PROJECT%20WORK%20ON%20WIRELESS%20NETWORKS

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PROJECT WORK ON WIRELESS NETWORKS      TOPIC   
MULTI-PATH FADING REDUCTION METHODS/TECHNIQUES 
   
SUPERVISOR PROF. LIJUN
QIAN              
STUDENT NANA K. AMPAH FALL, 2003

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NATURE OF PROBLEM

• Multi-path/multiple path fading
• One of the major challenges of wireless
  communication
• Its an effect of small-scale fading just like
  mobility
• Small-scale fading rapid fluctuation of radio
  signals over a short period of time or travel
  distance
• Effects of multi-path fading on signal
  transmission
• Presence of impulse noise over copper
  twisted-pair impedes smooth application of
  multi-carrier modulation techniques (ie.
  High-speed applications)
• Fading multi-path delay spread, and inter-symbol
  interference greatly deteriorate the performance
  (BER) of personal wireless communication
  channels/systems (PWCS)
• Reduces the transmission performance (reception)
  in sector division multiple access communication
  systems (SDMACS)

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NATURE OF PROBLEM CONTD

• Mechanisms of reflection diffraction and
  scattering from walls, ceilings, edges, corners
  or floor surfaces give rise to additional
  propagation paths beyond the direct optical
  line-of-sight (LOS) path between transmitter and
  receiver in indoor Police National Computer
  Systems
• Telecommunication Engineers are focused to come
  out with ways of combating the above problems

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MAIN OUTLINE OF PROJECT WORK

• Look for five methods for combating multi-path
  fading
• Analyze each of the five methods
• Compare all the five methods and choose the best
  one
• Verify whether some of the five methods can be
  applied together to achieve improved performance
• Make suggestions for improving existing methods

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IDENTIFIED METHODS FOR COMBATING MULTI-PATH FADING

• Time domain clipping method
• Sectorized beamspace adaptive diversity combiner
  method
• Simple multi-path delay spread estimation method
• Directive antenna or sector antenna diversity
  method
• Quasi-optical receiver with angle diversity

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TIME DOMAIN CLIPPING METHOD

• Simple time domain method
• Mainly applied in multi-carrier modulation
  systems (ADSL applications)
• Assumption that some received signals will be
  stronger than the received ADSL signal
• Decoder monitors sudden drop in automatic gain
  control (AGC) and clippings in analog-to-digital
  (ADC) receiver indicate presence of large
  voltage
• Decoder declares the presence of impulse and
  erases the corresponding multi-tone symbol if
  large voltage persists for several time domain
• Very effective method for network over which ADSL
  signal is severely attenuated compared to the
  impulse experienced

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TIME DOMAIN CLIPPING METHOD CONTD

• Large peak-to-average power ratio of multi-tone
  signal is a problem for this method
• To counter this, AGC is set very low to prevent
  clipping of receivers ADC due to peaks in the
  multi-tone itself

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SECTORIZED BEAMSPACE ADAPTIVE DIVERSITY COMBINER
METHOD

• Applied in general wireless communication
• Multiple receiving branches formed with multiple
  antenna beams of distinctive patterns
• Beams are synthesized to make fading phenomenon
  nearly uncorrelated
• Lack of adaptivity for cochannel interference
  (CCI) suppression is a disadvantage here
• Considerable CCI suppression can be achieved via
  nuling bolt with a set of judiciously chosen
  weight vectors
• There exist space diversity, time diversity,
  polarization diversity and beam (pattern)
  diversity
• Space diversity is the simplest due to ite simple
  and economical implementation and also no extra
  frequency bands are required here

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SECTORIZED BEAMSPACE ADAPTIVE DIVERSITY COMBINER
METHOD CONTD

• Beam diversity method is referred to as beam
  space version of space diversity method because
  decorrelation of fading phenomenon is performed
  using multiple beams pointed at different
  direction in space
• This method increases the capacity of
  communication channel allocated to the system
• Combiner means
• Strong CCI from outside the sector should be
  suppressed in the side lobe region of the
  combined beam
• The diversity beam encompassing a prescribed
  angular sector is combined such that the
  resulting SNR is maximized

