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32-430: (Redirected from Afh ) AFH or afh may refer to: Adaptive frequency-hopping spread spectrum , a radio technology Adult foster home, residence for elderly or physically disabled adults Afrihili language (ISO 639-3 code: afh ) Angiomatoid fibrous histiocytoma , a human tumour Architecture for Humanity , a charitable organization Action for Happiness ,

64-522: A charity in the United Kingdom Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title AFH . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=AFH&oldid=881077494 " Category : Disambiguation pages Hidden categories: Short description

96-560: A fixed-frequency transmission: Spread-spectrum signals are highly resistant to deliberate jamming unless the adversary has knowledge of the frequency-hopping pattern. Military radios generate the frequency-hopping pattern under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance. This key is generated by devices such as the KY-57 Speech Security Equipment. United States military radios that use frequency hopping include

128-559: A maximum dwell time for each channel. In 1899, Guglielmo Marconi experimented with frequency-selective reception in an attempt to minimise interference. The earliest mentions of frequency hopping in open literature are in US patent 725,605 , awarded to Nikola Tesla on March 17, 1903, and in radio pioneer Jonathan Zenneck 's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck writes that Telefunken had already tried it. Nikola Tesla doesn't mention

160-503: A method of transmitting signals on multiple frequencies in a random manner for secrecy, anticipating key features of later frequency hopping systems. A Polish engineer and inventor, Leonard Danilewicz , claimed to have suggested the concept of frequency hopping in 1929 to the Polish General Staff , but it was rejected. In 1932, U.S. patent 1,869,659 was awarded to Willem Broertjes, named "Method of maintaining secrecy in

192-463: A modulated radio signal is contained in the sidebands while the power in the carrier frequency component does not transmit information itself, so newer forms of radio communication (such as spread spectrum and ultra-wideband ), and OFDM which is widely used in Wi-Fi networks, digital television , and digital audio broadcasting (DAB) do not use a conventional sinusoidal carrier wave. Carrier leakage

224-552: A single coaxial cable , by modulating each television channel on a carrier wave of a different frequency, then sending all the carriers through the cable. At the receiver, the individual channels can be separated by bandpass filters using tuned circuits so the television channel desired can be displayed. A similar technique called wavelength division multiplexing is used to transmit multiple channels of data through an optical fiber by modulating them on separate light carriers; light beams of different wavelengths. The information in

256-424: Is different from Wikidata All article disambiguation pages All disambiguation pages Adaptive frequency-hopping spread spectrum Frequency-hopping spread spectrum ( FHSS ) is a method of transmitting radio signals by rapidly changing the carrier frequency among many frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter and receiver . FHSS

288-425: Is really the same. While providing no extra protection against wideband thermal noise , the frequency-hopping approach reduces the degradation caused by narrowband interference sources. One of the challenges of frequency-hopping systems is to synchronize the transmitter and receiver. One approach is to have a guarantee that the transmitter will use all the channels in a fixed period of time. The receiver can then find

320-405: Is transmitted across space by radio waves . At the sending end, the information, in the form of a modulation signal, is applied to an electronic device called a transmitter . In the transmitter, an electronic oscillator generates a sinusoidal alternating current of radio frequency ; this is the carrier wave. The information signal is used to modulate the carrier wave, altering some aspects of

352-440: Is used to avoid interference, to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications. The frequency band is divided into smaller sub-bands. Signals rapidly change ("hop") their carrier frequencies among the center frequencies of these sub-bands in a determined order. Interference at a specific frequency will affect the signal only during a short interval. FHSS offers four main advantages over

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384-786: The JTIDS/MIDS family, the HAVE QUICK Aeronautical Mobile communications system, and the SINCGARS Combat Net Radio, Link-16 . In the US, since the Federal Communications Commission (FCC) amended rules to allow FHSS systems in the unregulated 2.4 GHz band, many consumer devices in that band have employed various FHSS modes. eFCC CFR 47 part 15.247 covers the regulations in the US for 902–928 MHz, 2400–2483.5 MHz, and 5725–5850 MHz bands, and

416-997: The US Navy utilized Sylvania Electronic Systems Division's work during the Cuban Missile Crisis . A practical application of frequency hopping was developed by Ray Zinn , co-founder of Micrel Corporation. Zinn developed a method allowing radio devices to operate without the need to synchronize a receiver with a transmitter. Using frequency hopping and sweep modes, Zinn's method is primarily applied in low data rate wireless applications such as utility metering, machine and equipment monitoring and metering, and remote control. In 2006 Zinn received U.S. patent 6,996,399 for his "Wireless device and method using frequency hopping and sweep modes." Adaptive frequency-hopping spread spectrum ( AFH ) as used in Bluetooth improves resistance to radio frequency interference by avoiding crowded frequencies in

448-452: The air from the transmitter to the receiver. The term is also used for an unmodulated emission in the absence of any modulating signal. In music production , carrier signals can be controlled by a modulating signal to change the sound property of an audio recording and add a sense of depth and movement. The term carrier wave originated with radio. In a radio communication system, such as radio or television broadcasting, information

480-421: The available frequencies in consecutive order to communicate. Frequency hopping can be superimposed on other modulations or waveforms to enhance the system performance. Carrier frequency This carrier wave usually has a much higher frequency than the message signal does. This is because it is impractical to transmit signals with low frequencies. The purpose of the carrier is usually either to transmit

512-461: The carrier is suppressed (and in some forms of SSB, eliminated). The carrier must be reintroduced at the receiver by a beat frequency oscillator (BFO). Carriers are also widely used to transmit multiple information channels through a single cable or other communication medium using the technique of frequency division multiplexing (FDM). For example, in a cable television system, hundreds of television channels are distributed to consumers through

