Frequency-hopping spread spectrum
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 frequency-hopping code division multiple access (FH-CDMA) scheme.
FHSS is a wireless technology that spreads its signal over rapidly changing frequencies. Each available frequency band is subdivided 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 DSSS systems. A sub-type of FHSS used in Bluetooth is adaptive frequency hopping spread spectrum (AFH).
Main article: Spread-spectrum
A spread-spectrum transmission offers three main advantages over a fixed-frequency transmission:
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. U.S. military radios that use frequency hopping include the JTIDS/MIDS family, HAVE QUICK and SINCGARS
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 are 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 U.S. Federal Communications Commission (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.
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. Whilst 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 have a checksum for integrity 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 system in the 902–928 MHz and 2.4 GHz bands permits more power than non-spread-spectrum systems. Both frequency hopping and direct sequence systems can transmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. The Federal Communications Commission (FCC) prescribes a minimum number of channels and a maximum dwell time for each channel.
In a real multipoint 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.
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.
A Polish engineer, Leonard Danilewicz, came up with the idea in 1929. 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 invention of frequency hopping, though it came decades after others had come up with the concept and technologies making use of it were in existence, 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.
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. Such a situation can often happen in the scenarios that use unlicensed spectrum.
Chirp modulation can be seen as a form of frequency-hopping that simply scans through the available frequencies in consecutive order to communicate.