Post on 18-Nov-2014
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Wireless and Mobile Communications
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Channel or Communication Channel
−Channel is a medium used to convey information from a sender to a receiver.
−Channel is a path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths.
−In a communications system, the part that
connects a data source to a data sink is called Channel.
−A channel can be modelled physically by trying to calculate the physical processes which modify the transmitted signal.
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Examples of Channel
−Examples of communications channels include:
A connection between initiating and terminating nodes of a circuit.
A single path provided by a transmission medium via either physical separation, such as by multi pair cable or electrical separation, such as by frequency- or time-division multiplexing.
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Wireless Channel is Very Different!− Wireless channel “feels” very different from a wired channel as Wireless
Channel is . Not a point-to-point link Variable capacity, errors, delays Capacity is shared with interferers
− Characteristics of the channel appear to change randomly with time, which makes it difficult to design reliable systems with guaranteed performance.
− Cellular model vs reality:
Cellular system designs are interference-limited, i.e. the interference dominates the noise floor
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Mobile Radio Channel
−Effect of Mobility The mobile radio channel places fundamental
limitations on the performance of wireless communication systems.
Channel varies with user location and time Radio propagation is very complex Multipath scattering from nearby objects Shadowing from dominant objects Attenuation effects Results in rapid fluctuations of received power
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Radio Propagation
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Radio Propagation
−“Radio Propagation” is a term used to explain
“How radio waves behave when they are transmitted or are propagated from one point on the Earth to another”?
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Radio Propagation
− Radio propagation is somewhat unpredictable
− Radio waves at different frequencies propagate in different ways.
− A satellite link, though expensive, can offer highly predictable and stable line of sight coverage of a given area
− In free space, all electromagnetic waves obey the “Inverse-square law” which states that the power density of an electromagnetic wave is
proportional to the inverse of the square of the distance from the source
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transmittedsignal
receivedsignal
Ts
Radio Propagation Illustration
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Mobile Radio Propagation Environment
− Radio signals generally propagate according to three mechanisms; reflection, diffraction, and scattering. Reflections arise when the plane waves are incident upon a surface with dimensions that are very large compared to the wavelength. Diffraction occurs according to Huygen’s principle when there is an obstruction between the transmitter and receiver antennas, and secondary waves are generated behind the obstructing body. Scattering occurs when the plane waves are incident upon an object whose dimensions are on the order of a wavelength or less, and causes the energy to be redirected in many directions.
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−The relative importance of these three propagation mechanisms depends on the particular propagation scenario.
−As a result of the above three mechanisms, macrocellular radio propagation can be roughly characterized by three nearly independent phenomenon; Path loss variation with distance (Large Scale
Propagation )
Slow log-normal shadowing (Medium Scale Propagation )
Fast multipath fading. (Small Scale Propagation )
Mobile Radio Propagation Environment
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−Each of these phenomenon is caused by a different underlying physical principle and each must be accounted for when designing and evaluating the performance of a cellular system.
Mobile Radio Propagation Environment
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Transmission path between Transmitter and Receiver
Line-of-Sight (LOS)
Obstructed by buildings, mountains and foliage
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− The physical mechanisms that govern radio propagation are complex and diverse
− Generally attributed to the following four factors Direct Mode Reflection Ddiffraction Scattering.
− They have an impact on the wave propagation in a mobile communication system
− The most important parameter, “Received power” is predicted by Large Scale Propagation models based on the physics of reflection, diffraction and scattering
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Line Of Sight (LOS) Non Line Of Sight (NLOS)
Radio Propagation Mechanisms
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Radio Propagation mechanisms
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Direct Path (Line of Sight )
− Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other.
− This is probably the most common of the radio propagation modes at VHF and higher frequencies.
− Radio signals can travel through many non-metallic objects, radio can be picked up through walls.This is still line-of-sight propagation.
− Examples would include propagation between a satellite and a ground antenna or reception of television signals from a local TV transmitter.
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Direct Path (Line of Sight )
− The received signal is directly received at the receiver the effects such as reflection, diffraction and scattering doesn’t affect the signal reception that much.
