Spread Spectrum Technique

1

Spread Spectrum Spread Spectrum TechniquesTechniques

Prof.Sharada N. OhatkarC.C.O.E.W.,Pune

11/07/2012 Prof.Sharada N. Ohatkar

11/07/2012 Prof.Sharada N. Ohatkar 2

••PN Sequences, PN Sequences, ••SPREAD SPECTRUMSPREAD SPECTRUM

••DSSSDSSSDSSS DSSS with coherent with coherent BPSK, BPSK, Processing Gain, Processing Gain, Probability of error,Probability of error,

••FHSSFHSSFast FHSSFast FHSSSlow FHSSSlow FHSS

••Introduction to Multiple Access TechniquesIntroduction to Multiple Access Techniques--•• TDMA, TDMA, •• FDMA FDMA •• CDMACDMA..


PN Sequences

� PN generator produces periodic sequence that appears to be random

� PN Sequences � Generated by an algorithm using initial seed� Sequence isn’t statistically random but will pass many test of randomness

� Sequences referred to as pseudorandom numbers or pseudonoise sequences

� Unless algorithm and seed are known, the sequence is impractical to predict

DSSS

Feedback Shift Register

Logic Circuit

21 3m

Clock

Flip-Flops Output Sequence

Generation of PN Sequence

11/07/2012 4Prof.Sharada N. Ohatkar

Modulo-2 adder

Clock

1 2 3 4 5

1 0 0 0 0

1 0 0 0 0 0

0 0

0

0 1 0 0 0 00 0 1 0 0 0

1

1

0 0 0 1 0 ………..

……….……..

• PN Sequence = 0000100101100111110001101110101

• Since m = 5

Period = ( 25-1 ) = 31

Feedback Shift Register for m = 5

PN Sequence Example



Shift register sequence generator with Shift register sequence generator with

[5,2] configuration[5,2] configuration

Practically :Practically :55 stagestage,[5,4,3,2],[5,4,3,2] register with initial conditions register with initial conditions 1111111111 isis1 1 1 1 1 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 1 0 1 0 0 0 1 1 1 01 1 1 1 1 0 0 1 0 0 1 1 0 0 0 0 1 0 1 1 0 1 0 1 0 0 0 1 1 1 0And has a period And has a period 3131..


Important PN Properties

� Randomness� Uniform distribution

� Balance property

� Run property

� Correlation property

� Unpredictability


Properties of NProperties of N--Sequences Sequences N= 2N= 2nn--11

�� Property 1Property 1::� Has 2n-1 ones and 2n-1-1 zeros

�� Property 2:Property 2:� Sequence contains one run of ones, length n� One run of zeros, length n-1� One run of ones and one run of zeros, length n-2� Two runs of ones and two runs of zeros, length n-3� 2n-3 runs of ones and 2n-3 runs of zeros, length 1


Properties of NProperties of N--SequencesSequences

�� Property 3: Property 3: � The periodic autocorrelation of a ±1

n-sequence is

( )otherwise

... 2N, N,0,

11 =

−=

τ

N

R τ


Autocorrelation function of random data d(t) ,Autocorrelation function of random data d(t) ,having the values d=1having the values d=1


Autocorrelation of a PN sequenceAutocorrelation of a PN sequence

N=7 and Tc =1

0 1 2 3 4 5 6


1/NTc

SX(f)

-2/T -1/T 0 1/T 2/T

A2T

-2/Tc -1/Tc 0 1/Tc 2/Tc

Sc(f)

Comparison between a random binary wave (left) and

m-sequence (right).


Maximum length sequence of shift register lengths.


� SPREAD SPECTRUM is an RF communication system in which the base band signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency Signal.

� the band spread is accomplished by means of a code which is independent of the data, and

� synchronized reception with the code at the receive is used for de-spreading and subsequent data recovery.”

