engineering a spectrum revolution for 6G

6G is looking to achieve a broad range of goals in turn, requiring an extensive array of technologies. Like 5G, no single technology will define 6G. The groundwork laid out in the previous generation will serve as a starting point for the new one. As a distinct new generation though, 6G will also break free from previous ones, including 5G, by introducing new concepts. Among them, new spectrum technologies will help the industry achieve complete coverage for 6G.

Tapping into new spectrum

Looking back, every generation of cellular technology looks to leverage new spectrum. 6G won’t be an exception, with the emergence of new use cases and more demand for high-speed data. As a result, 6G needs to deliver much higher data throughputs than 5G, making millimeter-wave (mmWave) bands extremely attractive.

New frequency bands under consideration for 6G include 100 and 300 GHz, often called sub-terahertz (sub-THz) bands. There is also interest in the upper mid-band—the spectrum between 7 and 24 GHz—because of lower propagation loss compared to sub-THz bands, particularly between the 7 and 15 GHz frequencies.

This spectrum presents regulatory challenges though and is used by various entities including governments and satellite service providers. However, some bands could work for mobile communications with the implementation of more advanced spectrum sharing techniques. Figure 1 provides an overview of the frequencies allocated for mobile and wireless access in this spectrum.

Figure 1 An overview of frequency allocation for mobile and fixed wireless access in the upper mid-band. Source: Radio Regulations, International Telecommunication Union, 2020

While these frequencies have been used for a variety of applications outside of cellular, channel sounding is needed to characterize the use of this spectrum in 6G to ensure it provides the benefits for the targeted 6G application.

The 7 to 24 GHz spectrum is key area of focus for RAN Working Group 1 (RAN1) within the Third Generation Partnership Project (3GPP) for the purpose of Release 19, which will be finalized in late 2025 and facilitate the transition from 5G to 6G.

Scaling with ultra-massive MIMO

Over time, wireless standards have continued to evolve to maximize the bandwidth available in various frequency bands. Multiple-input multiple-output (MIMO) and massive MIMO technologies were major enhancements for radio systems with a significant impact for 5G. By combining multiple transmitters and receivers and using constructive and destructive interference to beamform information toward users, MIMO significantly enhanced performance.

6G can improve on this further. MIMO is expected to scale to thousands of antennas to provide greater data rates to users. Data rates are expected to grow from single gigabits per second to hundreds of gigabits per second. Ultra-massive MIMO will also enable hyper-localized coverage in dynamic environments. The target for localization precision in 6G is of 1 centimeter, a significant leap over 5G’s 1 meter.

Interacting with signals for better range and security

Reconfigurable intelligent surfaces (RIS) also represents a significant development for 6G. Currently, this technology is the focus of discussions at the 3GPP and the European Telecommunications Standard Institute (ETSI).

Using high-frequency spectrum is essential to achieve greater data throughputs but this spectrum is prone to interference. RIS technology will play a key role in addressing this challenge helping mmWave and sub-THz signals to overcome the high free space path loss and blockage of high-frequency spectrum.

RISs are flat, two-dimensional structures that consist of three or more layers. The top layer comprises multiple passive elements that reflect and refract incoming signals, enabling data packets to go around large physical obstacles like buildings, as illustrated in Figure 2.

Figure 2 RISs are two-dimensional multi-layer structures where the top layer consists of an array of passive elements that reflect/refract incoming signals, allowing the sub-THz signals used in 6G to successfully go around large objects. These elements can be programmed to control the phase-shift the signal to into a narrow beam directed at a specific location. Source: RIS TECH Alliance, March 2023

Engineers can program the elements in real time to control the phase shift enabling the RIS to reflect signals in a narrow beam to a specific location. With the ability to interact with the source signal, RISs can increase signal strength and reduce interference in dense multi-user environments or multi-cell networks, extending signal range and enhancing security.

Going full duplex

Wireless engineers have tried to enable simultaneous signal transmission and reception for years to drive a step-function increase in capacity for radio channels. Typically, radio systems employ just one antenna to transmit and receive signals, which requires the local transmitter to deactivate during reception or transmit on a different frequency to be able to receive a weak signal from a distant transmitter.

Duplex communication requires either two separate radio channels or splitting up the capacity of a single channel, but this is changing with the advent of in-band full duplex (IBFD) technology, which is currently under investigation in 3GPP Release 18. IBFD uses an array of techniques to avoid self-interference enabling the receiver to maintain a high level of sensitivity while the transmitter operates simultaneously on the same channel.

Introducing AI/ML-driven waveforms

New waveforms are another exciting development for 6G. Despite widespread use in cellular communications, the signal flatness of orthogonal frequency division multiplexing (OFDM) creates challenges with wider bandwidth signals in radio frequency amplifiers. Moreover, the integration of communication and sensing into a single system, known as joint communications and sensing (JCAS), also requires a waveform that can accommodate both types of signals effectively.

Recent developments in AI and machine learning (ML) offer the opportunity to reinvent the physical-layer (PHY) waveform that will be used for 6G. Integrating AI and ML into the physical layer could give rise to adaptive modulation, enhancing the power efficiency of communications systems while increasing security. Figure 3 shows how the physical layer could evolve to include ML for 6G.

Figure 3 The proposed migration to an ML-based physical layer for 6G to enhance both the power efficiency and security of the transmitter and receiver. Source: IEEE Communications Magazine, May 2021.

Towards complete coverage

6G is poised to reshape the communications landscape pushing cellular technology to make a meaningful societal impact. Today, the 6G standard is in its infancy with the first release expected to be Release 20, but research on various fronts is in full swing. These efforts will drive the standard’s development.

Predicting the demands of future networks and which applications will prevail is a significant challenge, but the key areas the industry needs to focus on for 6G have emerged, new spectrum technologies being one of them. New spectrum bands, ultra-massive MIMO, reconfigurable intelligent surfaces, full duplex communication, and AI/ML-driven waveforms will help 6G deliver complete coverage to users.

Jessy Cavazos is part of Keysight’s Industry Solutions Marketing team.

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Making waves: Engineering a spectrum revolution for 6G

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