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Line Of Sight (LOS) Non Line Of Sight (NLOS)
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Radio Propagation Models-Large Scale Path Loss
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Free Space Propagation Model
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Free Space Propagation Model
−Utility of Free Space Propagation Model
The Free Space Propagation Model is used to predict
• Received Signal Strength when the transmitter and receiver have a clear, unobstructed LoS between them.
• Satellite communication systems and microwave
line-of-sight radio links typically undergo free space propagation.
• As with most large-scale radio wave propagation models, the free space model predicts that
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Free Space Propagation Model
− Path Loss Signal attenuation as a positive quantity measured in
dB and defined as the difference (in dB) between the effective transmitter power and received power.
− Friis is an application of the standard “Free Space Propagation Model “
− It gives the Median Path Loss in dB ( exclusive of Antenna Gains and other losses )
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−The ideal conditions assumed for this model are almost never achieved in ordinary terrestrial communications, due to obstructions, reflections from buildings, and most importantly reflections from the ground.
−One situation where the equation is
reasonably accurate is in satellite communications when there is negligible atmospheric absorption;
Free Space Propagation Model
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Free Space Propagation Model
• Pr(d) = Power Received by a Receiver antenna separated by a Radiating Transmitter Antenna by a distance “ d”
(Function of the T-R separation ) • Pt= Transmitted power• Gt= Transmitter Antenna Gain (dimensionless)• Gr= Receiver Antenna Gain (dimensionless )• d = T-R separation distance in meters,• L = System loss.
The miscellaneous losses usually due to transmission line attenuation, filter losses, and antenna losses in the communication system.
(L greater than or equal to 1) A value of L = I indicates no loss in the system
hardware.
− The free space power received by a receiver antenna which is separated from a radiating transmitter antenna by a distance “d”, is given by the Friis free space equation,
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Free Space Propagation Model
− Conclusion of Friis free space equation
− The received power falls off as the square of the T-R separation distance.
− The Friis free space model is only a valid predictor for “Pr ” for values of “d” which are in the far-field of the “Transmitting antenna”
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Path loss with and without antenna gains
− Represents signal attenuation as positive quantity measured in dB, is defined as the difference (in dB) between the effective transmitted power and the received power
− The path loss for the free space model when antenna gains are included is given by
− When antenna gains are assumed to be unity,
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−Thus in practice, power can be measured at d0 and predicted at d using the relation
Free Space Propagation Model
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− if Pr is in units of dBm, the received power is given by
Free Space Propagation Model
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Free Space Propagation Model
−The effects of following can be included by adding additional factors due to impedance mismatch misalignment of the antenna pointing and
polarization and absorption can be included
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Summary
Modified free space equationPr(d) = Pr(d0)(d0/d)2
Modified free space equation in dB formPr(d) dBm = 10 log[Pr(d0)/0.001W] + 20 log(d0/d)
where d>= d0 >= df
df is Fraunhofer distance which complies:
df =2D2/where D is the largest physical linear dimension of the antenna
In practice, reference distance is chosen to be 1m (indoor) and 100m or 1km(outdoor) for low-gain antenna system in 1-2 GHz region.
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Example 3.1of Book (Wireless Communications (Principles and Practices ) by Theodore S.
Rappaport
−Find the far-field distance for an antenna with maximum dimension of 1 m and operating frequency of 900 MHz.
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Example 3.2 of Book (Wireless Communications (Principles and Practices ) by Theodore S. Rappaport
−If a transmitter produces 50 watts of power, express the transmit power in units of (a) dBm, and (b) dBW. If 50 watts is applied to a unity gain antenna with a 900 MHz carrier frequency, find the received power in dBm at a free space distance of 100 m from the antenna, What is Pr (10 km)? Assume unity gain for the receiver antenna.
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Non Line of Sight (NLoS)
− When the direct LOS between transmitter and receiver is lost the effects such as reflection, diffraction and scattering become very important as in the absence of direct path they become the main contributors to the received signal at the receiver.