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SIMPLE MULTI-PATH DELAY SPREAD ESTIMATION METHOD

• A simple and convenient method for in-service
  monitoring of delay spread BPSK and (pi/4
  shifted) QPSK
• Applied in mobile/portable/indoor/personal
  communication systems
• For BPSK its assumed information sequence is
  derived form the in-phase channel (I-ch) output
  of a differential detector so no signal appears
  at the quadrature channel (Q-ch) output (fig.1)
• With the presence of multi-path interference
  occurring with different delay time, non-zero
  signal start to appear and the time duration of
  non-zero output is shown to be proportional to
  the delay time
• It was realized that the average of Q-ch output
  is a good measure of an rsm delay spread after a
  detailed analysis

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Diagram 1 Block Diagram of Differential
Detector.
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SIMPLE MULTI-PATH DELAY SPREAD ESTIMATION METHOD
CONTD

• For (pi/4 shifted) QPSK, a simple measure of a
  difference of absolute values of I-ch and Q-ch
  detector outputs gives an estimate ofmulti-path
  delay which will be generally non-zero exactly
  over the duration of multi-path delay difference
• Through field tests, Q-ch detector output was
  confirmed to be a good measure of multi-path
  delay spread unless multi-path delay time exceeds
  a symbol duration, and a better measure of BER
  than delay spread

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DIRECTIVE ANTENNA OR SECTOR ANTENNA DIVERSITY
METHOD

• Applied in mobile/portable/indoor/personal
  communication systems
• A four-sector antenna diversity reception or four
  direction antenna pattern diversity reception
  used to reduce long-delayed multi-path waves
  which arrive in opposite directions, as well as
  fading (fig.2)
• Extensive field tests to measure BER of
  anti-multi-path modulation scheme direct shift
  keying DSK proved that directive antenna is a
  powerful means to mitigate the effect of
  long-delay multi-path propagation
• Measured spectral variations of BER of 256/512
  Kbps DSK signals from a transmitter antenna
  installed 27m high up a building is shown on
  fig.3

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Diagram 2 A Four-Direction Antenna Pattern.
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Diagram 3 Measured BER of DSK at 400 MHz Band.
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DIRECTIVE ANTENNA OR SECTOR ANTENNA DIVERSITY
METHODCONTD

• The cummulative distribution of BER improvement
  over the whole test course is summerized in fig.4
• Even BSPK shows a marked improvement if received
  by a directional antenna

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Diagram 4 Cumulative BER of DSK and BPSK.
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QUASI-OPTICAL RECEIVER WITH ANGLE DIVERSITY METHOD

• Applied in general communication systems
• The method is used in quasioptical receiver front
  end shown in fig.5
• A 10 Ghz lens amplifier antenna array and three
  self-oscillating grid mixers are used in this
  method
• Lens amplifies incomming plane waves incident
  from different directions and focuses them to
  three points where the mixers are placed
• Angle preserving nature of amplifier allows
  mixers to generate independent IF signals from
  three incident directions

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Diagram 5 A Quasi-Optical Receiver Consisting of
a Lens Amplifier and Self-
Oscillating Grid Mixers.
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QUASI-OPTICAL RECEIVER WITH ANGLE DIVERSITY
METHOD CONTD

• Resulting angle diversity help reduce multi-path
  fading in communication systems since the
  probability of fade from three different incident
  angles is low
• Efforts are still being made to integrate this
  method into transmitters and receivers

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CONCLUSION AND FUTURE WORK

• Conclusion
• From comparative table sectorized beam space
  adaptive diversity combiner method emerged as the
  best method with the highest weight
• Future work
• Verify whether some of the methods/techniques can
  be applied together to achieve improved
  performance of a given wireless network
• Identify the ways in which some of the
  methods/techniques can be improved on their own