Extremely high frequency is the International Telecommunication Union 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 millimeter, so it is also called the millimeter band and radiation in this band is called millimeter waves , sometimes abbreviated MMW or mmWave . Millimeter-length electromagnetic waves were first investigated by Jagadish Chandra Bose , who generated waves of frequency up to 60 GHz during experiments in 1894–1896.
69-772: The AN/APG-78 Longbow is a millimeter-wave fire-control radar (FCR) system for the AH-64D/E Apache attack helicopter . It was initially developed in the 1980s as the Airborne Adverse Weather Weapon System (AAWWS) as part of the Multi-Stage Improvement Program (MSIP) to enhance the AH-64A. By 1990, both AAWWS and MSIP were renamed Longbow. The radar is produced by Longbow LLC, a joint venture of Lockheed Martin and Northrop Grumman . The AN/APG-78 Longbow
138-487: A pseudonoise code (PN code) that is specific to each phone. As the user moves from one cell to another, the handset sets up radio links with multiple cell sites (or sectors of the same site) simultaneously. This is known as "soft handoff" because, unlike with traditional cellular technology , there is no one defined point where the phone switches to the new cell. In IS-95 inter-frequency handovers and older analog systems such as NMT it will typically be impossible to test
207-401: A cellular network, compared with a network with a single transmitter, comes from the mobile communication switching system developed by Amos Joel of Bell Labs that permitted multiple callers in a given area to use the same frequency by switching calls to the nearest available cellular tower having that frequency available. This strategy is viable because a given radio frequency can be reused in
276-487: A closed room, then deleted immediately upon search completion. Privacy advocates are concerned. "We're getting closer and closer to a required strip-search to board an airplane," said Barry Steinhardt of the American Civil Liberties Union. To address this issue, upgrades have eliminated the need for an officer in a separate viewing area. The new software generates a generic image of a human. There
345-426: A different area for an unrelated transmission. In contrast, a single transmitter can only handle one transmission for a given frequency. Inevitably, there is some level of interference from the signal from the other cells which use the same frequency. Consequently, there must be at least one cell gap between cells which reuse the same frequency in a standard frequency-division multiple access (FDMA) system. Consider
414-429: A few kilometers. Thus, they are useful for densely packed communications networks such as personal area networks that improve spectrum utilization through frequency reuse . Millimeter waves show "optical" propagation characteristics and can be reflected and focused by small metal surfaces and dielectric lenses around 5 to 30 cm (2 inches to 1 foot) diameter. Because their wavelengths are often much smaller than
483-513: A frequency reuse of 1. Since such systems do not spread the signal across the frequency band, inter-cell radio resource management is important to coordinate resource allocation between different cell sites and to limit the inter-cell interference. There are various means of inter-cell interference coordination (ICIC) already defined in the standard. Coordinated scheduling, multi-site MIMO or multi-site beamforming are other examples for inter-cell radio resource management that might be standardized in
552-399: A large number of active phones in that area. All of the cell sites are connected to telephone exchanges (or switches), which in turn connect to the public telephone network . In cities, each cell site may have a range of up to approximately 1 ⁄ 2 mile (0.80 km), while in rural areas, the range could be as much as 5 miles (8.0 km). It is possible that in clear open areas,
621-468: A limited, shared resource. Cell-sites and handsets change frequency under computer control and use low power transmitters so that the usually limited number of radio frequencies can be simultaneously used by many callers with less interference. A cellular network is used by the mobile phone operator to achieve both coverage and capacity for their subscribers. Large geographic areas are split into smaller cells to avoid line-of-sight signal loss and to support
690-650: A minimum of three channels, and three towers for each cell and greatly increases the chances of receiving a usable signal from at least one direction. The numbers in the illustration are channel numbers, which repeat every 3 cells. Large cells can be subdivided into smaller cells for high volume areas. Cell phone companies also use this directional signal to improve reception along highways and inside buildings like stadiums and arenas. Practically every cellular system has some kind of broadcast mechanism. This can be used directly for distributing information to multiple mobiles. Commonly, for example in mobile telephony systems,
