Lecture02 Spread Spectrum Communication By

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Lecture02 Spread Spectrum Communication By

• Engr. Muhammad Ashraf Bhutta

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Spread-Spectrum Communication

• Since 1940 is in use for military purposes
• excellent immunity to interference and
  intentional jamming
• This technique, as the name implies, spreads the
  signal over a broader spectrum of frequencies
  than is usual
• By using a smaller portion of a greater
  bandwidth, less interference is produced between
  competing signals

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Types of Spread-Spectrum Systems

• There are Three important types of
  spread-spectrum systems
• Frequency-hopping
• Direct-sequence
• Time hoping

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Frequency-Hopping Systems

• Frequency-hopping systems are the simpler of the
  three systems available
• A frequency generator is used that generates a
  carrier that changes frequency many times a
  second according to a programmed sequence of
  channels known as pseudo-random (PN) noise
  sequence
• It is called this because if the sequence is not
  known, the frequencies appear to hop about
  unpredictably
• Two Types I) Fast FH ii) Slow FH

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Direct-Sequence Systems

• The data to be transmitted is combined with the
  PN
• Direct-sequence systems inject pseudo-random
  noise (PN) into the bit stream that has a much
  higher rate than the actual data to be
  communicated
• The PN bits are inverted when real data is
  represented by a one and leave the bit stream
  unchanged when a data zero is transmitted
• The extra bits transmitted this way are called
  chips
• Most direct-sequence systems use a chipping rate
  of at least ten times the bit rate

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Direct-Sequence Spectrum

• The use of high-speed PN sequence results in an
  increase in the bandwidth of the signal,
  regardless of the modulation scheme used to
  encode the signal

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Reception of Spread-Spectrum Signals

• For a frequency-hopped signal, a conventional
  narrowband receiver is needed that hops in the
  same way and is synchronized to the transmitter
• The type of receiver used for spread-spectrum
  signals depends upon how the signal is generated
• One way to synchronize the signals is to transmit
  a tone on a prearranged channel at the start of
  each transmission before it begins hopping
• A more reliable method is to for the transmitter
  to visit several channels in a prearranged order
  before beginning a normal transmission

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Reception of Direct-Sequence Spread-Spectrum

• Direct-sequence spread-spectrum transmissions
  require a wideband receiver with autocorrelation
  incorporated into it
• Autocorrelation involves multiplying the the
  received signal by a signal generated at the
  receiver from the PN code
• When the input signal corresponds to the PN code,
  the output will be large at other times, the
  output will be small

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What is a CDMA?
Code division Multiple Access is a digital
technology pioneered by QUALCOMM that provides
crystal clear voice quality in new generation
wireless communication systems
CDMA provides better and most cost
effective Voice quality Privacy System
capacity Flexibility. SMS E-mail Internet
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Main Issue in a Communication System
Capacity Delay Error detection/Correction
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Transmission techniques
CDMA TDMA FDMA
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Classification of wireless Communication systems
Simplex( one direction e.g paging) Half-duplex(on
e way Comm.same frequency for TX and Rx , Push to
talk) Full Duplex i)FDD( two separate
Channel) ii)TDD(Same channel on different TS)
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Features and Advantages of CDMA
Universal Frequency Reuse Fast and accurate
Power Control Rake Receiver CDMA Hand off
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FDMA/TDMA Frequency Reuse Frequency Reuse
A reuse pattern of 7 (N7) is common in
FDMA/TDMA
Cellular Systems
Only
1/7 of a carrier's frequency allocation
is used in any one cell
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FDMA/TDMA Frequency Reuse
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CD CDMA MA Frequency Reuse
The principal attribute of a CDMA system is that
all subscribers can use the same
Universal frequency reuse applies
not only to users in the same cell,
but also to those in all other cells
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CDMA Frequency Reuse
Universal frequency reuse completely
eliminates the need for frequency planning
and gives
considerable increase in system capacity
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CDMA Power Control
without
CDMA will not work
an effective power control
because of the
near-far problem
Near far problem arises when
near a cell
jams
that is distant from the cell
a user
(assuming both are transmitting at the same power)
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Power Control
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Power Control
CDMA capacity is maximised
if all users are controlled
so that
their signals reach the base station
at approximately the same power level
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Power Control
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Power Control
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Direct Sequence Spread Spectrum
Direct Sequence
Spread Spectrum
CDMA
is the technique used in
Cellular Systems
narrowband less than 10 KHz
The information signal is
The energy from this narrowband signal is spread
over a
much larger bandwidth
by multiplying the information signal by a
wideband
spreading code
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Direct Sequence Spread Spectrum
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CDMA Rake Receiver
Signals sent over the air can take either
a direct path to the receiver
or
they can bounce off objects
and then travel to the receiver
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Multipaths
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Multipaths
multi-paths
These different paths called
can result in the receiver getting
several versions of the same signal
but at
slightly different times
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Multipaths
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CDMA RAKE Receiver
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CDMA RAKE Receiver
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CDMA RAKE Receiver
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CDMA Soft Handoff
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Soft Handoff
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Soft Handoff
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Soft Handoff Advantages
Soft handoff has several advantages
Fewer dropped calls
Less mobile transmit power
Increased Capacity
Increased Coverage
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Softer Handoff
Softer Handoff is a handoff between
two sectors of the
same cell