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Line Of Sight (LOS) Non Line Of Sight (NLOS)
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Reflection
−Occurs when waves impinges upon an obstruction that is much larger in size compared to the wavelength of the signal
−Example: reflections from earth and buildings
−These reflections may interfere with the original signal constructively or destructively
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Reflections
− Waves bouncing off of objects of large dimensions
Large buildings, earth surface etc.
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Reflections
Reflection occurs when RF energy is incident upon a boundary between two materials (e.g air/ground) with different electrical characteristics • Permittivity µ
– relates to a material's ability to transmit (or "permit") an electric field.
• Permeability ε – degree of magnetization of a material that
responds linearly to an applied magnetic field • Conductance σ
Reflecting surface must be smooth
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− Reflection from a Perfect Dielectric If the plane wave is incident on a perfect
dielectric, part of the energy is transmitted into the second medium and part of the energy is reflected back into the first medium, and there is no loss of energy in absorption.
− Reflection from a Perfect Conductor If the second medium is a perfect conductor,
then all incident energy is reflected back into the first medium without loss of energy.
Reflections
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− Perfect conductors reflect with no attenuation
− Dielectrics reflect a fraction of incident energy
r
t
Reflections
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Ground Reflection (2- ray) Model
−In a mobile radio channel, a single direct path between the base station and mobile is rarely the only physical path for propagation
−Hence the free space propagation model in most cases is inaccurate when used alone
−The 2- ray GRM is based on geometric optics
−It considers both- direct path and ground reflected propagation path between transmitter and receiver
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Ground Reflection (2-Ray) Model
− This was found reasonably accurate for predicting large scale signal strength over distances of several kilometers for mobile radio systems using tall towers ( heights above 50 m ),
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Accounting for Ground Reflection
Much more rapid path loss than expected due to free spacesMuch more rapid path loss than expected due to free spaces
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Diffraction
− Occurs when the radio path between sender and receiver is obstructed by an impenetrable body and by a surface with sharp irregularities (edges)
− Explains how radio signals can travel urban and rural environments without a line-of-sight path
− Obstacles with dimensions in order of lambda
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−Multipath fading Constructive/ Destructive combination of the
electromagnetic waves at the receive antenna
Diffraction
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(a) Constructive and (b) destructive forms of the Multipath phenomenon for sinusoidal signals
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Diffraction− Diffraction
Waves bending around sharp edges of objects
Obstacles with dimensions in order of lambda
− Diffraction occurs when waves hit the edge of an obstacle “Secondary” waves propagated into the
shadowed region Excess path length results in a phase
shift Fresnel zones relate phase shifts to the
positions of obstacles
TR
1st Fresnel zone
Obstruction
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Diffraction− RF energy can propagate:
around the curved surface of the Earth beyond the line-of-sight horizon Behind obstructions
− Although EM field strength decays rapidly as Rx moves deeper into “shadowed” or obstructed (OBS) region
− The field strength of a diffracted wave in the shadowed region is the vector sum of the electric field components of all the secondary wavelets in the space around the obstacles.
− The diffraction field has sufficient strength to produce a useful signal
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− Based on Huygen’s principle of wave propagation.
− Huygen’s principle says points on a wave front can be considered sources for additional wavelets.
Diffraction
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− Diffraction depends on
Geometry of the object λ Amplitude Phase Polarization of the incident wave
Diffraction
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Knife-edge Diffraction Model
− Simplest diffraction model
− When shadowing is caused by a single object such as a hill or mountain, the attenuation caused by diffraction can be estimated by treating the obstruction as a diffracting knife edge
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− Impossible to make very precise estimates of the diffraction losses
− Practically prediction is a process of theoretical approximation modified by necessary empirical corrections
Knife-edge Diffraction Model
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Knife-edge Diffraction Model
− A receiver at point R is located in the shadowed region (also called the diffraction zone).
− The field strength at point R is a vector sum of the fields due to all of the secondary Huygen's sources in the plane above the knife edge.
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Multiple Knife-edge Diffraction
−The propagation path may consist of more than one obstruction hilly terrain The total diffraction will be due to all of the
obstacles must be computed−Bullington Suggestion
The series of obstacles be replaced by a single equivalent obstacle so that the path loss can be obtained using single knife-edge diffraction model.