759-402: A mobile phone network manages handover). The most common example of a cellular network is a mobile phone (cell phone) network. A mobile phone is a portable telephone which receives or makes calls through a cell site (base station) or transmitting tower. Radio waves are used to transfer signals to and from the cell phone. Modern mobile phone networks use cells because radio frequencies are
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#1732772812186828-455: A number of desirable features: Major telecommunications providers have deployed voice and data cellular networks over most of the inhabited land area of Earth . This allows mobile phones and mobile computing devices to be connected to the public switched telephone network and public Internet access . Private cellular networks can be used for research or for large organizations and fleets, such as dispatch for local public safety agencies or
897-412: A number of frequency channels corresponding to a bandwidth of B/K , and each sector can use a bandwidth of B/NK . Code-division multiple access -based systems use a wider frequency band to achieve the same rate of transmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1, for example using a reuse pattern of 1/1. In other words, adjacent base station sites use
966-757: 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 , and scientific research. In a major new application of millimeter waves, certain frequency ranges near the bottom of the band are being used in the newest generation of cell phone networks, 5G networks. The design of millimeter-wave circuit and subsystems (such as antennas, power amplifiers, mixers and oscillators) also presents severe challenges to engineers due to semiconductor and process limitations, model limitations and poor Q factors of passive devices. Millimeter waves propagate solely by line-of-sight paths. They are not refracted by
1035-424: A small 100 MHz range has been reserved for space-borne radios, limiting this reserved range to a transmission rate of under a few gigabits per second. The band is essentially undeveloped and available for use in a broad range of new products and services, including high-speed, point-to-point wireless local area networks and broadband Internet access . WirelessHD is another recent technology that operates near
1104-399: A taxicab company, as well as for local wireless communications in enterprise and industrial settings such as factories, warehouses, mines, power plants, substations, oil and gas facilities and ports. In a cellular radio system, a land area to be supplied with radio service is divided into cells in a pattern dependent on terrain and reception characteristics. These cell patterns roughly take
1173-530: A time when invited by the base station operator. This is a form of time-division multiple access (TDMA). The history of cellular phone technology began on December 11, 1947 with an internal memo written by Douglas H. Ring , a Bell Labs engineer in which he proposed development of a cellular telephone system by AT&T. The first commercial cellular network, the 1G generation, was launched in Japan by Nippon Telegraph and Telephone (NTT) in 1979, initially in
1242-416: A user may receive signals from a cell site 25 miles (40 km) away. In rural areas with low-band coverage and tall towers, basic voice and messaging service may reach 50 miles (80 km), with limitations on bandwidth and number of simultaneous calls. Since almost all mobile phones use cellular technology , including GSM , CDMA , and AMPS (analog), the term "cell phone" is in some regions, notably
1311-408: A wide geographic area. This enables numerous portable transceivers (e.g., mobile phones , tablets and laptops equipped with mobile broadband modems , pagers , etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission. Cellular networks offer
1380-574: Is a millimeter-wave fire-control radar (FCR) target acquisition system and the Radar Frequency Interferometer (RFI), which are housed in a dome located above the main rotor. The radome 's raised position enables target detection while the helicopter is behind obstacles (e.g. terrain, trees or buildings). The APG-78 is capable of simultaneously tracking up to 128 targets and engaging up to 16 at once; an attack can be initiated within 30 seconds. A radio modem integrated with
1449-598: Is a telecommunications network where the link to and from end nodes is wireless and the network is distributed over land areas called cells , each served by at least one fixed-location transceiver (such as a base station ). These base stations provide the cell with the network coverage which can be used for transmission of voice, data, and other types of content. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed service quality within each cell. When joined together, these cells provide radio coverage over
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#17327728121861518-555: Is based on spread spectrum technology developed for military use during World War II and improved during the Cold War into direct-sequence spread spectrum that was used for early CDMA cellular systems and Wi-Fi . DSSS allows multiple simultaneous phone conversations to take place on a single wideband RF channel, without needing to channelize them in time or frequency. Although more sophisticated than older multiple access schemes (and unfamiliar to legacy telephone companies because it
