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Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is used as a multiple access method in the code division multiple access (CDMA) scheme frequency-hopping code division multiple access (FH-CDMA) .
FHSS is a wireless technology that spreads its signal over rapidly changing frequencies. Each available frequency band is divided into sub-frequencies. Signals rapidly change (“hop”) among these in a pre-determined order. Interference at a specific frequency will only affect the signal during that short interval. FHSS can, however, cause interference with adjacent direct-sequence spread spectrum (DSSS) systems. A sub-type of FHSS used in Bluetooth wireless data transfer is adaptive frequency hopping spread spectrum (AFH).
Spread-spectrum
Main article: Spread-spectrum
A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
Spread-spectrum signals are highly resistant to narrowband interference. The process of re-collecting a spread signal spreads out the interfering signal, causing it to recede into the background.
Spread-spectrum signals are difficult to intercept. A spread-spectrum signal may simply appear as an increase in the background noise to a narrowband receiver. An eavesdropper may have difficulty intercepting a transmission in real time if the pseudorandom sequence is not known.
Spread-spectrum transmissions can share a frequency band with many types of conventional transmissions with minimal interference. The spread-spectrum signals add minimal noise to the narrow-frequency communications, and vice versa. As a result, bandwidth can be used more efficiently.
Military use
Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary has knowledge of the spreading characteristics. Military radios use cryptographic techniques to generate the channel sequence under the control of a secret Transmission Security Key (TRANSEC) that the sender and receiver share in advance.
By itself, frequency hopping provides only limited protection against eavesdropping and jamming. Most modern military frequency hopping radios also employ separate encryption devices such as the KY-57 Speech Security Equipment. U.S. military radios that use frequency hopping include the JTIDS/MIDS family, the HAVE QUICK Aeronautical Mobile (OR) communications system, and the SINCGARS Combat Net Radio.
Civilian use
In the US, since the Federal Communications Commission (FCC) amended rules to allow frequency hopping spread spectrum systems in the unregulated 2.4 GHz band, many consumer devices in that band have employed various spread-spectrum modes.
Some walkie-talkies that employ frequency-hopping spread spectrum technology have been developed for unlicensed use on the 900 MHz band. Several such radios were marketed under the name eXtreme Radio Service (eXRS). Despite the name’s similarity to the FRS allocation, the system is a proprietary design, rather than an official FCC allocated service.
Motorola has deployed a business-banded, license-free digital radio that uses FHSS technology: the DTR series, models 410, 550 and 650.
Technical considerations
The overall bandwidth required for frequency hopping is much wider than that required to transmit the same information using only one carrier frequency. However, because transmission occurs only on a small portion of this bandwidth at any given time, the effective interference bandwidth is really the same. While providing no extra protection against wideband thermal noise, the frequency-hopping approach does reduce 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 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 channel sequences, so that once synchronized they can maintain communication by following the table. On each channel segment, the transmitter can send its current location in 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 frequency hopping and direct sequence systems can transmit at 1 Watt, a thousand-fold increase from the 1 milliwatt limit on non-spread-spectrum systems. The Federal Communications Commission (FCC) also prescribes a minimum number of channels and a maximum dwell time for each channel.
In a real multipoint spread spectrum radio system, space allows multiple transmissions on the same frequency to be possible using multiple radios in a geographic area. This creates the possibility of system data rates that are higher than the Shannon limit for a single channel. Spread spectrum systems do not violate the Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing of spectrum. This property is also seen in MIMO and DSSS systems. Beam steering and directional antennas also facilitate increased system performance by providing isolation between remote radios.
Multiple inventors
Perhaps the earliest mention of frequency hopping in the open literature is in radio pioneer Jonathan Zenneck’s book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken had already tried it.
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 the technology to follow the sequence.[1]
A Polish engineer and inventor, Leonard Danilewicz, came up with the idea in 1929.[2] Several other patents were taken out in the 1930s, including one by Willem Broertjes (U.S. Patent 1,869,659, issued Aug. 2, 1932).
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. However, SIGSALY was a top-secret communications system, so its existence did not become known until the 1980s.
The most celebrated use of frequency hopping was a patent awarded to actress Hedy Lamarr and composer George Antheil, who in 1942 received U.S. Patent 2,292,387 for their “Secret Communications System”. This intended early version of frequency hopping was supposed to use a piano-roll to change among 88 frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam, but there is no record of a working device ever being produced. The patent was rediscovered in the 1950s during patent searches when private companies independently developed Code Division Multiple Access, a non-frequency-hopping form of spread-spectrum, and has been cited numerous times since.
