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.

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. An FHSS signal simply appears as an increase in the background noise to a narrowband receiver. An eavesdropper would only be able to intercept the transmission if they knew the pseudorandom sequence.
# 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 utilized more efficiently.

Basic algorithm

Typically, the initiation of an FHSS communication is as follows
# The initiating party sends a request via a predefined frequency or control channel.
# The receiving party sends a number, known as a seed.
# The initiating party uses the number as a variable in a predefined algorithm, which calculates the sequence of frequencies that must be used. Most often the period of the frequency change is predefined, as to allow a single base station to serve multiple connections.
# The initiating party sends a synchronization signal via the first frequency in the calculated sequence, thus acknowledging to the receiving party it has correctly calculated the sequence.
# The communication begins, and both the receiving and the sending party change their frequencies along the calculated order, starting at the same point in time.

In some uses, most often military, a predefined frequency-hopping sequence is negotiated, and after completing the first step the procedure is continued from number 5.

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.

By itself, frequency hopping provides only limited protection against eavesdropping and jamming. To get around this weakness most modern military frequency hopping radios often employ separate encryption devices such as the KY-57. U.S. military radios that use frequency hopping include HAVE QUICK and SINCGARS.

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. Whilst providing no extra protection against wideband thermal noise, the frequency-hopping approach does reduce the degradation caused by narrowband interferers.

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 900MHz and 2.4GHz 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.

Multiple inventors of frequency hopping concepts

Perhaps the earliest mention of frequency hopping in the open literature is in radio pioneer Johannes Zenneck's book "Wireless Telegraphy" (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken had already tried it.

A Polish army officer, Leonard Danielewicz, came up with his own version of the idea in 1929. Several other patents were taken out in the 1930s, including one by Willem Broertjes (Germany 1929, U.S. Patent No. 1,869,659 (issued Aug. 2, 1932)).

During World War II, the US Army Signal Corp was inventing a communication system called SIGSALY, which incorporated spread spectrum in a single frequency context. However, SIGSALY was top secret communications system so its existence did not become known until the 1980s.

The most celebrated invention of frequency hopping was that of actress Hedy Lamarr and composer George Antheil, who in 1942 received [http://www.ncafe.com/chris/pat2/index.html patent number 2,292,387] for their "Secret Communications System."

This early version of frequency hopping used a piano-roll to change between 88 frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam. The patent was rediscovered in the 1950s during patent searches when private companies independently developed Code Division Multiple Access, a civilian form of spread-spectrum.

Variations

"Adaptive Frequency-hopping spread spectrum (AFH)" (as used in Bluetooth) improves resistance to radio frequency interference by avoiding using 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 hop only over the “good” frequencies, by avoiding the frequency channels that are experiencing radio frequency interference from a certain transmitter. 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.

In this case, there is a need to use strategies for dynamic adaptation of the frequency hopping pattern. [citepaper|title=STRATEGIES_FOR_ADAPTIVE_FREQUENCY_HOPPING_IN_THE_UNLICENSED_BANDS|url=http://kom.aau.dk/~petarp/papers/DAFH-AFR.pdf|publisher=IEEE Wireless Communications|accessdate=2008-03-02] 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.

ee also

* Amateur radio
* Direct-sequence spread spectrum
* N-sequence
* Orthogonal frequency-division multiplexing

References

External links

* [http://www.ferret.com.au/articles/7f/0c01f17f.asp More FHSS Information]
* [http://www.tolep.com/bluetooth/10/Frequency+Hopping+Spread+Spectrum+%28FHSS%29.html Bluetooth Frequency Hopping Spread Spectrum]
* [http://www.fcc.gov/oet/info/rules/part15/part15-2-16-06.pdf FCC Part 15 Rules that cover frequency hopping ]
* [http://kom.aau.dk/~petarp/papers/DAFH-AFR.pdf Frequency hopping in unlicensed spectrum] describes strategies for adaptive hopping in crowded spectrum, while considering the issues of radio etiquette and compliance with FCC Part 15 Rules