FAST FREQUENCY HOPPING & SLOW

FREQUENCY HOPPING SPREAD SPECTRUM

YASHNA SHARMAPOOJA MISHRA

M TECH(JOP), IIT DELHI

CONTENTSCONTENTS

INTRODUCTION TO FHSS FAST AND SLOW FREQUENCY HOPPING THE NEED FOR FHSS MATLAB SIMULATION OF FAST FHSS

MODULATOR & DEMODULATOR MATLAB SIMULATION OF SLOW FHSS

MODULATOR & DEMODULATOR ADVANTAGES AND DISADVANTAGES APPLICATIONS OF FHSS AND SHSS

FHSS-AN INTRODUCTIONFHSS-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 FHSSTHE 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 TYPES OF FREQUENCY HOPPINGHOPPING

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 FAST HOPPING-PARAMETERS USED IN MATLAB USED IN MATLAB PROGRAMMINGPROGRAMMING

Initially we have a BPSK System to modulate the data. The Bandwidth of the BPSK Signal is 2fb

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 FAST FREQUENCY HOPPINGHOPPING

0 500 1000 1500 2000 2500 3000

-1

0

1

X: 2403Y: -1

Original Bit Sequence

0 500 1000 1500 2000 2500 3000

-1

0

1

BPSK Modulated Signal

0 500 1000 1500 2000 2500 3000

-1

0

1

Spread Signal with 6 frequencies

0 500 1000 1500 2000 2500 3000

-1

0

1

Frequency Hopped Spread Spectrum Signal

0 500 1000 1500 2000 2500 3000

-1

0

1

X: 2403Y: -1

Original Bit Sequence

0 500 1000 1500 2000 2500 3000

-1

0

1

BPSK Modulated Signal

0 500 1000 1500 2000 2500 3000

-1

0

1

Spread Signal with 6 frequencies

0 500 1000 1500 2000 2500 3000

-1

0

1

Frequency Hopped Spread Spectrum Signal

FREQUENCY SPECTRUMFREQUENCY SPECTRUM

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

1.5 Frequency Hopped Spread Spectrum signal and its FFT

0 500 1000 1500 2000 2500 30000

50

100

150

200

BANDWIDTH

0 100 200 300 400 500 6000

10

20

30

40

50

60

X: 19.92Y: 0.06642

Power spectral density(BPSK SIGNAL)

Frequency (Hz)

0 100 200 300 400 500 6000

1

2

3

4

5

6

7

X: 119.5Y: 0.03645

Power spectral density(FHSS)

Frequency (Hz)

0 100 200 300 400 500 6000

10

20

30

40

50

60

X: 19.92Y: 0.06642

Power spectral density(BPSK SIGNAL)

Frequency (Hz)

0 100 200 300 400 500 6000

1

2

3

4

5

6

7

X: 119.5Y: 0.03645

Power spectral density(FHSS)

Frequency (Hz)

BPSK BPSK SIGNALSIGNAL

FHSS FHSS SIGNALSIGNAL

DEHOPPING & DEHOPPING & DEMODULATIONDEMODULATION

500 1000 1500 2000 2500 3000

-1

-0.5

0

0.5

1

OR

IGIN

AL

DA

TA

TIME UNITS

ORIGINAL BIT STREAM

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

TIME UNITS

FAS

T FH

SS

SIG

NA

L

FHSS SIGNAL

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

TIME UNITS

DE

MO

DU

LATE

D IN

TER

ME

DIA

TE

DEHOPPED SIGNAL

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

TIME UNITS

DE

MO

DU

LATE

D D

ATA

(UN

EN

VE

LOP

ED

)