1587-424: Is called the handover or handoff. Typically, a new channel is automatically selected for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel to the new channel and communication continues. The exact details of the mobile system's move from one base station to the other vary considerably from system to system (see the example below for how
1656-412: Is no anatomical differentiation between male and female on the image, and if an object is detected, the software only presents a yellow box in the area. If the device does not detect anything of interest, no image is presented. Passengers can decline scanning and be screened via a metal detector and patted down. According to Farran Technologies, a manufacturer of one model of the millimeter wave scanner,
1725-478: Is no problem with two cells sufficiently far apart operating on the same frequency, provided the masts and cellular network users' equipment do not transmit with too much power. The elements that determine frequency reuse are the reuse distance and the reuse factor. The reuse distance, D is calculated as where R is the cell radius and N is the number of cells per cluster. Cells may vary in radius from 1 to 30 kilometres (0.62 to 18.64 mi). The boundaries of
1794-454: Is particularly associated with the range of 40–70 GHz . This type of treatment may be called millimeter wave therapy or extremely high frequency therapy . This treatment is associated with eastern European nations (e.g., former USSR nations). The Russian Journal Millimeter waves in biology and medicine studies the scientific basis and clinical applications of millimeter wave therapy. Traffic police use speed-detecting radar guns in
1863-558: Is used commonly in flat terrain. The 71–76, 81–86 and 92–95 GHz bands are also used for point-to-point high-bandwidth communication links. These higher frequencies do not suffer from oxygen absorption, but require a transmitting license in the US from the Federal Communications Commission (FCC). There are plans for 10 Gbit/s links using these frequencies as well. In the case of the 92–95 GHz band,
1932-623: The Advanced Microwave Sounding Unit (AMSU) on one NASA satellite (Aqua) and four NOAA (15–18) satellites and the special sensor microwave/imager (SSMI/S) on Department of Defense satellite F-16 make use of this frequency range. In the United States, the band 36.0–40.0 GHz is used for licensed high-speed microwave data links, and the 60 GHz band can be used for unlicensed short range (1.7 km) data links with data throughputs up to 2.5 Gbit /s. It
2001-632: The UMTS system where it allows for low downlink latency in packet-based connections. In LTE/4G, the Paging procedure is initiated by the MME when data packets need to be delivered to the UE. Paging types supported by the MME are: In a primitive taxi system, when the taxi moved away from a first tower and closer to a second tower, the taxi driver manually switched from one frequency to another as needed. If communication
2070-572: The ionosphere nor do they travel along the Earth as ground waves as lower frequency radio waves do. At typical power densities they are blocked by building walls and suffer significant attenuation passing through foliage. Absorption by atmospheric gases is a significant factor throughout the band and increases with frequency. However, this absorption is maximum at a few specific absorption lines , mainly those of oxygen at 60 GHz and water vapor at 24 GHz and 184 GHz. At frequencies in
2139-484: The upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The International Telecommunication Union non-exclusive passive frequency allocation at 57–59.3 GHz 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. Currently operational U.S. satellite sensors such as
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2208-418: The "windows" between these absorption peaks, millimeter waves have much less atmospheric attenuation and greater range, so many applications use these frequencies. Millimeter wavelengths are the same order of size as raindrops , so precipitation causes additional attenuation due to scattering ( rain fade ) as well as absorption. The high free space loss and atmospheric absorption limit useful propagation to
2277-450: The 60 GHz range. Highly directional, "pencil-beam" signal characteristics permit different systems to operate close to one another without causing interference. Potential applications include radar systems with very high resolution. The Wi-Fi standards IEEE 802.11ad and IEEE 802.11ay operate in the 60 GHz ( V band ) spectrum to achieve data transfer rates as high as 7 Gbit/s and at least 20 Gbit/s , respectively. Uses of
2346-594: The Bell System , with cellular assets transferred to the Regional Bell Operating Companies . The wireless revolution began in the early 1990s, leading to the transition from analog to digital networks . The MOSFET invented at Bell Labs between 1955 and 1960, was adapted for cellular networks by the early 1990s, with the wide adoption of power MOSFET , LDMOS ( RF amplifier ), and RF CMOS ( RF circuit ) devices leading to
