FAST FREQU FREQUENCY ENCY HOPPING & SLOW FREQUENCY HOPPING SPREAD SPECTRUM YASHNA SHARMA POOJA MISHRA M TECH(JOP), IIT DELHI
CONTENTS INTRODUCTION TO FHSS FAST AND SLOW FREQUENCY F REQUENCY HOPPING THE NEED FOR FHSS MATLAB SIMULATION SIMULATION OF FAST FHSS MATLAB MODULATOR & DEMODULATOR MATLAB SIMULATION OF SLOW FHSS MODULATOR & DEMODULATOR ADVANTAGES AGES AND DISADVANT DISADVANTAGES AGES ADVANT APPLICATIONS OF FHSS AND SHSS APPLICATIONS
FHSS-AN INTRODUCTION
FREQUENCY HOPPED: Regular Hopping of Frequencies, at hop rates more , less or equal to the data rate of the modulated signal. SPREAD SPECTRUM : Spreading of the Modulated Signal with another modulation technique, to increase the bandwidth but reduce the Power Spectral Density
THE NEED FOR FHSS
Single Frequency Systems are vulnerable to jamming, interference, and multipath problems Repeated switching of frequencies during transmission according to specified techniques will minimize sensitivity of the system to jamming and interference Power Spectral Density Reduces as Bandwidth Increases, thus Signals get Hidden to Spurious Recievers.
TYPES OF FREQUENCY HOPPING THE FREQUENCY HOPPING RATE IS MORE THAN THE DATA RATE,I.E, THERE ARE MULTIPLE FREQUENCIES REPRESENTING ONE BIT OF INFORMATION
THE FREQUENCY HOPPING RATE IS LESS THAN THE DATA RATE,I.E, THERE MAY BE MULTIPLE BITS REPRESENTED BY ONE FREQUENCY
FAST HOPPING-PARAMETERS USED IN MATLAB PROGRAMMING
Initially we have a BPSK System to modulate the data. The Bandwidth of the BPSK Signal is 2f b We use 6 Frequencies for the Hopping Process The time periods are: Tb(Data Time Period)= 300 Tc(Period for which one Frequency Lasts)=120
Tb>Tc Hopping Rate> Data Rate= FAST FREQUENCY HOPPING
FAST FREQUENCY HOPPING Original Bit Sequence 1 0
X: 2403 Y: -1
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BPSK Modulated Signal 1 0 -1 0
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Original Bit Sequence
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BPSK Modulated Signal
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Spread Signal with 6 frequencies
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Frequency Hopped Spread Spectrum Signal
FREQUENCY SPECTRUM Frequency Hopped Spread Spectrum signal and its FFT 1.5 1 0.5 0 -0.5 -1 -1.5
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BANDWIDTH BPSK SIGNAL
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Power spectral density(BPSK SIGNAL)
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FHSS SIGNAL
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Power spectral density(FHSS)
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300 Frequency (Hz)
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DEHOPPING & DEMODULATION
ORIGINAL BIT STREAM
FHSS SIGNAL
1 1 L 0.5 A N G I S S S 0 H F T S A F -0.5
A 0.5 T A D L A 0 N I G I R O-0.5
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TIME UNITS
DEHOPPED SIGNAL 1 E T A I D E 0.5 M R E T N I D 0 E T A L U D -0.5 O M E D
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) D 1 E P O L E V 0.5 N E N U ( A T 0 A D D E T A -0.5 L U D O M E -1 D
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Original Bit Sequence FHSS WITH FSK-THE ACTUAL METHOD 1 0 -1 0
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BFSK Modulated Signal 1 0 -1 0
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Spread Signal with 6 frequencies 1 0 -1 0
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Frequency Hopped Spread Spectrum Signal 1 0 -1 0
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Frequency Hopped Spread Spectrum signal and its FFT 1.5 1 0.5 0 -0.5 -1 -1.5 0
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WHY FAST FREQUENCY HOPPPING?
