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 mmWave bands largely under-utilized in comparison with the sub-7 GHz 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 to 2027; 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.1
• 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.2
Figure 1 Notional 6G performance improvements compared to 5G.5
• 6G will most likely continue the trend of using higher and higher carrier frequencies beyond the mmWaves through THz bands and up to visible light, to provide high capacity point-to-point communication with an aim to achieve spectral efficiency 5x greater than 5G.3
6G will inevitably continue the expansion into higher frequencies, with a 100 to 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 are already allocated in the 141.8 to 275 GHz band.3 The Federal Communication Commission (FCC) has designated 21.2 GHz of spectrum for unlicensed use in the 116 to 123, 174.8 to 182, 185 to 190 and the 244 to 246 GHz bands.4
The basic 6G requirements for peak data rates are expected to be 50x those of 5G, with the user data speed experience at least 10x better than with 5G networks (see Figure 1). Additionally, 6G is to offer much higher area traffic capacity and connect an 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 key performance indicators (KPIs) is summarized in Table 1.
THz KEY ENABLERS
Achieving this next step in wireless communications evolution will require a much better understanding of technology limitations compared to the previous generations, i.e., 3G, 4G and 5G. The technology readiness levels will have a significant influence and impact on the timeline of 6G adoption, with its rollout expected to start between 2028 and 2030.
The success of 6G will rely on several enablers, described in the following paragraphs. We will concentrate only on those that expand and unlock additional spectrum 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 mmWave bands, also known as sub-THz, with frequencies ranging from 100 to 300 GHz. This will most likely be the most interesting band for research on new wireless communication systems.6 One thing to note, however, is that 6G will not go about providing enhancements over 5G by just employing new spectrum; it will do so by using 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 use of sub-THz bands will pose even greater challenges than has been the case for 5G, for RF engineering and device physics.2 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 several new technologies: in particular, III-V InP devices and Schottky diodes reaching the 1 THz mark.
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 standby activity, especially in case of IoT devices.1 6G in the sub-THz range will face challenges due to available transistor speeds in CMOS, SiGe and HBT, to ensure the available gain, output power and noise figure required to overcome the higher path loss.6,7 Integrated circuit technologies currently available are not yet sufficiently mature or economical for Tbps data transfers to 1 km distance (see Figure 2). Data transfer speeds of a few tens of Gbps have been reported below 120 GHz and within 10 m range using CMOS, while using InP and high directivity antennas enables comparable speeds to 1 km.8
Figure 2 IC technology demonstrations: range vs. operating frequency. Labels show the data rate, technology, modulation and antenna gain.8
Therefore, a stringent requirement is to develop semiconductor technology and devices that can supply enough RF power that will enable large array antenna systems to overcome path loss. The larger the antenna array, the more output power is required. For example, a 45 dBm EIRP from a handset equipped with a 16-element array requires a power amplifier (PA) delivering 16 to 20 dBm, while 75 dBm EIRP from a 256-element array at the base station requires a PA output power of 25 to 27 dBm.6 The best amplifier devices available are based on InP HBT technology, which is superior to CMOS in generating power above 100 GHz. InP PAs can deliver 23 and 18 dBm at 170 GHz and 220 GHz, respectively. Research to reduce the size of GaN transistors may enable this technology to be used in the range from 100 to 300 GHz.6 Such devices and systems should come on the market within the next few years.
At the receiver, sensitivity is mainly determined by the noise figure (NF) of the first element of the down-converter, i.e., the low noise amplifier. We can expect a receiver operating at 280 GHz will have a NF of at least 5 dB higher than that of its counterpart in a 28 GHz communications link.6 Additionally, integrated transceivers are currently limited to 10 to 12 percent of the fractional bandwidth; in practical terms, this means the RF front-end will have only 20 to 30 GHz of effective bandwidth when operating in the upper range of the sub-THz band.6 This bandwidth will still require adequate analog-to-digital and digital-to-analog converters with at least 6-bit resolution.7 Such requirements also pose a significant challenge to minimizing the power consumption.
In summary, due to the relatively low-cost and high level of integration, CMOS and SiGe technologies will perform well in applications up to 150 GHz. GaN and InP technologies will dominate applications where higher frequencies and higher output power are required, such as extreme capacity backhaul networks.2,6 Although at a very early stage of development, graphene-based electronics is a very promising technology for THz RF systems.1
Communication Channel Properties
While the communications channels in the sub-7 GHz and mmWave bands have been relatively well investigated and modeled, thanks to 5G development, the same is not true of the sub-THz range, where characterization activities have been scarce.6 Accurate understanding of the properties of THz channels, especially signal spreading loss and molecular absorption, is fundamental to implementing 6G technology. 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 kinetic energy that vibrates the molecules of various atmospheric gases (see Figure 3).
