Terahertz - To be or not to be in 6G?

THz – To be or not to be in 6G?

Introduction

While the story is still being written for 5G as the networks are being deployed in many parts of the world, this fifth-generation communication technology is already conditioning the path for what follows – 6G. With 5G still to deliver on its promises, and mm-wave bands largely underutilized in comparison with the Sub-7GHz range, the research community is already investigating the next generation of communication technology. The 6G specifications are expected to be developed and released around 2026-2027 and at the moment it is challenging to provide a clear and concise vision for 6G.

However, three things about 6G appear to be certain:

• 6G is expected to be more capable, intelligent, reliable, scalable and power efficient, satisfying all the requirements that cannot be realized at present with 5G [5].
• 6G will employ a combination of technologies already used in 5G and other previous generation wireless networks, as well as new technologies that were either deemed too immature for 5G or will be adopted or developed specifically for 6G [1].
• 6G will most likely continue the trend of using higher and higher carrier frequencies beyond the mm-waves through THz bands and up to visible light to provide high-capacity point-to-point communication with an aim to achieve spectral efficiency 5 times greater than 5G [6].

6G will inevitably continue the expansion into higher frequencies, with a 100 – 300 GHz range being considered as the first opportunity window where a number of services for radio astronomy, satellite earth exploration, mobile satellite and inter-satellite already allocated between in the 141.8 – 275 GHz band [6]. Federal Communication Commission (FCC10) has designated 21.2 GHz of spectrum for unlicensed use in the 116-123 GHz, 174.8-182 GHz, 185-190 GHz and the 244-246 GHz bands [7].

KPIs of 6G

The basic 6G requirements for peak data rate are expected to be 50x that of 5G, with the user data speed experience at least 10 times better than that with 5G networks. Additionally, 6G is to offer much higher area traffic capacity and connect even greater number of devices than 5G. With even lower latency and much improved reliability, 6G will truly address the needs of autonomous mobility, industrial automation and robotics. A detailed comparison of current 5G and expected 6G KPIs is provided in the table below.

KPIs of 6G

The basic 6G requirements for peak data rate are expected to be 50x that of 5G, with the user data speed experience at least 10 times better than that with 5G networks. Additionally, 6G is to offer much higher area traffic capacity and connect even greater number of devices than 5G. With even lower latency and much improved reliability, 6G will truly address the needs of autonomous mobility, industrial automation and robotics. A detailed comparison of current 5G and expected 6G KPIs is provided in the table below.?

Table 1: KPI comparison of 5G and 6G [1].

THz as a key enabler of 6G

Achieving this next step in wireless communications evolution will require much better understanding of technology limitations compared to previous generations – 3G, 4G and 5G. There are a number of key enablers that the success of 6G will rely on. For the purpose of this article, we will concentrate only on the enablers that expand and unlock additional spectrum bands for the purpose of wireless communications. While there are already plans to extend the upper limit of 5G to 71 GHz, the studies of 6G focus on upper millimeter wave bands (mm-waves), also known as sub-THz, with frequencies ranging from 100 to 300 GHz. This band will most likely be the most interesting band for research on new wireless communication systems [3]. One thing to note however is that 6G will not go about providing enhancements over 5G by just employing new parts of the spectrum, it will do so by utilizing legacy and new bands in a seamless and dynamic way to provide the required quality of service for the given use cases.

RF engineering and device physics

The development of 6G and utilization of sub- and THz bands will be met with even greater challenges than has been the case for 5G, where RF engineering and device physics is concerned [1]. Generation, modulation, detection and demodulation of THz signals in an energy efficient manner has always been very difficult and progress in this field over the last few decades has been relatively slow. In the last decade however, we have seen a number of new technologies, in particular III-V InP devices and Schottky diodes reaching the 1 THz mark.

The technology readiness levels will have a significant influence and impact on the timeline of 6G adoption, with its roll out expected to start in 2028-2030. 6G devices will require very high levels of integration and ultra-low energy consumption, and extensive capability for energy conservation and harvesting to maintain long periods of stand-by activity especially in case of IoT devices [5]. 6G in sub-THz range will face challenges due to available transistor speeds in CMOS, SiGe, HBT and degradation of available gain, output power and noise figure that are required to overcome the path loss [3], [9].

The integrated circuit technologies currently available are not yet sufficiently mature or economical for Tbps data transfers over and up to 1km distance. The data transfer speeds of a few tens of Gbps have been reported at frequencies below 120 GHz and within 10m range using CMOS, while using InP semiconductors and high directivity antennas allows to achieve comparable speeds of up to 1km range [10].

