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Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond



Pat Hindle, MWJ Editor
Pat Hindle is responsible for the editorial content, article review and special industry reporting for Microwave Journal magazine and its web site in addition to social media and special digital projects. Prior to joining the Journal, Mr. Hindle held various technical and marketing positions throughout New England, including Marketing Communications Manager at M/A-COM (Tyco Electronics), Product/QA Manager at Alpha Industries (Skyworks), Program Manager at Raytheon and Project Manager/Quality Engineer at MIT. Mr. Hindle graduated from Northeastern University - Graduate School of Business Administration and holds a BS degree from Cornell University in Materials Science Engineering.
Pat Hindle is responsible for the editorial content, article review and special industry reporting for Microwave Journal magazine and its web site in addition to social media and special digital projects. Prior to joining the Journal, Mr. Hindle held various technical and marketing positions throughout New England, including Marketing Communications Manager at M/A-COM (Tyco Electronics), Product/QA Manager at Alpha Industries (Skyworks), Program Manager at Raytheon and Project Manager/Quality Engineer at MIT. Mr. Hindle graduated from Northeastern University - Graduate School of Business Administration and holds a BS degree from Cornell University in Materials Science Engineering.

Professor Theodore S. Rappaport and colleagues at NYU WIRELESS recently published a new article entitled “Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond,” outlining the challenges and possible solutions for future communications using frequencies from 100 GHz to 3 THz. They also discuss future applications such as wireless cognition, hyper-accurate position location, sensing and imaging that will be possible with these new technologies.

The article discusses how within the global unlicensed bands of 60 GHz, there is about 7 GHz of bandwidth available which is not enough bandwidth to support data rates of 100 Gbps (that they are targeting for 6G) given today’s technology, so we must look beyond 100 GHz for the future. In March of 2019, the FCC opened up spectrum above 95 GHz for the first time in the US providing 21.2 GHz of unlicensed spectrum in the 116 t0 246 GHz range and is permitting experimental licensing up to 3 THz. This has opened the door for researchers to experiment with 6G and beyond technologies.

Wireless Communications and Applications Above 100 GHz: Opportunities and Challenges for 6G and Beyond


Frequencies from 100 GHz to 3 THz are promising bands for the next generation of wireless communication systems because of the wide swaths of unused and unexplored spectrum. These frequencies also offer the potential for revolutionary applications that will be made possible by new thinking, and advances in devices, circuits, software, signal processing, and systems. This paper describes many of the technical challenges and opportunities for wireless communication and sensing applications above 100 GHz, and presents a number of promising discoveries, novel approaches, and recent results that will aid in the development and implementation of the sixth generation (6G) of wireless networks, and beyond. This paper shows recent regulatory and standard body rulings that are anticipating wireless products and services above 100 GHz and illustrates the viability of wireless cognition, hyper-accurate position location, sensing, and imaging. This paper also presents approaches and results that show how long distance mobile communications will be supported to above 800 GHz since the antenna gains are able to overcome air-induced attenuation and present methods that reduce the computational complexity and simplify the signal processing used in adaptive antenna arrays, by exploiting the Special Theory of Relativity to create a cone of silence in over-sampled antenna arrays that improve performance for digital phased array antennas. Also, new results that give insights into power efficient beam steering algorithms, and new propagation and partition loss models above 100 GHz are given, and promising imaging, array processing, and position location results are presented. The implementation of spatial consistency at THz frequencies, an important component of channel modeling that considers minute changes and correlations over space, is also discussed. This paper offers the first in-depth look at the vast applications of THz wireless products and applications and provides approaches ...

The tremendous funding and research efforts invested in millimeter wave (mmWave) wireless communications, and the early success of 5G trials and testbeds across the world, ensure that commercial wide-spread 5G wireless networks will be realized by 2020 [1]. The use of mmWave in 5G wireless communication will solve the spectrum shortage in current 4G cellular communication systems that operate at frequencies below 6 GHz [2]. However, the increasing number of new applications such as virtual/augmented reality (VR/AR), autonomous driving, Internet of Things (IoT), and wireless backhaul (as a replacement for labor-intensive installation of optical fiber) [3], [4], as well as newer applications that have not been conceived yet, will need even greater data rates and less latency than what 5G networks will offer.

Today, within the global unlicensed wireless mmWave band of 60 GHz, there is ~7 GHz of bandwidth available [5], and in such a wide bandwidth, data rates on the order of 100 Gigabits per second (Gbps) can only be achieved with transmission schemes having a spectral efficiency of at least 14 bit/s/Hz, which requires symbol fidelity that is not feasible using currently known digital modulation techniques or transceiver components [6]–[7][8]. Therefore, data rates on the order of 100 Gbps or more will flourish at frequencies above 100 GHz, where the available spectrum is massively abundant [9].

