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Mobile Radio Propagation
Contents Free space propagation
Basic Propagation modelso Reflection
o Diffraction
o Scattering
Path Loss and Shadowing Models
Wave Propagation Radio, microwave, infrared and visible light portions of the spectrum can all be used to transmit information
o By modulating the amplitude, frequency, or phase of the waves.
The amount of information a wireless channel can carry is related to its bandwidth
Wavelength dictates the optimum size of the receiving antenna
Characteristics of Radio Waves Easy to generate
Can travel long distances
Can penetrate buildings
Used for both indoor and outdoor communication
Can be narrowly focused at high frequencies (greater than 100MHz) using parabolic antennas (like satellite dishes)
Subject to interference from other radio wave sources
Characteristics of Radio Waves (cont.)Properties of radio waves are frequency dependent
At low frequencies, they can pass through obstacles , but the power falls off sharply with increase in distance from the source
At high frequencies, they tend to travel in straight lines and bounce of obstacles (they can also be
absorbed by rain)LOS path
Reflected Wave
Communication ChannelsWired Channel
o Stationary
o Predictable
Wireless channel
o Random
o Typical to analyze
o Susceptible to noise, interference, other time varying channel impairments
Channel models for Wireless Communication Physical models: Considers exact profile of the propagation environment.
o Modes of propagation considered: Free-space or LOS, reflection, and diffraction.
Statistical models: Takes an empirical approach.
o The model is developed on measuring propagation characteristics in a variety of environments. They are easy to describe and use than physical models.
Need for Propagation modelsPropagations models can be used to determine
Coverage area of a transmitter
Transmit power requirement
Battery lifetime
Modulation and coding schemes required to improve the channel quality
Maximum achievable channel capacity of the system
Propagation models Large-scale propagation models
o Characterize signal strength for large T-R separation (several hundreds or thousands of meters)
o Compute local average received power by averaging signal measurements over a track of 5 to 40
o Received signal decrease gradually
o Useful for estimating the coverage area of transmitters
Small-scale propagation models
o Characterize rapid fluctuations in the received signal strength over very short travel distances (a few wavelengths)
o Signal is the sum of many contributors coming from different directions. Thus phases of received signals are random and the sum behave like a noise (Rayleigh fading)
o Received power may vary by as much as 3 or 4 orders of magnitude (30 or 40 dB)
Small-Scale and Large-Scale Fading
Free-Space Propagation Model Predict the received signal strength when transmitter and receiver have clear, unobstructed LOS path between them.
o Ex: Satellite communication system, microwave LOS system
The received power decays as a function of T-R separation raised to some power.
Free space power received by a receiver antenna is given by Friis free-space equation
𝑃𝑟(𝑑) = (𝑃𝑡𝐺𝑡𝐺𝑟𝜆2) / ((4𝜋)2𝑑2𝐿)
oPt is transmitted power
oGt, Gr is the Tx, Rx antenna gain
(dimensionless quantity)
oL is system loss factor not related to propagation (𝐿 ≥ 1). L = 1 indicates no loss in system hardware (we consider L = 1 in our calculations)
o Pr(d) is the received power
o d is T-R separation distance in meters
o is wavelength in meters
Free-Space Propagation Model (cont.) The gain of an antenna G is related to its affective aperture Ae by G = 4Ae /
2 where
o Ae is related to the physical size of the antenna
o is related to the carrier frequency ( = c/f = 2c / c ) where
Isotropic radiator generally considered an reference antenna in wireless systems; radiates power with unit gain uniformly in all directions.
Effective isotropic radiated power (EIRP) is the amount of power that a theoretical isotropic antenna emits to produce peak power density in the direction of maximum antenna gain.
EIRP = PtGt
Antenna gains are given in units of dBi (dB gain with respect to an isotropic antenna)
o f is carrier frequency in Hertz o c is speed of light in meters/sec
o c is carrier frequency in radians per second
What is Decibel (dB)A logarithmic unit used to describe a ratio between two values of a physical quantity (usually measured in units of power or intensity)
oThe ratio of two values 𝑃1 and 𝑃2 in dB is
10 log (𝑃1/𝑃2) dB
oExample: 𝑃1 = 100𝑊 and 𝑃2 = 1𝑊
The ratio is 10 log(100/1) = 𝟐𝟎 𝒅𝑩
dB unit is generally used to describe ratios of numbers with modest size.
dBm Indicates power ratio in dB with 1mW as the reference power
Example: Transmit power = 100𝑊 in dBm is
Transmit power (dBm) = 10 log(100𝑊 / 1 𝑚𝑊)
= 10 log(100,000)
= 50 𝑑𝐵𝑚
Similarly
1 mW = 0 dBm 1 W = 30 dBm 10 W = 40 dBm
100 W = 50 dBm 106 W = 90 dBm
dBW Indicates power ratio in dB with 1𝑊 as the reference power level.
