Presentation on fhss


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Presentation on fhss

  2. 2. • Principle of spread spectrum• Frequency hopping spread spectrum• Direct sequence spread spectrum• Direct sequence CDMA systems
  3. 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 widerbandwidth•Spread spectrum methods: Frequency hopping spread spectrum Direct sequence spread spectrum Time hopping spread spectrum
  4. 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. 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. 6. •Signal is broadcast over seemingly random seriesof radio frequencies A number of channels allocated for the FH signal Width of each channel corresponds to bandwidth of input signal•Signal hops from frequency to frequency at fixedintervals Transmitter operates in one channel at a time Bits are transmitted using some encoding scheme At each successive interval, a new carrier frequency is selected
  7. 7. •The frequency sequence is dictated by thespreading code•Receiver should hop synchronously with thetransmitter to be able to recover the message•Advantages Eavesdroppers hear only unintelligible blips Attempts to jam signal on one frequency succeed only at knocking out a few bits
  8. 8. •Slow-frequency-hop spread spectrum The hopping duration is larger or equal to the symbol duration of the modulated signal Tc >= Ts•Fast-frequency-hop spread spectrum The hopping duration is smaller than the symbol duration of the modulated signal Tc < Ts
  9. 9. •Large number of frequencies used•Results 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. 10. FHSS PMD takes the binary bits of information fromthe whitened PLCP service data unit (PSDU) andtransforms 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 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. 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. 12. Channel Hopping For use in the 2.4 GHz frequency band The 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IR IR PHY is one of the three PHY layers supported Differs from DSSS and FHSS because IR uses near-visible light as the transmission media IR communication relies on the light energy, which is by line-of-sight or reflected off objects Operation is restricted to indoor environments and cannot pass through walls
  13. 13. Modulation Transmits 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 PPM Modulation 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
  14. 14. 4-PPM symbol map for 2 Mbps Data bits 4-PPM symbol 00 0001 01 0010 11 0100 10 1000
  15. 15. •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
  16. 16. f5 f4 f3 f2 f11 2 3 4 5 6 7 8 9 10 11 12 TIME