3/7/2012 1 1 MOBILE PROPAGATION CHANNEL: LARGE-SCALE PATH LOSS 2 Introduction Mobile radio channel is an important controlling factor in wireless communication systems Transmission path between transmitter and receiver can vary in complexity LOS (Line-Of-Sigh t) - simplex Wired channels are stationary and predictable, radio channels are extremely random and have complex models Modeling of radio channels is done in statistical fashion based on measurements for each individual communication system or frequency spectrum
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Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
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8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Propagation Models To predict the average received signal
strength at a given distance from thetransmitter - large scale propagationmodels, hundreds or thousands of meters
To predict the variability of the signalstrength, at close spatial proximity to aparticular location -Small scale or fading
models
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Free Space Propagation Model
The free space propagation model is used topredict received signal strength when thetransmitter and receiver have a clear,unobstructed line-of-sight path between them
The free space model predicts that receivedpower decays as function of the transmitter-receiver (T-R) separation distance raised tosome power
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Free space power received by a receiver antennawhich is separated from a radiating transmittingantenna by a distance d (Friis free space equation):
P t is the transmitted power
P r (d) is the received powerG t , G R is the transmitter and receiver antenna giand is the T-R separation distance in metersL is the system loss factor not related to propagation (L ≥1)λ is the wavelength in meters
2
2 2
. . .( )
( 4 ) . .
t t r r
P G G P d
d L
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Free Space Propagation Model The gain of an antenna is related to its
effective aperture, A e
A e is related to the physical size of theantenna
λ is related to the carrier frequency
2
4 e AG
8
Free Space Propagation Model
The effective isotropic radiated power (EIRP )represents the maximum radiated power availablefrom a transmitter in the direction of maximumantenna gain, compared to an isotropic radiator
In practice, effective radiated power (ERP ) is used
instead of EIRP to demonstrate the maximumradiated power as compared to a half-wave dipoleantenna
t t IRP PG
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Free Space Propagation Model The Fraunhofer distance is given by
D is the largest physical linear dimension of the antenna
d f must satisfy , and
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f
Dd
f d Df d
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Free Space Propagation Model
Large-scale propagation models use a close-in distance,d 0 , as a known received power reference point
The received power, P r (d) , at any distance d > d 0 , maybe related to P r at P 0
The reference distance must be chosen such that it lies inthe far-field region, that is, d 0 ≥ d f , and d 0 is chosen to besmaller than any practical distance used in the mobilecommunication system
The received power in free space at a distance greaterthan d 0 is given by
2
00 0,r r f
d P d P d d d d
d
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Free Space Propagation Model Large dynamic range of received power
levels, often dBm or dBW units are usedto express received power levels
P r (d) is in dBm
P r (d 0 ) is in watts
( ) 10 log 20 log
0,001,r o o
r o f
P d d P d d d d
W d
14
Example
Given a transmitter produces 50 W of power. If this
power is applied to a unity gain antenna with 900MHz carrier frequency, find the received power at afree space distance of 100 m from the antenna. Whatis P r (10 km). Assume unity gain for the receiverantenna
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Receiving power is generally the mostimportant parameter predicted by large-scale propagation model based on thephysics of reflection, scattering, anddiffraction
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
reflection model is a useful propagationmodel that is based on geometrics optics, andconsiders both direct path and a groundreflected propagation path betweentransmitter and receiver
This model has been found to be reasonablyaccurate for predicting the large-scale signal
strength over distance of several kilometersfor mobile radio systems
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Ground Reflection (2-ray)Model
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
maximum T-R separation distance is atmost only a few tens of kilometers, andthe earth may be assumed to be flat
The total received E-field, E TOT , is thena result of the direct line-of-sight
component, E LOS , and the groundreflected component, E g
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Ground Reflection (2-ray)Model
If E 0 is the free space E-field (V/m) at thereference distance d0 from the transmitter,then for d > d 0 , the free space propagatingE-field is given by
where represents the envelope of
the E-field at d distance from the transmitter
0 0, cos c
E d d E d t t
d c
0 0( , )d t E d d
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Model The received power at the distance d from the
transmitter can be expressed as
Path loss 2-ray model (with antenna gains) can beexpressed in dB as
When θ Δ = Π , then d = (4h t h r )/λ is where the ground appears inthe first Fresnel zone between the transmitter and receiver
2 2
4( ) t r
r t t r
h h P d P G G
d
( ) 40log 10log 10log 20log 20logt r t r PL dB d G G h
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Example
A mobile is located 5 km away from a base station.and uses a vertical /4 monopole antenna with a gainof 2.55 dB to receive cellular radio signals. The Efield at 1 km from the transmitter is measured to be10-3 V/m. The carrier frequency used is 900 MHz.
(a) Find the length and gain of the receiving antenna
(b) Find the received power at the mobile using the
2-way ground model assuming the height of thetransmitting antenna is 50 m and receiving antenna
is 1.5 m above the ground.
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Geometry In practical diffraction problems, it is advantageous to reduce all
heights by a constant, so that the geometry is simplified withoutchanging the values of the angles
The concept of diffraction loss as a function of the pathdifference around an obstruction is explained by Fresnel zones.
