FHSS Presentation

8/7/2019 FHSS Presentation

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Frequency-Hopping

Spread SpectrumPresented By:

Eng.Hossam M. Al-Saket


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Outline

Introduction.

What is Frequency-Hopping?

Frequency-Hopping Notes. Frequency-Hopping Types.

System Block Diagram.

Frequency-Hopping Properties. Conclusion.


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Introduction

Spread spectrum is a means of transmission in which the signaloccupies a bandwidth in excess of the minimum necessary to sendthe information.

Advantages:

Anti-jamming. Anti-interference.

Low probability of intercept.

Multiple user random access communications with selective.

addressing capability.

High resolution ranging.

Accurate universal timing.

One of Spread spectrum types is FREQUENCY HOPPING.


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Frequency-Hopping Definition

It causes the carrier to hop from frequency to frequencyover a wide band according to a sequence defined bythe PRN.

Figure 1. Frequency-Hopping.

Dwell TimeHop Time


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Frequency-Hopping Pattern


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The transmitter and receiver are synchronized to the

same hop sequence.

It can be used to increase the security of thetransmission and also overcome jamming and signalfading problems.

The speed at which the hops are executed depends onthe data rate of the original information.

Frequency-Hopping Notes


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The bandwidth of a frequency-hopping signal is simply Ntimes the number of frequency slots available, where Nis the bandwidth of each hop channel.

The FHSS output is flat over the band of frequenciesused (see Figure 2).

Frequency-Hopping Notes

Figure 2. Spectrum-analyzer photo of a FHSS signal.


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The Effects Of Frequency Hopping


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1- Fast Frequency-Hopping (FFHSS) :- it is characterized by several hops within each data bit.

Symbol diversity.

Very resistant to jamming and interference, often used in

military systems. Use non coherent signal detection .

Usually BFSK or MFSK modulation .

Frequency-Hopping Types


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2 - Slow Frequency-Hopping (SFHSS) :

Several data symbols on each hop frequency

Code word diversitywith interleaving

Employ burst error control coding to restore loss of

multiple bits in one hop. (ARQ) Less complex

Coherent data detection is possible.

Frequency-Hopping Types


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SFHSS and FFHSS

Frequency shifted (hop) every Th seconds Duration of signal element is Ts seconds

Slow FHSS has Th Ts

Fast FHSS has Th < Ts


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Frequency-Hopping System Transmitter


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Frequency-Hopping System Receiver


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Slow Frequency Hopping ,MFSK (M=4, k=2)

= 2/ LT =2/

= 2


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Fast Frequency Hopping ,MFSK (M=4, k=2)

= 2/ =K 2

/

= 2


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Frequency-Hopping Properties

1. Power FH reduce the average power spectral density of a

signal.

FH signal appears to be a narrow band signal with high

spectral density. FH systems use FSK, and FSK is less power efficient

and SNR.

2. Interference

Narrowband interference will have severe impact on FH. the portion of signal lost may be recovered by spreading

the data energy out in time though forward error coding,but only if the FEC spans more than one hop time.


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3. Synchronization and Timing Difficult to synchronize the receiver to the transmitter because both

the time and frequency need to be in tune.

The system search procedure allows the mobile station to sit on anyfrequency and wait for a signal or beacon. If this is a bad frequency,

it may have to move to another and sit and wait. The system hasmany channels to search and it is not feasible to perform the searchin parallel.

4. Multipath Effects

In frequency domain, FHSS operates with narrow band signalslocated around different carrier frequencies. If at a specific moment,the FHSS system is using a carrier frequency significantly faded asa result of multipath, the FHSS receiver could not get enoughenergy to detect the signal.

Frequency-Hopping Properties


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Conclusion

FHSS provides only 3 Mbps capacity links, but it is avery robust technology, with excellent behavior in harshenvironment characterized by large areas of coverage,multiple collocated cells, noises, multipath, Bluetoooth

presence, etc. The technology allows easy cellular poin-to-multipoint deployment, providing excellent reliability.


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References

Maxim-ic ,An Introduction to Spread-SpectrumCommunications, feb. 2003.

William Stallings, Data and ComputerCommunications,7th Edition.

SorinM.SCHWARTZ , Frequency Hopping SpreadSpectrum (FHSS)

vs. Direct Sequence Spread Spectrum (DSSS) inBroadband Wireless Access (BWA) and Wireless

LAN (WLAN).

PICKHOLTZ, SCHILLING, MILSTEIN,Theory of

Spread-Spectrum Communications-A Tutorial, IEEETRANSACTIONS ON COMMUNICATIONS, VOLCOM-30, No.5, MAY 1982.

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FHSS Presentation