Spread Spectrum Techniques

1
AJAL.A.J
Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM
MAIL: ec2reach@gmail.com

08/01/14 2
Spread Spectrum
Spread spectrum is a communication technique that
spreads a narrowband communication signal over a wide range
of frequencies for transmission then de-spreads it into the
original data bandwidth at the receive.
Spread spectrum is characterized by:
wide bandwidth and
low power
Jamming and interference have less effect on Spread
spectrum because it is:
Resembles noise
Hard to detect
Hard to intercept

4
Spread Spectrum System Concept
DATA
SOURCE
JAMMER
L
I
N
K
S
E
L
E
C
T
O
R
TR
#1
TR
#2
TR
#K
COMMON CLOCKS
AND KEYS
RCV #1
RCV #2
RCV #K
D
I
V
E
R
SI
T
Y
C
O
M
B
I
N
E
R
DATA
USER
( )1n t  
( )2n t  
( )Kn t  
( )km t
( )2m t
( )1m t
( )J t

08/01/14 5
(The radio carrier signal is “spread out” on a specific channel )
Spread Spectrum
Frequency Hopping (FHSS)
( The radio carrier hops around the band. )
Direct Sequence (DSSS)

08/01/14 6
Rogoff 's noise wheel model used in spread spectrum
communication systems

08/01/14 7
Rogoff 's noise wheel model

8
Spectra in the Direct Sequence Spread Spectrum System

08/01/14 9
Direct Sequence Spread Spectrum
 Modulation technique ,Also known as
direct sequence code division multiple
access (DS-CDMA)
 The name 'spread spectrum' comes from the
fact that the carrier signals occur over the
full bandwidth (spectrum) of a device's
transmitting frequency.
 A RF carrier and pseudo-random pulse train
are mixed to make a noise like wide-band
signal.

08/01/14 10
DSSS (Direct Sequence Spread Spectrum)
• XOR the signal with pseudonoise (PN) sequence (chipping
sequence)
• Advantages
– reduces frequency selective
fading
– in cellular networks
• base stations can use the
same frequency range
• several base stations can
detect and recover the signal
• But, needs precise power control
user data
chipping
sequence
resulting
signal
0 1
0 1 10 1 0101 0 0 1 11
XOR
0 1 10 0 1011 0 1 0 01
=
Tb
Tc

08/01/14 11
user data
m(t)
chipping
sequence, c(t)
X
DSSS (Direct Sequence Spread
Spectrum)
modulator
radio
carrier
Spread spectrum
Signal y(t)=m(t)c(t) transmit
signal
Transmitter
demodulator
received
signal
radio
carrier
X
Chipping sequence,
c(t)
Receiver
integrator
products
decision
data
sampled
sums
correlator

12
Direct sequence contrasts with the other
spread spectrum process, known as
frequency hopping spread spectrum, in
which a broad slice of the bandwidth
spectrum is divided into many possible
broadcast frequencies.
In general, frequency-hopping devices use
less power and are cheaper, but the
performance of DS-CDMA systems is usually
better and more reliable.

08/01/14 14
Spread-spectrum
communications

08/01/14 15
DSSS Barker Code modulation
Source: Intersil

08/01/14 17
Direct Sequence
Data signal multiplied by Pseudo Random
Noise Code(PN Code)
• Low cross-correlation value
• Anti-jamming
• Main problem: Near-Far effect
–In cellular, it can do power control by BS
–In non-cellular, it need Frequency
Hopping

08/01/14 18
Detecting DS/SS PSK Signals
X
Bipolar, NRZ
m(t)
PN
sequence, c(t)
X
sqrt(2)cos(ωct + θ)
Spread spectrum
Signal y(t)=m(t)c(t) transmit
signal
transmitter
X
received
signal
X
c(t)
receiver
integrator
z(t)
decision
data
sqrt(2)cos(ωct + θ)
LPF
w(t)
x(t)

08/01/14 19
Optimum Detection of DS/SS PSK
• Recall, bipolar signaling (PSK) and white noise
give the optimum error probability
• Not effected by spreading
– Wideband noise not affected by spreading
– Narrowband noise reduced by spreading

08/01/14 20
Signal Spectra
• Effective noise power is channel noise
power plus jamming (NB) signal power
divided by N
10Processing Gain 10logss ss b
c
B B T
N
B B T
 
= = = ÷
 
Tb
Tc

08/01/14 21
Multiple Access Performance
• Assume K users in the same frequency
band,
• Interested in user 1, other users interfere
4
1
3
5
2
6

08/01/14 22
Comparison of Spectrum
30 kHz
Analog Cellular Voice Channel
6 MHz
TV Channel
28 - 100 MHz
Unlicensed Spread Spectrum Devices
1000 - 3000 MHz Ultra-Wideband Devices

08/01/14 23
A DSSS generator:
• To generate a spread spectrum signal one
requires:
1. A modulated signal somewhere in the
RF spectrum
2. A PN sequence to spread it

08/01/14 24
original RF carrier = ω0
ωs = is the sequence clock

08/01/14 29
Pseudo-Noise (PN) sequence
• A pseudo-noise (PN) sequence is a
periodic binary sequence with a noise-like
waveform.
• It is generated by using linear feedback
shift register.
• The main advantages of using PN
sequences are
antijamming,
multipath protection,
multiple access,
message privacy,
identification … etc.

