3.9 Predictable Link Budget Design using Path Loss Models

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3.9 Predictable Link Budget Design using Path
Loss Models
Most RF propagation models are derived from
combined (i) analytical studies (ii)
experimental methods

• Empirical Approach measured data is fitted to a
  curve or an
• analytical expression
• uses field measurements
• implicitly accounts for all factors (known and
  unknown)
• model generally not valid for all frequencies or
  environments

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• Median Path Loss Determination
• estimate receive power at distance d from
  transmitter

? total received electrical field (V/m) ?d
electric field of equivalent direct path N
number of paths between T and R Lk relative
loss of kth path ?k relative phase shift of
kth path
if LOS exists ? L0 1 and ?0 0
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(1) Average Large Scale Path Loss Model

• distance dependent mean path loss - over
  significant distances

d0 close in reference distance, often
determined emperically d transmitter - receiver
separation n path loss exponent - indicates
rate of path loss increase with d0
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• Free Space Reference Distance, d0,
• always in antennas far-field - eliminate near
  field effects for
• reference path
• must be specified for different environments

Environment d0
large cellular ?1km
microcell 1m-10m
Reference Path Loss, PL(d0) calculated using
either (i) free space path loss (eqn 3.5)
(ii) field measurements at d0
Path Loss Exponent, n
Environment n Environment n
free space 2 In building LOS 1.6-1.8
Urban-cellular 2.7-3.5 Obstructed in Building 4-6
Shadowed Urban Cellular 3-5 Obstructed in Factories 2-3
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3.9.2 Log Normal Shadowing

• surrounding clutter isnt considered by log
  distance model
• averaged received power (eqn 3.68) is
  inconsistent with measured data
• measured PL(d) at any location is random, with
  log normal distribution
• about (normal distribution of
  log10() )

• antenna gains included in PL(d)
• ?? zero-mean Gaussian distributed random
  variable (in dB)
• ? standard deviation of ?

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• Log Normal Distribution - describes random
  shadowing effects
• for specific Tx-Rx, measured signal levels have
  normal distribution
• about distance dependent mean (in dB)
• occurs over many measurements with same Tx-Rx
  different
• clutter standard deviation, ? (also measured
  in dB)

• Lognormal Model For Local Shadowing
• typically, ?dB ranges from 5-12

• let u median path loss (dB) at distance d from
  transmitter
• ? distribution xdB of observed path loss has
  pdf given by

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• Log Normal Graph Pr(xdB gt x) vs Gain/Loss
  Relative to Median Path Loss
• shown for ?dB 4,6,8, 12

• median ? 50 of samples expected to be gt median
  
• 50 expected to be lt median
• all curves intersect at median

?dB loss relative to median path time
6dB gt 10dB 1
12dB gt 10dB 10
4dB gt 7dB 1
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• ? n are derived from measurements using
  linear regression
• minimizes difference between measured
  estimated path loss
• minimized in a mean-square sense over many
  measurements ds

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Q(z)
(ii) Pr(d) gt ? or Pr(d) lt ? is determined
from CDF
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3.9.3 Determination of Coverage Area

• in a given coverage area, let ? desired
  receive signal level
• could be determined by receiver sensitivity (or
  visa versa)
• random shadowing effects cause some locations at
  d to have
• received power, Pr(d) lt ?

• Determine boundary coverage vs area covered
  within a boundary,
• assuming
• a circular coverage area with radius R from base
  station
• likelihood of coverage at cell boundary is known
  (given)
• d r represents radial distance from transmitter

useful service area (coverage area) U(?)
area with Pr(d) gt ?
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Left Axis area with Pr(r) gt ? (coverage
area-use 3.73) Right Axis PrPr(r) gt ?
(boundary coverage-use 3.68)
PrPr(r) gt ? ?/n U(?)
0.95 2 0.99
0.70 2 0.9
0.60 2 0.82
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• 3.10 Outdoor Propagation Model
• estimating PL(d) requires terrain profile for
  propagation over
• irregular terrain such as
• simple curved earth profile
• highly mountainous
• obstacles trees, building,
• all models predict Pr(d) at given point or small
  area (sector)
• wide variations in approach, complexity,
  accuracy
• most based on systematic interpretation of
  empirical data

- Longely Rice - Durkins Model - Okumura Model -
Hata Model - Wideband PCS Microcell - PCS
Extension to Hata Model - Walfisch Bertoni Model
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• 3.10.1 Longely Rice Model (ITS irregular terrain
  model)
• used for point-point systems under different
  types of terrain
• frequency ranges from 40MHz-100GHz

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• Longely Rice Model available as a computer
  program
• calculates large scale median transmission loss
  over irregular terrain for
• frequencies between 20MHz-10GHz
• input parameters include
• transmission frequency,
• path length antenna heights,
• polarization,
• surface refractivity
• earth radius climate
• ground conductivity ground dielectric constant
• path specific parameters antennas horizon
  distance, horizon elevation
• angle, trans-horizon distance, terrain
  irregularity

