Extremely high frequency

Extremely high frequency (EHF) is the designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 GHz, lying between the high frequency band and the far infrared band, which is the lower part of terahertz band. Radio waves in this band have wavelengths from ten to one millimeter, and are therefore also called millimeter band and radiation in this band is called millimeter waves (mmWaves). Millimeter-length electromagnetic waves were first investigated by Indian physicist Jagadish Chandra Bose during 1894-1896, when he reached up to 60 GHz in his experiments.

EHF waves are prone to atmospheric attenuation, making them of little use over long distances. The radio waves in the EHF band, or millimeter band, are absorbed by gases in the atmosphere, which reduces their range, and has made them best suited for short range terrestrial communication over about a kilometer. They mainly propagate solely by line-of-sight paths and are not reflected by the ionosphere nor do they travel along the Earth as ground waves. At typical power densities, they are blocked by building walls and suffer significant attenuation passing through foliage.

In particular, signals in the 57-64 GHz region are subject to a resonance of the oxygen and are severely attenuated. Even over relatively short distances, rain fade is a serious problem, which is caused when a signal is absorbed by the rain and reduces the strength of that signal. This also means that in climates other than deserts, absorption due to humidity has an impact on signal propagation.

Absorption by atmospheric gases is maximized at specific frequencies, such as the absorption around 60 GHz by oxygen, or absorption of water vapor at 24 GHz and 184 GHz. At frequencies in the valleys between these absorption peaks, millimeter waves have much less atmospheric attenuation and a greater range. The frequencies in these valleys tend to be used in many applications.

The short propagation range of EHF waves, or mmWaves, allows smaller frequency reuse distances than lower frequencies. And the short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential. The mmWaves have been used for military fire-control radar, airport security scanners, short range wireless networks, and scientific research. And a relatively new application of mmWaves has been the use of the lower frequency ranges of the EHF range in 5G networks. Although the design of millimeter-wave circuit and subsystems (such as antennas, power amplifiers, and oscillators) also presents sever challenges to engineers due to semiconductor and process limitations, and model limitations.

The EHF band has commonly been used in radio astronomy and remote sensing. Ground-based radio astronomy has been limited to high altitude sites such as Kitt Peak and Atacama Large Millimeter Array (ALMA) due to atmospheric absorption issues. Satellite-based remote sensing near 60 GHz have been used to determine temperature distributions in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The International Telecommunication Union (ITU) nonexclusive passive frequency allocation at 57-59.3 is used for atmospheric monitoring in meteorological and climate sensing applications, and is important for these purposes due to the properties of oxygen absorption and emission in Earth's atmosphere.

In the United States, the 38.6-40.0 GHz band has been used for licensed high-speed microwave data links, and the 60 GHz band can be used for unlicensed short range data links and data throughputs up to 2.5 gigabytes/second.

The 71-76, 81-86, and 92-95 GHz bands have also been used for point-to-point high-bandwidth communication links. These frequencies, as opposed to the 60 GHz frequency, require a transmitting license in the Unite States from the FCC, through they do not suffer from the effects of oxygen absorption as the 60 GHz does. There have been plans for 10 GB/s links using these frequencies as well. In the case of the 92-95 GHz band, a small 100 MHz range has been reserved for space-borne radios, making this reserved range limited to a transmission rate of under a few GB/s.

In addition, because of the shorter wavelengths this band permits, it offers a chance to use smaller antennas that can achieve high directivity and high gain. This allows for a more efficient use of frequencies for point-to-multipoint applications, which means a greater number of directive antennas can be placed in a given area, and offer a higher density of users due to a greater frequency reuse. This allows the EHF band to serve some applications that would otherwise use fiber-optic communications, for very short links such as a circuit board and for vehicular communication for semi-autonomous or fully-autonomous vehicular communication.

In telecommunications, the EHF or mmWave band has been suggested to be used for 5G propagation, especially given its ability to deliver low-latency, high-bandwidth communications. This has been considered in several 5G-enabled scenarios, such as enhanced mobile broadband, massive machine-type communications, and ultra-reliable and low latency communications.

For enhanced mobile broadband (eMBB), mmWaves offers a chance to deal with increased data rates, high user density, and high traffic capacity for hotspot scenarios and seamless coverage for high mobility scenarios with improved data rates over current signal propagation technology. eMBB is expected to be able to support several sub-use cases such as office use, gaming us, virtual or augmented reality, three-dimensional or ultra-high-definition video streaming.

