Propagation effects in WiMAX systems

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Propagation effects in WiMAX systems

• Sharmini Enoch
• Dr.Ifiok Otung

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Contents

• WiMAX
• Propagation effects in WiMAX
• Signal to noise ratio performance
• Conclusions

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WiMAX

• WiMAX- Worldwide Interoperability for Microwave
  Access
• Technology based on IEEE 802.16 standard
• Use wireless links with microwave or millimetre
  wave radios
• Use licensed spectrum (typically)
• Are metropolitan in scale
• Provide public network service to fee-paying
  customers (typically)
• Use point-to-multipoint architecture with
  stationary rooftop or tower-mounted antennas
• Provide efficient transport of heterogeneous
  traffic supporting quality of service (QoS)

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Propagation effects in WiMAX

• 2-66 GHz frequency range affected by impairments
• Propagation loss in terrestrial LOS relative to
  free space loss is sum of different contributions
  such as
• (1) Rain
• (2) Atmospheric gases
• (3) Fog
• (4) Atmospheric multi-path
• (5) Diffraction
• (6) Snow
• In NLOS additional attenuation introduced by
• (1) Shadowing
• (2)Vegetation
• Simulation carried out using ITU-R models

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Rain attenuation

• Prediction of rain attenuation in LOS and NLOS
  terrestrial links is essential above 2 GHz
  frequency band
• Above 10 GHz temporal variation in path loss is
  due to rain attenuation the process depending
  on instantaneous rainfall rate
• ITU-R 838 predicts rain attenuation

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Attenuation due to atmospheric gases

• Propagation loss has to be calculated for
  absorption due to water vapor and oxygen
• Using ITU-R 676, the calculation is performed

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Fog and Cloud attenuation

• Typical particle diameter is between ten µm (fog)
  and several ten µm (water vapor) and number of
  particles per cm3, is between 100 and 500
• Incidence of fog with visibility less than 200 m
  in UK is typically in the range of 1 to 3 of
  the year
• Attenuation due to thick fog is around 0.1 dB
  which is quite negligible

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Atmospheric Multipath fading

• A particularly severe form of frequency selective
  fading occurs when beam spreading of the direct
  signal combines with a surface reflected signal
  to produce multi-path fading
• ITU-R 530 and ITU-R 453 are used in finding the
  average annual percentage distribution

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Diffraction fading

• When the atmosphere is sufficiently
  sub-refractive (large positive values of the
  gradient of refractive index, low k-factor
  values), the ray paths will be bent in such a way
  that the earth appears to obstruct the direct
  path between transmitter and receiver, giving
  rise to the kind of fading called diffraction
  fading
• Diffraction fading is the factor that determines
  the antenna heights

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Attenuation due to Snow and Dust

• Attenuation due to snow or dust is predominantly
  a function of the moisture content of the
  particles
• From ITU-R 839, there will be no additional
  attenuation due to melting layer if the following
  condition is satisfied
• hlink lt hrainm - 3600
• hlink -gt rain height at the centre of the path
  link
• hrainm -gt median rain height
• In our calculation the above equation is
  satisfied

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Shadowing

• Shadowing is an important effect in wireless
  networks
• It causes the received SINR to vary dramatically
  over long time scales
• The Walfish-Ikegami (W-I) model applies to
  smaller cells
• It is recommended by WiMAX forum for modelling
  microcellular environments
• The model assumes an urban environment with a
  series of buildings

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Vegetation attenuation

• Attenuation due to vegetation varies due to
• (1) Irregular nature of medium
• (2) Wide range of species
• (3) Densities
• (4) Water content
• Specific attenuation through trees in leaf is 20
  greater than for leafless trees above 1 GHz

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Signal to noise ratio performances

• For all percentages of time the signal is
  subjected to outage. Such a link is not
  acceptable
• BER is very high in the value of 0.2 for all
  percentages of time

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Improvement of BER

• By reducing bit rate and coverage distance
• No significant improvement in BER with decreasing
  distance and lower data rate

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Conclusions

• Our study shows that availability of 99.99 is
  difficult to achieve under realistic propagation
  impairments conditions
• WiMAX theoretical data rates between 1 Mbps to 75
  Mbps is not possible as BER is higher at high
  data rates
• Future work is aimed at reducing fading using
  space time block coding and increasing data rates
  using MIMO techniques

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Thank you for your attention!!!