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.”
Variations of FHSS
Adaptive Frequency-hopping spread spectrum (AFH) (as used in Bluetooth) improves resistance to radio frequency interference by avoiding crowded frequencies in 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, by avoiding the “bad” frequency channels—perhaps those “bad” frequency channels are experiencing frequency selective fading, or perhaps some third party is trying to communicate on those bands, or perhaps those bands are being actively jammed. Therefore, AFH should be complemented by a mechanism for detecting good/bad channels.
However, if the radio frequency interference is itself dynamic, then the strategy of “bad channel removal”, applied in AFH might not work well. For example, if there are several colocated frequency-hopping networks (as Bluetooth Piconet), then they are mutually interfering and the strategy of AFH fails to avoid this interference.
The problem of dynamic interference, gradual reduction of available hopping channels and backward compatibility with legacy bluetooth devices was resolved in version 1.2 of the Bluetooth Standard (2003). Other Strategies for dynamic adaptation of the frequency hopping pattern have been reported in the literature.[3] 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 the available frequencies in consecutive order to communicate.
1932 – Izzy Asper, Canadian lawyer, businessman, and politician, founded Canwest (d. 2003)
Israel Harold “Izzy” Asper, OC OM QC (August 11, 1932 – October 7, 2003) was a Canadian tax lawyer and media magnate. He was the founder of the now defunct CanWest Global Communications Corp[1] and father to its former CEO and President Leonard Asper, former director and corporate secretary Gail Asper, as well as former Executive Vice President David Asper.[2] He was also the leader of the Manitoba Liberal Party from 1970 to 1975 [1] and is credited with the idea and vision to establish the Canadian Museum for Human Rights.
Life and career
Israel Asper was born to a Jewish family in Minnedosa, Manitoba, the son of musicians[3] Leon Asper and Cecilia Swet,[4] who had emigrated from Ukraine.[5] Asper attended the University of Manitoba. In 1957 he received his law degree from the University of Manitoba, and was called to the bar shortly thereafter. He founded the firm of Asper, Freedman & Co. in 1959,[1] and was also a partner and co-founder of the firm Buchwald, Asper, Henteleff (now Pitblado LLP)along with Harold Buchwald and Yude Henteleff. In 1970 he wrote The Benson Iceberg: A critical analysis of the White Paper on Tax Reform in Canada.[2]
He married Ruth Miriam “Babs” Asper on May 27, 1956[6] at Shaarey Zedek Synagogue (Winnipeg).[7]
Also in 1970, Asper was elected leader of the Manitoba Liberal Party (defeating university professor John Nesbitt). Asper represented a right-libertarian strain within the party. In the Manitoba election of 1973, he promoted a laissez-faire economy, and advocated the elimination of the welfare state. He also advocated the public financing of election campaigns, to ensure that politics would not be dominated entirely by monied interests. His Liberals won only five seats, and Asper was elected in Wolseley by only four votes. He resigned as party leader and MLA in 1975, though he continued to support the Manitoba Liberal Party in later years.[1]
His media empire started with the Winnipeg television station CKND-TV in 1975. CanWest grew to encompass the Global Television Network, the daily newspaper National Post and over 60 other Canadian newspapers.[1]
Asper was noted for his fierce loyalty to Manitoba, refusing enticements to move east to Toronto. The faculty of management at the University of Manitoba renamed itself the Asper School of Business in 2000. He was also a noted philanthropist, making major donations to the arts and education; in 2001 he donated $5 million CAD to the St. Boniface Hospital & Research Foundation in Winnipeg. Asper became an Officer of the Order of Canada in 1995.[8] Also in 1995, he was inducted into the Canadian Broadcast Hall of Fame.[1]
He was a prominent member of Canada’s Jewish community, and a vocal supporter for Israel.[5]
Asper was also a close friend of many of Canada’s prominent political and business elite, including Jean Chrétien and Paul Martin.
Controversially, Asper’s newspaper chain fired journalist Russell Mills when he wrote an article which was critical of Jean Chretien and demanded he resign.[9]
In 1956, he married Ruth Miriam Bernstein known as “Babs.”[10] Asper died in St. Boniface Hospital at the age of 71[3] after suffering a heart attack.[11]
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