Spread SpectrumSpread Spectrum


ADVANTAGES OF SS

1. Resists intentional and non-intentional interferences.

2. Eliminate the effect of multi path fading and selective fading.

3. Share the same frequency band with other users.

4. Privacy due to PN sequence.5. Require low PSD.


Application of Spread Spectrum Application of Spread Spectrum

� Global Positioning System (GPS)� each satellite has a unique spreading code� enhanced resolution

� Cordless Phones

� Wireless LANs

� Wireless Audio Speakers

� Home Security Systems

� Medical Monitoring Devices

� CDMA

(military & commercial, mobile communication, radar & navigation)


Types of spread spectrum:

1.1. Direct sequence spread spectrum Direct sequence spread spectrum (DSSS):(DSSS):

Original

Data Signal

Narrowband

Filter

Other

SS Users

Demodulator

Filtering

ISI

Modulated

Data

Data Signal

with Spreading

Narrowband

Interference

Other

SS Users

Receiver

Input

ISI

8C32810.117-Cimini-7/98


Di(f-fc)

Dj(f-fc)

1 3 2 4

1 2 34

2. Frequency Hopped spread spectrum (FHSS):2. Frequency Hopped spread spectrum (FHSS):


Direct Sequence Spread Spectrum Direct Sequence Spread Spectrum (DSSS)(DSSS)

� Each bit in original signal is represented by multiple bits (PN) in the transmitted signal

� Spreading code spreads signal across a wider frequency band � Spread is in direct proportion to number of bits used

� combines digital information stream with the spreading code bit stream using exclusive-OR.


DSSS:Advantages1.multipath fading and selective fading effects reduced due to spreading of the signal bandwidth.

2. PN sequence have low autocorrelation, multi path which is delayed more than a chip will appear as noise.

3. soft capacity limit on number of users.4. advantage of voice activity patterns which cannot be exploited in FDMA &TDMA.

5.SECURITY.


DSSS: Disadvantages

1. Implementation complexity.2. Expensive receivers.3. Self jamming due to non-perfect

orthogonality of spreading codes.4. Other users signals will appear as noise

and reduce SNR of desired signal.5. Near-far problem.


DSSS transmitter system and spectra

DSSS

Wave forms at the transmitter..

Tb

t

(a)

(b)

NTc

t

Tc

t

+1

0

-1

Time (c) Frequency

f

Tb

-Rb Rb

-Rc Rc

f

Tc

-Rc Rc

f

Tc

+1

0

-1

+1

0

-1

A Notion of Spread Spectrum (SS)

(b) PN signal.

(c) Product signal

(a) Data signal.



DSS receiver system


DSSS base band transmission.DSSS base band transmission.

transmittertransmitter channelchannel

receiverreceiver

narrowband wideband

Baseband spread-spectrum system. (a) Transmitter. (b) Channel. (c) Receiver.

( ) ( ) ( )tctbtm = ( ) ( ) ( )

( ) ( ) ( )titctb

titmtr

+=

+= ( ) ( ) ( )

( ) ( )[ ] ( )

( ) ( ) ( ) ( )tctitctb

tctitm

tctrtz

+=

+=

=

2

A Notion of Spread Spectrum (SS)


DSSS

Model of a direct-sequence spread BPSK system.

( ) ( ) ( )

( ) ( ) ( )titstc

titxty

+=

+= ( ) ( ) ( )

( ) ( ) ( ) ( )

( ) ( ) ( )titcts

titctstc

tytctu

+=

+=

=

2

DSSS With BPSK



DSSS band pass channel transmission.DSSS band pass channel transmission.

PolarityPolarityb(t) at time tb(t) at time t+ + --

+ 0 + 0 ππππππππ-- ππππππππ 00

PolarityPolarityc(t) at c(t) at

time ttime ttransmitter


receiverreceiver


1. Average power of interference j(t) Jammer, ∑∫

−

=

==1

0

2

0

2

b

2)(

T

1J

N

k

k

b

T

jT

dttjb

=

c

be

JT

EerfcP

2

1

c

b

T

TPG =2, Processing Gain: PG= BW (spreaded signal)

BW (unspreaded signal)

= 1/Tc

1/Tb

= Tb

Tc

3.Probability of error:


=

O

be

N

EerfcP

2

1

2oN

22

cO JTN=

bb PTE =

==

J

P

T

T

JT

PT

N

E

c

b

c

b

O

b

O

b

NE

PG

P

J=

4.Antijam characteristics:

BPSK ,

Interference treated as wideband noise of PSD

P—average signal power

( )min

10log10)(._

−=

O

bdBdB

N

EPGmarjammig

Jamming margin,

Minimum value needed to support a minimum prescribed average error probability.