Figure 3 Spreading (dashed lines) and molecular absorption (solid lines) losses for frequencies from 0.1 to 1.4 THz and distances of 1, 10 and 100 m.2
Several transmission windows occur on the molecular absorption characteristic of the THz band, in which the effective susceptibility of molecular gases to vibration is limited and much lower than the spreading loss. For the lower range, these windows are between 120 to 140 GHz and at 240 and 300 GHz.2 Additionally, outdoor THz wireless communication can differ significantly under various meteorological conditions, with snow and rain introducing additional losses in the signal propagation. With indoor communication, the walls, plants, animals and humans affect the propagation properties, causing the signals to be absorbed, reflected, transmitted and diffracted, making long distance communication challenging. High gain antennas can compensate for the high propagation losses, and ultra-massive MIMO (UM-MIMO) antenna systems are emerging as practical means of solving the range issues.9
For all these reasons, the THz channels must be empirically characterized using channel sounding techniques, which are far more challenging than at cellular bands. This is due to the high attenuation of THz signals in both indoor and outdoor environments and the high directionality of THz waves. These factors impose limitations on the channel sounder system architecture and, consequently, on the availability of measurement equipment. While at sub-7 GHz, complex wideband channel sounders with more than 50 dual-polarized antenna element arrays can be used, the channel sounders for THz tend to rely on traditional high directivity horn antennas and single receiver architectures to preserve high dynamic range and measurement fidelity. Achieving high directional resolution, large bandwidth and high phase stability comes at the expense of system complexity and its associated cost.3 The need for mechanical positioners slows down the measurement and currently makes channel sounding impractical and unfeasible. For these reasons, few radio channel models are available for the upper mmWave bands; the available models often rely heavily on simulations and are valid for very specific indoor scenarios. More work is required to model industrial settings, accounting for construction materials, mechanical and electrical noise and the presence of robots and heavy machinery.3
Energy Consumption and Sustainability
The required leap into the sub-THz frequencies with its abundant bandwidth cannot be achieved by merely increasing the carrier frequency.6 Analog RF electronics operating at these high frequencies with large bandwidth has limited power-added efficiency (PAE). Currently, high frequency PAs can only deliver PAEs of 7 and 4 percent at 170 and 220 GHz, respectively. The PAE of a transceiver would be expected to fall well under 10 percent.
The digitalization of large bandwidths and the speed of digital signal processing become major issues. Sampling frequencies of A/D converters and their associated power consumption remain inadequate for efficient signal processing and the energy consumption demands that will be imposed on 6G networks. Energy efficiency is increasingly important as the world shifts toward sustainability.5 6G infrastructure energy consumption is expected to be at the level of 4G networks irrespective of the number of terminals, so the expected growth in data traffic volume does not result in a comparable growth in energy consumption.10
Packaging the Front-End and Antennas
Component and system level packaging and antenna array integration pose significant challenges at 5G. With additional transmission line losses from sub-THz propagation, higher integration will be required. Resolving these issues may require novel 3D packaging and structures with the chips and antennas stacked on each other to reduce interconnect lengths, footprint and tight antenna array (UM-MIMO) integration.6 The combination of tight integration and low efficiency of the RF circuitry poses yet another major challenge: thermal management.
TECHNOLOGY CHALLENGES
A new generation of communications technology occurs when two driving forces align: one that stems from societal needs, the other when technologies are mature enough to address the need. Wireless communication that operates in the sub-THz and THz bands will only make sense for those use cases that are not focused on cost and energy efficiency. These likely include indoor communications in data centers, where massive amounts of data could be transported without a complex and costly cable infrastructure. With outdoor communications, backhaul will emerge as an application where fiber-like data transfers will be achievable over 0.1 to 1 km links. The lower sub-THz band, with frequencies in the 100 to 200 GHz range, seems like a good candidate for such applications. Researchers and engineers are already conducting studies and developing devices and systems in D- (110 to 170 GHz) and G-Band (140 to 220 GHz).
There is still significant progress to be achieved in developing more efficient modulation. Supported by artificial intelligence and machine learning, new modulation schemes will emerge, tailored for specific use cases, throughput and latency.