Fig. 1 illustrates the applications and range of frequencies available from the sub-THz regime up through and beyond the optical spectrum and shows how mmWaves and THz frequencies are three and two orders of magnitude, respectively, below the frequencies of visible light. At optical and infrared frequencies, issues like the impact of atmospheric and water absorption on the signal propagation, ambient sunlight, required low transmission power budget due to eye-safety limits, and high diffusion losses on rough surfaces limit their use for wireless communication systems [10]. Ionizing radiation, which includes ultraviolet, x-rays, galactic radiation, and gamma-rays, is dangerous since it is known to have sufficiently high particle energy to dislodge electrons and create free-radicals that can lead to cancer [11], [12] and is believed to be a major health risk for interplanetary travel [12], [13]. The adverse health effects of ionizing radiation may be negligible, however, if used with care [14]. Ionizing radiation can be used for gauging the thickness of metals, Roentgen Stereophotogrammetry, astronomy, nuclear medicine, sterilizing medical equipment, and pasteurizing certain foods and spices [15]. Unlike ionizing radiation, mmWave and THz radiation are nonionizing because the photon energy is not nearly sufficient (0.1 to 12.4 meV, which is more than three orders of magnitude weaker than ionizing photon energy levels) to release an electron from an atom or a molecule, where typically 12 eV is required for ionization [11], [12], [16]. Since ionizing radiation is not known to be a concern at mmWave and THz frequencies, and heating is believed to be the only primary cancer risk [11],[12] the Federal Communications Commission (FCC) and International Commission on Non-Ionizing Radiation Protection (ICNIRP) standards [17], [18] are designed principally to protect against thermal hazards, particularly for the eyes and skin where these tissues are most sensitive to heat from radiation due to lack of blood flow. However, we must point out that with the likelihood of THz sources becoming more widely available, there should be careful work done to understand the biological and molecular impact of THz radiation on human health [12], since, even though THz is more than two orders of magnitude lower in frequency than ionizing radiation, it would be prudent to know with certainty that heating is the only health concern at THz [11].

Figure 1

The electromagnetic spectrum, and various applications as a function of frequency.

Figure 2

Surface scattering across the EM spectrum a) Most surfaces appear smooth at microwave frequencies (specular scattering), b) Same surfaces exhibit significant considerable roughness in optical spectrum (diffuse scattering) c) in THz regime most building surfaces exhibit significant diffuse scattering and strong specular reflections.

Figure 3

Leveraging mmWave imaging and communications can position devices with centimeter accuracy, even in NLOS scenarios. Figure (a) shows the experimental setup adopted by [50] to illustrate the capability of mmWave imaging-based positioning. In Figure (b) the user location is projected on the constructed mmWave image.

Figure 4

(a) A spatially-oversampled Nx×Ny uniform rectangular antenna array (URA). The antenna spacing is λm/(2Kx) and λm/(2Ky) along the x -and y-axes, respectively. Here λm is the electromagnetic wavelength, Kx1 and Ky1 are the spatial oversampling factors along x and y, respectively, and assume Kx=Ky=Ku for convenience. (b) The ROS of waves received by the 2-D URA (green) consists of a narrow light cone that is sparse in the 3-D spacetime frequency domain.

Figure 5

An overview of a space-time ΔΣ ADC where spatiotemporal noise-shaping occurs along with both spatial- and temporal-frequency domains. The analog filtering module inside the multi-port ADC couples the inputs and output ports in such a manner that the quantization noise arising from the 1-bit quantizers are shaped to be outside the Ku -compressed “cone of silence” of the array. The plane-wave signals of interest, on the other hand, now lie inside the “cone of silence” and can be extracted by a sharp digital beamformer whose minor lobes will significantly attenuate the quantization noise of the system.

While 5G, IEEE 802.11ay, and 802.15.3d [19], [20] are being built out for the mmWave spectrum and promise data rates up to 100 Gbps, future 6G networks and wireless applications are probably a decade away from implementation, and are sure to benefit from operation in the 100 GHz to 1 THz frequency bands where even greater data rates will be possible [3], [7], [10]. The short wavelengths at mmWave and THz will allow massive spatial multiplexing in hub and backhaul communications, as well as incredibly accurate sensing, imaging, spectroscopy, and other applications described subsequently in this paper [21]–[22][23][24]. The THz band, which we shall describe as being from 100 GHz through 3 THz, can also enable secure communications over highly sensitive links, such as in the military due to the fact that extremely small wavelengths (orders of microns) enable extremely high gain antennas to be made in extremely small physical dimensions [25]. Although we note the formal definition of the THz region is 300 GHz through 3 THz, some have begun to use the terms “sub-THz” or “sub-mmWave” (e.g. using frequency or wavelength) to define the 100–300 GHz spectrum.

