Mobile Communication Systems 1 Prof. Carlo Regazzoni Prof. Fabio Lavagetto

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Mobile Communication Systems 1Prof. Carlo
RegazzoniProf. Fabio Lavagetto

DIST
DIBE
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Basics of radio propagation

• Introduction characteristics of radio
  propagation
• Attenuation
• Antenna
• Fading effects
• Multipath fading
• Doppler effect
• Frequency selective and non-selective fading
• Conclusion.

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Basics of radio propagation

• The free space attenuation
• It is the attenuation, only due to the path
  length and in presence of a free space
  propagation conditions (no obstacles between the
  transmitter and the receiver)
• The free space introduce the following
  attenuation term
• The following expression is defined as available
  loss for the radio link
• Where the two last terms represent the antennas
  gain.

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Basics of radio propagation

• Antennas
• There are two main types of antennas
• Isotropic antenna
• Directional antenna.
• The first one irradiates its energy in all
  spatial directions in the same manner.
• The second one irradiates the signal in a
  particular direction.
• The antenna gain is defined as the ratio between
  the power radiated by a directional antenna and
  the one radiated by an isotropic antenna.
• In general

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Introduction

• Propagation effects
• There are four phenomena (reflection, refraction,
  diffraction, scattering) associated with the
  propagation of wireless signals which give rise
  to
• Multipath
• Fading
• Delay spread
• Doppler shift.
• The wireless channel is considered to be
  gaussian, additive, and band-limited (AWGN).
  Whereas in real world the channel exhibits non
  gaussian characteristic (not AWGN).

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The radio channel multipath propagation

• Usually studied channels are characterized by a
  time-invariant impulse response
• Instead multipath channel is characterised by a
  time-variant impulse response
• On the transmission of a single impulse (ideally
  a dirac delta), the response would be typically a
  time variant impulse train of impulses dispersed
  in time (defined as time spread, t) with
  different attenuations.

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Multi-path propagation the channel impulse
response
Let the transmitted signal be represented by s(t)
and received signal as x(t).
The received signal can be represented as
Where l means the equivalent low pass response.
c(t t) represents the channel response by
choosing time t as the reference time where ?
represents the delay with respect to the origin
of time axis.
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Multi-path effects

• If the propagating channel is slowly time
  variant, the value of ?n(t) oscillates with time
  and its variability has small effects
• However, ?n(t) can vary up to 2? in a time
  interval if ?n(t) varies along a factor 1/fc,
  which is a very small value. This can be true for
  bandpass signals modulated around fc
• ? n(t) is a very sensitive parameter that
  characterizes the time-variant channel even if it
  has small oscillation
• Moreover, the propagation delay related to each
  path can be often assumed to change in a complete
  random uncorrelated way (thus it is considered as
  a random process )
• This means that the received signal cannot be
  modelled as Gaussian random process.

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The received signal envelope modelling

• The received signal is dispersed in time with
  varying attenuation and phase. The signal is
  amplified (if constructive interference occurs)
  and attenuated (if destructive interference
  occurs). When the channel impulse response can be
  modeled as a Gaussian random process the envelope
  of the received signal can be modeled as
• Rician if the Gaussian process has non-zero
  mean. Practically, the channel can be modeled
  with a Line on Sight (LoS) path and other
  non-line of sight paths.
• Rayleigh if the Gaussian process has zero mean.
  Practically, the channel can be modeled with
  non-line of sight paths
• Nakagami it is a general case which can be
  expressed for both Rician and Rayleigh fading
  with unequal fading amplitudes.

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Channel correlation functions
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The relation between various considered fourier
function
Time-variant correlation function of the channel
IFT
FT
Time-variant correlation function of the channel
Scattering function
FT
IFT
Doppler power spectrum
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The delay spread

• Delay spread
• In general, the delayed paths are longer than the
  LoS path
• As consequence the delayed paths arrive at the
  receiving antenna with different delays and in
  different time instants
• The delay spread can be computed according the
  following relation

• Smax distance covered by the longest path.
• Smin distance covered by the shortest path.

• The major effect due to the delay spread is the
  presence of Intersymbol Interference (ISI)

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Narrowband and wideband channel and coherence
bandwidth

• A channel is defined as narrowband if
• Where T is the symbol time duration
• A channel is defined as wideband if
• The coherence bandwidth is defined according to
  the following relations

T gt Tm
T lt Tm
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Channel frequency selectivity and temporal fading

• If
• the channel is frequency selective otherwise is
  not frequency selective.

Channel frequency selectivity
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Channel frequency selectivity and temporal fading

• Temporal fading
• The temporal fluctuations of the amplitude of the
  received paths are combined at the receiver
  antenna
• This combination can be destructive or
  constructive

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Channel frequency selectivity and temporal fading

• Channel frequency selectivity and fading
• The frequency selectivity and the temporal fading
  are two different types of distortion that are
  usually present on the same channel
• On a wideband channel both the effects of
  frequency selectivity and temporal fading are
  present
• On a narrowband channel the temporal fading is
  present

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Time variance of the channel

• Temporal variation speed of fading depends on the
  spreading of frequencies due to the time varying
  nature of the environment
• Phase time varying of replicas provides a
  spectral increase in a transmitted carrier
• This phenomena is characterized by doppler
  spectrum defined previously
• The doppler spread depends on
• the relative velocity of the receiver with
  respect to the transmitter
• the movements of the objects between the
  transmitter and the receiver.

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Time variance of the channel

• In both cases the doppler spread can be computed
  as

Path 1
Path 2

• The coherence time is defined as

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Slow and Fast Fading

• If
• If
• The channel is defined as slowly fading channel
  and in this case

Slow fading
Fast fading
Underspread channel
Overspread channel
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Conclusion transmission scheme over fading
channels
Transmission technique Type of channel
Narrowband digital modulation underspread
Diversity techniques CDMA OFDM MC CDMA overspread