Realistically, it is difficult to imagine 6G communication networks using the bands beyond 300 GHz. Not only is the free space loss often greater than 100 dB—reducing the range of a wireless link to a few tens of meters, at best—but semiconductor devices, materials and integration technologies are not developed to support Tbps connectivity above 300 GHz. While RF electronics will improve with time and, when combined with advanced antenna beamforming techniques, will mitigate the higher signal losses and limited link distances, it is unreasonable to assume the required maturity of RF technology is simply a matter of time. It is uncertain if CMOS and SiGe BiCMOS will provide adequate performance at THz frequencies by the time 6G is implemented. Additionally, nano- and meta-materials, as well as graphene-based electronics, need time to mature and prove viability. Will the trend of ever-increasing speed of analog and digital CMOS and BiCMOS based electronics, so fundamental to large scale communications, continue? Or is it a time for other semiconductor technologies such as InP HBT to form the foundation for 6G?
Also uncertain: will the complexities of RF circuit parallelization, antenna design and fabrication, high speed and power components, efficiency, power consumption and heat dissipation and system integration challenges can be resolved in time for the 6G rollout. Even assuming that all the RF electronics and technology issues can be resolved, with stringent requirements on power consumption and energy efficiency for 6G devices and networks, data processing may become the bottleneck. Allowing for the most optimistic assumptions about RF circuit performance at sub-THz, the applicability of such systems will be very limited for any battery-powered devices.
True THz communication beyond 300 GHz may come, but it may only be in time for 7G. The biggest challenge is the achievable link distance, as transceiver available output power and sensitivity are and will remain low for the foreseeable 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 to meeting and collaborating virtually, communications have become as vital as water and electricity. We are constantly 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.10 The vision for 6G is to provide an affordable and scalable network with great coverage everywhere, so it ends the digital divide and truly provides an all-connected world.
TO BE OR NOT?
Sub-THz 6G faces significant unresolved challenges. For the RF circuits, higher output power and efficiency, lower NF and phase noise must be achieved, with even more advanced antenna beamforming solutions used to combat signal losses and limited link distance. Propagation channels at THz frequencies for both indoor and outdoor communications remain uncharacterized and largely depend on simulation rather than measurements, as channel sounding techniques and measurement equipment are primitive. The energy consumption and sustainability requirements expected for 6G networks are daunting, which reveals the need for novel energy harvesting and energy transfer for devices and networks to meet stringent bit-per-joule requirements. Packaging and integration techniques that largely rely on rectangular waveguide and milling technique must be simplified to support volume manufacturing and reduce cost.
The immaturity of the technologies and fabrication techniques make the challenge of developing 6G even greater than it was for 5G, which piggy-backed on a more mature foundation. For 6G to deliver THz communication and enable many new use cases in the typical decade timeframe is more difficult. Considering the current maturity of the enabling technologies and the required development, THz communication in 6G seems an unlikely scenario. A more realistic bet is for THz communication to arrive in time for 7G.
References
- Ch. de Alwis, A. Kalla, Q.-V. Pham, P. Kumar, K. Dev, W.-J. Hwang and M. Liyanage, “Survey of 6G Frontiers: Trends, Application, Requirements, Technologies and Future Research,” IEEE Open Journal of the Communications Society, April 2021, pp. 836–886.
- N. Rajatheva, I. Atzeni, S. Bicais, E. Björnson et. al., “Scoring the Terabit/s Goal: Broadband Connectivity in 6G,” IEEE Open Journal of Communication Society, August 2020.
- H. Tataria, M. Shafi, A. F. Molisch, M. Dohler, H. Sjöland and F. Tufvesson, “6G Wireless Systems: Vison, Requirements, Challenges, Insights, and Opportunities,” IEEE Proceedings, Vol. 109, Issue 7, July 2021.
- “Spectrum Horizons,” ET Docket No. 18-21, RM-11795, FCC 19-19, March 2019.
- “6G Vision – The Next Hyper-Connected Experience for All,” Samsung Research, July 2020.
- “White Paper on RF Enabling 6G – Opportunities and Challenges from Technology to Spectrum,” 6G Flagship, 6G Research Vision, No. 13, April 2021.
- “Key Drivers and Research Challenges for 6G Ubiquitous Wireless Intelligence,” 6G Flagship, 6G Research Visions 1, September 2019.
- E. C. Strinati, S. Barbarossa, J. L. Gonzales-Jimenez, D. Ktenas, N. Cassiau, L. Maret and C. Dehos, “6G: The Next Frontier,” IEEE Vehicular Technology Magazine, Vol. 14, Issue 3, Sept. 2019.
- A. Faisal, H. Sarieddeen, H. Dahrouj, T. Y. Al-Naffouri and M.-S. Alouini, “Ultra-Massive MIMO Systems at Terahertz Bands: Prospects and Challenges,” IEEE Vehicular Technology Magazine, Vol. 15, Issue 4, December 2020.
- “European Vision for the 6G Network Ecosystem,” 5G Infrastructure Association, Version 1, June 2021.