Frequency Hopping CDMA

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Frequency-hopping spread spectrum

From Wikipedia, the free encyclopedia

(Redirected from Frequency-hopping CDMA)

Frequency-hopping spread spectrum (FHSS) is a method

of transmitting radio signals by rapidly switching a carrieramong many frequency channels, using a pseudorandomsequence known to both transmitter and receiver. It isutilized as a multiple access method in the frequency-hopping code division multiple access (FH-CDMA)scheme.

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

1. Spread-spectrum signals are highly resistant to

narrowband interference. The process of re-collecting a spreadsignal spreads out the interfering signal, causing it to recedeinto the background.

2. Spread-spectrum signals are difficult to intercept. An FHSSsignal simply appears as an increase in the background noise toa narrowband receiver. An eavesdropper would only be able tointercept the transmission if they knew the pseudorandomsequence.

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 tothe narrow-frequency communications, and vice versa. As a result, bandwidth can be utilizedmore efficiently.

Modulation techniques

Analog modulation

AM SSB QAM FM PM SM

Digital modulation

FSK ASK OOK PSK QAMMSK CPM PPM TCM OFDM

Spread spectrum

CSS DSSS FHSS THSS

See also: Demodulation, modem

Multiplextechniques

Circuit mode(constant bandwidth)

TDM FDM WDM

Polarization multiplexing

Spatial multiplexing (MIMO)

Statistical multiplexing(variable bandwidth)

Packet mode Dynamic TDM

FHSS DSSS OFDMA

Related topics

Channel access methods

Media Access Control (MAC)

Contents

1 Basic algorithm 2 Military use 3 Technical considerations 4 Multiple inventors 5 Variations of FHSS

6 See also 7 Notes 8 References 9 External links

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Basic algorithm

Typically, the initiation of an FHSS communication is as follows

1. The initiating party sends a request via a predefined frequency or control channel.2. The receiving party sends a number, known as a seed.

3. The initiating party uses the number as a variable in a predefined algorithm, which calculatesthe sequence of frequencies that must be used. Most often the period of the frequency changeis predefined, as to allow a single base station to serve multiple connections.

4. The initiating party sends a synchronization signal via the first frequency in the calculatedsequence, thus acknowledging to the receiving party it has correctly calculated the sequence.

5. The communication begins, and both the receiving and the sending party change theirfrequencies 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 aftercompleting the first step the procedure is continued from number 5.

Military use

Spread-spectrum signals are highly resistant to deliberate jamming, unless the adversary hasknowledge of the spreading characteristics. Military radios use cryptographic techniques to generatethe channel sequence under the control of a secret Transmission Security Key (TRANSEC) that thesender 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 separateencryption devices such as the KY-57. U.S. military radios that use frequency hopping includeHAVE QUICK and SINCGARS.

Technical considerations

The overall bandwidth required for frequency hopping is much wider than that required to transmitthe same information using only one carrier frequency. However, because transmission occurs onlyon a small portion of this bandwidth at any given time, the effective interference bandwidth is reallythe 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 oftime. The receiver can then find the transmitter by picking a random channel and listening for validdata on that channel. The transmitter's data is identified by a special sequence of data that is unlikelyto occur over the segment of data for this channel and the segment can have a checksum for integrityand further identification. The transmitter and receiver can use fixed tables of channel sequences sothat once synchronized they can maintain communication by following the table. On each channelsegment, 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 powerthan non-spread spectrum systems. Both frequency hopping and direct sequence systems cantransmit at 1 Watt. The limit is increased from 1 milliwatt to 1 watt or a thousand times increase. TheFederal 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




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possible using multiple radios in a geographic area. This creates the possibility of system data ratesthat are higher than the Shannon limit for a single channel. Spread spectrum systems do not violatethe Shannon limit. Spread spectrum systems rely on excess signal to noise ratios for sharing ofspectrum. This property is also seen in MIMO and DSSS systems. Beam steering and directionalantennas also facilitate increased system performance by providing isolation between remote radios.

Multiple inventorsPerhaps the earliest mention of frequency hopping in the open literature is in radio pioneer JohannesZenneck's bookWireless 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 fixedcommand 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, Leonard Danilewicz, came up with the idea in 1929.[2] Several other patents were

taken out in the 1930s, including one by Willem Broertjes (Germany 1929, U.S. Patent 1,869,659(issued Aug. 2, 1932)).

During World War II, the US Army Signal Corp was inventing a communication system calledSIGSALY, which incorporated spread spectrum in a single frequency context. However, SIGSALYwas a 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 composerGeorge Antheil, who in 1942 received U.S. Patent 2,292,387 for their "Secret CommunicationsSystem." This early version of frequency hopping used a piano-roll to change between 88frequencies, 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 companiesindependently developed Code Division Multiple Access, a civilian form of spread-spectrum.

Variations of FHSS

Adaptive Frequency-hopping spread spectrum (AFH) (as used in Bluetooth) improves resistance toradio frequency interference by avoiding using crowded frequencies in the hopping sequence. Thissort 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" frequencychannels -- 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 beingactively jammed. Therefore, AFH should be complemented by a mechanism for detecting good/badchannels.

However, if the radio frequency interference is itself dynamic, then the strategy of bad channelremoval, applied in AFH might not work well. For example, if there are several colocatedfrequency-hopping networks (as Bluetooth Piconet), then they are mutually interfering and thestrategy 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.[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 tocognitive radio, where the networks and the devices should exhibit frequency-agile operation.




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Chirp modulation can be seen as a form of frequency-hopping that simply scans through theavailable frequencies in consecutive order.

See also

Direct-sequence spread spectrum Maximum length sequence Orthogonal frequency-division multiplexing Dynamic frequency hopping Spread spectrum List of multiple discoveries

Notes

1. ^ Denis Winter,Haig's Command - A Reassessment2. ^ 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 inconstant changes of transmitter frequency. The commission did, however, see fit to grant me 5,000

zotych for executing a model and as encouragement to further work." Cited in Wadysaw Kozaczuk,Enigma: How the German Machine Cipher Was Broken, and How It Was Read by the Allies in WorldWar II, 1984, p. 27.

3. ^STRATEGIES_FOR_ADAPTIVE_FREQUENCY_HOPPING_IN_THE_UNLICENSED_BANDS. IEEEWireless Communications. http://kom.aau.dk/~petarp/papers/DAFH-AFR.pdf. Retrieved 2008-03-02.

References

Wadysaw Kozaczuk,Enigma: How the German Machine Cipher Was Broken, and How It

Was Read by the Allies in World War Two, edited and translated by Christopher Kasparek,Frederick, MD, University Publications of America, 1984, ISBN 0-89093-547-5.

External links

More FHSS Information

FCC Part 15 Rules that cover frequency hopping 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 RulesRetrieved from "http://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum"Categories: Channel access methods | Applications of cryptography | Multiplexing | Radio frequencypropagation | Radio modulation modes | Radio resource management

This page was last modified on 22 October 2009 at 16:06. Text is available under the Creative Commons Attribution-ShareAlike License; additional

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