An  Introduc+on  to    Frequency-­‐Hopping  

Spread-­‐Spectrum  (FHSS)  Data  Communica+on  Techniques  

Jason  S.  Skinner  

Outline  

I.  Introduc4on  to  spread-­‐spectrum  communica4ons  

II.  Introduc4on  to  frequency  hopping  and  terminology  

III.  Par4al-­‐band  interference  IV.  Techniques  to  overcome  par4al-­‐band  

interference  V.  Summary  of  current  technology  using  

frequency-­‐hop  spread  spectrum  

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What  is  Spread  Spectrum?  

•  Given  an  electrical  signal  to  be  transmiFed  –  Fourier  analysis:  

•  “Any  signal  can  represented  as  a  linear  combina4on  of  many  sinusoidal  signals  at  different  frequencies.”  

•  Range  of  frequencies  =  Spectrum  – Width  of  spectrum    =  Bandwidth  

•  Apply  techniques  to  deliberately  spread  the  spectrum  of  the  original  signal  – New  signal  with  wider  bandwidth  

•  Two  main  techniques:  – Direct-­‐Sequence  Spread  Spectrum  (DSSS)    –  Frequency-­‐Hopping  Spread  Spectrum  (FHSS)  

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What  is  Frequency  Hopping?  

•  Early/simple  wireless  communica4on  systems:  –  Converted  an  analog  voice/data  message  into  an  electrical  signal  

–  Electrical  signal  was  placed  onto  a  carrier  signal  •  Frequency  of  carrier  signal  much  larger  than  bandwidth  of  electrical  signal  

•  Process  called  MODULATION  

–  Receiver  extracted  electrical  signal  from  received  signal  •  Process  called  DEMODULATION  

–  Electrical  signal  converted  back  into  analog  voice/data  message  

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Early/Simple  Wireless  Communica4on  Systems  

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Receiver

Transmitter

Modulator

Channel

Demodulator

DataSource

Destination

Problems  in  Military/Tac4cal  Environment:    Eavesdropping  

•  Enemy  forces:  – Scan  frequency  spectrum  

– Look  for  “spikes”  in  the  spectrum  •  “Spike”  indicates  presence  of  carrier  signal  

– Determine  carrier  signal  frequency  – Set  receiver  to  same  carrier  frequency  – Can  listen  in  on  communica4ons  

– Can  intercept  – Can  pretend  to  be  part  of  friendly  forces  

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Eavesdropping  

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Problems  in  Military/Tac4cal  Environment:    Jamming  

•  Enemy  forces:  – Scan  frequency  spectrum  

– Look  for  “spikes”  in  the  spectrum  •  “Spike”  indicates  presence  of  carrier  signal  

– Determine  carrier  signal  frequency  – Set  transmiFer  to  same  carrier  frequency  – Can  disrupt  communica4ons  

– Can  block  communica4ons  

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Jamming  

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Communica4on  Systems  with  Frequency  Hopping  

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Receiver

Transmitter

Modulator

Channel

Demodulator

DataSource

Destination

Frequency Hopper

Frequency Dehopper

What  is  Frequency  Hopping?  

•  Carrier  signal  frequency  changed  (hopped)  – Predetermined  sequence  (hopping  paGern)  •  Pseudorandom  sequence  known  to  both  transmiFer  and  receiver  

•  Times  at  which  the  carrier  frequency  is  changed  are  hop  epochs  

•  Time  interval  between  consecu4ve  hop  epochs  is  hop  interval  

•  Data  transmission  4me  interval  (dwell  interval)  –  Dwell  interval  ≤  Hop  interval      

•  Available  radio  frequency  spectrum  (band)  –  Divided  into  q  sub-­‐bands  (frequency  slots)  

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Slow  Frequency  Hop  vs.  Fast  Frequency  Hop  

•  Fast  Frequency  Hop  (FFH)  – Data  symbol  transmission  requires  at  least  2  dwell  intervals  •  Digital:    Carrier  frequency  changes  in  middle  of  bit  period  

– Hopping  rate  >  Data  rate  

•  Slow  Frequency  Hop  (SFH)  – One  or  more  data  symbols  transmiFed  per  dwell  

•  Digital:    Mul4ple  bits  of  informa4on  per  dwell  

– Hopping  rate  ≤  Data  rate  

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Slow  Frequency  Hop  Signal  

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Frequency

Time

1        2        3  

Eavesdropping   7        8        9  

10    11    12  

13    14    15  

4        5        6  

Jamming  

Good  vs.  Bad  for  FHSS  

•  Provides  NO  protec4on  for  individual  symbols  if  frequency  slot  contains  interference  

•  Provides  frequency  diversity  – Data  symbols  transmiFed  at  different  carrier  frequencies  experience  different  channel  effects  

– Taking  advantage  of  frequency  diversity  effec4vely  can  overcome  interference  

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Par4al-­‐Band  Interference  

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Frequency

Time

1        2        3  

4        5        6  

7        8        9  

10    11    12  

13    14    15  

Par4al-­‐Band  Interference  (PBI)  

•  Sources  – Hos4le  jamming,    – Mul4ple-­‐access  interference  – Other  radio  frequency  interference  

•  ρ  =  frac4on  of  RF  band  that  contains  PBI  •  Typically  modeled  as  white  Gaussian  noise  – Stronger  than  thermal  noise  

•  Frequency  occupancy  may  change  – Typically  much  slower  than  hopping  rate  

•  Symbols  in  dwell  with  PBI  are  hit  16  

Technique  1:  Error-­‐Control  Coding  (ECC)  