544-408: The carrier, to impress the information on the wave. The alternating current is amplified and applied to the transmitter's antenna, radiating radio waves that carry the information to the receiver 's location. At the receiver, the radio waves strike the receiver's antenna, inducing a tiny oscillating current in it, which is applied to the receiver. In the receiver, the modulation signal is extracted from

576-431: The hopping sequence. This sort of adaptive transmission is easier to implement with FHSS than with DSSS . The key idea behind AFH is to use only the "good" frequencies and avoid the "bad" ones—those experiencing frequency selective fading , those on which a third party is trying to communicate, or those being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good and bad channels. But if

608-401: The information through space as an electromagnetic wave (as in radio communication ), or to allow several carriers at different frequencies to share a common physical transmission medium by frequency division multiplexing (as in a cable television system). The term originated in radio communication, where the carrier wave creates the waves which carry the information (modulation) through

640-430: The modulated carrier wave, a process called demodulation . Most radio systems in the 20th century used frequency modulation (FM) or amplitude modulation (AM) to add information to the carrier. The frequency spectrum of a modulated AM or FM signal from a radio transmitter is shown above. It consists of a strong component (C) at the carrier frequency f C {\displaystyle f_{C}} with

672-458: The modulation contained in narrow sidebands (SB) above and below the carrier frequency. The frequency of a radio or television station is considered to be the carrier frequency. However the carrier itself is not useful in transmitting the information, so the energy in the carrier component is a waste of transmitter power. Therefore, in many modern modulation methods, the carrier is not transmitted. For example, in single-sideband modulation (SSB),

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704-461: The patent to the U.S. Navy . Frequency-hopping ideas may have been rediscovered in the 1950s during patent searches when private companies were independently developing direct-sequence Code Division Multiple Access , a non-frequency-hopping form of spread-spectrum. In 1957, engineers at Sylvania Electronic Systems Division adopted a similar idea, using the recently invented transistor instead of Lamarr's and Antheil's clockwork technology. In 1962,

736-512: The phrase "frequency hopping" directly, but certainly alludes to it. Entitled Method of Signaling , the patent describes a system that would enable radio communication without any danger of the signals or messages being disturbed, intercepted, interfered with in any way . The German military made limited use of frequency hopping for communication between fixed command points in World War I to prevent eavesdropping by British forces, who did not have

768-439: The radio frequency interference is itself dynamic, then AFH's strategy of "bad channel removal" may not work well. For example, if there are several colocated frequency-hopping networks (as Bluetooth Piconet ), they are mutually interfering and AFH's strategy fails to avoid this interference. The problem of dynamic interference, gradual reduction of available hopping channels and backward compatibility with legacy Bluetooth devices

800-409: The requirements for frequency hopping. Some walkie-talkies that employ FHSS technology have been developed for unlicensed use on the 900 MHz band. FHSS technology is also used in many hobby transmitters and receivers used for radio-controlled model cars, airplanes, and drones. A type of multiple access is achieved allowing hundreds of transmitter/receiver pairs to be operated simultaneously on

832-403: The same band, in contrast to previous FM or AM radio-controlled systems that had limited simultaneous channels. The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency . But because transmission occurs only on a small portion of this bandwidth at any given time, the instantaneous interference bandwidth

864-425: The table. In the US, FCC part 15 on unlicensed spread spectrum systems in the 902–928 MHz and 2.4 GHz bands permits more power than is allowed for non-spread-spectrum systems. Both FHSS and direct-sequence spread-spectrum (DSSS) systems can transmit at 1 watt, a thousandfold increase from the 1 milliwatt limit on non-spread-spectrum systems. The FCC also prescribes a minimum number of frequency channels and

896-484: The technology to follow the sequence. Jonathan Zenneck's book Wireless Telegraphy was originally published in German in 1908, but was translated into English in 1915 as the enemy started using frequency hopping on the front line. In 1920, Otto B. Blackwell, De Loss K. Martin, and Gilbert S. Vernam filed a patent application for a "Secrecy Communication System", granted as U.S. Patent 1,598,673 in 1926. This patent described

928-494: The transmission of wireless telegraphic messages", which describes a system where "messages are transmitted by means of a group of frequencies... known to the sender and receiver alone, and alternated at will during transmission of the messages". During World War II , the US Army Signal Corps was inventing a communication system called SIGSALY , which incorporated spread spectrum in a single frequency context. But SIGSALY

960-471: The transmitter by picking a random channel and listening for valid data on that channel. The transmitter's data is identified by a special sequence of data that is unlikely to occur over the segment of data for this channel, and the segment can also have a checksum for integrity checking and further identification. The transmitter and receiver can use fixed tables of frequency-hopping patterns, so that once synchronized they can maintain communication by following

992-409: Was a top-secret communications system, so its existence was not known until the 1980s. In 1942, actress Hedy Lamarr and composer George Antheil received U.S. patent 2,292,387 for their "Secret Communications System", an early version of frequency hopping using a piano-roll to switch among 88 frequencies to make radio-guided torpedoes harder for enemies to detect or jam. They then donated

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1024-494: Was resolved in version 1.2 of the Bluetooth Standard (2003). Such a situation can often happen in the scenarios that use unlicensed spectrum . In addition, dynamic radio frequency interference is expected to occur in the scenarios related to cognitive radio , where the networks and the devices should exhibit frequency-agile operation. Chirp modulation can be seen as a form of frequency-hopping that simply scans through

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