There are tremendous challenges ahead for creating commercial transceivers at THz frequencies, but global research is addressing the challenges. For example, the DARPA T-MUSIC program is investigating SiGe HBT, CMOS/SOI, and BiCMOS circuit integration, in hopes of achieving power amplifier threshold frequencies ft of 500–750 GHz [26]. A survey of power amplifier capabilities since the year 2000 is given in [27]. It should be clear that the semiconductor industry will solve these challenges, although new architectures for highly dense antenna arrays will be needed, due to the small wavelengths and physical size of RF transistors in relation to element spacing in THz arrays. Section III provides some promising design approaches for future digital arrays.

Since there is very high atmospheric attenuation at THz band frequencies, especially at frequencies above 800 GHz (see Fig. 6), highly directional “pencil” beam antennas (antenna arrays) will be used to compensate for the increased path loss due to the fact that the gain and directivity increase by the square of the frequency for a fixed physical antenna aperture size [6], [28]. This feature makes THz signals exceedingly difficult to intercept or eavesdrop [4], [10], [25], [29]. However, a narrow pencil-like beam does not guarantee immunity from eavesdropping, and physical-layer security in THz wireless networks and transceiver designs that incorporate new counter-measures for eavesdropping will be needed [30].

Figure 6

Atmospheric absorption beyond the natural Friis free space loss of electromagnetic waves at sea level versus frequency under different humidity conditions [101], [104].

Energy efficiency is always important for communication systems, especially as circuitry moves up to above 100 GHz, and a theoretical framework to quantify energy consumption in the presence of the vital device, system, and network trade-offs was presented in [31], [32]. The theory called the consumption factor theory (CF, with a metric measured in bps/W), provides a means for enabling quantitative analysis and design approaches for understanding power trade-offs in any communication system. It was shown in [31], [32] that the efficiency of components of a transmitter closest to the output, such as the antenna, have the largest impact on CF [31]. The power efficiency increases with increasing bandwidth when most of the power used by components that are “off”, e.g., ancillary, to the signal path (e.g., the baseband processor, oscillator, or a display) is much greater than the power consumed by the components that are in line with the transmission signal path (e.g., power amplifier, mixer, antenna) [31], [32]. For a very simple radio transmitter, such as one that might be used in low-cost IoT or “smart dust” applications where the power required by the ancillary baseband processor and the oscillator is small compared to the delivered radiated power, the power efficiency is independent of the bandwidth [31]. Thus, contrary to conventional wisdom, the CF theory proves that for antennas with a fixed physical aperture, it is more energy efficient to move up to mmWave and THz frequencies which yield much wider bandwidth and better power efficiency on a bits per second per watt (bps/W) basis, as compared to the current, sub-6 GHz communication networks.

Global regulatory bodies and standard agencies such as the FCC [33], the European Telecommunication Standards Institute (ETSI) [34], and the International Telecommunication Union (ITU) [35], are seeking comments to allocate frequency bands above 95 GHz for point-to-point use, broadcasting services, and other wireless transmission applications and use cases [36]–[37][38][39]. In fact, in March 2019, the FCC voted to open up spectrum above 95 GHz for the first time ever in the USA, and provided 21.2 GHz of spectrum for unlicensed use shown in Table 1, and permitted experimental licensing up to 3 THz [40]. The mmWave coalition [41], which is a group of innovative companies and universities united in the objective of removing regulatory barriers to technologies using frequencies ranging from 95 GHz to 275 GHz in the USA, submitted comments to the FCC and to the National Telecommunications and Information Administration (NTIA) for developing a sustainable spectrum strategy for America’s future, and urged NTIA to facilitate greater access to spectrum above 95 GHz for non-Federal use in January 2019 [41]. The Institute of Electrical and Electronics Engineers (IEEE) formed the IEEE 802.15.3d [20] task force in 2017 for global Wi-Fi use at frequencies from 252 GHz to 325 GHz, creating the first worldwide wireless communications standard for the 250–350 GHz frequency range, with a nominal PHY data rate of 100 Gbps and channel bandwidths from 2 GHz to 70 GHz [20]. The use cases for IEEE 802.15.3d include kiosk downloading (dubbed the “Information Shower” by an author of this paper) [42], intra-device radio communication [43], connectivity in data centers, and wireless fiber for front haul and backhaul [20], [41], [44]. As indicated in [41], an interesting issue is the system simulation and fabrication of antennas to ensure that ground-based fixed and mobile sub-THz systems do not interfere with satellites and space-based sensors operating in the same sub-THz bands above the earth.