For massive machine-type communication (mMTC), in combination with IoT technologies, which require low power consumption and low data rates for large numbers of connected services, mMTC and the use of mmWaves offers long-range communication with energy efficiency and asynchronous access. And in the case of ultra-reliable and low-latency communications (URLLC), mmWaves can be used to cater to safety-critical and mission-critical applications of 5G, such as industrial automation and robotics, autonomous driving, drone-based delivery, and remote medical assistance.

In the defense industry, extremely high frequency and mmWave have been used in the Lightweight Integrated MMW Sensor (LIMS) and the Advanced Fire Control Radar (AFCOR) systems. The AFCOR is a dual-frequency (Ku and 94 GHz bands) coherent pulse doppler radar which offers improved tracking of low-angle airborne targets. The LIMS is a 94 GHz experimental sensor developed to optimize target detection and tracking capabilities while also maximizing the range at which a target can be detected and tracked.

The LIMS system is a signal processor integrated with a coherent, solid-sate 94 GHz transceiver having a 1 GHz instantaneous bandwidth. The sensor's waveform offers good range resolution and uses a high duty cycle for extended range detection compatible with peak power limitations of MMW solid-state sources. These systems are capable of being carried on various weapon systems and can be used for stationary target discrimination and classification, as well as target tracking and weapon aiming.

Further, the United States Air Force has been reported to have used extremely high frequency bands to develop a nonlethal weapon systems called Active Denial System (ADS), which emits a beam of radiation with a wavelength of 3 mm. The weapon is reportedly not painful, but makes the target feel as if their clothes will catch fire. The military version of this anti-personnel weapon has an output power of 100 kilowatts (kW) while a smaller version of the system intended for use by law enforcement, also known as the Silent Guardian, has an output power of 30 kW.

Another development of mmWave has been its use in security applications. This is capable as clothing and other organic materials are translucent in some mmWave atmospheric windows. This would allow an airport or related travel terminal to screen groups or passengers and individuals using mmWave beams and see if they are carrying or hiding anything on their person without having to search them. Privacy advocates have raised concerns about the use of this technology, as people are seen without clothing, but most applications remove the face and other identifying features during a scan. These systems have been installed in airports in the United States and in Amsterdam.

EHF is also being used for future satellite services, such as high quality TV broadcasting and multimedia content delivery. This has been aimed at surveying opportunities and challenges of EHF exploration for broadband satellite applications in the broadcast framework, with theoretical capacity evaluation suggesting the EHF satellite links can offer unprecedented data-rates superior than that offered by Ku and Ka-bands. However, the issues of tropospheric propagation impairments and appropriate waveform design still need to be solved.

In the case of the United States Air Force's Advanced Extremely High Frequency (AEHF) satellite systems, the systems have used super high-frequency (SHF) for long range communications and to help with tropospheric impairments. The AEHF is a next-generation military strategic and tactical satellite relay system, which is intended to provide survivable, protected communications to U.S. Forces and selected allies worldwide, that would consist of four crosslinked satellites, a ground mission control and user terminals.

AEHF is designed to support a wide range of missions, such as strategic nuclear and defense operations, special operations, theatre missile defense, space operations, and intelligence operations. The systems achieved initial operation capability in July 2018, and expanded to six satellites in orbit by March 2020.

For many radio wave frequencies and their propagation, which is largely affected by environmental and climatic factors which impact the performance of radio communication systems, availability and reliability of those systems, and the use of congested electromagnetic systems, have pushed towards the development of flexible solutions to solve these challenges. Instead of, what has been previously used, explicit programming for these challenges, machine learning has been proposed as a solution, as the model can be trained and can continue to learn from data and make inferences on new observations.

Shallow machine learning algorithms have been applied to the design of intelligent antenna systems, reliable radio propagation models, and spectrum usage prediction techniques, and in these cases the use of machine learning has shown superior performance compared to traditional approaches. In the case of mmWave, or EHF, these frequencies can support high communication rates, and facilitate the use of multiple-input-multiple-output (MIMO) techniques to increase wireless capacity. However, MIMO technology can result in path loss and channel uncertainty and has a high energy consumption cost. The use of machine learning could provide an opportunity for channel modeling and offer greater reliability in mmWave devices. Further, the application of machine learning could be used to optimize frequencies to reduce atmospheric propagation loss and increase the overall range of mmWave devices.

The use of machine learning has been demonstrated in non-EHF or mmWave spectrum management, while for mmWave spectrum the use of algorithmic machine learning has been proposed and tested for UAV-based wireless communications, wireless cellular systems, and for wearable device networks.