SPREAD SPECTRUM

THE CONCEPT OF SPREAD

SPECTRUM

Spread Spectrum

Is important form of encoding method designed

for wireless communications.

Can be used to transmit either analog or digital

data using analog signal

Concept or basic idea

To modulate a signal so as to increase significantly

the bandwidth (spread spectrum) of the signals to be

transmitted

Makes jamming and interception harder.

Two approaches, both in use:

Frequency hopping spread spectrum

Direct sequence spread spectrum

SPREAD SPECTRUM

Spreading basically handles Spread Spectrum: also combines signals from different sources to fit into a larger bandwidth though with a different goal of privacy and anti jamming to be achieved.

It is designed to be used in wireless applications LANs and WANs. The challenge is to have air as a medium transmitting information/ signal without interception (by an eavesdropper) or being subject to jamming (by malicious intruder, military operations).

To achieve this spread spectrum spreads original spectrum needed for each station as well as adding redundancy. In that, if the required bandwidth for each station, spread spectrum expands it to Bss in that Bss>>B. Extended bandwidth allows the source to wrap its message in a protective manner for more secure transmission.

Ways in which spread spectrum achieves this:

The bandwidth allocated to each station needs to be by far larger than what is needed so as to enhance redundancy (by adding redundancy, the spread spectrum spreads the original spectrum needed for each station).

Spreading of signal after it is created by the source. That is, expanding of the original bandwidth B to Bss must be done by a process independent of the original bandwidth.

Spread spectrum

Figure 1

General Model of Spread Spectrum System

Figure 2

Pseudorandom Numbers

Generated by a deterministic algorithm using an

initial value called a seed.

Algorithm produces a sequence of numbers

The numbers are not statistically random

But if algorithm good, results pass reasonable tests of

randomness

Starting from an initial seed

Need to know algorithm and seed to predict

sequence

Hence only receiver can decode signal

Spread Spectrum - Advantages

Several advantages can be gained from this

apparent waste of spectrum by this approach

Immunity from noise and multipath distortion

Can hide / encrypt signals

Several users can share same higher bandwidth with

little interference

CDM/CDMA mobile telephones

FREQUENCY-HOPPING SPREAD

SPECTRUM

Frequency hopping spread spectrum

(FHSS)

Frequency Hopping Spread Spectrum:

technique uses M different carrier frequencies

that are modulated by the source signal. At one

moment, the signal modulates one carrier

frequency; at the next moment, the signal

modulates another carrier frequency.

Frequency Hopping Spread Spectrum (FHSS)

Signal is broadcast over seemingly random series

of frequencies

Receiver hops between frequencies in sync with

transmitter

Eavesdroppers hear unintelligible blips

Jamming on one frequency affects only a few bits

Frequency Hopping - Example

Figure 3

FHSS (Transmitter)

Figure 4

FHSS Transmitter

For transmission, binary data is fed into a

modulator using some digital-to-analog encoding

scheme such as Frequency Shift Keying (FSK) or

Binary Phase Shift Keying (BPSK)

The resulting signal is centered on some base

frequency

Pseudonoise (PN) or pseudorandom number,

source serves as an index into a table of

frequencies

Each k bits of the PN source specifies one of the 2k

carrier frequencies

At each successive interval, each k PN bits, a new

carrier frequency is selected.

FHSS Receiver

On reception, the spread spectrum signal is demodulated using the

same sequence of PN-derived frequencies and then demodulated to

produce the output data.

Figure 5

Frequency selection in FHSS

Figure 6

Frequency selection in FHSS

The pattern for this station is 101, 111, 001, 000, 010, all,

100. Note that the pattern is pseudorandom it is repeated

after eight hoppings.

This means that at hopping period 1, the pattern is 101.

The frequency selected is 700 kHz; the source signal

modulates this carrier frequency.

The second k-bit pattern selected is 111, which selects the

900-kHz carrier; the eighth pattern is 100, the frequency is

600 kHz.

After eight hoppings, the pattern repeats, starting from

101 again.

Figure shows how the signal hops around from carrier to

carrier. We assume the required bandwidth of the original

signal is 100 kHz.

FHSS cycles

Figure 7

FHSS cycles

It can be shown that this scheme can accomplish

the previously mentioned goals.

If there are many k-bit patterns and the hopping

period is short, a sender and receiver can have

privacy. If an intruder tries to intercept the

transmitted signal, she can only access a small piece

of data because she does not know the spreading

sequence to quickly adapt herself to the next hop.

The scheme has also an antijamming effect. A

malicious sender may be able to send noise to jam the

signal for one hopping period (randomly), but not for

the whole period.

Bandwidth sharing FDM & FHSS

In FDM, each station uses the

bandwidth, but the allocation is

fixed.

In FHSS, each station uses the

bandwidth, but the allocation

changes hop to hop.

Figure 8

Slow and Fast FHSS

Commonly use multiple FSK (MFSK)

Have frequency shifted every TC seconds

Duration of signal element is TS seconds

Slow FHSS has TC TS

Fast FHSS has TC < TS

FHSS quite resistant to noise or jamming

With fast FHSS giving better performance

Slow MFSK FHSS

Figure 9

Fast MFSK FHSS

Figure 10

DIRECT SEQUENCE SPREAD

SPECTRUM

Direct Sequence Spread Spectrum:

Technique expands the bandwidth of the original

signal. It replaces each data bit with n bits using

a spreading code.

Direct Sequence Spread Spectrum (DSSS)

Each bit is represented by multiple bits using a

spreading code

This spreads signal across a wider frequency

band

Has performance similar to FHSS

DSSS

Figure 11

DSSS example

Figure 12

Direct Sequence Spread Spectrum (Example)

Figure 13

Direct Sequence Spread Spectrum System

Figure 14

DSSS Example Using BPSK

Figure 15

Approximate Spectrum of DSSS Signal

Figure 16

CODE DIVISION MULTIPLE ACCESS

Code Division Multiple Access

(CDMA)

A multiplexing technique used with spread

spectrum

Given a data signal rate D

Break each bit into k chips according to a fixed

chipping code specific to each user

Resulting new channel has chip data rate kd

chips per second

Can have multiple channels superimposed

CDMA - Example

Figure 17

CDMA for DSSS

Figure 18

Summary

Looked at use of spread spectrum techniques:

FHSS

DSSS

CDMA

References:

By Behrouz A. Forouzan, Data communications

and networking, chapter 6.