An Introduc+on to Frequency- Hopping Spread- Spectrum (FHSS) Data Communica+on Techniques. Jason S. Skinner

Transcription

1 An Introduc+on to Frequency- Hopping Spread- Spectrum (FHSS) Data Communica+on Techniques Jason S. Skinner

2 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 2

3 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) 3

4 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 4

5 Early/Simple Wireless Communica4on Systems Data Source Modulator Transmitter Channel Destination Demodulator Receiver 5

6 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 6

7 Eavesdropping 7

8 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 8

9 Jamming 9

10 Communica4on Systems with Frequency Hopping Data Source Modulator Transmitter Frequency Hopper Channel Destination Demodulator Receiver Frequency Dehopper 10

11 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) 11

12 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 12

13 Slow Frequency Hop Signal Frequency Eavesdropping 7 Jamming Time 13

14 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 14

15 Par4al- Band Interference Frequency Time 15

16 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

17 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 E s, the energy per informa4on bit is E b = E s /r = ne s /k 17

18 FHSS with ECC Data Source Encoder Modulator Transmitter Frequency Hopper Channel Destination Decoder Demodulator Receiver Frequency Dehopper 18

19 Error- Control Coding (ECC) Hamming Codes Reed- Solomon Codes Used in CDs and DVDs Convolu4onal Codes Examples Used for NASA deep- space communica4ons Used in IEEE (Wi- fi) devices Used in cellular phones Turbo Codes and Turbo Product Codes Used in satellite communica4ons Low- Density Parity- Check Codes 19

20 Hamming Codes Discovered in late 1940 s Codes: (2 m - 1, 2 m - 1- m), for some m 3 Examples: (7,4), (15,11), (31,26), (63,57) Rates: 0.571, 0.733, 0.838, Can correct ONE error in each received codeword 20

21 Par4al- Band Interference Frequency Time 21

22 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) 22

23 Interleaving Example Before Interleaving: , , , , , , ALer Interleaving: , , , , , ,

24 Standard FHSS Block Diagram Data Source Encoder Interleaver Modulator Transmitter Frequency Hopper Channel Destination Decoder Deinterleaver Demodulator Receiver Frequency Dehopper 24

25 Par4al- Band Interference Frequency Time 25

26 Used in: Headphones/Microphones/Headsets Mobile phones/telephones Laptops/PCs Printers GPS receivers Digital cameras Video game consoles 26

27 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

28 HAVE QUICK 28

29 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 29

30 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) 30

31 SINCGARS 31

32 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 32

33 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 33

34 Ques+ons? Jason S. Skinner