Oversimplifies the calculations and often provides very optimistic estimates of the received signal strength.
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Multiple Knife-edge Diffraction
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−Millington Suggestion More rigorous treatment wave-theory solution for the field behind two
knife edges in series. Prediction of diffraction losses due to two
knife edges. Extending this to more than two knife edges
becomes a formidable mathematical obstruction
Multiple Knife-edge Diffraction
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− Obstacles with size in the order of the wavelength of the signal or less
− Foliage, lamp posts, street signs, walking pedestrian, etc. cause scattering
− They are produced by small objects, rough surfaces and other irregularities on the channel
− Causes the transmitter energy to be radiated in many directions
Scattering
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Scattering− Scattering
Waves traveling through a medium with small objects in it
foliage, street signs, lamp posts, etc. or reflecting off rough surfaces Obstacles with size in the order of the wavelength
of the signal or less− The EM wave incident upon a rough or complex
surface is scattered in many directions and provides more energy at a receiver energy that would have been absorbed is instead
reflected to the Rx.− Scattering is caused by trees, lamp posts, towers,
etc. flat surface → EM reflection (one direction) rough surface → EM scattering (many directions)
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Scattering
− Generally difficult to model because the environmental conditions that cause it are complex
− Modeling “position of every street sign” is not feasible.
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Scattering
−Nearby metal objects (street signs, etc.) Usually modelled statistically
−Large distant objects Analytical model:
• Radar Cross Section (RCS)
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Radar Cross Section model
− The radar cross section of a scattering object is defined as the ratio of the power density of the signal scattered in the direction of the receiver to the power density of the radio wave incident upon the scattering object, and has units of square meters.
− Why do we require this?− In radio channels where large, distant
objects induce scattering, the physical location of such objects can be used to accurately predict scattered signal strengths.
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Radar Cross Section (RCS) Model
RCS (Radar Cross Section) =
Power density of scattered wave in direction of receiver
Power density of radio wave incident
on the scattering object
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Radar cross section model:
− For urban mobile radio systems ,− Models based on the bistatic radar equation
is used to compute the received power due to scattering in the far field.
− The bistatic radar equation − It describes the propagation of a wave
traveling in free space which impinges on a distant scattering object, and is the reradiated in the direction of the receiver, given by
RT2
TTR 20logd -20logd - )30log(4-]RCS[dBm)20log((dBi)G(dBm)P(dBm)P
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− Where dT and dR are the distance from the scattering object to the transmitter and receiver respectively.
− Scattering object is assumed to be in the(far field) Fraunhofer region of both the transmitter and receiver
− Useful for predicting receiver power which scatters off large objects such as buildings
Radar cross section model:
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Radar Cross Section (RCS) Model
PR = PT • GT • 2 • RCS
(4)3 • dT • dR
Where,PT = Transmitted Power
GT = Gain of Transmitting antenna
dT = Distance of scattering object from Transmitter
dR = Distance of scattering object from Receiver
PR = PT • GT • 2 • RCS
(4)3 • dT • dR
Where,PT = Transmitted Power
GT = Gain of Transmitting antenna
dT = Distance of scattering object from Transmitter
dR = Distance of scattering object from Receiver
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Radio Propagation Mechanisms
s
s
l
Diffraction/Shadowing: “bending” around sharp edges,
Scattering: small objects, roughsurfaces (<): foilage, lamposts, street signs
Reflection/Refraction: large objects (>>)
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We have studies
Propagation in free space always like light (straight line) Received power proportional to 1/d²
(d = distance between sender and receiver) Receiving power additionally influenced by
fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges
reflection scattering diffractionshadowing refraction
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Signal Propagation in the “Real World”
a wave can be absorbed
reflectreflect
penetratepenetrate
bend
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The Cluttered World of Radio Waves
walls
hallwayswindows
trees
vehicles
rain
hills
girders
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Mobile movement through a coverage area
− As a mobile moves through a coverage area, different propagation mechanisms have an impact on the instantaneous received signal strength.
