Degradation In a Cellular Communication Environment

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Degradation In a Cellular Communication
Environment

• Transferring knowledge to future leaders

Presented by Professor Johnson I Agbinya
jagbinya_at_uwc.ac.za
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Why Look at Degradation In Cellular Networks?

• Signal degradation affects system performance and
  capacity
• During design and planning of a network we must
  provide for effects of degradation
• Need to understand how to model them to develop
  software to handle system design and planning
• Needed for tuning and optimizing networks
• For providing professional consultancy to the
  telco industry
• To provide the basis for understanding cellular
  communication standards on noise performance

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Types of Degradation In Cellular Networks

• Noise
• Multiple Access Interference (MAI)
• Fading

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Focus In This Lecture

• Signal Fading Categories
• Multipath Propagation
• Propagation Models
• Loss Formulae
• Link Budget

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Essential Definitions

• Reflection A change in the direction of a signal
  without penetrating the object. Occurs when the
  path of a signal is obstructed. The dimensions of
  the obstructing object is larger than the
  wavelength of the signal
• Diffraction An object with large dimension
  blocks the path of a wave.
• Scattering An object in the path of a wave
  causes it to spread or scatter in different
  directions. Occurs when the dimensions of the
  object are comparable to the wavelength of the
  signal.

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Fading - Multipath Propagation

• Multipath
• Signals on transmission take many paths to arrive
  at a receiver (multipath)
• The strongest component arrives from the direct
  path
• Multipath Effects cause
• time variations due to multiple delays
• random frequency modulations due to Doppler
  shifts
• random changes in signal strengths over short
  periods
• Multipath delay causes the signal to appear
  noise-like in amplitude

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Multipath Model

• Multipath is modeled as a linear time varying
  filter with impulse response h(t,t)
• where t is the multipath delay in the channel for
  a fixed time t
• a low pass filter approximation is used in
  practice
• signal components are modeled relative to the
  component that arrived first with delay to 0
• components arriving latter are separated at
  discrete times with delays in N equally spaced
  time intervals of width Dt
• components in bin with delay ti i Dt are thus
  lumped together as one

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Detecting Multipath Signals

• Techniques
• channel sounding through direct pulse
  measurements
• spread spectrum sliding correlator
• swept-frequency channel analyser
• Measured parameters
• dispersion parameters (mean excess delay, maximum
  excess delay at some given signal to noise ratio
  and rms delay spread)
• coherence bandwidth
• Doppler spread or spectral broadening

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Doppler Shifts

• Doppler Effect
• A moving object causes the frequency of a
  received wave to change
• In a cellular communication environment the
  measured frequency increases as the mobile moves
  towards a base station
• As it moves away from the base station, the
  frequency decreases
• Effects of Doppler shifts
• bandwidth of the signal could increase or
  decrease leading to poor and/or missed reception
• For a mobile phone in an object (eg. car) moving
  at a speed of v m/s, the Doppler shift is
• where q is the angle made by the signal path to
  the base station and the ground plane

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Effects of Doppler Frequency Shift

• 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 expressions
• where fm 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 t

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Delay Spread

• Definition
• The standard deviation of the distribution of
  multipath signal amplitudes is called delay
  spread, st.
• Delay spread varies with the terrain with typical
  values for rural, urban and suburban areas

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Coherence Bandwidth

• Doppler frequency shift causes the signal
  bandwidth to change. What then is the real
  bandwidth of the signal?
• The concept of coherence bandwidth is used to
  address this question
• Definition Coherence bandwidth is 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

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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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Categories of Fading

• There are two major categories of fading
• (1) small-scale fading - caused by
• superposition of multipath signals
• speed or RX or TX
• bandwidth of transmitted signal
• (2) large-scale fading -called path loss and
  depends on the distance between TX and RX
• also known as log-normal fading or shadowing

