Download - Spread Spectrum
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.