Therefore, there is a stringent requirement to develop semiconductor technology and devices that could supply enough RF power that will allow for implementation of large array antenna systems and overcome path loss. The larger the antenna array the more output power is required, for example a 45 dBm EIRP required from a handset equipped with 16 element array requires a power amplifier (PA) that delivers 16-20 dBm, while to achieve 75 dBm EIRP from a 256-element array at the base station requires an output power from the PA in a range of +25 – +27 dBm [3]. Best amplifier devices currently available on the market are based on InP HBT technology, which is superior to CMOS in generating power above 100 GHz, and can now deliver +23 dBm and +18 dBm at 170 GHz and 220 GHz, respectively. Attempts are being made to reduce the size of GaN transistors to allow for the application of this technology in the upper mm-wave range (100-300 GHz) [3]. Such devices and systems should come on the market within the next few years.

On the other hand, at the receiver end of the system, the receiver sensitivity is mainly determined by the noise figure (NF) performance of the first element of the downconverter – the low noise amplifier (LNA). Here we can expect that a receiver operating at 280 GHz would exhibit NF that at the very least is 5 dB higher than that of its counterpart implemented to work in a 28 GHz communications link [3].

Additionally, integrated transceiver implementations are currently limited to 10-12 % of fractional bandwidth which in practical terms means that only 20-30 GHz of effective bandwidth can be made available by the RF front-end operating in the upper range of the sub-THz band [3]. Such bandwidth would still require however adequate A/D and D/A converters with at least 6 bits or more resolution [9]. Additionally, such requirements also pose a significant challenge from the power consumption point of view.

In summary, due to their relatively low cost and high level of integration, it is expected that CMOS and SiGe technologies?will perform well in applications up to 150 GHz. Whereas GaN and InP technologies will dominate applications where higher frequencies and higher output power will be required such as extreme capacity backhaul networks [1], [3]. Graphene based electronics although at a very early stage is also a very promising technology for THz RF systems [5].

Communication channel properties

While the communications channels in the sub-7GHz band in particular and mm-wave bands, thanks to 5G development, have been relatively well investigated and modeled, the same cannot be said about the sub-THz range where the characterization activities have been scarce [3].

Accurate understanding of properties of THz channel, especially when it comes to signal spreading loss and molecular absorption, is fundamental to development and implementation of 6G technology. The signal spreading loss is a phenomenon associated with wave spreading that occurs when an electromagnetic wave passes through the medium, while molecular absorption is associated with the loss that occurs when a portion of the energy of the electromagnetic wave is converted into the kinetic energy that makes molecules of various atmospheric gases vibrate.

Figure 1: Spreading loss (dashed lines) and molecular absorption loss (solid lines) for frequencies from 0.1 to 1.4 THz and three different distances 1 m, 10 m and 100m [1].

There are a number of transmission windows defined on the molecular absorption characteristic of THz band, in which the effective susceptibility of molecular gases to vibration is limited and much lower than the spreading loss. For lower THz range these windows are defined in 120-140 GHz, 240 GHz and 300 GHz bands [1]. Additionally, the range of the outdoor THz wireless communication can differ significantly under various meteorological conditions with snow and rain introducing additional losses in the path of the signal propagation. While when it comes to indoor communication, additional effects of walls, plants, animals and humans affect the propagation properties as the signals are absorbed, reflected, transmitted and diffracted, which makes long-distance communication very challenging. High gain antennas can be used to compensate for high propagation losses and ultra-massive multiple-input multiple-output (UM-MIMO) antenna systems are emerging as practical means of combating the range issues [4].

All the reasons above require that the THz channels are empirically characterized by means of channel sounding techniques, which are far more challenging than at cellular bands. This is mainly due to high attenuation of THz signals in both indoor and outdoor environments, and high directionality of THz waves. These factors impose limitations on channel sounder system architecture and consequently on the availability of measurement equipment. While at sub-7GHz band, complex wideband channel sounders with more than 50 dual-polarized antenna element arrays can be used, the channel sounders for THz range tend to rely on traditional high directivity horn antennas and single receiver architectures that allow preserving high dynamic range and measurement fidelity. Achieving high directional resolution, large bandwidth and high phase stability comes at the expense of system complexity and associated cost [6].

The need for mechanical positioners slows down the measurement and currently makes the whole channel sounding activity unpractical and often unfeasible. For those reasons there are very few actual radio channel models available for upper mm-wave bands and these often still rely heavily on simulations. Those models that are available are valid for very specific indoor scenarios. Much more work is required to model industrial settings taking into account construction materials, mechanical and electrical noise, and presence of robotic and heavy machinery equipment [6].