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Small-Scale Fading

• Also known by other names such as
• Fading multipath and Rayleigh fading
• Rayleigh fading is a result of constructive and
  destructive interference between several versions
  of the same signal at the receiver, leading to
  attenuation of signal power or amplitude
• Usually over a fraction of the signal wavelength
• Attenuation between 20 to 30 dB
• multipath fading manifests as time spreading or
  time variation of the signal (due to motion,
  foliage, reflections and scattering)

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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 s2 is the total 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
• s2 is the power of the line of sight signal and
  I0 is a Bessel function of the first kind

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Characteristics of Small-Scale Fading

• Small-scale fading occurs as either of 4 types
• frequency selective fading in which the bandwidth
  of the signal is greater than the coherence
  bandwidth and the delay spread is greater than
  the symbol rate Signals at some frequency
  components experience more fading than others -
  (caused by multipath delay spread)
• flat fading when the bandwidth of the signal is
  less than the coherence bandwidth and the delay
  spread is less than the symbol rate - (caused by
  multipath delay spread)
• fast fading when the Doppler spread is high and
  the coherence time is less than the symbol period
  and
• slow fading with a low Doppler spread and
  coherence time is greater than the symbol period
  - (caused as well by Doppler spread)

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Summary of Small-scale fading

• Correct for small-scale fading with
• adaptive equalizers
• modulation techniques such as spread spectrum

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Propagation of Cellular Communication Signals

• Cellular communication is mostly land based.
• There are a few applications on ships and
  airlines using networks in a box and satellites
• Propagation considerations are therefore based on
  
• urban, rural, suburban and
• in a few cases water and desert terrain
• sky or space propagation (satellites)

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Theoretical Propagation Model

• Radio frequency (RF) sources are modeled as
  isotropic sources of energy
• radiates microwave energy uniformly in all
  directions
• radiates into the so-called spherical volume
• a half-wave dipole source is used in practice to
  model effective radiated power (ERP)
• The practical measure for radiating source is
  effective isotropic radiated power (EIRP)
• a half-wave dipole is used
• EIRP ERP 2.15 dB
• The additional dB corrects for the fact that the
  source is not isotropic

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Free Space Propagation (Friis Formula)

• The medium separating the TX and RX is assumed to
  be free space
• The model accounts for the gains of the TX and RX
  antennas, the power transmitted and the volume of
  space under consideration
• It is given by Friis Formula as
• Where Pr, Pt, Gr and Gt are the receiver power,
  transmitter power, receiver antenna and
  transmitter antenna gains respectively
• and radiation is into a spherical space of radius
  d surrounding the antenna

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Loss Between TX and RX

• The dielectric medium between the TX and RX is
  usually a wire, air, optic fibre or some liquid.
  The loss in the medium is modelled as
• In practice the loss is expressed in decibels
• d is distance in km and f is frequency in MHz

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Correction to Free Space Loss

• For UHF mobile communication, a correction is
  needed to the above expression
• The correction is due to clutter between the TX
  and RX and is corrected for with the expression

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Terrestrial Propagation

• RF Propagation of under natural settings is
  referred to as terrestrial propagation
• For mobile communications, we would like to
  introduce the heights of the RX and TX antenna
  into the loss equations
• We also need to account for losses due to the
  terrain, buildings and other man made structures
• The theoretical starting point is a two ray
  propagation model
• Two Ray Propagation
• Consists of a direct and ground reflected paths

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Two Rays Propagation

• The path difference between the direct and
  reflected rays is
• the path difference is proportional to the
  product of the heights of the antennas and
  inversely proportional the distance between RX
  and TX
• destructive reflections from ground surface can
  be avoided when the path difference is
• When RF waves propagate, they form wavefronts, or
  concentric circles called Fresnel zones, one
  wavelength apart

