RF Enabling 6G Opportunities and Challenges from Technology to Spectrum

Executive summary

Many visions of 6G and the evolution of data rates in wireless systems in general are directing their vectors toward Tbps communications by 2030. This target, with the important goal of sustainable development in the future world, will inevitably lead to many questions within the RF community about whether this will be feasible at all, and how. As the same question is also raised by other experts from various fields, this paper will examine the challenges and opportunities before us from many perspectives. It is evident that 6G will not only be extremely high-speed communications. For example, the existing wireless infrastructure and new solutions in ultra-low power communications will also live and evolve, further enriching the fabric of wireless systems serving an even wider spread of applications.

6G is also expected to merge communications and sensing in a new way, and the wide bandwidth needed for data will also benefit many high-precision sensing applications. Multi-use, scalability, and energy efficiency will be of the utmost importance, and the tradeoffs will be neither straightforward nor easy to implement, or even understand, without a deep dive into key technologies. These are evolving mostly from the well-established solutions in the market, but at the same time, the need for something completely new is evident. The gap between radio and optical communications is also narrowing, potentially answering some of the concerns.

This white paper will not only discuss the opportunities but also the serious challenges that are foreseeable in the technology community. These may slow down, or in the worst case even block, some of the development paths. How we cope with such threats and use the existing opportunities in the engineering process from early research to final products in the current landscape is one of the key questions that will be asked by the RF community. At the end of this paper, a list of refined questions will be shared. These are not for the RF com munity alone, but for experts working in various disciplines and stakeholders that are together building the future of 6G enabled by radio technologies.

Introduction

Wireless data rates have doubled every eighteen months over the last three decades, as predicted by Edholm’s law. Direct extrapolation from [KP13] predicts that Terabit-per-second (Tbps) link speed will even be achieved before 2030, facilitating capacity well beyond current optical networks. To support the prediction, standardization especially in Wireless Local Area Networks (WLAN i.e. 802.11 family) and high-rate Wireless Speciality Networks (WSN i.e. 802.15.3 family) is moving gradually from tens of Gbps beyond 100 Gbps. However, practical trials have shown that state-of-the-art radio links can approach the 100 Gbps milestone only at a 1–2 m distance over fixed links without beam steerability [RLG20]. This means that taking the next 1,000x increase in link capacity from 1 Gbps to 1 Tbps in practical networks will be a far from straightforward challenge for the next 5 to 10 years, especially for wide area but even for shorter-range mobile coverage.

Physical and financial constraints are setting strict boundaries, and as the continuum of Moore’s law, Edholm’s law, relying strongly on the former, requires the favorable and rapid development of core technologies from semiconductor processes to complete chipsets and other associated components like antennas to keep the past trend moving forward. In addition to the availability of spectrum, facilitating the development is essential. As radio communications is approaching the Tbps challenge by going to higher frequencies, the other trend is that optical communications is gradually expanding from wireline to wireless solutions, examining the problem from a different direction.

Advanced physical layer solutions and more importantly, new spectral bands and hardware (HW) technologies to provide adequate performance, will be required to support these extremely high data rates. In this context, the Terahertz region is envisioned as a key wireless technology enabler to satisfy this demand, by alleviating spec trum scarcity and capacity limitations of current wireless systems, and enabling a plethora of long-awaited applications in diverse fields.

The definition of the THz band seems to vary in the literature, although the ITU definition of the THF (tremendously high frequency) region holds that the scientific definition of the THz band is from 0.3 THz to 3 THz [WIK20]. One should note that the definition of the THz band in many recent papers spans the frequencies between 0.1 THz and 10 THz, and it remains one of the least explored frequency bands for communications [AJH14]. To avoid unnecessary conflicts with ITU definitions, we call the higher end of the EHF (extremely high frequency) band the upper mmW band or region. The band covers frequencies of 100–300 GHz, and it will probably be the most interesting band in the coming years for the research of new radio communications systems. The region provides a much larger portion of spectrum than the lower mmW region (30–100 GHz). The latter has also already been extensively adopted by many standards, including 3 GPP 5G NR (new radio), 802.11, and various radar systems.

Wireless technologies that include a lower mmW region are unable to support Tbps links. On the one hand, advanced digital modulations, e.g. Orthogonal Frequency Division Multiplexing (OFDM), and sophisticated communication schemes, e.g. very large-scale Multiple Input Multiple Output (MIMO) systems, are being used to achieve a very high spectral efficiency at frequencies below 5 GHz. However, the scarcity of the available bandwidth limits the achievable data rates. For example, in Long-Term Evolution Advanced (LTE-A) networks, peak data rates in the order of 1 Gbps are possible when using a four-by-four MIMO scheme over a 100 MHz aggregated bandwidth. These data rates are three orders of magnitude below the anticipated 1 Tbps. However, millimeter wave (mm-Wave) communication systems such as those at 60 GHz can support data rates in the order of 10 Gbps within one meter [RMG11], or nowadays even more. While this is one path to follow, this data rate is still two orders of magnitude below the expected demand.

This paper covers a wide range of aspects in the path toward Tbps communications and sensing, from spectrum and radio channel to enabling HW technologies, including semiconductor circuits, antennas, packaging, and the testing of transceivers approaching the THz region. Opportunities in optical communications will also be discussed to close the “THz gap” from both ends of the spectrum. The broad scope of radio frequency (RF) and optical technologies deserves a more extensive view of the digital signal processing aspects, including the future of Moore’s law, because broadband communications depend heavily on this evolution. Unfortunately, space is limited here, so the reader is advised to examine other sources like [MA17] and [DFS20].

Precise targets for 6G are not yet defined, but visions, use cases, and key values are being extensively discussed globally, in addition to relevant and potential technologies [HUH21]. They already indicate major needs to develop RF technologies well beyond the current state of the art. Of course, much will also be reused from the existing generations, but we need at the same time to address the challenges in which current technologies cannot meet the future demands, and the roadmap is not obvious, with many alternative scenarios for the technical solutions. This imposes major challenges for the RF community to overcome the challenges, while keeping the system complexity manageable.