1

AJAL.A.J

Assistant Professor –Dept of ECE,

Federal Institute of Science And Technology (FISAT) TM 

MAIL: ec2reach@gmail.com

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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

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Spread Spectrum System Concept

DATASOURCE

JAMMER

LINK

SELECTOR

TR#1

TR#2

TR#K

COMMON CLOCKS AND KEYS

RCV #1

RCV #2

RCV #K

DIVERSITY

COMBINER

DATA

USER

1n t

2n t

Kn t

km t

2m t

1m t

J t

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(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)

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 Rogoff 's noise wheel model used in spread spectrum communication systems

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Rogoff 's noise wheel model

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Spectra in the Direct Sequence Spread Spectrum System

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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.

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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

resultingsignal

0 1

0 1 10 1 0101 0 0 1 11

XOR

0 1 10 0 1011 0 1 0 01

=

Tb

Tc

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user datam(t)

chippingsequence, c(t)

X

DSSS (Direct Sequence Spread Spectrum)

modulator

radiocarrier

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

signal

Transmitter

demodulator

receivedsignal

radiocarrier

X

Chipping sequence, c(t)

Receiver

integrator

products

decision

datasampledsums

correlator

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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.

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DS Modulation

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Spread-spectrum communications

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DSSS Barker Code modulation

Source: Intersil

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DSSS properties

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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

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Detecting DS/SS PSK Signals

XBipolar, NRZm(t)

PNsequence, c(t)

X

sqrt(2)cos(ct + )

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

signal

transmitter

X

receivedsignal

X

c(t)

receiver

integrator

z(t)

decisiondata

sqrt(2)cos(ct + )

LPF

w(t)

x(t)

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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

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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 TN

B B T

Tb

Tc

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Multiple Access Performance

• Assume K users in the same frequency band,

• Interested in user 1, other users interfere

4

13

5

2

6

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Comparison of Spectrum

30 kHzAnalog Cellular Voice Channel

6 MHzTV Channel

28 - 100 MHzUnlicensed Spread Spectrum Devices

1000 - 3000 MHz Ultra-Wideband Devices

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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

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original RF carrier = ω 0

ω s = is the sequence clock

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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

The most widely used PN sequence is the maximum-length shift register sequence or m-sequence.

The other PN sequences are Gold sequence and

Kasami sequence.

antijamming,multipath protection, multiple access, message privacy, identification … etc.

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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()

-1/n Tc

-nTcnTc

+Output

31RF - Cellcom course\Dr. Moshe Ran

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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, . . .]

mStages connected to modulo-2 adder

21,2

31,3

41,4

51,4

61,6

81,5,6,7

+Output

11/1

01

1

1

LmL

m

ccL

mRL

nmnnc

Problems with Problems with mm-sequences-sequences

Cross-correlations with other Cross-correlations with other mm-sequences -sequences generated by different input sequences can be generated by different input sequences can be quite highquite high

Easy to guess connection setup in 2Easy to guess connection setup in 2m m samples samples so not too secureso not too secure

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

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Figure - Signals used to modulate the carrier in FHSS and DSSS (Dwell time in FHSS is represented )

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Systems Behavior• The following issues will be studied in parallel

for FHSS and DSSS systems:

1.- Systems Collocation2.- Noise and Interference Immunity 3.- The Near / Far problem4.- Multipath Immunity5.- Time and frequency diversity6.- Throughput7.- Security8.- Bluetooth interference

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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

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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.

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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.

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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.

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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.)

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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

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7.- Security

• The issue: Protecting the transmission against eavesdropping

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8.- Bluetooth interference

• FHSS are significantly less sensitive to Bluetooth interference.

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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)

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