Mobile Communications

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Mobile Communications
Part IV- Propagation Characteristics Multi-path
Propagation - Fading
Professor Z Ghassemlooy School of Computing,
Engineering and Information Sciences, University
of Northumbria U.K. http//soe.unn.ac.uk/ocr
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Contents

• Channel Model
• Fading
• Doppler Shift
• Dispersion
• Summary

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Multipath Fading Components

• Complex low-pass representation of impulse
  response

C. D. Charalambous et al
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Band-pass Representation of Impulse Response

• Band-pass representation of impulse response

C. D. Charalambous et al
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Representation of Additive Noise Channel

• Low-pass and band-pass representation of received
  signal

C. D. Charalambous et al
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Fading

• Is due to multipath propagation.
• With respect to a stationary base station,
  multipath propagation creates a stochastic
  standing wave pattern, through which the mobile
  station moves.
• Caused by shadowing
• when the propagation environment is changing
  significantly, but this fading is typically much
  slower than the multipath fading.
• Modem design is affected mainly by the faster
  multipath fading, which can be normally assumed
  to be locally wide-sense stationary (WSS).

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Multipath Propagation - Fading
a b are in phase
a b are out of phase by ?
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Multipath Propagation - contd.

• Assuming receiver is stationary and there is no
• direct path, the received signal can be
  expressed as a sum
• of delayed components or in terms of phasor
  notation

A single pulse
Where ai is the amplitude of the scattered
signal, p(t) is the transmitted signal (pulse)
shape, ti is the time taken by the pulse to
reach the receiver, N is the number of different
paths taken by the signal to reach receiver, and
fc is the carrier frequency
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Fading - Types

• Slow (Long) Term

• Fast (Short) Term (Also known as Rayleigh fading)

Exact representation of fading characteristics
is not possible, because of infinite number
of situation.
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Fading - Slow (Long) Term

• Slower variation in mean signal strength
  (distance 1-2 km)
• Produced by movement over much longer distances
• Caused by
• Terrain configuration (hill, flat area etc.)
  Results in local mean (long term fading)
  attenuation and fluctuation.
• The built environment (rural and urban areas
  etc.), between base station and the mobile unit
  Results in local mean attenuation

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Fading - Slow (Long) Term
C. D. Charalambous et al
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Fading- Fast (Short) Term

• Describes the constant amplitude fluctuations in
  the received
• signal as the mobile moves.
• Caused by multipath reflection of transmitted
  signal by local
• scatters (houses, building etc.)
• Observed over distances ?/2
• Signal variation up to 30 dB.
• Is a frequency selective phenomenon.
• Can be described using Rayleigh statistics, (no
  line of sight).
• Can be described using Rician statistics, (line
  of sight).
• Causes random fluctuations in the received
  power, and also
• distorts the pulse carrying the information.

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Fading- Fast (Short) Term - contd.
A received signal amplitude is given as the sum
of delayed components. In terms of phasor
notation it is given as
In-phase
Quadrature
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Fading- Fast (Short) Term - contd.
The phase?i can be assumed to be uniformly
distributed in the range (0, 2?), provided the
locations of buildings etc. are completely
random. This for large N, the amplitude of the
received signal is
where
X and Y are independent, identically distributed
Gaussian random variables.
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Fading- Fast (Short) Term - contd.
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Rayleigh Distribution

• If the impulse response h(t, t) of the mobile
  radio station is time invariant and has zero
  mean, then the envelope of the impulse response
  has a Rayleigh distribution given as
• where r gt0
• where s2 is variance of A (the total received
  power in the multipath signal).

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Rice Fading

• If however the impulse response has a non zero
  mean then there is a significant component of the
  direct path (line of sight, specular component)
  signal and the magnitude of the impulse response
  has a Ricean distribution
• Ricean distribution is the combination of
  Rayleigh signal with the direct line of sight
  signal. The distribution is
• ?2 is the power of the line of sight signal and
  I0 is a Bessel function of the first kind

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Fast Fading Cases 1 Stationary Mobile
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Fast Fading Cases 1

• The number of fading depends on
• Traffic flow
• Distance between the mobile and moving cars
• The received signal at the MU is

Number of path
propagation time for the ith path
path attenuation factor for the ith path
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Fast Fading Cases 1
is additional relative delay (positive or
negative)
where
and
Thus
envelope
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Fast Fading Cases 2 Non-stationary Mobile
The received signal at the mobile is
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Fast Fading Cases 2 Doppler Frequency

• A moving object causes the frequency of a
  received wave to change
• Substituting for ? and x, the expression for the
  received signal is

The Doppler frequency
The received signal frequency
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Fast Fading Cases 2 Doppler Frequency

• When ? 0o (mobile moving away from the
  transmitter)

• When ? 90o (I.e. mobile circling around)

• When ? 180o (mobile moving towards the
  transmitter)