Prediction Modes for Longely Rice 1. point-point
mode used when detailed terrain profile or path
specific parameters are known 2. area mode
prediction uses estimated path specific
parameters
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• 3.10.2 Durkins Model similar to Longly-Rice
• predicts field strength contours over irregular
  terrain
• adopted by UK joint radio committee
• consists of two parts
• (1) ground profile
• reconstructed from topographic data of proposed
  surface along
• radial joining transmitter and receiver
• models LOS diffraction derived from obstacles
  local scatters
• assume all signal received along radial (no
  multipath)
• (2) expected path loss calculated along the
  radial
• move receiver location to deduce signal strength
  contour
• pessimistic in narrow valleys
• identifies weak reception areas well

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• 3.10.3 Okumura Model wholly based on measured
  data - no analytical explanation
• among the simplest best for in terms of path
  loss accuracy in
• cluttered mobile environment
• disadvantage slow response to rapid terrain
  changes
• common std deviations between predicted
  measured path loss ?
• 10dB - 14dB
• widely used for urban areas
• useful for
• - frequencies ranging from 150MHz-1920MHz
• - frequencies can be extrapolated to 3GHz
• - distances from 1km to 100km
• - base station antenna heights from 30m-1000m

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• Okumura developed a set of curves in urban areas
  with quasi-smooth terrain
• effective antenna height
• - base station hte 200m
• - mobile hre 3m
• gives median attenuation relative to free space
  (Amu)
• developed from extensive measurements using
  vertical omni-
• directional antennas at base and mobile
• measurements plotted against frequency

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Estimating path loss using Okumura Model 1.
determine free space loss, Amu(f,d), between
points of interest 2. add Amu(f,d) and correction
factors to account for terrain
L50(dB) LF Amu(f,d) G(hte) G(hre) GAREA
(3.80)
L50 50 value of propagation path loss
(median) LF free space propagation
loss Amu(f,d) median attenuation relative to
free space G(hte) base station antenna
height gain factor G(hre) mobile antenna
height gain factor GAREA gain due to
environment
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• model corrected for
• ?h terrain undulation height
• isolated ridge height
• average terrain slope
• mixed land/sea parameter

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Median Attenuation Relative to Free Space
Amu(f,d) (dB)
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Correction Factor GAREA(dB)
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3.10.4 Hata Model empirical model of graphical
path loss data from Okumura - predicts median
path loss for different channels - valid over
UHF/VHF band from 150MHz-1.5GHz - charts used to
characterize factors affecting mobile land
propagation - standard formulas for approximating
urban propagation loss - correction factors for
some situations - compares closely with Okumura
model as d gt 1km ? large mobile systems
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Parameter Comment
L50 50th value (median) propagation path loss (urban)
fc frequency from 150MHz-1.5GHz
hte, hre Base Station and Mobile antenna height
? (hre) correction factor for hre , affected by coverage area
d Tx-Rx separation
L50 (urban)(dB) 69.55 26.16log10 fc 13.82
log10 hte ?(hre) (44.9-6.55hte)log10 d
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Mobile Antenna Height Correction Factor for Hata
Model
? (hre) Comment
(1.1log10 fc - 0.7)hre (1.56log10 fc - 0.8)dB Medium City 3.83
8.29(log10 1.54hre)2 1.1 dB Large City (fc ? 300MHz) 3.84a
3.2(log10 11.75hre)2 4.97 dB Large City (fc gt 300MHz) 3.84b
L50 (dB) Comment
L50 (urban) - 2log10 (fc/28)2 5.4 Suburban Area 3.85
L50 (urban) - 4.78(log10 fc)2 - 18.33log10 fc - 40.98 Rural Area 3.86
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• Valid Range for Parameters
• 150MHz lt fc lt 1GHz
• 30m lt hb lt 200m
• 1m lt hm lt 10m
• 1km lt r lt 20km

• Propagation losses increase
• with frequency
• in built up areas

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Example Tables for Okumura-Hata Model

• Terrain Legend
• Urban
• Suburban
• Open

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• 3.10.5 PCS Extension to Hata Model
• European Co-operative Scientific Technical
  (EUROCOST)
• formed COST-231
• extend Hatas model to 2GHz

L50 (urban)(dB) 46.3 33.9logfc 13.82 loghte
?(hre)
(44.9-6.55hte)logd CM
fc frequency from 1500MHz - 2 GHz hte
30m-200m hre 1m-10m d 1km-20km
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3.10.6 Walfisch Bertoni Model
path loss S P0Q2P1
(3.89)
P0 free space path loss between isotropic
antennas Q2 reduction in rooftop signal due to
row of buildings that immediately
shadow hill P1 based on diffraction ?
determines signal loss from roof top to
street
S (dB) L0 Lrts Lms
(3.91)
L0 free space loss Lrts roof-to-street
diffraction scatter loss Lms multi-screen
diffraction loss from rows of building
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3.10.7 Wideband PCS Microcell Model
Feuerstien Measured cellular systems in Bay
Area - 20MHz pulsed transmitter at 1900 MHz -
base station antenna heights 3.7m, 8.5m, 13.3m -
mobile antenna heights 1.7m