Example: A spread spectrum communication system has following parameters,Information bit duration, = 4.095msPN chip duration, =1µs

Find 1. PG 2. 3. jamming margin

bT

cT

eP

Solution: 40951

095.4===

s

ms

T

TPG

c

b

µ

Required period of PN sequence is N=4095,shift register length =12,

For satisfactory reception, we assume not to exceed eP

510−

10=O

b

N

E For coherent BPSK.

=

O

be

N

EerfcP

2

1= 0.387* 10^-5

1.

2.


( ) ( )10log10)4095log10(._ 1010 −= dBdBmarjammig

=36.1-10=26.1dB.

i.e. ,information bits at the receiver o/p can be i.e. ,information bits at the receiver o/p can be detected reliably even when the noise or interference detected reliably even when the noise or interference at the receiver i/p is up to 409.5 times the received at the receiver i/p is up to 409.5 times the received signal power.signal power.

3.


Direct Sequence Spread Spectrum (DSSS)Direct Sequence Spread Spectrum (DSSS)


Frequency Hopped Spread SpectrumFrequency Hopped Spread SpectrumFHSSFHSS

Carrier hops randomly from one frequency to anotherCarrier hops randomly from one frequency to anotherJamming on one frequency affects only a few bits

i.e. improved resistance to jamming

FH is spread sequentially than instantaneously as in FH is spread sequentially than instantaneously as in DSSS.DSSS.


Advantages

1. Immunity to frequency selective fades.

2. Security.3. Soft capacity.4. Immune to the near-far problem.

Disadvantages:Multi-user interference possible if two or more users simultaneously occupy a frequency channel.

FHSS


Frequency hoped Frequency hoped M M ––aryary FSK TRANSMITTER.FSK TRANSMITTER.


Frequency hoped Frequency hoped M M ––aryary FSK RECEIVER.FSK RECEIVER.


1.1. SLOW frequency hopping(several symbols per hop.)SLOW frequency hopping(several symbols per hop.)

Symbol rate Rs of MFSK is integral multiple of hop rate Rh.

Several symbols are transmitted on each frequency hop.

2. FAST frequency hopping(several hops per symbol)2. FAST frequency hopping(several hops per symbol)

Rh is integral multiple of Rs.

Carrier frequency will change or hop several times during transmission of one symbol.

TWO BASIC CHARACTERIZATION of FHSSTWO BASIC CHARACTERIZATION of FHSS


Consider a slow hop spread spectrum system with binary FSK, two symbols per frequency hop , and a PN sequence generator with outputs with the binary message of 0 1 1 0 1 1 0 1 1 0 0 0 .The message is transmitted using the following PN sequence with k=3: {010,110,101,100,000,101,011,001,001,111,011,001,110,101,101,001,110,001,011,111,100,000.110,110} ,plot the output frequencies for the input message.

001

111

110

101

100

011

010

1/Rs1/Rh

Rs

Fre

quency W

c=8r b

1/Rb

i/p binary data 0 10 1

PN Sequence 001

Time

000fc0

fc1

fc2

fc3

fc4

fc5

fc6

fc7

Carrier frequency

1 01 0111

1 11 1011

0 10 1001

1 01 0110

0 00 0101



Consider a fast hop spread spectrum system with binary FSK, two hops per symbol, and a PN sequence generator with outputs with the binary message of 0 1 1 0 1 1 0 1 1 0 0 0 .The message is transmitted using the following PN sequence : {010,110,101,100,000,101,011,001,001,111,011,001,110,101,101,001,110,001,011,111,100,000.110,110} ,plot the output frequencies for the input message.

001

111

110

101

100

011

010

1/Rs1/Rh

Rs

Frequency Wc=8Rb

1/Rb

i/p binary data 0 10 1

PN Sequence010,110,101,100, 000,101,011,001, 001,111,011,001, 110,101,101,001, 110,001, 011,111,100,000.110,110 001

Time

000fc0

fc1

fc2

fc3

fc4

fc5

fc6

fc7

Carrier frequency

1 01 0 1 11 1 0 10 1 1 01 0 0 00 0






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Spread Spectrum Technique