CHAPTER 9 SPREAD SPECTRUM - ULisboa · SPREAD SPECTRUM • Input is fed into a channel encoder...

Wireless Communication

Networks and Systems 1st edition

Cory Beard, William Stallings

© 2016 Pearson Higher

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CHAPTER 9

SPREAD

SPECTRUM

Spread Spectrum 9-1

SPREAD SPECTRUM

• Input is fed into a channel encoder

– Produces analog signal with narrow bandwidth

• Signal is further modulated using sequence of digits

– Spreading code or spreading sequence

– Generated by pseudonoise, or pseudo-random number

generator

• Effect of modulation is to increase bandwidth of

signal to be transmitted

Spread Spectrum 9-2

9.1 GENERAL MODEL OF SPREAD SPECTRUM DIGITAL

COMMUNICATION SYSTEM Spread Spectrum 9-3

SPREAD SPECTRUM

• On receiving end, digital sequence is used to

demodulate the spread spectrum signal

• Signal is fed into a channel decoder to recover data

Spread Spectrum 9-4

SPREAD SPECTRUM

• What can be gained from apparent waste of spectrum?

– Immunity from various kinds of noise and multipath distortion

– Can be used for hiding and encrypting signals

– Several users can independently use the same higher bandwidth with very little interference

Spread Spectrum 9-5

FREQUENCY HOPING SPREAD

SPECTRUM (FHSS)

• Signal is broadcast over seemingly random series of radio frequencies

– A number of channels allocated for the FH signal

– Width of each channel corresponds to bandwidth of input signal

• Signal hops from frequency to frequency at fixed intervals

– Transmitter operates in one channel at a time

– Bits are transmitted using some encoding scheme

– At each successive interval, a new carrier frequency is selected

Spread Spectrum 9-6

9.2 FREQUENCY HOPPING EXAMPLE

Spread Spectrum 9-7

9.3 FREQUENCY HOPPING SPREAD SPECTRUM SYSTEM

Spread Spectrum 9-8

FREQUENCY HOPING SPREAD

SPECTRUM

• Channel sequence dictated by spreading code

• Receiver, hopping between frequencies in

synchronization with transmitter, picks up message

• Advantages

– Eavesdroppers hear only unintelligible blips

– Attempts to jam signal on one frequency succeed only at

knocking out a few bits

Spread Spectrum 9-9

FHSS USING MFSK

• MFSK signal is translated to a new frequency every

Tc seconds by modulating the MFSK signal with the

FHSS carrier signal

• For data rate of R:

– duration of a bit: T = 1/R seconds

– duration of signal element: Ts = LT seconds

• Tc ≥ Ts - slow-frequency-hop spread spectrum

• Tc < Ts - fast-frequency-hop spread spectrum

Spread Spectrum 9-10

9.4 SLOW-FREQUENCY-HOP SPREAD SPECTRUM USING MFSK

M = 4, K = 2 Spread Spectrum 9-11

9.5 FREQUENCY-HOP SPREAD SPECTRUM USING MFSK M = 4, K = 2

Spread Spectrum 9-12

FHSS PERFORMANCE

CONSIDERATIONS

• Large number of frequencies used

• Results in a system that is quite resistant to

jamming

– Jammer must jam all frequencies

– With fixed power, this reduces the jamming power

in any one frequency band

Spread Spectrum 9-13

HEDY LAMARR (GUSTAV MACHATÝ, “EKSTASE”, 1933)

DIRECT SEQUENCE SPREAD

SPECTRUM (DSSS)

• Each bit in original signal is represented by multiple

bits in the transmitted signal

• Spreading code spreads signal across a wider

frequency band

– Spread is in direct proportion to number of bits used

• One technique combines digital information stream

with the spreading code bit stream using exclusive-

OR or exclusive-NOR (Figure 9.6)

Spread Spectrum 9-15

9.6 EXAMPLE OF DIRECT SEQUENCE SPREAD SPECTRUM

Spread Spectrum 9-16

9.7 DIRECT SEQUENCE SPREAD SPECTRUM SYSTEM

Spread Spectrum 9-17

DSSS USING BPSK

• Multiply BPSK signal,

sd(t) = A d(t) cos(2 fct)

by c(t) [takes values +1, -1] to get

s(t) = A d(t)c(t) cos(2 fct)

• A = amplitude of signal

• fc = carrier frequency

• d(t) = discrete function [+1, -1]

• At receiver, incoming signal multiplied by c(t)

– Since, c(t) x c(t) = 1, incoming signal is recovered

Spread Spectrum 9-18

9.8 EXAMPLE OF DIRECT SEQUENCE SPREAD SPECTRUM USING BPSK

Spread Spectrum 9-19

9.9 APPROXIMATE SPECTRUM OF DIRECT SEQUENCE SPREAD

SPECTRUM SIGNAL Spread Spectrum 9-20

BARKER CODES (E.G., IEEE 802.11)

Known Barker codes

Length Codes

2 +1 −1 +1 +1

3 +1 +1 −1

4 +1 +1 −1 +1 +1 +1 +1 −1

5 +1 +1 +1 −1 +1

7 +1 +1 +1 −1 −1 +1 −1

11 +1 +1 +1 −1 −1 −1 +1 −1 −1 +1 −1

13 +1 +1 +1 +1 +1 −1 −1 +1 +1 −1 +1 −1 +1

CODE-DIVISION MULTIPLE ACCESS

(CDMA)