TABLE 1 Unlicensed Spectrum Proposed by FCC [33]
Table 1

This paper is organized as follow: Section II provides insights into mmWave and THz applications. Section III explores the possibility of using array signal processing techniques for improved front-end performance at frequencies above 100 GHz. Section IV demonstrates viability for true time delay (TTD) beamformers for wideband digital arrays. Section V reviews key wireless propagation fundamentals and present measurements and research above 100 GHz which could be used for many novel applications, including future 6G communications. Channel sounding systems and measured results at above 100 GHz are given in Section VI, respectively. 28, 73 and 142 GHz channels are compared in Section VII. Scattering mechanism at mmWave and THz frequencies are explained in Section VIII. Section IX investigates positioning approaches (e.g., position location that can be used in navigation) at THz frequencies that promise unprecedented accuracy. Section X discusses the implementation of spatial consistency, an important component of channel modeling at THz frequencies, and Section XI concludes the paper and summarizes key areas that warrant future research.

mmWave and THz Applications

The ultra-high data rates facilitated by mmWave and THz wireless local area and cellular networks will enable super-fast download speeds for computer communication, autonomous vehicles, robotic controls, the information shower [42], high-definition holographic gaming, entertainment, video conferencing, and high-speed wireless data distribution in data centers [10]. In addition to the extremely high data rates, there are promising applications for future mmWave and THz systems that are likely to evolve in 6G networks, and beyond. These applications can be categorized into main areas such as wireless cognition, sensing, imaging, wireless communication, and position location/THz navigation (also called localization, or positioning), as summarized in Table 2.

TABLE 2 Promising Applications at mmWave and THz
Table 2

A. Wireless Cognition

Wireless cognition is the concept of providing a communication link that enables massive computations to be conducted remotely from the device or machine that is doing real-time action [46]. For example, a lightweight drone fleet may not have the power or weight budget to conduct massive computations on board the apparatus, but with a wide enough channel bandwidth and sufficiently fast data rate, real-time computations for extremely complex tasks, such as contextual awareness, vision, and perception may be carried out at a fixed base station or edge server that is in wireless connection and supporting real-time cognition for the drone fleet. Robots, autonomous vehicles, and other machines may be similarly designed to exploit cognitive processing performed remotely from the machine using wireless, with the ability to perform tasks without the benefit of local cognition on the platform [45], [46].

When one considers the growth of computational power provided by Moore’s law, it can be seen that the modest price of 1000 USD (the cost of today’s smartphone) will likely be able to purchase a computer with computational capabilities that are on the order of the human brain by the year 2036. This observation stems from results in [54], [55] which extrapolate the increase in computational capabilities over time since 1965 [56]. The following analysis shows that Terahertz frequencies will likely be the first wireless spectrum that can provide the real-time computations needed for wireless remoting of human cognition [52].

There are about 100 billion (1011) neurons in the human brain, each of which can fire 200 times per second (5 ms update rate), and each neuron is connected to about 1000 others, resulting in a computation speed of 20×1015 floating-point operations per second (flops) [54], [55], which, if each operation is assumed to be binary, will require a data rate of 20,000 Tbps:

===Human Brain flops (Computation Speed)1011 neurons×200 flop/sec×103/neuron20×1015 flop/sec=20 petaflops/sec×1 bit/flop20,000 Tbps.(1)
View SourceRight-click on figure for MathML and additional features.

Each neuron has to write access to 1000 bytes resulting in memory size of the human brain of 100 Terabytes [54], [55]:

Storage==1011 neurons×103 bytes/neuron1014 bytes=100 TB.(2)
View SourceRight-click on figure for MathML and additional features.

Today’s state-of-the-art 1000 USD computer technology performs one trillion (e.g., 1012) computations/sec, which is four orders of magnitude less than the speed of the human brain. Future wireless generations (e.g., 6G or 7G) are likely to allocate up to 10 GHz RF channels for each user in the THz regime, and by assuming that each user is able to exploit 10 bits/symbol modulation methods and 1000 times increase in channel capacity using yet-to-be-invented concepts beyond cooperative multipoint (CoMP) and Massive-MIMO, it is readily seen in (3) that data rates of 100 Terabytes/sec will be achieved.

R==10 GHz channel×10 bits/(sec c dot Hz)×103100 Tbps(3)
View SourceRight-click on figure for MathML and additional features.From (1) and (2), it is clear that a 100 Tbps link is plausible in a 10 GHz channel bandwidth, providing 0.5% of real-time human computational power. Ambitiously, if 100 GHz channel bandwidths are used, 1 Petabit/sec of information, or 5% of the real-time computational power of the human brain, could be carried over wireless [52].

[the full article is HERE

source: https://ieeexplore.ieee.org

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