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Fast Fading Cases 3 Moving MU Stationary
Scatterer
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Fast Fading Cases 3
Received signal at the MU
and for q 0
Incident signal
Reflected signal
Fading with zero amplitude occurs when
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Fast Fading Cases 4 Moving MU and Scatterers
The resultant received signal is the sum of all
the scattered waves from different angles qi
depending upon the momentary attitude of the
various scatterers.
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Effects of Doppler shifts

• Bandwidth of the signal could increase or
  decrease leading to poor and/or missed reception.
• The effect in time is coherence time variation
  and signal distortion
• Coherence time is the time duration over which
  two signals have strong potential for amplitude
  correlation
• Coherence time is expressed by
• where fD-max is the maximum Doppler shift, which
  occurs when q 0 degrees
• To avoid distortion due to motion in the channel,
  the symbol rate must be greater than the inverse
  of coherence time.

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Coherence Distance

• Coherence distance is the minimum distance
  between points in space for which the signals are
  mostly uncorrelated.
• This distance is usually grater than 0.5
  wavelengths, depending on antenna beamwidth and
  angle of arrival distribution.
• At the BTS, it is common practice to use spacing
  of about 10 and 20 wavelengths for low-medium and
  high antenna heights, respectively (120o sector
  antennas).

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Coherence Bandwidth (Bc)

• Defined to be the statistical measure of the
  range of frequencies over which the channel is
  considered constant or flat. It is the bandwidth
  over which two frequencies have a strong
  potential for amplitude correlation

Effect of frequency selective fading on the
received signal spectrum
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Estimation of Coherence Bandwidth

• Coherence bandwidth is estimated using the value
  of delay spread of the channel, st
• For correlation gt 0.9
• For correlation gt 0.5
• Typical values of delay
• spreads for various types
• of terrain

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Channel Classification
C. D. Charalambous et al
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Fading in Digital Mobile Communications

• If Bsgtgt Bc, then a notch appears in the
  spectrum. Thus
• resulting in inter-symbol interference (ISI).
• - To overcome this, an adaptive equaliser (AE)
  with
• inverse response may be used at the receiver.
• Training sequences are transmitted to update AE.

• If Bsltlt Bc, then flat fading occurs, resulting
  in a
• burst of error.
• - Error correction coding is used to overcome
  this
• problem.

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Multipath Delay Spread

• The standard deviation of the distribution of
  multipath signal amplitudes is called delay
  spread. For directive antenna is characterized by
  the rms (RMS delay spread of the entire delay
  profile) which is defined as
• where
• ?avg Sj Pj ? j ,
• ? j is the delay of the j th delay component of
  the profile
• Pj (power in the j th delay component) / (total
  power in all components
• Delay spread varies with the terrain with typical
  values for rural, urban and suburban areas

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Multipath Propagation - Dispersion

• Dispersion over time Interference with
  neighbor symbols, resulting in Inter Symbol
  Interference (ISI)
• The signal arrived at the receiver directly and
  phase shifted
• Distorted signal depending on the phases of the
  different parts

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Mitigation Techniques for the Multipath Fading
Channel

• Space diversity
• Signals at the same frequency using two or three
  antennas located several wavelengths a part.
• Antennas are connected to two or three radio
  receivers.
• The receiver will the strongest signal is elected
• Disadvantage Uses two or more antennas,
  therefore the need for a large site.
• Frequency diversity
• Signals at different frequencies received by the
  same antenna very rarely fade simultaneously.
  Thus the use of several carrier frequencies or
  the use of a wideband signal to combat fading.
• A single aerial connected to a number receiver,
  each tuned to a different frequency, whose
  outputs are connected in parallel. The receiver
  with the strongest instantaneous signal will
  provide the output.
• Disadvantage Uses two or more frequencies to
  transmit the same signal.

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Mitigation Techniques for the Multipath Fading
Channel

• Time diversity Spread out the effects of errors
  through interleaving and coding
• Multipath diversity
• Consider the tapped delay line model of a channel
  shown previously
• If multipaths can be put together coherently at
  the receiver, diversity improvement results
• This is what the RAKE receiver does (see next
  viewgraph)

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RAKE Multipath Signal Processing
R.E. Ziemer 2002
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System Design and Performance Prediction

• Base station placement dependent on
• Propagation environment
• Anticipated geographic distribution of users
• Economic considerations (minimize number of base
  stations)
• Political and public opinion considerations
• Traffic types (3G)
• Performance figure of merit
• Spectrum efficiency for voice ?v voice
  circuits/MHz/base station
• Spectrum efficiency for information ?i
  bps/MHz/base station
• Dropped call rate fraction of calls ended
  prematurely

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Summary

• The random fluctuations in the received power
  are due to
• fading.

• If there is a relative motion between
  transmitter and receiver
• (mobile) the result is Doppler shift

• If maximum Doppler shift is less than the data
  rate, there
• is slow fading channel.

• If maximum Doppler shift is larger than the data
  rate, there
• is fast fading channel.

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Questions and Answers

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