EEE440 Modern Communication Systems

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EEE440 Modern Communication Systems

• Wireless and Mobile Communications

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Indoor Propagation

• Propagation impairments in an indoor radio
  channel are caused mainly by
• reflection from, and diffraction around,
  objects (including walls and floors) within the
  rooms
• transmission loss through walls, floors and
  other obstacles
• channelling of energy, especially in corridors
  at high frequencies
• motion of persons and objects in the room,
  including possibly one or both ends of the radio
  link,

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Indoor wireless propagation

• Like the outdoor propagation, the indoor
  propagation also experience multipath dispersion
  caused by a large number of reflectors and
  scatterers
• In a typical indoor environment, a fixed antenna
  (usually ceiling-mounted) serves a number of
  mobile radios
• In narrow-band, multipath causes fluctuations in
  the received signal envelope and phase
• There are critical differences from the outdoor
• The outdoor channel is stationary in time and
  nonstationary in space.
• Signal dispersion is mainly caused by large fixed
  objects.
• Because the base antenna is elevated and mobile
  antenna are at relatively low level, the effects
  of people and moving vehicle are negligible.
• The indoor channel is neither stationary in space
  nor in time
• The motion of people and equipments with a
  low-level ceiling mounted antenna is not
  negligible

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Indoor wireless propagation

• The indoor channel has higher path losses and
  sharper changes in the mean signal level compared
  to the outdoor channel
• Applicability of a simple negative exponent
  distance-dependent path loss model is not
  universally accepted for indoor.
• Rapid motion and high velocities are absent.
• The Doppler shift is negligible

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Indoor wireless propagation

• Maximum delay of multipath signals is typically
  less than one microsecond
• Delay spread is usually less than 100 nanoseconds
  
• Frequency selective fading occurs at higher
  bandwidth

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Modelling indoor wireless propagation

• Modelling indoor wireless propagation is highly
  complex
• For the multipath dispersion, a simple
  geometrical model is used
• The signal reaches the mobile via one or more
  main waves
• These main waves consist of a LOS ray and several
  rays reflected or scattered by main structures
  such as outer walls, floors, ceilings etc

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Modelling indoor wireless propagation

• The resulting paths for each main wave arrive
  with very close delays, experience about the same
  attenuation, but have different phase values due
  to different path lengths
• There are three types of variations in the
  channel
• Small-scale, midscale and large-scale variations
• Small-scale variations-Within a site the channel
  is quite similar since the channel structure does
  not change considerably over short distance. This
  is equivalent to the correlated fading in the
  outdoor channel within a short distance.
• Midscale-in local areas with similar antenna
  separation, there may be great differences in
  signal levels equivalent to shadowing in the
  outdoor
• Large-scale the channel structure change
  drastically due to increase in the number of
  objects. Equivalent to path loss in the outdoor.
  But cannot be modeled simply by the same outdoor
  model.

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Modelling indoor wireless propagation

• The impulse response approach can be used to
  characterise the channel
• For each point in the 3-dimensional space the
  channel is a linear time-varying filter with the
  impulse response given by

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Modelling indoor wireless propagation

• In reality a mobile phone moving through the
  indoor channel experinces a space-varying fading
  phenomenon, so the impulse response varies at
  different points

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Modelling indoor wireless propagation

• The number of multipath components, N in each
  impulse response is a random variable
• The mean value of N differs for different types
  of building
• The path variable sequences are random sequences
  and depend on the shape, size and construction of
  the building.
• The arrival times can be modelled by the modified
  Poisson distribution.

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Modelling indoor wireless propagation

• Distribution of the pulse amplitudes
• The difference in time delay of a number of paths
  is much less than the reciprocal of the
  transmission bandwidth and the paths cannot be
  resolved as distinct pulses
• The unresolvable pulses add up and the envelope
  of their sum is observed
• The envelope is a random variable

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Modelling indoor wireless propagation

• The amplitude fading may follow different
  distribution depending on the conditions
• Various distribution have been reported
  Rayleigh, Rician, Nakagami, Weibull, Lognormal,
  Suzuki

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Modelling indoor wireless propagation

• The distribution of the arrival time
• The Standard Poisson distribution cannot be used
  because indoor scatterers are not located with
  total randomness
• The modified Poisson distribution has better fit
• Other distribution may also apply depending on
  conditions

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Modelling indoor wireless propagation

• The distribution of the path phases
• No empirical model for path phases because of
  difficulties to measure the phase of individual
  multipath components
• Uniform distribution is used
• The signal phase is critically sensitive to path
  length
• As the path length changes by a wavelength (eg.
  30cm at 1gHz), the phase changes by 2pi.
• Moderate changes in the position of the mobile
  result in great phase difference

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Indoor wireless propagation

• Interdependencies within path variables
• Adjacent multipath components of the same impulse
  response profile are likely to be correlated
  because a number of scattering objects that
  produce them may be the same
• Correlation between the arrival times is due to
  the grouping property of local structures.
• The amplitude sequence is correlated with the
  arrival time sequence because later paths of a
  profile experience higher attenuation due to
  greater path lengths and possibly multiple
  reflections.

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Indoor wireless propagation

• Temporal variations of the channel
• Due to motion of people and equipments, the
  channel statistics changes even when the
  transmitter and receiver are fixed
• Motion can introduce deep fade (sometimes as high
  as 20dB)
• Different magnitude and duration of fade for
  different kind of buildings.
• Large-scale path losses
• Greater than outdoor
• Large variation can occur over very small
  distance
• Complicated. Cannot use the conventional path
  loss model
• No single model is appropriate for all conditions

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Indoor wireless propagation

• Delay spread
• Depends on size and types of buildings, existence
  of a clear LOS, room sizes, antenna separation
• Determines whether frequency selective fading
  occurs or not
• Generally less than the outdoor so can use higher
  bitrate
• Penetration Loss
• Power lost as signal enters the buildings varies
  depending on types of building materials and
  various other factors
• The difference between the received signal inside
  a building and around the parameter of that
  building
• Important for frequency reuse
• Depends on the construction materials, building
  orientation with respect to transmitter, internal
  layout etc.