− When a mobile has a clear LoS path to the base-station Diffraction and scattering will not dominate the
propagation.
− When a mobile is at a street level without LOS Diffraction and scattering will dominate the propagation.
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− No direct LoS path between Transmitter and Receiver
− Presence of high-rise buildings causing severe diffraction loss.
− Due to multiple reflections from various objects, the electromagnetic waves travel along different paths of varying lengths.
− The interaction between these waves causes multipath fading at a specific location,
− Strengths of the waves decrease as the distance between the transmitter and receiver increases.
Urban cellular radio systems
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Radio Propagation Models
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−The design of spectrally efficient wireless communication systems requires a detailed understanding of the radio propagation environment. The characteristics of the radio channel vary greatly with the operating frequency, and the mode of propagation, e.g., line-of-sight (LoS) radio links, diffraction/scatter, and satellite links.
Radio Propagation Models
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− A typical cellular radio system consists of a collection of fixed base stations (BSs) that define the radio coverage areas or cells.
− The height and placement of the BS antennas affects the proximity of local scatterers at the BS.
− In a macrocellular environment, the BS antennas are usually well elevated above the local terrain and relatively free of local scatterers.
− Typically, a non-line-of sight (NLoS) radio propagation path will exist between a BS and mobile station (MS), because of natural and man-made objects that are situated between the BS and MS.
Radio Propagation Models
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Radio Propagation Models
− As a consequence the radio waves must propagate via reflections, diffraction and scattering.
− At the MS, plane waves arrive from many different directions and with different delays, as shown in Figure . This property is called multipath propagation.
− The multiple plane waves combine vectorially at the receiver antenna to produce a composite received signal.
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− Small changes in the differential propagation delays due to MS mobility will cause large changes in the phases of the individually arriving plane waves.
− Hence, the arriving plane waves arriving at the MS and BS antennas will experience constructive and destructive addition depending on the location of the MS.
− If the MS is moving or there are changes in the scattering environment, then the spatial variations in the amplitude and phase of the composite received signal will manifest themselves as time variations, a phenomenon called envelope fading.
− As we will see later, the time rate of envelope fading depends on the velocity of the MS.
Radio Propagation Models
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− Radio channels are reciprocal in the sense that if a propagation path exists, it carries energy equally well in both directions.
− However, the spatial distribution of arriving plane waves may be significantly different in each direction.
− A Mobile Station in a typical macrocellular environment is usually surrounded by local scatterers so that the plane waves will arrive from many directions without a direct LoS component.
− Two-dimensional isotropic scattering where the arriving plane waves arrive in from all directions with equal probability is a very commonly used scattering model for the forward channel in a macrocellular system.
− For this type of scattering environment the received envelope is Rayleigh distributed at any time, and is said to exhibit Rayleigh fading.
Radio Propagation Models
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Radio Propagation Models
− The BSs in macrocells are relatively free from local scatterrs so that the plane waves tend to arrive from one direction with a fairly small angle of arrival (AoA) spread as shown. These differences in the scattering environment for the forward and reverse channels cause difference in the spatial correlation properties of their respective faded envelopes.
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−In a microcellular environment, the BS antennas are often placed below the skyline of buildings and are surrounded by local scatterers, such that the plane waves will arrive at the BS with a larger AoA spread. Furthermore, a LoS path will sometimes exist between the MS and BS, while at others times there is no LoS path.
Radio Propagation Models
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− Even in the absence of LoS propagation conditions, there often exists a dominant reflected or diffracted path between the MS and BS.
− The LoS or dominant reflected or diffracted path produces the specular component and the multitude of weaker secondary paths contribute to the scatter component of the received envelope.
− In this type of propagation environment, the received signal envelope still experiences fading. However, the presence of the specular component changes the received envelope distribution, and very often a Ricean distributed envelope is assumed.
− In this case the received envelope is said to exhibit Ricean fading.
Radio Propagation Models
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−If the envelope or squared-envelope is measured and averaged over a spatial distance of 20 to 30 wavelengths, the mean envelope or mean squared-envelope can be obtained.
−Sometimes, this quantity is called the local mean because it corresponds to the mean value a particular locality.
