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Spread spectrum


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  • 1. Spread Spectrum Technologies By- Rohit Dhongde (B-41) Swapnil Dondal(B-49) Vaibhav Deshmukh(B-52) Sushant Burde(B-48) Pratik Morey(B-39)
  • 2. Spread Spectrum multiple access (SSMA) 2 Spread Spectrum multiple access (SSMA) uses signals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth. A PN sequence converts a narrowband signal to a wideband noise-like signal before transmission. Two main types of SSMA: 1. Frequency hopped multiple access (FH) 2. Direct sequence multiple access (DS) Code division multiple access(CDMA) Advantage: 1. Immune to multipath interference and robust multiple access capability. 2. Efficient in a multiple user environment
  • 3. General Model of Spread Spectrum System
  • 4. Frequency Hopping Spread Spectrum l Carrier changes frequency (HOPS) according to a pseudorandom Sequence. 4 Pseudorandom sequence is a list of frequencies. The carrier hops through this lists of frequencies. 4 The carrier then repeats this pattern. 4 During Dwell Time the carrier remains at a certain frequency. 4 During Hop Time the carrier hops to the next frequency. 4 The data is spread over 83 MHz in the 2.4 GHz ISM band. 4 This signal is resistant but not immune to narrow band interference.
  • 5. 6.5 Frequency hopping spread spectrum (FHSS)
  • 6. Frequency Hopping Spread Spectrum (FHSS)  Signal broadcast over seemingly random series of frequencies  Receiver hops between frequencies in sync with transmitter  Eavesdroppers hear unintelligible blips  Jamming on one frequency affects only a few bits
  • 7. Basic Operation  Typically 2k carriers frequencies forming 2k channels  Channel spacing corresponds with bandwidth of input  Each channel used for fixed interval  300 ms in IEEE 802.11  Some number of bits transmitted using some encoding scheme  May be fractions of bit (see later)  Sequence dictated by spreading code
  • 8. Frequency Hopping Example
  • 9. Direct Sequence Spread Spectrum February 2005Copyright 2005 All Rights Reserved 9 l Spread spectrum increases the bandwidth of the signal compared to narrow band by spreading the signal. l There are two major types of spread spectrum techniques: FHSS and DSSS. 4 FHSS spreads the signal by hopping from one frequency to another across a bandwidth of 83 Mhz. 4 DSSS spreads the signal by adding redundant bits to the signal prior to transmission which spreads the signal across 22 Mhz. * The process of adding redundant information to the signal is called Processing Gain . * The redundant information bits are called Pseudorandom Numbers (PN).
  • 10. Direct Sequence Spread Spectrum Using BPSK Example