Energy consumption and sustainability

The required leap forward into the sub-THz frequencies with abundance of bandwidth cannot be achieved by merely enhancing the carrier frequency [3]. Analog RF electronics operating at high frequencies and providing large bandwidths has its limitations when power-added efficiency (PAE) is concerned. Currently high frequency PAs can only deliver PAE of 7% and 4% at 170 GHz and 220 GHz, respectively. The PAE of a whole transceiver would therefore be expected to fall well under the 10% mark.

The digitalization of large bandwidths and the speed of digital signal processing become also major issues. Sampling frequency of A/D converters and their associated power consumption remain inadequate for an efficient signal processing and energy consumption demands that will be imposed on the 6G networks. Energy efficiency in particular becomes increasingly important as the world shifts toward sustainability [2]. The 6G infrastructure energy consumption is expected to be at the level of 4G networks irrespective of the number of terminals, so that the expected growth in volume of data traffic does not result in a comparable growth in energy consumption [8].

Packaging of RF front-ends and antennas

Component and system level packaging as well as antenna array integration pose significant challenges already at 5G. With additional transmission line losses associated with the sub-THz propagation even higher level of integration will be required. Resolving these issues may require novel 3D packaging techniques and structures with the chips and antennas stacked on top of each other to allow for reduction of interconnect lengths, footprint and tight antenna array (UM-MIMO) integration [3].

The combination of a very tight integration and rather low efficiency of the RF circuitry will become a major challenge.

Conclusions

The new generation of communication technology arises when two driving forces come together: one that stems from societal needs and the other that brings along technologies that are mature enough to enable it. Wireless communication that truly operates in the upper mm-wave (sub-THz) and THz band will only make sense in those use cases that are not particularly focused on cost and energy efficiency. These would most likely include indoor communication of data centers, where massive amount of data could be conveniently moved around without a need for complex and costly cable infrastructure. In the case of outdoor communication, backhaul will emerge as an application where fiber-like data transfers will be achievable over short to medium distances (0.1 – 1 km). Lower sub-THz band with frequencies in 100 – 200 GHz range seems like a good candidate for such use cases. Researchers and engineers are already conducting studies and development for devices and systems in D (110-170 GHz) and G (140 – 220 GHz) bands.

There is still significant progress to be achieved with regard to more efficient modulation schemes. Supported by artificial intelligence and machine learning techniques, new forms of modulation schemes will emerge and will be tailored for specific use cases, throughputs and latencies.

Realistically, it is difficult to imagine 6G communication networks to utilize bands beyond 300 GHz. Not only is the free space loss often in excess of 100 dB reducing the range of wireless links to at best a few tens of meters, but also semiconductor devices, materials and integration technologies are not yet here today to support Tbps connectivity above 300 GHz. While it is reasonable to expect that RF electronics will improve over time, and in combination with advanced antenna beamforming techniques, will offer an acceptable solution to signal losses and limited link distances, it is unreasonable to assume that the required maturity of RF technology is a simple matter of time. It is highly uncertain if CMOS and SiGe BiCMOS based electronics will provide adequate performance in THz band by the time 6G is being implemented. Additionally, nano- and meta-materials as well as graphene-based electronics need more time to mature and be considered suitable for THz communications.

Will the trend of ever-increasing speed of analog and digital CMOS and BiCMOS based electronics that has been fundamental to large scale communications continue? Or is it a time for other semiconductor technologies, such as InP HBT, to come to the fore and form the foundation upon which 6G is built?

Also, it remains to be seen if the complexities of RF circuitry parallelization, antenna design and fabrication, high signal speed and power components, heat dissipation and power consumption, and efficiency as well as integration challenges can be resolved in time for the 6G rollout. Even assuming that all the RF electronics and technology issues could be resolved in time, with such stringent requirements on power consumption and energy efficiency of 6G devices and networks, processing the data may quickly become the bottleneck. Allowing for even the most optimistic assumptions with regard to the RF electronics performance at sub-THz band, the applicability of such systems will be very limited for the battery power devices.

True THz communication (beyond 300 GHz) may certainly come in the future, but it may as well be in time for 7G more so than 6G. The biggest challenge is the achievable link distance as the transceiver available output power and sensitivity are and will remain low in the nearest future. There is a strong possibility however that the sub-THz bands will be employed in 6G for all those applications, and use cases where the sheer amount of data transfer capacity will be the main KPI and can be justified by the business case

As people have moved meeting and collaborating with others into the virtual space, communications have become as vital as water and electricity. Today we are constantly being bombarded with a vision of the future in which billions of humans, things and connected vehicles, robots and drones will share zettabytes of data in the all-connected world [8]. The vision for 6G is to be an affordable and scalable network with great coverage everywhere so that it ends the digital divide and provide a truly all-connected world.