−Usually, the local mean will also experience slow variations over distances of several tens of wavelengths due to the presence of large terrain features such as buildings and hills.
−.
Radio Propagation Models
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− This phenomenon is known as shadow fading or shadowing. Experimental observations have confirmed that the shadow fades follow a log-normal distribution .
− This log-normal distribution applies to both macrocellular and microcellular environments.
− If the local mean is averaged over sufficiently large spatial distances (to average over the shadows), the area mean is obtained.
− The area mean is the average signal strength that is received to/from a MS over a large area that lies at (approximately) the same distance from the BS.
− The area mean is directly related to the path loss, which predicts how the area mean varies with the distance between the BS and MS.
Radio Propagation Models
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−Early studies by Okumura and Hata yielded empirical path loss models for urban, suburban, and rural areas that are accurate to within 1 dB for distances ranging from 1 to 20 km. These studies concentrated on macrocellular systems.
−More recent work has considered path loss prediction in microcells.
−The COST231 study resulted in the COST231-Hata and COST231-Walfish-Ikegami models for urban microcellular path loss prediction.
Radio Propagation Models
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Radio Propagation Models− “Radio Propagation model” or “Radio Wave Propagation
Model” or “Radio Frequency Propagation Model”
− It is an “Empirical Mathematical Formulation”
− Developed on the basis of large collections of data gathered for the specific scenario
− Used for the characterization of radio wave propagation as a function of Frequency Distance Others.
− A single model is developed to predict the behavior of propagation for all similar links under similar constraints.
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Radio Propagation Models
− Propagation models have traditionally focused on
predicting the “Average Received Signal strength” at a given distance from the transmitter
the goal of formalizing the way , radio waves are propagated from one place to another
the variability of the signal strength in close spatial proximity to a particular location
Predicting path loss along a link or the effective coverage area of a transmitter.
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Why propagation models are designed ?
− Modeling radio channel is important for determining :
Propagation characteristics
Coverage area of a transmitter
Transmitter power requirement
Battery lifetime
Modulation and coding schemes to improve the channel quality
The maximum channel capacity
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Why different Radio Propagation Models ?
−Radio channels are extremely random and do not offer easy analysis (Unlike wired channels that are stationary and
predictable)
− Each individual telecommunication link has to encounter different Terrain Path Obstructions Atmospheric conditions and other phenomena
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Why different Radio Propagation Models ?
−It is intractable to formulate the exact loss for all telecommunication systems in a “single mathematical equation”.
−Different models exist for different types of radio links under different conditions.
−The models rely on computing the “Median Path loss” for a link under a certain probability that the considered conditions will occur.
−Impacts of the speed of motion how rapidly the signal level fades as a mobile
terminal moves in space.
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Limitations of Radio Propagation Model
− For any model, the collection of data has to be sufficiently large to provide enough likeliness (or enough scope) to all kind of situations that can happen in that specific scenario.
− Like all empirical models, radio propagation models do not point out the exact behavior of a link. They predict the most likely behavior the link may exhibit under the specified conditions.
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Types of models for radio propagation
− Different models have been developed to meet the needs of realizing the propagation behavior in different conditions.
Models for outdoor applications
Models for indoor applications
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Outdoor Propagation Models
− There are a number of mobile radio propagation models to predict path loss over irregular terrain.
− These methods generally aim to predict the signal strength at a particular sector. But they vary widely in complexity and accuracy.
− These models are based on systematic interpretation of measurement data obtained in the service area.
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Indoor Propagation Models
− Indoor radio channel differs from traditional mobile radio channel in: −distances covered are much smaller−variability of the environment is greater for a
much smaller range of T-R separation distances
− It is strongly influenced by specific features, such as −layout of the building−construction materials−building type
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Fading Processes
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Power
distance A Large scale effect - Path Loss
B Medium scale effect - shadowing
C Small scale effect - rapid fluctuations in signal amplitude
CB
A
Mobile movement through a coverage area
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Wireless Channel and Fading Processes
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Wireless Channel and Fading Processes
Pr ~ 1/r2
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Propagation Models− Maxwell's equations
− These are too complex to model the propagation.