• CDMA (Code Division Multiple Access)

– all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel

– each sender has a unique random number, the sender XORs the signal with this random number

– the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function

• Disadvantages:

– higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal)

– all signals should have the same strength at a receiver

• Advantages:

– all terminals can use the same frequency, no planning needed

– huge code space (e.g. 232) compared to frequency space

– interferences (e.g. white noise) is not coded

– forward error correction and encryption can be easily integrated

Spread Spectrum 9-22

CODE-DIVISION MULTIPLE ACCESS

(CDMA)

• Basic Principles of CDMA

– D = rate of data signal

– Break each bit into k chips

• Chips are a user-specific fixed pattern

– Chip data rate of new channel = kD

Spread Spectrum 9-23

CDMA EXAMPLE

• If k=6 and code is a sequence of 1s and -1s

– For a ‘1’ bit, A sends code as chip pattern • <c1, c2, c3, c4, c5, c6>

– For a ‘0’ bit, A sends complement of code • <-c1, -c2, -c3, -c4, -c5, -c6>

• Receiver knows sender’s code and performs electronic decode function

• <d1, d2, d3, d4, d5, d6> = received chip pattern

• <c1, c2, c3, c4, c5, c6> = sender’s code

Spread Spectrum 9-24

665544332211 cdcdcdcdcdcddSu

9.10 CDMA EXAMPLE

Spread Spectrum 9-25

CDMA IN THEORY

• Sender A – sends Ad = 1, key Ak = 010011 (assign: “0”= -1, “1”= +1) – sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

• Sender B – sends Bd = 0, key Bk = 110101 (assign: “0”= -1, “1”= +1) – sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

• Both signals superimpose in space – interference neglected (noise etc.) – As + Bs = (-2, 0, 0, -2, +2, 0)

• Receiver wants to receive signal from sender A – apply key Ak bitwise (inner product)

• Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6 • result greater than 0, therefore, original bit was “1”

– receiving B • Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. “0”

Spread Spectrum 9-26

CDMA EXAMPLE

• User A code = <1, –1, –1, 1, –1, 1>

– To send a 1 bit = <1, –1, –1, 1, –1, 1>

– To send a 0 bit = <–1, 1, 1, –1, 1, –1>

• User B code = <1, 1, –1, – 1, 1, 1>

– To send a 1 bit = <1, 1, –1, –1, 1, 1>

• Receiver receiving with A’s code

– (A’s code) x (received chip pattern)

• User A ‘1’ bit: 6 -> 1

• User A ‘0’ bit: -6 -> 0

• User B ‘1’ bit: 0 -> unwanted signal ignored

Spread Spectrum 9-27

CDMA ON SIGNAL LEVEL I

data A

key A

signal A

data key

key

sequence A

Real systems use much longer keys resulting in a larger distance

between single code words in code space.

1 0 1

1 0 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

0 1 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As

CDMA ON SIGNAL LEVEL II

signal A

data B

key B

key

sequence B

signal B

As + Bs

data key

1 0 0

0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1

1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1

Bd

Bk

Bs

As

CDMA ON SIGNAL LEVEL III

Ak

(As + Bs)

* Ak

integrator

output

comparator

output

As + Bs

data A

1 0 1

1 0 1 Ad

CDMA ON SIGNAL LEVEL IV

integrator

output

comparator

output

Bk

(As + Bs)

* Bk

As + Bs

data B

1 0 0

1 0 0 Bd

CDMA ON SIGNAL LEVEL V

comparator

output

wrong

key K

integrator

output

(As + Bs)

* K

As + Bs

(0) (0) ?

9.11 CDMA IN A DSSS ENVIRONMENT

Spread Spectrum 9-33

RAKE RECEIVER

• RAKE receiver

– Multiple versions of a signal arrive more than one

chip interval apart

– RAKE receiver attempts to recover signals from

multiple paths and combine them

• This method achieves better performance than

simply recovering dominant signal and treating

remaining signals as noise

Spread Spectrum 9-34

9.12 PRINCIPLE OF RAKE RECEIVER

Spread Spectrum 9-35

CATEGORIES OF SPREADING

SEQUENCES

• Spreading Sequence Categories

– PN sequences

– Orthogonal codes

• For FHSS systems

– PN sequences most common

• For DSSS systems not employing CDMA

– PN sequences most common

• For DSSS CDMA systems

– PN sequences

– Orthogonal codes

Spread Spectrum 9-36

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

Spread Spectrum 9-37

OVSF (ORTHOGONAL VARIABLE

SPREADING FACTOR) CODING

Spread Spectrum 9-38

1

1,1

1,-1

1,1,1,1

1,1,-1,-1

X

X,X

X,-X 1,-1,1,-1

1,-1,-1,1

1,-1,-1,1,1,-1,-1,1

1,-1,-1,1,-1,1,1,-1

1,-1,1,-1,1,-1,1,-1

1,-1,1,-1,-1,1,-1,1

1,1,-1,-1,1,1,-1,-1

1,1,-1,-1,-1,-1,1,1

1,1,1,1,1,1,1,1

1,1,1,1,-1,-1,-1,-1

SF=1 SF=2 SF=4 SF=8

SF=n SF=2n

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