Frequency-hopping spread spectrum

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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).

Transmitter Electronic device that emits radio waves

In electronics and telecommunications a transmitter or radio transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves.

Contents

Each available frequency band is divided into sub-frequencies. Signals rapidly change ("hop") among these in a predetermined 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.

Direct-sequence spread spectrum Modulation technique to reduce signal interference

In telecommunications, direct-sequence spread spectrum (DSSS) is a spread spectrum modulation technique used to reduce overall signal interference. The spreading of this signal makes the resulting wideband channel more noisy, allowing for greater resistance to unintentional and intentional interference.

Adaptive frequency-hopping spread spectrum (AFH), a specific type of FHSS, is used in Bluetooth wireless data transfer.

Bluetooth is a wireless technology standard used for exchanging data between fixed and mobile devices over short distances using short-wavelength UHF radio waves in the industrial, scientific and medical radio bands, from 2.400 to 2.485 GHz, and building personal area networks (PANs). It was originally conceived as a wireless alternative to RS-232 data cables.

Spread-spectrum

A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:

  1. 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.
  2. 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.
  3. 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.

Radio jamming is the deliberate jamming, blocking or interference with authorized wireless communications. In the United States, radio jamming devices are illegal and their use can result in large fines.

Military Organization primarily tasked with preparing for and conducting war

A military is a heavily-armed, highly organised force primarily intended for warfare, also known collectively as armed forces. It is typically officially authorized and maintained by a sovereign state, with its members identifiable by their distinct military uniform. It may consist of one or more military branches such as an Army, Navy, Air Force and in certain countries, Marines and Coast Guard. The main task of the military is usually defined as defence of the state and its interests against external armed threats.

Cryptography Practice and study of techniques for secure communication in the presence of third parties.

Cryptography or cryptology is the practice and study of techniques for secure communication in the presence of third parties called adversaries. More generally, cryptography is about constructing and analyzing protocols that prevent third parties or the public from reading private messages; various aspects in information security such as data confidentiality, data integrity, authentication, and non-repudiation are central to modern cryptography. Modern cryptography exists at the intersection of the disciplines of mathematics, computer science, electrical engineering, communication science, and physics. Applications of cryptography include electronic commerce, chip-based payment cards, digital currencies, computer passwords, and military communications.

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, Link-16.

KY-57 portable, tactical cryptographic device

The Speech Security Equipment (VINSON), TSEC/KY-57, is a portable, tactical cryptographic device in the VINSON family, designed to provide voice encryption for a range of military communication devices such as radio or telephone.

United States Federal republic in North America

The United States of America (USA), commonly known as the United States or simply America, is a country comprising 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe. Most of the country is located in central North America between Canada and Mexico. With an estimated population of over 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the most populous city is New York City.

HAVE QUICK frequency-hopping system

HAVE QUICK is a ECM resistant / frequency-hopping system used to protect military Aeronautical mobile (OR) radio traffic.

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.

Federal Communications Commission Independent agency of the U.S. Government

The Federal Communications Commission (FCC) is an independent agency of the United States government created by statute to regulate interstate communications by radio, television, wire, satellite, and cable. The FCC maintains jurisdiction over the areas of broadband access, fair competition, radio frequency use, media responsibility, public safety, and homeland security.

Spread spectrum Spreading the frequency domain of a signal

In telecommunication and radio communication, spread-spectrum techniques are methods by which a signal generated with a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference, noise and jamming, to prevent detection, and to limit power flux density.

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.

Frequency-hopping spread spectrum technology is also used in many hobby RC (radio controlled) transmitters and receivers used for model cars, airplanes, and drones. A type of multiple access is achieved allowing hundreds of transmitter/receiver pairs to be operated simultaneously on the same band in contrast to previous FM or AM RC systems that had limited simultaneous channels. [1]

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 the possibility of multiple transmissions on the same frequency 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

In 1899 Guglielmo Marconi experimented with frequency-selective reception in an attempt to minimise interference. [2]

The earliest mentions of frequency hopping in the open literature are in US patent 725,605 awarded to Nikola Tesla in March 17, 1903 and 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. Nikola Tesla doesn’t mention 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. [3]

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. [4]

A Polish engineer and inventor, Leonard Danilewicz, came up with the idea in 1929. [5] 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.

In 1942, actress Hedy Lamarr and composer George Antheil 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

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. [6] Such a situation can often happen in the scenarios that use unlicensed spectrum.

The combination FHSS and OFDM (N-OFDM) FHSS+N-OFDM.jpg
The combination FHSS and OFDM (N-OFDM)

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.

The other concept also can use the combination of frequency hopping spread spectrum with OFDM or non-orthogonal frequency-division multiplexing (N-OFDM) of signals. It can increase data performance. [7]

See also

Notes

  1. "Most RC model aircraft airborne simultaneously". Guinness World Records. Retrieved 2018-02-16.
  2. eetimes.com: A short history of spread spectrum
  3. Denis Winter, Haig's Command - A Reassessment
  4. Danilewicz later recalled: "In 1929 we proposed to the General Staff a device of my design for secret radio telegraphy which fortunately did not win acceptance, as it was a truly barbaric idea consisting in constant changes of transmitter frequency. The commission did, however, see fit to grant me 5,000 złotych for executing a model and as encouragement to further work." Cited in Władysław Kozaczuk, Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in World War II, 1984, p. 27.
  5. Petar Popovski; Hiroyuki Yomo; Ramjee Prasad (December 2006). "Strategies For Adaptive Frequency Hopping In The Unlicensed Bands" (PDF). IEEE Wireless Communications. Archived from the original (PDF) on 2009-02-27. Retrieved 2008-03-02.Cite journal requires |journal= (help)
  6. 1 2 Патент Украины на полезную модель № 122771. МПК H04B 3/60 (2006.01), H04B 1/58 (2006.01), H04B 1/56 (2006.01). Способ повышения скорости передачи данных сигналами с псевдослучайной перестройкой частоты./ Слюсар В.И.- Заявка на выдачу патента Украины на полезную модель № u201707800 от 25.07.2017.- Патент опубл. 25.01.2018, бюл. № 2.

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