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Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band, and the far infrared band, the lower part of which is the terahertz band. Radio waves in this band have wavelengths from ten to one millimetre, so it is also called the millimetre band and radiation in this band is called millimetre waves, sometimes abbreviated MMW or mmW or mmWave. Millimetre-length electromagnetic waves were first investigated by Bengali Indian physicist Jagadish Chandra Bose during 1894–1896, when he reached up to 60 GHz in his experiments.

Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere. Therefore, they have a short range and can only be used for terrestrial communication over about a kilometer. Absorption increases with frequency until at the top end of the band the waves are attenuated to zero within a few meters. Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances. However the short propagation range allows smaller frequency reuse distances than lower frequencies. The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential. Millimeter waves are used for military fire-control radar, airport security scanners, short range wireless networks, weapon system LRAD, and scientific research.

“Fifth Generation” technology standard for cellular networks, which cellular phone companies began deploying worldwide in 2019, frequencies near the low end of this band are designated for use in the newest generation of cell phone networks.

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In telecommunications,  fifth generation technology standard for cellular networks, which cellular phone companies began deploying worldwide in 2019, the planned successor to the 4G networks which provide connectivity to most current cellphones. Like its predecessors, 5G networks are cellular networks, in which the service area is divided into small geographical areas called cells. All 5G wireless devices in a cell are connected to the Internet and telephone network by radio waves through a local antenna in the cell. The main advantage of the new networks is that they will have greater bandwidth, giving faster download speeds, eventually up to 10 gigabits per second (Gbit/s). Due to the increased bandwidth, it is expected that the new networks will not just serve cellphones like existing cellular networks, but also be used as general internet service providers for laptops and desktop computers, competing with existing ISPs such as cable internet, and also will make possible new applications in IoT and M2M areas. Current 4G cellphones will not be able to use the new networks, which will require new 5G enabled wireless devices.

The increased speed is achieved partly by using higher frequency radio waves than current cellular networks. However, higher frequency microwaves have a shorter range than the frequencies used by previous cell phone towers, requiring smaller cells. So to ensure wide service, 5G networks operate on up to three frequency bands, low, medium, and high. A 5G network will be composed of networks of up to 3 different types of cell, each requiring different antennas, each type giving a different tradeoff of download speed vs distance and service area. 5G cellphones and wireless devices will connect to the network through the highest speed antenna within range at their location:

Low-band 5G uses a similar frequency range as current 4G cellphones, 600 – 700 MHz giving download speeds a little higher than 4G: 30-250 megabits per second (Mbit/s). Low-band cell towers will have a similar range and coverage area to current 4G towers. Mid-band 5G uses microwaves of 2.5-3.7 GHz, currently allowing speeds of 100-900 Mbit/s, with each cell tower providing service up to several miles radius. This level of service is the most widely deployed, and should be available in most metropolitan areas in 2020. Some countries are not implementing low-band, making this the minimum service level. High-band 5G uses frequencies of 25 – 39 GHz, near the bottom of the millimeter wave band, to achieve download speeds of 1 – 3 gigabits per second (Gbit/s), comparable to cable internet. However millimeter waves (mmWave or mmW) only have a range of about 1 mile (1.6 km), requiring many small cells, and have trouble passing through some types of building walls. Due to their higher costs, current plans are to deploy these cells only in dense urban environments, and areas where crowds of people congregate such as sports stadiums and convention centers. The above speeds are those achieved in actual tests in 2020, speeds are expected to increase during rollout.