Spread Spectrum

1

Spread Spectrum
2

DS SS has good interference rejection ,PG is
the parameter that expresses the performance
advantage of SS system over a narrowband
system. Both PG and spread factor N are equal
to Tb/Tc. Hence longer the length of PN sequence
larger will be PG.
3

• Bit rate of binary data entering the transmitter
  input
• is
• 2. The bandwidth of PN sequence c(t) , of main
  lobe
• is Wc

4
Probability of error To calculate probability of
error, we consider output component v of coherent
detector as sample value Of random variable
V Eb is signal energy per bit and Vcj is noise
component Decision rule If detector output
exceeds a threshold of zero volts received bit
is symbol1 else decision is favored for zero.

5

• Average probability of error Pe is nothing but
  conditional
• Probability which depends on random variable Vcj.
  
• As a result receiver makes decision in favor of
  symbol 1
• When symbol 0 transmitted and vice versa
• RV Vcj is sum of N such random variables. Hence
  for
• Large N , the RV Vcj can assume Gaussian
  distribution .
• As mean and variance has already been discussed ,
  
• zero mean and variance JTc/2

6
Click to edit Master title style

Probability of error can be calculated from
simple formula for DS/BPSK system Antijam
Characteristics Consider error probability of
BPSK

• Click to edit Master text styles
• Second level
• Third level
• Fourth level
• Fifth level

6
7

Comparing both probabilities Since bit
energy Eb PTb , P average signal power. We can
express bit energy to noise density ratio as
8
or
The ratio J/P is termed jamming margin Jamming
Margin is expressed in decibels as
Where is minimum bit energy to
noise ratio needed to support a prescribed
average probability of error.
9
Jamming Margin J/P The ratio J/P is figure of
merit that provides a measure of how invulnerable
a system is to interference. It indicates how
much noise power relative to signal power is
required to degrade system performance. The
system with larger J/P has greater noise
rejection capability. J/P decibels is referred
to as Antijam Margin, a safety margin against
interference.
10
Frequency Hop Spread Spectrum

• In DS SS system, PN code is used to modulate BPSK
  
• Signal. Performance of this system is evaluated
  by
• Processing gain, which is function of PN sequence
  length
• If PG is made larger, Chip duration Tc reduces
  further which
• Permits more chips transmission per bit and hence
  
• Bandwidth increases.
• Further there is practical limitation on devices
  to generate
• Larger length PN sequence with large PG which may
  not be
• Sufficient to combat jammer.

11
In conventional FSK system, data symbol modulates
a fixed carrier. In FHSS system, data symbol
modulates carrier whose frequency is pseudo
randomly varied. Jammer is forced to cover a
wider range of spectrum because data modulated
carrier is randomly hopping into different
frequencies. Generally employs M-ary Frequency
shift keying-FH/MFSK , where k log2M
information bits are used to determine which one
of M frequencies is to be transmitted. FH/MFSK
occupies very wide band in order of several giga
Hz range.
12
spread transmit signal
transmitter
narrowband signal
user data
MFSK modulator
Mixer
PN code sequence
frequency synthesizer
PN code Generator
receiver
narrowband signal
received signal
data
MFSK Detector
mixer
PN code generator
frequency synthesizer
13
Frequency Hop Spread Spectrum- Transmitter

• Incoming data is subjected to MFSK modulator.
• Modulated wave and output from digital frequency
• synthesizer are applied to mixer which has
  multiplier
• and filter.
• 3.Filter selects required sum frequency component
  
• k-bit PN sequence drive frequency synthesiser
  which
• hops the carrier over 2k values.
• 4. As a result for range of 2k hops FH/MFSK
  demands
• more BW may be in the order of GHz.
• 5. But coherent detection is possible for single
  hops ,
• as frequency synthesiser is unable to
  maintain phase
• coherence .