• assume flat ground reflection model
• let df 1st Fresnel zone clearance

• Model for Average Path Loss - LOS channel
• double regression model with regression
  breakpoint at 1st
• Fresnel zone clearance
• fits measured data well
• model assumes omni-directional vertical antennas

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Average Path Loss PCS Microcell
e.g. at 1900MHz ? p1 38.0dB
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n OBS path loss exponent related to
transmitter height
? log normal shadowing component from distance
dependent mean (3.10.2)
Transmit Antenna Height 1900 MHz LOS n1 n2 ?(dB) 1900 MHz OBS n ?(dB)
low (3.7m) 2.18 3.29 8.76 2.58 9.31
med (8.5m) 2.17 3.36 7.88 2.56 7.67
high(13.3m) 2.07 4.16 8.77 2.69 7.94
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• 3.11 Indoor Propagation Model
• smaller Tx-Rx separation distances than outdoors
• higher environmental variability for much small
  Tx-Rx separation
• - conditions vary from doors open/closed,
  antenna position,
• - variable far field radiation for receiver
  locations antenna types
• strongly influenced by building features,
  layout, materials

• Dominated by same mechanisms as outdoor
  propagation (reflection,
• refraction, scattering)
• Classified as either LOS or OBS
• Surveyed by Mol91, Has93

- Partition Losses Same Floor - Partition
Losses Different Floor - Log-distance path loss
model - Ericsson Multiple Breakpoint Model -
Attenuation Factor Model
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• Partition Losses Same Floor
• hard partitions immovable, part of building
• soft partitions movable, lower than the ceiling

Partition Losses Different Floor dependent on
external building dimensions, structural
characteristics materials
Log-distance path loss model accurate for many
indoor paths

• n depends on surroundings and building type
• ?? normal random variable in dB having std
  deviation ?
• identical to log normal shadowing mode (3.69)

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• (1) Ericsson Multiple Breakpoint Model
  measurements in multi-floor office building
• uses uniform distribution to generate path loss
  values between
• minimum maximum range, relative to distance
• 4 breakpoints consider upper and lower bound on
  path loss
• assumes 30dB attenutation at d0 1m
• - accurate for f 900MHz unity gain anntenae
• provides deterministic limit on range of path
  loss at given distance

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• (2) Attenuation Factor Model (Seidel92b)
• includes effect of building type variations
  caused by obstacles
• reduces std deviation for path loss to ? ? 4dB
• std deviation for path loss with log distance
  model ? ?13dB

nSF exponent value for same floor measurement
must be accurate FAF floor attenuation factor
for different floor PAF partition attenuation
factor for obstruction encountered by primary
ray tracing
primary ray tracing single ray drawn between Tx
Rx yields good accuracy with good computational
efficiency
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Replace FAF with nMF exponent for multiple
floor loss
? decreases as average region becomes
smaller-more specific

• Building Path Loss obeys free space loss
  factor (?) (Dev90b)
• loss factor increases exponentially with d
• ? (dB/m) attenuation constant for channel

4-story bldg
2-story bldg
f ?
850MHz 0.62
1.7GHz 0.57
f ?
850MHz 0.48
1.7GHz 0.35
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Path Loss Exponent Standard Deviation for
Typical Building
Location n s (dB) number of points
same floor 2.76 12.9 501
through 1 floor 4.19 5.1 73
through 2 floor 5.04 6.5 30
through 3 floor 5.22 6.7 30
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(3) Simple Indoor Path Loss Model

• r distance between transmitter receiver
• r0 nominal reference distance (typically 1m)
• WAF(p) is wall attenuation factor, for P floors
• FAF(q) is floor attenuation factor, for Q floors
• n ? 2 for close distances, larger for greater
  distances

material loss at 900MHz loss at 1700MHz
plaster wall ? 5dB ? 11dB
concrete wall ? 10dB ? 17dB

• more accurate when P and Q are small
• model neglects angle of incidence effect of
  distance on n

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• 3.12 Signal Penetration into Buildings
• no exact models
• signal strength increases with height
• lower levels are affected by ground clutter
  (attenuation
• penetration)
• higher floors may have LOS channel ? stronger
  incident signal on
• walls

RF Penetration affected by - frequency - height
within building - antenna pattern in elevation
plain
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• penetration loss
• decreases with increased frequency
• loss in front of windows is 6dB greater than
  without windows
• penetration loss decreases 1.9dB with each floor
  when lt 15th
• floor
• increased attenuation at gt15 floors
  shadowing affects from
• taller buildings
• metallic tints result in 3dB to 30dB
  attenuation
• penetration impacted by angle of incidence

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Penetration Loss vs Frequency for two different
building
(1)
(2)
Frequency (MHz) Attenuation (dB)
441 16.4
896.5 11.6
1400 7.6
Frequency (MHz) Attenuation (dB)
900 14.2
1800 13.4
2300 12.8

• Ray Tracing Site Specific Models
• rapid acceleration of computer visualization
  capabilities
• SISP site specific propagation models
• GIS graphical information systems
• - support ray tracing
• - augmented with aerial photos architectural
  drawings