spread spectrum communication systems

7/28/2019 Spread Spectrum communication systems

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Spread Spectrum Dickerson EE422 1

Todays Schedule

Reading: Lathi 9.2 (Spread Spectrum Intro) Quiz 3 Mini-Lecture 1:

Spread spectrum


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Introduction to Spread Spectrum

Problems such as capacity limits, propagationeffects, synchronization occur with wireless

systems Spread spectrum modulation spreads out themodulated signal bandwidth so it is muchgreater than the message bandwidth

Independent code spreads signal attransmitter and despreads signal at receiver


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Multiplexing in 4 dimensions space (s i) time (t)

frequency (f) code (c)

Goal: multiple useof a shared medium

Important: guard spaces needed!

s 2

s 3

s 1

Multiplexing

f

t

c

k2 k3 k4 k5 k6 k1

f

tc

f

tc

channels k i


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Frequency multiplex

Separation of spectrum into smaller frequency bands Channel gets band of the spectrum for the whole time

Advantages: no dynamic coordination needed works also for analog signals

Disadvantages: waste of bandwidth

if traffic distributed unevenly inflexible guard spaces

k3 k4 k5 k6

f

t

c


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f

t

ck2 k3 k4 k5 k6 k1

Time multiplex

Channel gets the whole spectrum for a certainamount of time

Advantages: only one carrier in the

medium at any time throughput high even

for many users

Disadvantages: precise

synchronizationnecessary


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f

Time and frequency multiplex

A channel gets a certain frequency band for acertain amount of time (e.g. GSM)

Advantages: better protection against tapping protection against frequency

selective interference higher data rates compared to

code multiplex Precise coordination

requiredt

c

k2 k3 k4 k5 k6 k1


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Code multiplex

Each channel has unique code All channels use same spectrum at same time Advantages:

bandwidth efficient no coordination and synchronization good protection against interference

Disadvantages: lower user data rates more complex signal regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6 k1

f

t

c


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Spread Spectrum Technology

Problem of radio transmission: frequencydependent fading can wipe out narrow bandsignals for duration of the interference

Solution: spread the narrow band signal into a broad band signal using a special code

detection atreceiver

interference spread

signal

signal

spreadinterference

f f

power power


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Spread Spectrum Technology

Side effects: coexistence of several signals without

dynamic coordination

tap-proof Alternatives: Direct Sequence (DS/SS),

Frequency Hopping (FH/SS)

Spread spectrum increases BW of messagesignal by a factor N , Processing Gain

10Processing Gain 10 log ss ss B B N

B B


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Effects of spreading andinterference

P

f i)

P

f ii)

sender P

f iii)

P

f iv)

receiver

f v)

user signalbroadband interferencenarrowband interference

P


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Spreading and frequencyselective fading

frequency

channelquality

1 23

4

5 6

Narrowbandsignal

guard space

22222

frequency

channelquality

1

spread

spectrum

narrowbandchannels

spread spectrumchannels


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DSSS (Direct Sequence SpreadSpectrum) I

XOR the signal with pseudonoise (PN) sequence(chipping sequence)

Advantages reduces frequency selective

fading in cellular networks

base stations can use thesame frequency range

several base stations candetect and recover the signal

But, needs precise power control

user data

chippingsequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

Tb

Tc


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DSSS (Direct Sequence SpreadSpectrum) II

Xuser datam(t)

chippingsequence, c(t)

modulator

radiocarrier

Spread spectrumSignal y(t)=m(t)c(t) transmit

signal

transmitter

demodulator receivedsignal

radiocarrier

X

Chipping sequence,

c(t)

receiver

integrator

products

decisiondata

sampled

sums

correlator


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DS/SS Comments III

Pseudonoise(PN) sequence chosen so thatits autocorrelation is very narrow => PSD

is very wide Concentrated around t < T c Cross- correlation between two users codes is

very small


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DS/SS Comments IV

Secure and Jamming Resistant Both receiver and transmitter must know c(t)

Since PSD is low, hard to tell if signal present Since wide response, tough to jam everything

Multiple access If c i(t) is orthogonal to c j(t), then users do not interfere

Near/Far problem Users must be received with the same power


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FH/SS (Frequency HoppingSpread Spectrum) I

Discrete changes of carrier frequency sequence of frequency changes determined via PN sequence

Two versions Fast Hopping : several frequencies per user bit (FFH) Slow Hopping : several user bits per frequency (SFH)

Advantages frequency selective fading and interference limited to short period uses only small portion of spectrum at any time

Disadvantages not as robust as DS/SS simpler to detect


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FHSS (Frequency HoppingSpread Spectrum) II

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

Tb

0 1 1 t

f

f 1

f 2

f 3

t

Td

f

f 1

f 2

f 3

t

Td

Tb: bit period T d: dwell time


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FHSS (Frequency HoppingSpread Spectrum) III

modulator user data

hoppingsequence

modulator

narrowbandsignal

Spread transmitsignal

transmitter

receivedsignal

receiver

demodulator data

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer


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Applications of Spread Spectrum