2415-764: The Ka-band (33.4–36.0 GHz). ELF 3 Hz/100 Mm 30 Hz/10 Mm SLF 30 Hz/10 Mm 300 Hz/1 Mm ULF 300 Hz/1 Mm 3 kHz/100 km VLF 3 kHz/100 km 30 kHz/10 km LF 30 kHz/10 km 300 kHz/1 km MF 300 kHz/1 km 3 MHz/100 m HF 3 MHz/100 m 30 MHz/10 m VHF 30 MHz/10 m 300 MHz/1 m UHF 300 MHz/1 m 3 GHz/100 mm SHF 3 GHz/100 mm 30 GHz/10 mm EHF 30 GHz/10 mm 300 GHz/1 mm THF 300 GHz/1 mm 3 THz/0.1 mm Frequency reuse A cellular network or mobile network
2484-709: The US, used interchangeably with "mobile phone". However, satellite phones are mobile phones that do not communicate directly with a ground-based cellular tower but may do so indirectly by way of a satellite. There are a number of different digital cellular technologies, including: Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), cdmaOne , CDMA2000 , Evolution-Data Optimized (EV-DO), Enhanced Data Rates for GSM Evolution (EDGE), Universal Mobile Telecommunications System (UMTS), Digital Enhanced Cordless Telecommunications (DECT), Digital AMPS (IS-136/TDMA), and Integrated Digital Enhanced Network (iDEN). The transition from existing analog to
2553-471: The basis of 3G ). With FDMA, the transmitting and receiving frequencies used by different users in each cell are different from each other. Each cellular call was assigned a pair of frequencies (one for base to mobile, the other for mobile to base) to provide full-duplex operation. The original AMPS systems had 666 channel pairs, 333 each for the CLEC "A" system and ILEC "B" system. The number of channels
2622-580: The beam to feel an intense burning pain, as if their skin is going to catch fire. The military version had an output power of 100 kilowatts (kW), and a smaller law enforcement version, called Silent Guardian that was developed by Raytheon later, had an output power of 30 kW. Clothing and other organic materials are transparent to millimeter waves of certain frequencies, so a recent application has been scanners to detect weapons and other dangerous objects carried under clothing, for applications such as airport security. Privacy advocates are concerned about
2691-405: The case of a taxi company, where each radio has a manually operated channel selector knob to tune to different frequencies. As drivers move around, they change from channel to channel. The drivers are aware of which frequency approximately covers some area. When they do not receive a signal from the transmitter, they try other channels until finding one that works. The taxi drivers only speak one at
2760-493: The cells can also overlap between adjacent cells and large cells can be divided into smaller cells. The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/K (or K according to some books) where K is the number of cells which cannot use the same frequencies for transmission. Common values for the frequency reuse factor are 1/3, 1/4, 1/7, 1/9 and 1/12 (or 3, 4, 7, 9 and 12, depending on notation). In case of N sector antennas on
2829-418: The centers of the cells and were omnidirectional, a cellular map can be redrawn with the cellular telephone towers located at the corners of the hexagons where three cells converge. Each tower has three sets of directional antennas aimed in three different directions with 120 degrees for each cell (totaling 360 degrees) and receiving/transmitting into three different cells at different frequencies. This provides
AN/APG-78 Longbow - Misplaced Pages Continue
2898-507: The desired service including mobility management, registration, call set-up, and handover . Any phone connects to the network via an RBS ( Radio Base Station ) at a corner of the corresponding cell which in turn connects to the Mobile switching center (MSC). The MSC provides a connection to the public switched telephone network (PSTN). The link from a phone to the RBS is called an uplink while
2967-615: The development and proliferation of digital wireless mobile networks. The first commercial digital cellular network, the 2G generation, was launched in 1991. This sparked competition in the sector as the new operators challenged the incumbent 1G analog network operators. To distinguish signals from several different transmitters, a number of channel access method s have been developed, including frequency-division multiple access (FDMA, used by analog and D-AMPS systems), time-division multiple access (TDMA, used by GSM ) and code-division multiple access (CDMA, first used for PCS , and
3036-599: The digital standard followed a very different path in Europe and the US . As a consequence, multiple digital standards surfaced in the US, while Europe and many countries converged towards the GSM standard. A simple view of the cellular mobile-radio network consists of the following: This network is the foundation of the GSM system network. There are many functions that are performed by this network in order to make sure customers get
3105-507: The equipment that manipulates them, the techniques of geometric optics can be used. Diffraction is less than at lower frequencies, although millimeter waves can be diffracted by building edges. At millimeter wavelengths, surfaces appear rougher so diffuse reflection increases. Multipath propagation , particularly reflection from indoor walls and surfaces, causes serious fading. Doppler shift of frequency can be significant even at pedestrian speeds. In portable devices, shadowing due to
