Multiple Access Techniques Spread Spectrum

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Multiple Access TechniquesSpread Spectrum
MCS.07
Maj JW Paul
Refs lecture notes LFTSP 2003 Dr Smain Amari
lecture notes ACS 2003 Mr Josh Dore
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Quod malum posset futurum
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Review

• Why is AM radio better than FM at night?

4
Review

• Draw a time domain plot for white noise

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Todays Class

• Multiple Access Techniques
• Spread Spectrum in detail

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MultipleAccessTechniques
Time
Frequency
FDMA
Time
Frequency
TDMA
Time
Frequency
CDMA
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Question

• The EM spectrum is a limited resource
• How can we share it?
• Time
• Space
• Frequency
• Polarization
• Spread Spectrum - use a wider bandwidth?

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Multiple Access techniques

• allow many users to simultaneously share a
  communications resource

• Goal
• Time Division Multiple Access (TDMA)
• Space Division Multiple Access (SDMA)
• Frequency Division Multiple Access (FDMA)
• Polarization Division Multiple Access (PDMA)
• Code Division Multiple Access (CDMA)
• aka Spread Spectrum

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Key Issue

• separate the signals at the receiver to extract
  your information
• Two methods
• Do not mix the signals in the first place
• can use space or time (SDMA or TDMA)
• Use distinctive properties of each signal as a
  means to identify
• Frequency spectrum (FDMA)
• Polarization of waves (PDMA)
• code sequence attached to each message (CDMA)

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TDMA Illustration

• Divides the radio spectrum into time slots, and
  in each slot only one user is allowed to either
  transmit or receive

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TDMA Details

• The incoming data from each source are briefly
  buffered and scanned to to form a composite
  digital data stream mc ( t ) .

m1 ( t )
U1
Buffer
m2 ( t )
Frame
Frame
U2
Buffer
mc ( t )
...
1
2
preamble
1
2
N
preamble
N
mN ( t )
information
UN
Time slot
Buffer
Each slot may be empty or occupied. has
preamble guard bits
Scan operation
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TDMA Pros and Cons

• Advantages
• flexible bit rate
• channels may have varying data rates
• efficient use of channels

• Disadvantages
• Synchronization
• must lock on to your time slot (signal
  processing)
• Overhead
• processing required for buffering...

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SDMA Space Division Multiple Access

• Use highly directional Ae
• The receiver selects the beam that provides the
  greatest signal enhancement and interference
  reduction

• Smart antenna systems can adjust their antenna
  pattern to enhance the desired signal, null or
  reduce interference.

Desired Signal Direction
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SDMA Pros and Cons

• Advantages
• BW increases with km2
• Simple system

• Disadvantages
• Restricted Geometry
• terminals in same direction cannot share
• May have unused BW
• if no terminals in given zone, bw not used

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FDMA Illustration

• assigns individual channels or bands to
  individual users, which can only be used by that
  user for the period of the call

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FDMA Pros and Cons

• Advantages
• Simple to implement
• Continuous tx
• minimal overhead bits

• Disadvantages
• Nonlinear Effects
• sharing of Ae causes spreading at saturation
• Inefficient
• once assigned, cannot be reassigned even if not
  in use

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PDMA Polarization Division Multiple Access

• Two methods
• Two antennas with orthogonal polarizations
• an antenna with dual-polarization (SATCOM)
• Each polarization provides one separate channel

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PDMA Pros and Cons

• Advantages
• doubles BW

• Disadvantages
• Large specialized Ae

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Sic Pilum Iactum Est
Literally Thats the way the spear is thrown
Figuratively Why do I always get the shaft
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Spread Spectrum

• CDMA - FHMA - DSMA - SSMA

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Definition - Spread Spectrum

• The transmission bandwidth must be much larger
  than the information bandwidth
• The resulting RF bandwidth is determined by a
  function other than the information being sent

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Spread Spectrum - illustrated
Power Density
Conventional Transmission
PDi
Spread Spectrum Transmission
PDSS
f
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How

• Two main methods
• Frequency Hopped Multiple Access (FHMA)
• Direct Sequence Multiple Access (DSMA)
• THMA does exist, but not common
• Both depend on pseudo random orthogonal codes
  remember part 2 of definition...
• often called pseudo noise

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FHSS Frequency Hopping Multiple Access

• message is "cut" into small "chunks"
• Each chunk is modulated by a different fc
  (determined by pseudo-random code)
• A band pass filter accepts the signals that
  follow the hopping sequence and rejects all other
  requires synchronization
• note - some early systems used short predictable
  patterns

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FHSS - illustrated
Frequency
Frequency
Hop
Tune
Time
Dwell
Time
Time
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DSMA Direct Sequence Multiple Access

• Each bit is chipped
• Example - time domain

0.1 ms
Data
1 bit
0.1 ?s
Chips
1000 chips
Requires much wider bandwidth
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Cross Correlation

• Mathematical process used to determine the
  similarity between two signals

111101011001000
15-bit Code
011110101100100
Received Signal
100011110101100
Modulo-2 sum
Correlation -1/15 (very poor)

• to determine start of code
• to lock onto correct code

Used for despreading
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Pseudo Random Orthogonal...

• Different sequences are said to be orthogonal if
  they do not interfere with one another (ie have
  low cross correlation)
• A sequence is pseudo random if it is orthogonal
  with a time shifted version of itself
• note - this significantly reduces the number of
  codes available ltlt 2 n -1

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Spreading Process
Noise
Info
Info Signal
Baseband Signal
Transmitted (Coded) Signal
Before spreading
After spreading
How can you recover signal lt noise
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SNR gain of spread spectrum

• The ratio of the SNR out to the SNR into the
  demodulator (? spreading factor).

SNRout BWRF

GP
SNRin Rinfo
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Example
Given 1 Mcps PN code
1 kbps information data signal
BW RF 2 MHz
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G 2 x 10 2000 33 dB
p
3
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This means that after de-spreading, signal is 33
dB (2000 times) bigger than the noise.
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CDMA Pros and Cons

• Advantages
• security
• channel capacity
• 4xTDMA, 20xFDMA
• One channel for all
• lt fading in wideband
• LPE, LPI, LPD
• low probability of
• exploitation
• interception
• detection

• Disadvantages
• self jamming
• near far problem
• power issues

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Final

• Most modern systems use a combination of TDMA,
  SDMA, FDMA, CDMA
• Take advantage of best features of each

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Review
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Comparison SDMA/TDMA/FDMA/CDMA
Approach

SDMA

TDMA

FDMA

CDMA

Idea





Terminals





Signal





separation

Advantages





Dis
-

advantages




Comment








From Prof. Dr.-Ing. Jochen Schiller,
http//www.jochenschiller.de/ MC SS02
36
Review

• Why is AM radio better than FM at night?

• FM 100MHz
• AM 0.9MHz

37
Review

• Draw a time domain plot for white noise