Increased Resistance to Jamming/Interference due to multiple hops Resistance to Multipath Fading by providing Diversity L= Tsymbol/ Thop ; Order of Diversity Low probability of Intercept: Used for assuring security
SLOW HOPPING- PARAMETERS USED IN MATLAB PROGRAMMING
Initially we have a BPSK System to modulate the data. The bandwidth of the BPSK Signal is 2f b We use 6 Frequencies for the Hopping Process. The time periods are: Tb(Data Time Period)= 60 Tc(Period for which one Frequency Lasts)=120
Hopping Rate
Data Rate= SLOW FREQUENCY HOPPING
SLOW FREQUENCY HOPPING Original Bit Sequence 1 0 -1 0
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BPSK Modulated Signal 1 0 -1 0
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Spread Signal with 6 frequencies 1 0 -1 0
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Frequency Hopped Spread Spectrum Signal 1 0 -1 0
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Original Bit Sequence 1 0 -1 0
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BPSK Modulated Signal 1 0 -1 0
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Spread Signal with 6 frequencies 1 0 -1 0
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Frequency Hopped Spread Spectrum Signal 1 0 -1 0
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BANDWIDTH Power spectral density(BPSK SIGNAL) 30
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300 400 Frequency (Hz) Power spectral density (FHSS)
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DEHOPPING AND DEMODULATION Original bit sequence
Slow FHSS signal 1
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Time units
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Dehopped signal
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Original Bit Sequence 1 0 -1 0
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BFSK Modulated Signal 1 0 -1 0
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Spread Signal with 6 frequencies 1 0 -1 0
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Frequency Hopped Spread Spectrum Signal 1 0 -1 0
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ADVANTAGES AND LIMITATIONS OF USING SLOW FREQUENCY HOPPING
FHSS provides only 3Mbps capacity but it is a very robust technology with little influence from noises, reflections and from other radio stations. Coherent data detection is easily possible through slow frequency hopping. It is also useful in burst error control coding to restore loss of bits in one hop. SFH uses a less expensive synthesizer with a worst near far rejection and it also need for more powerful error correction scheme.
APPLICATIONS OF FHSS It is useful in cellular deployments for fixed Broadband Wireless Access. Bluetooth radio also uses FHSS operating in the 2.4 GHz band, with a dwell time of 0.625ms (1,600 hops per second).The amount of time spent on each hop is known as dwell time. This can be explained further as: When DSSS executes a transmission, it is using 22 MHz for the duration of the transmitted frame. When FHSS executes a transmission, it is using 1 MHz for the duration of the transmitted frame.
APPLICATIONS(Continue) Thus the chances of having Bluetooth “hitting” the 22 MHz band used by DSSS are higher than the chances of it hitting the 1 MHz band used by FHSS.
It is also used in radar systems.
It means the hop rate is an integer multiple of the pulse repetition rate i.e., the pulse carrier frequency will change or hop several times during the transmission of one pulse.
REFERENCES
Sami A. Mostafa, “FREQUENCY HOPPING TRANSCEIVER SYSTEM WITH APPLICATION TO RADAR” Filol, Riley ,“ An Interpolated Frequency-Hopping Spread-Spectrum Transceiver ” Berder, Bouder, “Identification of Frequency Hopping Communications” Zoran Spasojevic, John Burns, “Performance Comparison Of Frequency Hopping And Direct Sequence Spread Spectrum Systems In The 2.4 GHz Range” Rapport, “Wireless Communication”, Second Edition
THANK YOU
A LOOK AT THE MODULATION PROCESS TO ANTENNA BPSK MODULATED DATA I F
RF LO
FREQUENC Y SYNTHESIZ ER HOPPING CODE GENERATO R
GENERATION OF THE HOPPING SEQUENCE
A LOOK AT THE DEMODULATION PROCESS BAND PASS FILTER
RF LO
I F
FREQUENC Y SYNTHESIZ ER HOPPING CODE GENERATO R
IF FILTER
FREQ. DISCRIMINATO R
DEMODULATED DA