3174-445: The form of regular shapes, such as hexagons, squares, or circles although hexagonal cells are conventional. Each of these cells is assigned with multiple frequencies ( f 1 – f 6 ) which have corresponding radio base stations . The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent cells, which would cause co-channel interference . The increased capacity in
3243-574: The future. Cell towers frequently use a directional signal to improve reception in higher-traffic areas. In the United States , the Federal Communications Commission (FCC) limits omnidirectional cell tower signals to 100 watts of power. If the tower has directional antennas, the FCC allows the cell operator to emit up to 500 watts of effective radiated power (ERP). Although the original cell towers created an even, omnidirectional signal, were at
3312-484: The gases in the atmosphere. 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
3381-535: The human body is a problem. Since the waves penetrate clothing and their small wavelength allows them to reflect from small metal objects they are used in millimeter wave scanners for airport security scanning. This band is commonly used in radio astronomy and remote sensing . Ground-based radio astronomy is 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 can determine temperature in
3450-507: The metropolitan area of Tokyo . Within five years, the NTT network had been expanded to cover the whole population of Japan and became the first nationwide 1G network. It was an analog wireless network . The Bell System had developed cellular technology since 1947, and had cellular networks in operation in Chicago and Dallas prior to 1979, but commercial service was delayed by the breakup of
3519-444: The millimeter wave bands include point-to-point communications, intersatellite links , and point-to-multipoint communications . In 2013 it was speculated that there were plans to use millimeter waves in future 5G mobile phones. In addition, use of millimeter wave bands for vehicular communication is also emerging as an attractive solution to support (semi-)autonomous vehicular communications. Shorter wavelengths in this band permit
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#17327728121863588-410: The most important use of broadcast information is to set up channels for one-to-one communication between the mobile transceiver and the base station. This is called paging . The three different paging procedures generally adopted are sequential, parallel and selective paging. The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where
3657-517: The net result is greater frequency reuse , and higher density of users. The high usable channel capacity in this band might allow it to serve some applications that would otherwise use fiber-optic communication or very short links such as for the interconnect of circuit boards. Millimeter wave radar is used in short-range fire-control radar in tanks and aircraft, and automated guns ( CIWS ) on naval ships to shoot down incoming missiles. The small wavelength of millimeter waves allows them to track
3726-536: The number of subscribers per cell site, greater data throughput per user, or some combination thereof. Quadrature Amplitude Modulation (QAM) modems offer an increasing number of bits per symbol, allowing more users per megahertz of bandwidth (and decibels of SNR), greater data throughput per user, or some combination thereof. The key characteristic of a cellular network is the ability to reuse frequencies to increase both coverage and capacity. As described above, adjacent cells must use different frequencies, however, there
3795-431: The other way is termed downlink . Radio channels effectively use the transmission medium through the use of the following multiplexing and access schemes: frequency-division multiple access (FDMA), time-division multiple access (TDMA), code-division multiple access (CDMA), and space-division multiple access (SDMA). Small cells, which have a smaller coverage area than base stations, are categorised as follows: As
3864-508: The phone is located (this group of cells is called a Location Area in the GSM or UMTS system, or Routing Area if a data packet session is involved; in LTE , cells are grouped into Tracking Areas). Paging takes place by sending the broadcast message to all of those cells. Paging messages can be used for information transfer. This happens in pagers , in CDMA systems for sending SMS messages, and in
3933-441: The phone user moves from one cell area to another cell while a call is in progress, the mobile station will search for a new channel to attach to in order not to drop the call. Once a new channel is found, the network will command the mobile unit to switch to the new channel and at the same time switch the call onto the new channel. With CDMA , multiple CDMA handsets share a specific radio channel. The signals are separated by using
