Transcript Direct Sequence Spread-Spectrum with Frequency Hopping

Direct Sequence SpreadSpectrum with
Frequency Hopping
What is spread spectrum?
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Methods by which energy generated in a
particular bandwidth is deliberately
spread in the frequency domain, resulting
in a signal with a wider bandwidth.
used for a variety of reasons, including
the establishment of secure
communications, increasing resistance to
natural interference and jamming, and to
prevent detection.
Why do we need this?
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In some situations it is required that a
communication signal be difficult to
detect, and difficult to demodulate even
when detected. Here the word ‘detect’ is
used in the sense of ‘to discover the
presence of’.
In other situations a signal is required that
is difficult to interfere with, or ‘jam’.
Commercial use
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Initial commercial use of spread spectrum
began in the 1980s in the US with three
systems: Equatorial Communications System's
very small aperture (VSAT) satellite terminal
system for newspaper newswire services, Del
Norte Technology's radio navigation system
for navigation of aircraft for crop dusting and
similar applications, and Qualcomm's
OmniTRACS system for communications to
trucks
Spread-spectrum telecommunications
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a technique in which a (telecommunication)
signal is transmitted on a bandwidth
considerably larger than the frequency content
of the original information.
a signal structuring technique that employs
direct sequence, frequency hopping or a hybrid
of these, which can be used for multiple access
and/or multiple functions.
This technique decreases the potential
interference to other receivers while achieving
privacy.
Forms of spread spectrum
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Frequency-hopping spread spectrum(FHSS)
direct-sequence spread spectrum (DSSS)
time-hopping spread spectrum (THSS)
chirp spread spectrum (CSS)
and combinations of these techniques
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Each of these techniques employs pseudorandom
number sequences — created using pseudorandom
number generators .
What is pseudorandom number sequences ?
>is a sequence of numbers that has been computed
by some defined arithmetic process but is effectively
a random number sequence for the purpose for
which it is required.
To determine and control the spreading pattern of the
signal across the allotted bandwidth
Benefits
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Resistance to intended or unintended
jamming.
Sharing of a single channel among
multiple users.
Determination of relative timing between
transmitter and receiver.
Uses
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The United States GPS, Russian Glonass, and
European Galileo satellite navigation systems.
CDMA cellular phones.
Cordless phones operating in the 2.4 and 5.8
GHz bands.
The 802.11 and 802.11b Wi-Fi standards. (The
faster modes in 802.11g use OFDM, not spread
spectrum, although it can fall back to the
slower 802.11b modes.)
ZigBee / 802.15.4
Automatic meter reading
Spread Spectrum modulation
techniques present two major
advantages for Wireless Local
Area Networks (WLAN):
(ranges allocated for use with Spread Spectrum
technology is 2.4 GHz to 2.4835 GHz)
Low power density relates to the fact that
the transmitted energy is spread over a
wide band, and therefore, the amount of
energy per specific frequency is very low.
The effect of the low power density of the
transmitted signal is that such a signal
will not disturb (interfere with) the
activity of other systems' receivers in the
same area.
Redundancy relates to the fact that the
message is (or may be) present on
different frequencies from where it may
be recovered in case of errors. The effect
of redundancy is that Spread Spectrum
systems present a high resistance to
noises and interference, being able to
recover their messages even if noises are
present on the medium.
A Comparison of Frequency
Hopping and Direct Sequence
Spread Spectrum
Why we need to compare the performance of
this technology? (WLAN’s)
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possibility to collocate systems
noise and interference immunity
operation in environments generating radio
reflections
data transfer capacity (throughput)
size
power consumption (relevant for battery based
note books)
price.
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 wideband signal.
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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.
DS Modulation
Frequency Hopping spread
spectrum
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It is the repeated switching of frequencies
during radio transmission, often to
minimize the effectiveness of "electronic
warfare" - that is, the unauthorized
interception or jamming of
telecommunications.
It also is known as frequency- hopping
code division multiple access (FHCDMA).
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Transmitting on one frequency for a
certain time, then randomly jumping to
another, and transmitting again.
FH-CDMA devices use less power and
are generally cheaper, but the
performance of DS-CDMA systems is
usually better and more reliable.
The biggest advantage of frequency
hopping lies in the coexistence of several
access points in the same area, something
not possible with direct sequence.
FH Modulation
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Frequency hopping has two benefits. Electrical
noise—random electromagnetic signals which
are not part of any communications signal—
will only affect a small part of the signal. Also,
the effects of any other forms of radio
communications operating in narrow bands of
the spectrum will be minimized. Any such
interference that occurs will result in only a
slightly reduced quality of voice transmission,
or a small loss of data. Since data networks
acknowledge successful receipt of data, any
missing pieces will trigger a request to
transmit the lost data.
Hybrid DS/FH Spread Spectrum
Communication System
Hybrid DS/FH Spread Spectrum
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The DS/FH Spread Spectrum technique is
a combination of direct-sequence and
frequency-hopping. One data bit is
divided over frequency-hop channels
(carrier frequencies). In each frequencyhop channel one complete PN-code of
length is multiplied with the data signal
(see figure, where is taken to be 5).
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As the FH-sequence and the PN-codes
are coupled, an address is a combination
of an FH-sequence and PN-codes. To
bound the hit-chance (the chance that two
users share the same frequency channel in
the same time) the frequency-hop
sequences are chosen in such a way that
two transmitters with different FHsequences share at most two frequencies
at the same time (time-shift is random).
Hybrid System: DS/(F)FH
Additional
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CDMA use technique Spread Spectrum
How spread spectrum generated
Additional
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occupies a larger bandwidth than
necessary
use of a code which is independent of the
transmitted data
Additional
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
Additional
Frequency Hopping
Additional
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Disadvantage:
 a high processing-gain is hard. There is
need for a frequency-synthesizer able
perform fast-hopping over the carrierfrequencies.