Cell phones IS-95 (DS/SS)

GSM Global Positioning System (GPS) Wireless LANs

802.11b


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Performance of DS/SS Systems

Pseudonoise (PN) codes Spread signal at the transmitter

Despread signal at the receiver Ideal PN sequences should be

Orthogonal (no interference) Random (security) Autocorrelation similar to white noise (high at

t =0 and low for t not equal 0)


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PN Sequence Generation

Codes are periodic and generated by a shiftregister and XOR

Maximum-length (ML) shift register

sequences, m- stage shift register, length: n =2 m 1 bits R( t )

-1/n Tc

t ->

-nTc nTc

+Output


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Generating PN Sequences

Take m=2 =>L=3 cn=[1,1,0,1,1,0, . . .],

usually written as bipolar c n=[1,1,-1,1,1,-1, . . .]

m Stages connected tomodulo-2 adder

2 1,2

3 1,3

4 1,4

5 1,46 1,6

8 1,5,6,7

+Output

-- 11/1

01

1

1

Lm L

m

cc L

m R L

nmnnc


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Problems with m -sequences

Cross-correlations with other m -sequencesgenerated by different input sequences can

be quite high Easy to guess connection setup in 2 msamples so not too secure

In practice, Gold codes or Kasamisequences which combine the output of m-sequences are used.


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Detecting DS/SS PSK Signals

XBipolar, NRZm(t)

PNsequence, c(t)

X

sqrt(2)cos (w ct + q )

Spread spectrumSignal y(t)=m(t)c(t) transmit

signal

transmitter

X

receivedsignal

X

c(t)

receiver

integrator

z(t)

decision data

sqrt(2)cos (w ct + q )

LPF

w(t)

x(t)


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Optimum Detection of DS/SSPSK

Recall, bipolar signaling (PSK) and whitenoise give the optimum error probability

Not effected by spreading Wideband noise not affected by spreading Narrowband noise reduced by spreading

2 bb

E P Q


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Signal Spectra

Effective noise power is channel noise power plus jamming (NB) signal power divided by N

10Processing Gain 10 log ss ss b

c

B B T N

B B T

T b

Tc


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Multiple Access Performance

Assume K users in the same frequency band,

Interested in user 1, other users interfere4

13

5

2

6


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Signal Model

Interested in signal 1, but we also getsignals from other K-1 users:

At receiver,

2 cos2 cos

k k k k k c k k

k k k k c k k k c k

x t m t c t t m t c t t

t t w t q t t w q w t

- - - - - -

12

K

k k

x t x t x t


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Interfering Signal

After mixing and despreading (assume t 1=0)

After LPF

After the integrator-sampler

1 12 cos cosk k k k k c k c z t m t c t c t t t t t w w q - -

1 1cosk k k k k k w t m t c t c t t t q - - -

1 10cosbT

k k k k k k I m t c t c t dt q t t - - -


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At Receiver

m(t) =+/-1 (PSK), bit duration T b Interfering signal may change amplitude at t k

At User 1:

Ideally, spreading codes are Orthogonal:

1 1 1 0 10cos k bk T

k k k k k k I b c t c t dt b c t c t dt t

t q t t -

- - -

1 1 1 10bT I m t c t c t dt

1 1 10 0 0b bT T

k k c t c t dt A c t c t dt t -


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Multiple Access Interference(MAI)

If the users are assumed to be equal power interferers, can be analyzed using thecentral limit theorem (sum of IID RVs)

1

1 3 2b

b

P Q

K N E

-


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Example of PerformanceDegradation

N=8 N=32


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Near/Far Problem (I)

Performance estimates derived using assumptionthat all users have same power level

Reverse link (mobile to base) makes thisunrealistic since mobiles are moving Adjust power levels constantly to keep equal

1k


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Near/Far Problem (II)

K interferers, one strong interfering signaldominates performance

Can result in capacity losses of 10-30%

1

( ) (1) (1)2

1

3 2b

K k b b bk

P Q

E E N E


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Multipath Propagation


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RAKE Receiver

Received signal sampled at the rate 1/Ts> 2/Tc for detectionand synchronization

Fed to all M RAKE fingers. Interpolation/decimation unit provides a data stream on chiprate 1/Tc

Correlation with the complex conjugate of the spreadingsequence and weighted (maximum-ratio criterion)summationover one symbol


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RAKE Receiver

RAKE Receiver has to estimate: Multipath delays

Phase of multipath components Amplitude of multipath components Number of multipath components

Main challenge is receiver synchronizationin fading channels


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S d S Di k EE422 38

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Student Presentations 13.3 on Optimal Receivers for FSK and

MSK systems

spread spectrum communication systems

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