Presentation on fhss

  • View
    3.074

  • Download
    3

Embed Size (px)

Transcript

  • 1. FREQUENCY HOPPING SPREAD SPECTRUMBy:- Tanu Jaswal

2. Principle of spread spectrum Frequencyhopping spreadspectrum Direct sequence spread spectrum Direct sequence CDMA systems 3. Spread spectrum techniques are methods bywhich electromagnetic energy generated in aparticular bandwidth is deliberately spread in thefrequency domain, resulting in a signal with a widerbandwidthSpread spectrum methods:Frequency hopping spread spectrumDirect sequence spread spectrumTime hopping spread spectrum 4. 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 5. At the receiving end:digit sequence is used to demodulate the spread spectrum signal Signal is fed into a channel decoder to recover data 6. Signal is broadcast over seemingly random seriesof radio frequenciesA number of channels allocated for the FH signalWidth of each channel corresponds to bandwidth of inputsignalSignal hops from frequency to frequency at fixedintervalsTransmitter operates in one channel at a timeBits are transmitted using some encoding schemeAt each successive interval, a new carrier frequency isselected 7. The frequency sequence is dictated by thespreading codeReceiver should hop synchronously with thetransmitter to be able to recover the messageAdvantagesEavesdroppers hear only unintelligible blipsAttempts to jam signal on one frequency succeed only atknocking out a few bits 8. Slow-frequency-hop spread spectrum The hopping duration is larger or equal to the symbol duration of the modulated signal Tc >= TsFast-frequency-hop spread spectrum The hopping duration is smaller than the symbol duration of the modulated signal Tc < Ts 9. Large number of frequencies usedResults in a system that is quite resistant to jamming Jammer must jam all frequencies With fixed power, this reduces the jamming power in any one frequency band 10. FHSS PMD takes the binary bits of information fromthe whitened PLCP service data unit (PSDU) andtransforms them into RF signalsPSDU Data Whitening Datawhitening is applied to the PSDU before transmission to minimize bias on the data if long strings of 1s or 0s appear in the PSDU The PHY stuffs a special symbol every 4 octets of the PSDU in a PPDU frame A 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 11. Modulation IEEE 802.11 version released in 1997 uses two-level GFSK in the FHSS PMD to transmit the PSDU at the basic rate of 1 Mbps The PLCP preamble and PLCP header are always transmitted at 1 Mbps Four-level GFSK is an optional modulation method defined in the standard that enables the whitened PSDU to be transmitted at a higher rate GFSK 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) 12. Channel HoppingFor use in the 2.4 GHz frequency bandThe channels are evenly spaced across the band over a span 83.5MHzHop channels differs from country to countryChannel hopping is controlled by the FHSS PMDIRIR PHY is one of the three PHY layers supportedDiffers from DSSS and FHSS because IR uses near-visible light as thetransmission mediaIR communication relies on the light energy, which is by line-of-sightor reflected off objectsOperation is restricted to indoor environments and cannot passthrough walls 13. ModulationTransmits binary data at 1 and 2 MbpsUses PPM modulation to reduce the optical powerrequiredSpecific data rate is dependent upon the type of PPMModulation for 1 Mbps operation is 16-PPM, while it is4-PPM for 2 MbpsPPM is a modulation technique that keeps the amplitude,pulse width constant, and varies the position of the pulsein time 14. 4-PPM symbol map for 2 Mbps Data bits 4-PPM symbol00 000101 001011 010010 1000 15. Signal broadcast over seemingly random seriesof frequenciesReceiver hops between frequencies in syncwith transmitterEavesdroppers hear unintelligible blipsJamming on one frequency affects only a fewbits 16. f5 f4f3 f2f11 2 3 4 5 67 8 9 10 11 12TIME