08/01/14 30
PN Sequence Generation
• Codes are periodic and generated by a shift register and
XOR
• Maximum-length (ML) shift register sequences, m-stage
shift register, length: n = 2m
– 1 bits
R(τ)
-1/n
Tc
τ −>
-nTc
nTc
+
Output

31RF - Cellcom courseDr. Moshe Ran08/01/14
Generating PN Sequences
• Take m=2 =>L=3
• cn=[1,1,0,1,1,0, . . .],
usually written as
bipolar cn=[1,1,-1,1,1,-1,
. . .]
m Stages connected
to modulo-2 adder
2 1,2
3 1,3
4 1,4
5 1,4
6 1,6
8 1,5,6,7
+
Output
( )



−≤≤−
=
=
= ∑=
+
11/1
01
1
1
LmL
m
cc
L
mR
L
n
mnnc

Problems withProblems with mm-sequences-sequences
Cross-correlations with otherCross-correlations with other mm-sequences-sequences
generated by different input sequences can begenerated by different input sequences can be
quite highquite high
Easy to guess connection setup in 2Easy to guess connection setup in 2mm samplessamples
so not too secureso not too secure
In practice, Gold codes or Kasami sequencesIn practice, Gold codes or Kasami sequences
which combine the output of m-sequences arewhich combine the output of m-sequences are
used.used.

08/01/14 38
Figure - Signals used to modulate the carrier in FHSS and
DSSS (Dwell time in FHSS is represented)

08/01/14 39
Systems Behavior
• The following issues will be studied in
parallel for FHSS and DSSS systems:
1.- Systems Collocation
2.- Noise and Interference Immunity
3.- The Near / Far problem
4.- Multipath Immunity
5.- Time and frequency diversity
6.- Throughput
7.- Security
8.- Bluetooth interference

08/01/14 40
1.- Systems Collocation
• The issue: How many independent
systems may operate simultaneously
without interference?
In DSSS systems, collocation could be based
on the use of different spreading codes (sequences(
for each active system

08/01/14 41
2.- Noise and Interference
Immunity
• The issue: Capability of the system to operate
without errors when other radio signals are
present in the same band.
• FHSS systems operate with SNR (Signal to
Noise Ratio) of about 18 dB.
• DSSS systems, because of the more
efficient modulation technique used (PSK), can
operate with SNR as low as 12 dB.

08/01/14 42
3.- Near / Far problem
• The issue: The problems generated to a
FH / D SSS receiver by other active
transmitters located in its proximity, are
known as Near / Far problems.

08/01/14 43
4.- Multipath
• The issue: Environments with reflective
surfaces (such as buildings, office walls,
etc.) generate multiple possible paths
between transmitter and receiver and
therefore the receiver receives multiple
copies of the original (transmitted) signal.

08/01/14 44
5.- Time and frequency diversity
• Both DSSS and FHSS retransmit lost
packets, until the receiving part
acknowledges correct reception. A packet
could be lost because of noises
FHSS systems use “frequency diversity" .
)packets are retransmitted on different frequencies /
hops(.

08/01/14 45
6.- Throughput
• The issue: What amount of data is actually
carried by the system (measured in bps).
6.1.- Single system throughput
6.2.-Aggregate throughput of collocated systems

08/01/14 46
7.- Security
• The issue: Protecting the transmission
against eavesdropping

08/01/14 47
8.- Bluetooth interference
• FHSS are significantly less sensitive to
Bluetooth interference.

48
Frequency Hopping Vs. Direct Sequence
FH systems use a radio carrier that “hops” from
frequency to frequency in a pattern known to both
transmitter and receiver
– Easy to implement
– Resistance to noise
– Limited throughput (2-3 Mbps @ 2.4 GHz)
DS systems use a carrier that remains fixed to a
specific frequency band. The data signal is spread
onto a much larger range of frequencies (at a
much lower power level) using a specific
encoding scheme.
– Much higher throughput than FH (11 Mbps)
– Better range
– Less resistant to noise (made up for by redundancy – it
transmits at least 10 fully redundant copies of the
original signal at the same time)

Spread Spectrum Techniques

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