− Propagation Models − They are normally used to predict the “average signal
strength” at a given distance from the transmitter.
– Large Scale or Path Loss propagation model
– Propagation models that predict the mean signal strength for an arbitrary T-R separation distance are useful in estimating the radio coverage area.
– This is called the Large Scale or Path Loss propagation model (several hundreds or thousands of meters);
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Small Scale Fading
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Radio Propagation Models-Small Scale Fading– Small Scale or Fading models.
– Propagation models that characterize the rapid fluctuations of the received signal strengths over very short distance or short duration (few seconds) are called Small Scale or Fading models.
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Radio Propagation Models-Small Scale Fading− As the mobile moves over small distances, the instantaneous
received signal will fluctuate rapidly giving rise to small-scale fading
− The reason is that the signal is the sum of many contributors coming from different directions and since the phases of these signals are random, the sum behave like a noise (Rayleigh fading).
− In small scale fading, the received signal power may change as much as 3 or 4 orders of magnitude (30dB or 40dB), when the receiver is only moved a fraction of the wavelength.
− Even when mobile is stationary, the received signals may fade due to movement of surrounding objects!
− Small scale fades Multipath effects:
• Rapid changes in signal strength over a small area or time interval• Random frequency modulation due to varying• Doppler shifts on different Multipath signals• Time dispersion (echoes) caused by Multipath propagation delays
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−In telecommunications, fading is a change in
the attenuation of a communications channel.
−Propagation models that characterize the rapid fluctuations of the received signal strength over
• very short travel distances (a few wavelengths)
• or short time durations (on the order of
seconds)
Radio Propagation Models-Small Scale Fading
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− As a mobile moves over very small distances, the instantaneous received signal strength may fluctuate rapidly giving rise to small-scale fading.
− The reason for this is that the received signal is a sum
of many contributions coming from different directions.
− In small-scale fading, the received signal power may vary by as much as three or four orders of magnitude when the receiver is moved by only a fraction of a wavelength.
− Since the phases are random, the sum of the contributions varies widely. For example, obeys a Rayleigh fading distribution.
Radio Propagation Models-Small Scale Fading
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−Figure 3.1 illustrates small-scale fading and the slower large-scale variations for an indoor radio communication system.
−Notice in the figure that the signal fades rapidly as the receiver
moves but the local average signal changes much
more slowly with distance.
−Small-scale or fading models describes methods to measure and model Multipath in the mobile radio environment.
Radio Propagation Models-Small Scale Fading
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Medium Scale Fading
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Medium Scale Fading (Shadowing)− Shadowing: It is the term given to the slow variations in received signal power as the
user moves through the environment, especially behind large buildings or near by hills. These variations occur approx. 1 -2 times per second, that’s why Slow Fading! Reflected Scattered Path Diffracted Path
− Shadowing Behind mountains Large buildings
− Shadows: signals blocked by obstructing structures
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Large Scale Fading
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Radio Propagation Models-Large Scale Path Loss
− As the mobile moves away from the transmitter over larger distances, the local average received signal will gradually decrease. This is called large-scale path loss.
− Typically the local average received power is computed by averaging signal measurements over a measurement track.
− The models that predict the mean signal strength for an arbitrary-receiver transmitter (T-R) separation distance are called large-scale propagation models
− Useful for estimating the coverage area of transmitters
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Radio Propagation Models-Large Scale Path Loss
−Characterize signal strength over large T-R separation distances (several hundreds or thousands of meters).
−In free space, received power attenuates like 1/r2.
−With reflections and obstructions, signal can attenuate even more rapidly with distance.
−Large Scale Path Loss Models are important for cell site planning
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Path Loss
−Represents signal attenuation as a positive quantity measured in dB,
−Defined as the difference (in dB) between the “Effective transmitted power” and the “Received power”, and may or may not include the effect of the antenna gains
−Path loss encountered along any radio link
−Path Loss serves as the dominant factor for characterization of propagation for the link
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Small scale, Medium scale and large scale fading
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Small scale, Medium scale and large scale fading