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Fresnel Zones

• The Fresnel zones are propagation break points
• At the first Fresnel zone (n1) no reflections of
  waves can take place and
• The distance to this point is
• Until this point, the propagation is assumed to
  be free space and rays travel is direct (point to
  point) with no reflections
• Free space and terrestrial propagation models are
  used for design of microcells and also for in
  building coverage or solutions
• when the distance is less than the first Fresnel
  zone, none of the models is adequate and
  empirical design is used

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Propagation Over Specular Ground

• RF propagation over ideal or the so-called
  specular ground is modeled by the modified
  free-space model
• or
• As a result of this fourth power dependence on
  distance, every time we double the distance, we
  lose 12 dB of signal energy. Consequently
• frequency reuse should be done at shorter
  distances
• The path loss exponent varies from terrain to
  terrain

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Path Loss Exponent

• The path loss exponent for various terrain is
  given below

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Propagation - Practical Models

• Propagation out door is difficult to predict and
  as such, empirical models, without real
  analytical basis are applied
• Most of the models used are accurate to within 10
  to 14 decibels in urban and suburban areas
• They tend to be less accurate in rural areas
  because most of the data used may have been
  collected in the urban and suburban areas
• In practice there are huge variations in the
  types of terrain and environment to cover.
• Heights of antenna, clutter, tree density,
  beamwidth, wind speed, season and multipath, vary
  widely and affect mobile phone waves.
• Hence complex models are required for such
  situations. They are used to predict propagation
  loss.

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Propagation - Practical Models

• Popular propagation models used in design of
  cellular networks include
• Okamura model
• Walfisch-Ikegami model
• Hata model
• Cost 231 model
• Egli, Lee, Carey, Longley-Rice and
  Ibrahim-Parsons model
• Each of these model is an adaptation for specific
  terrain and frequency ranges
• Most of these models are used in GSM, CDMA, and
  IMT-2000 standards for planning of cellular radio
  networks

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Hata - Okamura Model

• Used for modelling path loss in suburban areas
• Valid in the 150 to 1500 MHz range (GSM and NMT)
• Expects receivers greater than a km from base
  station (BS)
• Base station antenna heights greater than 30m
• Therefore model targets 2G (GSM and PCS) systems
  in the 900 to 1800 MHz range
• Could be extended to 2GHz systems with
  modifications with higher base stations not
  including hilly and wooded areas
• Path loss regions are divided into 3 regimes A, B
  and C

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Hata - Okamura Model (1)

• The Path Loss categories considered
• A (maximum path loss) hilly terrain with
  moderate-to-heavy tree densities
• B (intermediate path loss) terrain conditions
  between category A and C
• C (minimum path loss) mostly flat terrain with
  light tree densities
• The median path loss at 1.9 GHz for a distance do
  from a base station is given by
• where , l is the
  wavelength in metres, s is shadowing effect and n
  is the path loss exponent

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Corrections and Path Loss Exponent

• The Path Loss exponent can be estimated from the
  expression
• The constants a, b and c depend on terrain
  category
• The height of the base station hb is between 10m
  and 80m, and do 100m
• The corrections to path loss due to terrain are
  given
• Table Path loss (terrain correction) variables

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Corrections and Path Loss

• Shadowing effects follow a log-normal
  distribution with typical standard deviation
  between 8.2 and 10.6 dB
• Shadowing effects also depends on the terrain and
  tree density type
• In general correction terms are used to account
  for antenna height and frequency region.
• For the model to apply to frequencies outside the
  range of specification (2GHz), and for receive
  antenna heights between 2m and 10m, correction
  terms are specified.

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Coarse for of Path Loss Model

• Has 3 correction terms (Lp, Lf and Lh)
• (in dB) is the frequency (in MHz) correction
  term given by the expression
• The correction term (categories A and B) for
  antenna height is
• and for categoriy C
• and the height of the receive antenna is in the
  range 2m lt h lt 10m

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Cost 231 Hata Loss Model

• Is applicable to urban areas (flat suburban)

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Questions and Answers

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