•  Include  addi4onal  symbols  –  Redundancy  –  Parity  symbols  

–  Complex  code  symbols  

•  Input:    informa4on  block  has  size  k  

•  Output:    encoded  block  has  size  n  •  Labeled  (n,k)  code  –  Rate  of  code  is  r  =  k/n  

•  If  energy  per  symbol  is  Es,  the  energy  per  informa4on  bit  is  Eb  =  Es/r  =  nEs/k  

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FHSS  with  ECC  

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Receiver

Transmitter

Modulator

Channel

Demodulator

DataSource

Destination

Frequency Hopper

Frequency Dehopper

Encoder

Decoder

Error-­‐Control  Coding  (ECC)  Examples  

•  Hamming  Codes  

•  Reed-­‐Solomon  Codes  –  Used  in  CDs  and  DVDs  

•  Convolu4onal  Codes  –  Used  for  NASA  deep-­‐space  communica4ons  –  Used  in  IEEE  802.11  (Wi-­‐fi)  devices  

–  Used  in  cellular  phones  

•  Turbo  Codes  and  Turbo  Product  Codes  – Used  in  satellite  communica4ons  

•  Low-­‐Density  Parity-­‐Check  Codes  19  

Hamming  Codes  

•  Discovered  in  late  1940’s  •  Codes:    (2m-­‐1,  2m-­‐1-­‐m),  for  some  m≥3  – Examples:    (7,4),    (15,11),    (31,26),    (63,57)  

– Rates:              0.571,      0.733,          0.838,        0.905  

•  Can  correct  ONE  error  in  each  received  codeword  

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Par4al-­‐Band  Interference  

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Frequency

Time

Interleaving  

•  Reordering  of  symbols  in  block  of  data  •  Provides  protec4on  against  bursts  of  errors  in  a  symbol  stream  

•  Example  techniques:  –  Block  interleaving    

•  (used  in  some  cellular  standards)  

– Helical  interleaving    •  (typically  used  with  turbo  product  codes)  

–  S-­‐random  interleaving    •  (complex,  typically  used  with  other  turbo  codes)  

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Interleaving  Example  

1  2  3  4  5  6  7,  8  9  10  11  12  13  14,  15  16  17  18  19  20  21,  22  23  24  25  26  27  28,  29  30  31  32  33  34  35,  36  37  38  39  40  41  42,  43  44  45  46  47  48  49  

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1  8  15  22  29  36  43,  2  9  16  23  30  37  44,  3  10  17  24  31  38  45,  4  11  18  25  32  39  46,  

5  12  19  26  33  40  47,  6  13  20  27  34  41  48,  

7  14  21  28  35  42  49  

ALer  Interleaving:  

Before  Interleaving:  

Standard  FHSS  Block  Diagram    

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Receiver

Transmitter

Modulator

Channel

Demodulator

DataSource

Destination

Frequency Hopper

Frequency Dehopper

Encoder

Decoder

Interleaver

Deinterleaver

Par4al-­‐Band  Interference  

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Frequency

Time

•  Used  in:  – Headphones/Microphones/Headsets  

– Mobile  phones/Telephones  – Laptops/PCs  – Printers  – GPS  receivers  – Digital  cameras  – Video  game  consoles  

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•  Uses  frequency  hopping  •  Uses  up  to  79  different  frequencies  – From  unlicensed  Industrial,  Scien4fic,  and  Medical  (ISM)  2.4  GHz  short-­‐range  frequency  spectrum  

•  Low  power  consump4on  

•  Short  ranges  – 1  m,  10  m,  100m  opera4ons  

•  Gross  data  rate  of  1Mbps  27  

HAVE  QUICK  

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HAVE  QUICK  I  

•  HAVE  QUICK  I  – Combat-­‐net  radio  

– Uses  frequency  hopping  – Used  by  U.S.  and  allied  military  forces  – Nearly  all  U.S.  military  aircrap  use  it  

– Protects  communica4ons  in  the  UHF  band  •  From  225  MHz  to  400  MHz  

– Replaced  Vietnam-­‐era  single-­‐frequency  radios  

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HAVE  QUICK  II  

•  Second  genera4on  of  HAVE  QUICK  – Used  more  complex  frequency  hopping  

– Used  by  U.S.  and  allied  military  forces  •  Army  Avia4on,  Air  Traffic  Services,  Rangers  

•  Air  Force,  Navy,  and  NATO  forces  

•  Radios  being  phased  out  – HAVE  QUICK  waveforms  included  in  Joint  Tac4cal  Radio  System  (JTRS)  

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SINCGARS  

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SINCGARS  

•  Combat-­‐Net  Radio  – Used  by  U.S.  military  forces  –  ITT  received  contract  in  1983  –  First  equipped  in  1990  –  Replaced  Vietnam-­‐era  single-­‐frequency  radios  

•  Voice  and  data  communica4ons  •  Form  factors  –  Vehicle-­‐mount  –  Backpack  – Airborne  – Handheld  

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SINCGARS  

•  Two  modes:  – Single  frequency  – Frequency  hopping  •  Slow  frequency  hopping  

•  Uses  25  kHz  channels  in  the  VHF  FM  band  – From  30  MHz  to  88  MHz  

•  SINCGARS  radios  being  phased  out  – SINCGARS  waveform  included  in  Joint  Tac4cal  Radio  System  (JTRS),  along  with  HAVE  QUICK  waveforms  

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Ques+ons?  

Jason  S.  Skinner