Presentation on fhss

FREQUENCY HOPPING SPREAD SPECTRUM

By:- Tanu Jaswal

PRESENTATION ON

• Principle of spread spectrum• Frequency hopping spread

spectrum• Direct sequence spread spectrum• Direct sequence CDMA systems

Spread Spectrum

•Spread spectrum techniques are methods by which electromagnetic energy generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth

•Spread spectrum methods:Frequency hopping spread spectrumDirect sequence spread spectrumTime hopping spread spectrum

What is Spread Spectrum?

•At the transmitter side:─Input is fed into a channel encoder

•Produces analog signal with narrow bandwidth─ Signal is further modulated using sequence of digits

• Spreading code or spreading sequence • Generated by pseudonoise, or pseudo-random number generator

• Effect of modulation is to increase bandwidth of signal to be transmitted

SPREAD SPECTRUM

At the receiving end:

−digit sequence is used to demodulate the spread spectrum signal− Signal is fed into a channel decoder to recover data

SPREAD SPECTRUM

SPREAD SPECTRUM

•Signal is broadcast over seemingly random series of radio frequencies

A number of channels allocated for the FH signalWidth of each channel corresponds to bandwidth of input signal

•Signal hops from frequency to frequency at fixed intervals

Transmitter operates in one channel at a timeBits are transmitted using some encoding schemeAt each successive interval, a new carrier frequency is selected

Frequency Hopping Spread Spectrum (FHSS)

•The frequency sequence is dictated by the spreading code

•Receiver should hop synchronously with the transmitter to be able to recover the message

•AdvantagesEavesdroppers hear only unintelligible blipsAttempts to jam signal on one frequency succeed only at knocking out a few bits

Frequency Hopping Spread Spectrum (FHSS)

Frequency Hopping Spread Spectrum (FHSS)

Frequency Hopping Spread Spectrum (FHSS)

•Slow-frequency-hop spread spectrumThe hopping duration is larger or equal to the symbol duration of the modulated signalTc >= Ts

•Fast-frequency-hop spread spectrumThe hopping duration is smaller than the symbol duration of the modulated signalTc < Ts

Frequency Hopping Spread Spectrum (FHSS)

Slow Frequency-Hop SS

Fast Frequency-Hop SS

•Large number of frequencies used

•Results in a system that is quite resistant to jamming

Jammer must jam all frequenciesWith fixed power, this reduces the jamming power in any one frequency band

FHSS Performance Considerations

FHSS PMD takes the binary bits of information from the whitened PLCP service data unit (PSDU) and transforms them into RF signals PSDU Data Whitening

Data whitening is applied to the PSDU before transmission to minimize bias on the data if long strings of 1’s or 0’s appear in the PSDUThe PHY stuffs a special symbol every 4 octets of the PSDU in a PPDU frameA 127-bit sequence generator using the polynomial S(x) = x7 + x4 + 1 and 32/33 bias-suppression encoding algorithm are used to randomize and whiten the data

Frequency Hopping Spread Spectrum (FHSS)

ModulationIEEE 802.11 version released in 1997 uses two-level GFSK in the FHSS PMD to transmit the PSDU at the basic rate of 1 MbpsThe PLCP preamble and PLCP header are always transmitted at 1 MbpsFour-level GFSK is an optional modulation method defined in the standard that enables the whitened PSDU to be transmitted at a higher rateGFSK is a modulation technique used by the FHSS PMD shifts the frequency either side of the carrier hop frequency Four-level GFSK is similar to two-level GFSK and modulator combines two binary bits into symbol pairs (10, 11, 01, 00)

Frequency Hopping Spread Spectrum (FHSS)

Channel Hopping For use in the 2.4 GHz frequency bandThe channels are evenly spaced across the band over a span 83.5 MHz Hop channels differs from country to country Channel hopping is controlled by the FHSS PMD

IRIR PHY is one of the three PHY layers supportedDiffers from DSSS and FHSS because IR uses near-visible light as the transmission mediaIR communication relies on the light energy, which is by line-of-sight or reflected off objectsOperation is restricted to indoor environments and cannot pass through walls

Frequency Hopping Spread Spectrum (FHSS)

ModulationTransmits binary data at 1 and 2 Mbps Uses PPM modulation to reduce the optical power required Specific data rate is dependent upon the type of PPMModulation for 1 Mbps operation is 16-PPM, while it is 4-PPM for 2 Mbps PPM is a modulation technique that keeps the amplitude, pulse width constant, and varies the position of the pulse in time

Frequency Hopping Spread Spectrum (FHSS)

4-PPM symbol map for 2 Mbps

Frequency Hopping Spread Spectrum (FHSS)

Data bits 4-PPM symbol

00 0001

01 0010

11 0100

10 1000

Frequency hopping spread spectrum (FHSS)

Frequency selection in FHSS

FHSS cycles

Bandwidth sharing

•Signal broadcast over seemingly random seriesof frequencies•Receiver hops between frequencies in syncwith transmitter•Eavesdroppers hear unintelligible blipsJamming on one frequency affects only a fewbits

Frequency Hopping Spread Spectrum (FHSS)

Frequency Hopping Spread Spectrum (FHSS) • FSK DATA MODULATION •PERIODIC CHANGES IN THE CARRIER FREQUENC SPREADS THE SIGNAL • CARRIER FREQUENCY CHANGES AT A SPECIFIED HOP RATE • CARRIER FREQUENCY HOPS AFTER A PRESCRIBED TIME • TOTAL SYSTEM BANDWIDTH INCLUDES ALL OF THE CHANNEL FREQUENCIES USED IN HOPPING

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