4002-483: The receiving end to produce a somewhat normal-sounding voice at the receiver. TDMA must introduce latency (time delay) into the audio signal. As long as the latency time is short enough that the delayed audio is not heard as an echo, it is not problematic. TDMA is a familiar technology for telephone companies, which used time-division multiplexing to add channels to their point-to-point wireline plants before packet switching rendered FDM obsolete. The principle of CDMA
4071-415: The same base station site, each with different direction, the base station site can serve N different sectors. N is typically 3. A reuse pattern of N/K denotes a further division in frequency among N sector antennas per site. Some current and historical reuse patterns are 3/7 (North American AMPS), 6/4 (Motorola NAMPS), and 3/4 ( GSM ). If the total available bandwidth is B , each cell can only use
4140-462: The same frequencies, and the different base stations and users are separated by codes rather than frequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one sector, but rather that the entire cell bandwidth is also available to each sector individually. Recently also orthogonal frequency-division multiple access based systems such as LTE are being deployed with
4209-465: The sensor suite allows data to be shared with ground units and other Apaches, allowing them to fire on targets detected by a single helicopter. The updated AN/APG-78 radar for the AH-64E Guardian has overwater capabilities, potentially enabling naval strikes. Millimeter-wave Compared to lower bands, radio waves in this band have high atmospheric attenuation : they are absorbed by
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#17327728121864278-403: The stream of outgoing bullets as well as the target, allowing the computer fire control system to change the aim to bring them together. With Raytheon the U.S. Air Force has developed a nonlethal antipersonnel weapon system called Active Denial System (ADS) which emits a beam of millimeter radio waves with a wavelength of 3 mm (frequency of 95 GHz). The weapon causes a person in
4347-547: The technology exists to extend the search area to as far as 50 meters beyond the scanning area which would allow security workers to scan a large number of people without their awareness that they are being scanned. Recent studies at the University of Leuven have proven that millimeter waves can also be used as a non-nuclear thickness gauge in various industries. Millimeter waves provide a clean and contact-free way of detecting variations in thickness. Practical applications for
4416-519: The technology focus on plastics extrusion , paper manufacturing , glass production and mineral wool production . Low intensity (usually 10 mW/cm or less) electromagnetic radiation of extremely high frequency may be used in human medicine for the treatment of diseases . For example, "A brief, low-intensity MMW exposure can change cell growth and proliferation rates, activity of enzymes , state of cell genetic apparatus, function of excitable membranes and peripheral receptors." This treatment
4485-400: The use of smaller antennas to achieve the same high directivity and high gain as larger ones in lower bands. The immediate consequence of this high directivity, coupled with the high free space loss at these frequencies, is the possibility of a more efficient use of frequencies for point-to-multipoint applications. Since a greater number of highly directive antennas can be placed in a given area,
4554-423: The use of this technology because, in some cases, it allows screeners to see airport passengers as if without clothing. The TSA has deployed millimeter wave scanners to many major airports. Prior to a software upgrade the technology did not mask any part of the bodies of the people who were being scanned. However, passengers' faces were deliberately masked by the system. The photos were screened by technicians in
4623-622: Was expanded to 416 pairs per carrier, but ultimately the number of RF channels limits the number of calls that a cell site could handle. FDMA is a familiar technology to telephone companies, which used frequency-division multiplexing to add channels to their point-to-point wireline plants before time-division multiplexing rendered FDM obsolete. With TDMA, the transmitting and receiving time slots used by different users in each cell are different from each other. TDMA typically uses digital signaling to store and forward bursts of voice data that are fit into time slices for transmission, and expanded at
4692-438: Was interrupted due to a loss of a signal, the taxi driver asked the base station operator to repeat the message on a different frequency. In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically without interruption and without a base station operator or manual switching. This
4761-471: Was not developed by Bell Labs ), CDMA has scaled well to become the basis for 3G cellular radio systems. Other available methods of multiplexing such as MIMO , a more sophisticated version of antenna diversity , combined with active beamforming provides much greater spatial multiplexing ability compared to original AMPS cells, that typically only addressed one to three unique spaces. Massive MIMO deployment allows much greater channel reuse, thus increasing
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