14
Frequency Hop Spread Spectrum- Receiver

• Incoming FH/MSK wave is received and de-hopped by
• Mixer .Local PN used is in synchronism with
  the one
• at transmitter.
• 2. Output is then band-pass filtered processed by
  
• Non-coherent MFSK detector which employs
  matched
• filters.
• 3. Estimate of original symbol transmitted is
  obtained by
• selecting largest filter output.

15
Advantages frequency selective fading and
interference is limited to short period simple
implementation uses only small portion of
spectrum at any time Disadvantages not as robust
as DSSS simpler to detect Two versions Fast
Hopping several frequencies hops per
symbol Slow Hopping several symbols transmitted
per frequency hop
16
An individual FH/MFSK tone of shortest duration
is chip The chip rate Rc is given by Rh hop
rate Rs symbol rate
For slow hop, bit rate Rb of data sequence,
symbol rate Rs of MFSK and chip rate Rc are
related as K log2M
17
Assuming jammer has average power J over entire
frequency hopped spectrum of W Hz. It is
equivalent to PSD of AWGN N0/2.
Symbol Energy to noise ratio Jamming Margin
2k


10log102k 3k
in decibels



18
Hopping for 8FSK
19
Fast hop
slow hop
20
Fast hop 3hops/symbol Hop rate Rh is integer
multiple of MFSK symbol rate Rs
bits
1
0
time
bits
10
01
11
01
11
10
slow hop 2bits/hop MFSK Symbol rate Rs is
integer multiple of hop rate Rh
time
21
(No Transcript)
22
Illustration Let us see variation of slow
FH/MFSK signal with time for one complete PN
sequence. The period of PN is 15. Parameters of
FH/MFSK are Number of bits per MFSK symbol K
2 Number of MFSK tones M 2K 4 Length of PN
segment per hop k 3 Total number of hops 2k
8 Let Binary sequence 01111110001001111010 PN
sequence generated is 001110011001001 Given MFSK
tones 4 f000, f101, f210, f311 Data
symbols (tw0 bits) 01,11,11,10,00,10,01,11,10,10
Length of PN per hop 3 001,110,011,001,001
23
Binary sequence 01111110001001111010 PN
sequence generated is 001110011001001
Hop frequency fc
Frequency band
Slow Frequency hopping
Rs
0 Input binary sequence
Rb
Rh
PN sequence
001
110
011
001
24
Fast frequency hopping
25
f3
f3
f3
f3 f2 f1 f0
f2
f2
f1
f1
f0
De-hop pattern
26
Comparison Direct sequence spread spectrum and
Frequency hop spread spectrum
FH signals lower their average power spectral
density by hopping over many channel A DS
spectrum exhibits discrete spectral lines that
are related to the length of the sequence used
for the spreading.
27
When it comes to the relative merits of
(DS) versus frequency hop (FH) spread
spectrum modulation schemes, Choices depend on
the particular implementation scenario
28

• DS signal can achieve higher data rates by
  increasing the modulation complexity or
  increasing the clock rates.
• FH has few options for data rate increases.
• DS, being more power-efficient require less
  transmit power.
• FH system requires a significant boost in
  transmit power,
• FH can be handled with a simple analog
  limiter/discriminator receiver while DS requires
  complex base-band processing.

29

• Commercial Applications
• An example of commercial spread spectrum systems
  are systems that are designed to be used in
  so-called unlicensensed bands, such as the
  Industry, Scientific, Medical (ISM) band around
  2.4 GHz.
• Typical applications are here cordless
  telephones, wireless LANs, and cable replacement
  systems as Bluetooth.
• Since the band is unlicensed, there is no central
  control over the radio resources,
• interference is from other communication systems
  and other electrical and electronic equipment
  (e.g., microwave ovens, radars, etc.). Here the
  jamming is not intentional, but the interference

30
Code-division multiple access systems (CDMA
systems) use spread spectrum techniques to
provide communication to several concurrent
users. CDMA is used in one second generation
(IS-95) and several third generation wireless
cellular systems (e.g., cdma2000 and WCDMA). One
advantage of using jamming-resistant signals in
these applications is that the radio resource
management is significantly reduced.