The Fresnel zones represent successive regions wheresecondary waves have a path length from the transmitter toreceiver which are nλ/2 greater than the total path length of aline-of-sight path
The successive Fresnel zone have the effect of alternatelyproviding constructive and destructive interference to the totalreceived signal
The radius of the nth Fresnel zone circle is denoted by r n and beexpressed in terms of n , λ, d 1 , and d 2
1 21 2
1 2
,n n
n d d r d d r
d d
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Diffraction – Fresnel ZoneGeometry
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Geometry In mobile communication systems, diffraction loss
occurs from the blockage of secondary waves suchthat only a portion of the energy is diffracted aroundan obstacle
That is, an obstruction causes a blockage of energyfrom some of the Fresnel zones, thus allowing onlysome of the transmitted energy to reach the receiver.
Depending on the geometry of the obstruction, thereceived energy will be a vector sum of the energy
contributions from all unobstructed Fresnel zones.
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Diffraction – Fresnel ZoneGeometry
If an obstruction does not block the volumecontained within the first Fresnel zone, thediffraction loss will be minimal, and diffractioneffects may be neglected
In fact, a rule of thumb used for design of line-of-sight microwave links is that as longas 55% of the first Fresnel zone is kept clear,then further Fresnel zone clearance does notsignificantly alter the diffraction loss
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
It is impossible to make very precise estimates of thediffraction losses
In practice, prediction is a process of theoreticalapproximation modified by necessary empiricalcorrections
The limiting case of propagation over a knife-edgevies good insight into the order of magnitude of diffraction loss
When shadowing is caused by a single object such asa hill or mountain, the attenuation caused bydiffraction can be estimated by treating theobstruction as a diffracting knife-edge
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
In many practical situations, especially in hill terrain,the propagation path may consist of more than oneobstruction, in which case the total diffraction lossdue to all of the obstacles must be computed
Bullington suggested that the series of obstacles bereplaced by a single equivalent obstacle so that thepath loss can be obtained using single knife-edge
diffraction model This method oversimplifies the calculations and often
provides very optimistic estimates of received signalstrength
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
surface, the reflected energy is spreadout (diffused) in all directions due toscattering
The actual received signal is oftenstronger than what is predicted by
reflection and diffraction models alone
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Practical Link Budget Design
Most radio propagation models are derived using acombination of analytical and empirical models
Empirical approach is based on fitting curves oranalytical expressions that recreate a set of measured data Advantages: Takes into account all propagation factors, both
known and unknown
Disadvantages: New models need to be measured fordifferent environment or frequency
Over many years, some classical propagation modelshave been developed, which are used to predictlarge-scale coverage for mobile communicationsystem design
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Log-normal Shadowing Since PL(d) is a random variable with a normal
distribution in dB about the distance dependentmean, so is P r (d) , and Q -function or error function(erf ) amy be used to determine the probability thatthe received signal level will exceed a particular level
The Q -function2
21 1
( ) 1
2 2 2
x
z
z Q z e d x e r f
( ) 1 ( )Q z Q z
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Log-normal Shadowing
The probability that the received signal level willexceed a certain value γ can be calculated from thecumulative density function
Similarly, the probability that the received signal levelwill be belowγ is given by
( )
Pr ( ) r r
P d P d Q
( )
Pr ( ) r r
P d P d Q
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
of Coverage Area For a circular coverage area having radius R
from a base station, let there be somedesired received signal threshold γ
We are interested in computing U(γ) , thepercentage of useful service area, i.e. thepercentage of area with a received signal thatis equal or greater than γ
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Determination of Percentageof Coverage Area
Let d = r represent the radial distance from thetransmitter
It can be shown that Pr[P r (r) > γ] is the probabilitythat the random received signal at d = r exceeds thethreshold γ within an incremental area dA
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
(b) Calculate the standard deviation aboutthe mean value
(c) Estimate the received power at d = 2 kmusing the resulting model
(d) Predict the likelihood that the receivedsignal at 2 km will be greater than -60dBm.
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Example
The MMSE estimate may be found using the followingmethod: Let p i be the received power at a distance d i and let be the estimate for pi using the (d/d 0 )
n
path loss model. The sum of squared errors betweenthe measured and estimated is given by
The value of n which minimizes the mean squareerror can be obtained by equating the derivative of
J (n ) to zero, and then solving by n
i p̂
k
i
ii p pn J
1
2ˆ)(
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Longley-Rice Model Applicable to point-to-point communication systems
in frequency range from 40MHz to 100GHz, overdifferent kind of terrain
Longley-Rice propagation prediction model is alsoreferred to as the ITS irregular terrain model
Longley-Rice model is also available as a computerprogram calculate large-scale median transmissionloss relative to free space loss over irregular terrainfor frequencies between 20MHz to 10GHz
There have been many modifications and corrections
to the Longley-Rice model to deal with radiopropagation in urban area – this is particularlyrelevant to mobile radio
84
Okumura Model
It is one of the most widely used models forsignal prediction in urban area.