0 500 1000 1500 2000 2500 3000

-1

0

1

Original Bit Sequence

0 500 1000 1500 2000 2500 3000

-1

0

1

BFSK Modulated Signal

0 500 1000 1500 2000 2500 3000

-1

0

1

Spread Signal with 6 frequencies

0 500 1000 1500 2000 2500 3000

-1

0

1

Frequency Hopped Spread Spectrum Signal

FHSS WITH FSK-THE ACTUAL METHOD

0 500 1000 1500 2000 2500 3000-1.5

-1

-0.5

0

0.5

1

1.5 Frequency Hopped Spread Spectrum signal and its FFT

0 500 1000 1500 2000 2500 30000

100

200

300

WHY FAST FREQUENCY WHY FAST FREQUENCY HOPPPING?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 SLOW HOPPING- PARAMETERS USED IN MATLAB USED IN MATLAB PROGRAMMING PROGRAMMING

Initially we have a BPSK System to modulate the data. The bandwidth of the BPSK Signal is 2fb

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

Tb ˂ Tc

Hopping Rate ˂ Data Rate= SLOW FREQUENCY HOPPING

SLOW FREQUENCY SLOW FREQUENCY HOPPINGHOPPING

0 500 1000 1500 2000 2500 3000

-1

0

1

Original Bit Sequence

0 500 1000 1500 2000 2500 3000

-1

0

1

BPSK Modulated Signal

0 500 1000 1500 2000 2500 3000

-1

0

1

Spread Signal with 6 frequencies

0 500 1000 1500 2000 2500 3000

-1

0

1

Frequency Hopped Spread Spectrum Signal

0 500 1000 1500 2000 2500 3000

-1

0

1

Original Bit Sequence

0 500 1000 1500 2000 2500 3000

-1

0

1

BPSK Modulated Signal

0 500 1000 1500 2000 2500 3000

-1

0

1

Spread Signal with 6 frequencies

0 500 1000 1500 2000 2500 3000

-1

0

1

Frequency Hopped Spread Spectrum Signal

BANDWIDTH

0 100 200 300 400 500 6000

10

20

30Power spectral density(BPSK SIGNAL)

Frequency (Hz)

0 100 200 300 400 500 6000

2

4

6

8Power spectral density(FHSS)

Frequency (Hz)

DEHOPPING AND DEHOPPING AND DEMODULATIONDEMODULATION

500 1000 1500 2000 2500 3000

-1

-0.5

0

0.5

1

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

0 500 1000 1500 2000 2500 3000-1

-0.5

0

0.5

1

Original bit sequence

Demodulated signal

Dehopped signal

Slow FHSS signal

Time units

Time units

Time units Time units

0 500 1000 1500 2000 2500 3000

-101

Original Bit Sequence

0 500 1000 1500 2000 2500 3000

-101

BFSK Modulated Signal

0 500 1000 1500 2000 2500 3000

-101

Spread Signal with 6 frequencies

0 500 1000 1500 2000 2500 3000

-101

Frequency Hopped Spread Spectrum Signal

ADVANTAGES AND LIMITATIONS ADVANTAGES AND LIMITATIONS OF USING SLOW FREQUENCY OF USING SLOW FREQUENCY

HOPPINGHOPPING

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 Schiller, “Mobile Communications”, Second Edition

THANK YOUTHANK YOU

A LOOK AT THE A LOOK AT THE MODULATION PROCESSMODULATION PROCESS

FREQUENCFREQUENCY Y

SYNTHESISYNTHESIZERZER

HOPPING HOPPING CODE CODE

GENERATGENERATOROR

BPSK MODULATED DATATO ANTENNA

GENERATION OF THE HOPPING SEQUENCE

IF

RF

LO

A LOOK AT THE A LOOK AT THE DEMODULATION PROCESSDEMODULATION PROCESS

FREQUENCFREQUENCY Y

SYNTHESISYNTHESIZERZER

HOPPING HOPPING CODE CODE

GENERATGENERATOROR

IF

LO

BAND BAND PASS PASS

FILTERFILTER

IF IF FILTERFILTER

RF

FREQ. FREQ. DISCRIMINATDISCRIMINAT

OROR

DEMODULATED DATA