Closing Comments

6G technology operating at THz spectrum faces a number of challenges that remain unresolved. When it comes to the RF electronics higher output power and efficiency, lower phase noise and noise figure must be achieved and even more advanced antenna beamforming solutions have to be employed to combat signal losses and limited link distance. Propagation channels at THz frequencies for both indoor and outdoor communications still remain largely uncharacterized and depend vastly on simulations rather than measurements as channel sounding techniques and equipment remain relatively primitive. The energy consumption and sustainability requirements that are expected to be imposed on 6G networks seem even more challenging and this opens up a need for novel energy harvesting and energy transfer schemes for the devices and networks to comply with very stringent bit-per-joule requirements. Not to mention the packaging and integration techniques that for sub-THz and THz bands are still largely based on rectangular waveguides and milling techniques.

?

It would not be unreasonable to assume that all those challenges and immaturity of technologies and techniques make the challenge to bring 6G even greater than 5G, which has piggy-backed on much more mature foundations to start with. Today it seems that for 6G to deliver THz communication and enable so many new use cases in the given timeframe is a much more difficult task. Considering the current maturity status of various enabling technologies and the rate of progress that is required from them going forward, THz communication in 6G seems like a very unlikely scenario. A much safer and realistic bet would be on THz communication supporting 7G.

Farran | Millimeter wave solutions, made simple.

Farran has been supporting the research & development of 6G technology since the conceptual works began. Whether for millimeter-waves or sub-THz and THz bands, we have supplied test & measurement equipment required to perform the early phase studies of path losses and channel sounding. We support our customers in their efforts to characterize materials, devices and subsystems in an on-wafer, bench-top and over-the-air measurement scenarios from 40 GHz to 500 GHz. As the 6G technology is starting to take shape and our customers are at the early stage of development of devices and technologies, we support them with cost effective and application specific standard products and custom solutions that fulfill their needs in terms of cost, test time and reliability of measurements.

Contact us today to discover how we can help you chose a product or a solution that suits your 6G needs. Our knowledgeable and helpful team will be happy to take you through our existing product lines or invite you to a call with our engineering team who will be delighted to advise on a specification and functionality of a custom solution for you.

References:

[1] N. Rajatheva, I. Atzeni, S. Bicais, E. Bj?rnson et. al.“Scoring the Terabit/s Goal: Broadband Connectivity in 6G”, IEEE Open Journal of Communication Society, Aug. 2020.

[2] “6G Vision – The Next Hyper-Connected Experience for All”, Samsung Research, Jul. 2020.

[3] “White Paper on RF Enabling 6G – Opportunities and Challenges from Technology to Spectrum”, 6G Flagship, 6G Research Vison, No. 13, April 2021.

[4] A. Faisal, H. Sarieddeen, H. Dahrouj, T. Y. Al-Naffouri, M.-S. Alouini, “Ultra-Massive MIMO Systems at Terahertz Bands: Prospects and Challenges”,?IEEE Vehicular Technology Magazine, Vol. 15,?Issue 4, Dec. 2020

[5] Ch. de Alwis, A. Kalla, Q.-V. Pham, P. Kumar, K. Dev, W.-J. Hwang, M. Liyanage, “Survey of 6G Frontiers: Trends, Application, Requirements, Technologies and Future Research, IEEE Open Journal of the Communications Society, Apr. 2021, pp. 836-886

[6] H. Tataria, M. Shafi, A. F. Molisch, M. Dohler, H. Sj?land, F. Tufvesson, “6G Wireless Systems: Vison, Requirements, Challenges, Insights, and Opportunities, IEEE Proceedings, Vol. 109, Issue 7, Jul. 2021.

[7] “Spectrum Horizons”, ET Docket No. 18-21, RM-11795, FCC 19-19, March 15, 2019.

[8] “European Vision for the 6G Network Ecosystem”, 5G Infrastructure Association, June 2021, Ver. 1.

[9] “Key Drivers and Research Challenges for 6G Ubiquitous Wireless Intelligence”, 6G Flagship, 6G Research Visions 1, Sep. 2019.

[10] E. C. Strinati, S. Barbarossa, J. L. Gonzales-Jimenez, D. Ktenas, N. Cassiau, L. Maret, C. Dehos, “6G: The Next Frontier”,?IEEE Vehicular Technology Magazine, Vol. 14,?Issue?3, Sept. 2019.