This model is applicable for frequencies inrange 10MHz to 1920MHz (although it istypically extrapolated up to 3000MHz) anddistance of 1Km to 100Km
It can be used for base station antennaheight ranging from 30m to 1000m
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Okumura Model Okumura developed a set of curves giving the
median attenuation relative to free space ( A m u) , inan urban area over a quasi-smooth terrain with abase station effective antenna height (h te ) of 200mand a mobile antenna height (h r e) of 3 m
To determine path loss using Okumura’s model, thefree space loss between the points of interest is firstdetermined, and then the value of A mu(f,d) (as readfrom the curves) is added to it along with correction
factors to account for the type of terrain.
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Okumura Model
The model can be expressed as
Where L 50 is the 50th percentile (i.e. median) value of propagation path loss
L F is the free space path loss
A mu is the median attenuation relative to free space
G (h re ) is the base station antenna height gain factor
G (h re ) is the mobile antenna height gain factor
G area is the gain due to the type of environment
50( ) ( , ) ( ) ( )
F mu te re area L dB L A f d G h G h G
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Okumura Model The major disadvantage with the model
is its slow response to rapid changes interrain, therefore the model is fairlygood in urban and suburban area, butnot good as in rural area
Common standard deviations between
predicted and measured path lossvalues are around 10dB to 14dB
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Example
Find the median path loss using Okumura’smodel for d = 50Km, h t e = 100m, h re = 10m inan urban environment. If the base stationradiated an EIRP of 1KW at a carrierfrequency of 900MHz. Find the power at thereceiver (assume a unity gain receiving
antenna)
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Hata Model Although Hata’s model does not have any of
the path specific corrections which areavailable in Okumura’s model, theexpressions have significant practical value
The predictions of Hata model compare veryclose with the original Okumura model, aslong as d exceeds 1Km.
This model is well suited for large cell mobilesystems, but no personal communicationssystems (PCS) which have cells on the orderof 1Km radius
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PCS Extension to Hata Model
European Co-operative for Scientific andTechnical research (EURO-COST) formed theCOST-231 working committee to develop anextended version of Hata model
COST-231 proposed the following formula toextend Hata model to 2GHz
The COST-231 extension of the Hata model is
restricted to the following parameters: f : 1500MHz – 2000MHz h te : 30m to 200m h re : 1m to 10m d : 1Km to 10Km
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Indoor Propagation Models The indoor radio channel differs from the traditional
mobile radio channel in two aspects – the distancescovered are much smaller, and the variability of theenvironment is much greater for a much smaller of T-R separation distances.
Propagation within buildings is strongly influenced byspecific features such as the layout of the building,construction materials and the building type
Indoor radio propagation is dominated by the samemechanisms as outdoor: reflection, diffraction, and
scattering In general, indoor channels may be classified either
as line-of-sight (LOS) or obstructed (OBS) withvarying degrees of clutter
106
Partition Losses (Same Floor)
Buildings have a wide variety of partitionsand obstacles which form the internal andexternal structure
Partitions that are formed as part of thebuilding structure are called hard partitions,and partitions that may be moved and whichdo not span to the ceiling are called soft
partition Partitions vary widely in their physical and
electrical characteristics, making it difficult toapply general models to specific indoorinstallation
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
The losses between floors of a building aredetermined by the external dimensions andmaterials of the building, as well as, the typeof construction used to create the floors andthe external surroundings
The following table illustrated values for floorattenuation factor (FAF) in three buildings
It can be seen that for all three buildings, theattenuation between one floor of the buildingis greater than the incremental attenuationcaused by each additional floor
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
The Ericsson radio system model obtained bymeasurement in a multiple floor office building
The model has four breakpoints and considers bothan upper and lower bound on the path loss
The model also assumes that there is 30dBattenuation at d0 = 1m, which can be shown to beaccurate for f = 900MHz and unity gain antenna
Rather than assuming a log-normal shadowingcomponent, the Ericsson model provides adeterministic limit on the range of path loss at aparticular distance
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
An in-building propagation model thatincludes the effect of building type as well asthe variations caused by obstacles
This model provides flexibility and was shownto reduce the standard deviation betweenmeasured and predicted path loss to around4dB, as compare to 13dB when only a log-distance model was used in two differentbuildings
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Devasirvatham, et. al. found that in-building pathloss obeys free space plus an additional loss factorwhich increase exponentially with distance as infollowing table
8/2/2019 Mobile Ppropagation Channel - Large-scale Path Loss (2 Slides)
Buildings The signal strength inside of a building due to an external
transmitter is important for wireless systems that sharefrequencies with neighboring buildings or with outdoor systems
As with propagation measurements between floors, it is difficultto determine exact models for penetration as only a limitednumber of experiments have been published, and they aresometimes difficult to compare
Some generalizations can be made from the literaturemeasurements reported: signal strength received inside abuilding increase with height. At the lower floors of a building,the urban clutter induces greater attenuation and reduces thelevel of penetration. At the higher floor, a LOS path may exist,
thus causing a stronger incident signal at the exterior walls of the building