Example: Transmit power = 100𝑊 in dBW is
Transmit power (𝑑𝐵𝑊) = 10 log(100𝑊 / 1𝑊)
= 10 log 100
= 20 𝑑𝐵𝑊
Similarly
1 mW = -30 dBm 1 W = 0 dBm 10 W = 10 dBm
100 W = 20 dBm 106 W = 60 dBm
Free-Space Path Loss Path loss is defined as the difference (in dB) between the effective transmitted power and the received power
Free-space path loss is defined as the path loss of the free-space model
𝑃𝐿(𝑑𝐵) = 10 𝑙𝑜𝑔(𝑃𝑡/𝑃𝑟) = −10 𝑙𝑜𝑔[(𝐺𝑡𝐺𝑟𝜆2)/(4𝜋)2𝑑2]
Friis equation holds when distance 𝑑 is in the far-field of the transmitting antenna
The far-field or Fraunhofer region of a transmitting antenna is defined as the region beyond the far-field distance 𝑑𝑓 given by: o 𝑑𝑓 = 2𝐷2/𝜆 , 𝐷 is the largest physical dimension of the antenna
oAdditionally 𝑑𝑓 >> 𝐷 𝑎𝑛𝑑 𝑑𝑓 >> 𝜆
Reference Distance, 𝑑0 Friis free space eq. does not hold for 𝑑 = 0 Received power reference point, 𝑑0 is used
𝑑𝑓 ≤ 𝑑0 ≤ 𝑑𝑑0 should be smaller than any practical distance a mobile system uses
The power received in free space at a distance greater than d0 is
Pr(𝑑) = Pr(𝑑0)(𝑑0/𝑑)𝑛 where 𝑑𝑓 ≤ 𝑑0 ≤ 𝑑
Reference distance d0 for practical systems: o For frequncies in the range 1 to 2 GHz
1 m in indoor environments
100 m to 1 km in outdoor environments
Radio propagation mechanisms
Source: Radio Frequency and Wireless Communications - Scientific Figure on Research Gate
Source: https://physicsweekly.weebly.com/uploads/2/5/8/4/25849299/6320607.jpg?332
Reflection Reflection occurs when wave impinges upon an obstruction much larger in size compared to the wavelength of the signalo Example: reflections from earth and buildings
Reflected waveform may interfere with the original signal constructively or destructively
Reflection (cont.) When a radio wave propagating in one medium impinges upon another medium having different electrical properties, the wave is partially reflected and partially transmitted
oPerfect dielectric:
Part of the energy is transmitted into the second medium and part of the energy is reflected back into the first medium
no loss of energy in absorption
oPerfect conductor:
All incident energy is reflected back into the first medium
No loss of energy.
The fraction that is reflected is described by the Fresnel equation and is dependent upon the incoming light's polarization and angle of incidence.
Reflection (cont.) EM waves are transmitted in two orthogonal dimensions, referred to as polarizations. Two commonly used orthogonal sets of polarizations are
o Horizontal and Vertical polarization
Vertical polarization is commonly used in terrestrial mobile radio communication. In VHF band, vertical polarization produces a higher field strength near the ground. Also, mobile antennas for vertical polarization are more robust and convenient to implement.
o Left-hand and right-hand circular polarization
Often used in satellite communication. Can be used together for well-designed communication links to double the transmission capacity in a given frequency band.
Ground Reflection (Two-Ray) ModelIn a mobile radio channel, a single direct path between the BS and a mobile is seldom the only physical means for propagation and the Free space propagation model is inaccurate in most cases when used alone. Two-ray model is
Based on geometric optics and it considers both the direct path and a ground reflected propagation path
Reasonably accurate for predicting the large scale signal strength over distances of several kilometers for mobile radio systems that use tall towers.
Ground Reflection Model (cont.) The total received E-field, ETOT
is a result of the direct LOS component 𝐸𝐿𝑂𝑆 and the ground reflected component 𝐸𝑟
𝐸𝑇𝑂𝑇 =𝐸𝐿𝑂𝑆 + 𝐸𝑟
Ground Reflection Model (cont.) Using the method of images, path difference between LOS and ground reflected path can be calculated.
For 𝑑 ≫ ℎ𝑡 + ℎ𝑟, path difference Δ is
Δ = 𝑑′′ − 𝑑′ ≈2ℎ𝑡ℎ𝑟𝑑
Phase difference 𝜃∆ between the two E-field components and the time delay between arrival of the two components is
𝜃∆ =2𝜋∆
𝜆≈4𝜋ℎ𝑡ℎ𝑟𝑑𝜆
Ground Reflection Model (cont.)
For large distance 𝑑 ≫ ℎ𝑡ℎ𝑟
𝑃𝑟 = 𝑃𝑡𝐺𝑡𝐺𝑟ℎ𝑡2ℎ𝑟
2
𝑑4
The received power falls off with distance raised to the fourth power, or at a rate of 40 dB/decade
This is much more rapid path loss than expected due to free space
Diffraction Diffraction occurs when radio wave is obstructed by an impenetrable body or a surface with sharp irregularities (edges)
Due to bending of radio waves it enables communication between devices with no line-of-sight path
Diffraction (cont.) Secondary waves are present throughout the space including the space behind the obstacle due to bending of waves around the obstacle.
Enables communication even when a line of sight path does not exist between transmitter and receiver.
At high frequencies, diffraction depends on the geometry of the object, as well as the amplitude, phase and polarization of the incident wave at the point of diffraction
DiffractionHuygens’ Principal
All points on a wavefront can be considered as point sources for
producing secondary wavelets
Secondary wavelets combine to produce new wavefront in the
direction of propagation
Diffraction arises from propagation of secondary wavefront into
shadowed area
Field strength of diffracted wave in shadow region = electric field
components of all secondary wavelets in the space around the obstacle
Diffraction (cont.) Consider a transmitter-receiver pair in
free space
Obstacle of effective height h with
infinite width is placed between Tx and
Rx o distance from transmitter = d1
o distance from receiver = d2
LOS distance between transmitter & receiver is 𝑑 = 𝑑1 + 𝑑2
Diffraction (cont.) Excess Path Length is the difference between the direct and the diffracted path Δ = Δ𝑑 – (𝑑1+ 𝑑2), where Δ𝑑 = Δ𝑑1+ Δ𝑑2
Δ𝑑𝑖 = ℎ2 + 𝑑𝑖2
Thus, excess path length is
Δ = ℎ2 + 𝑑12 + ℎ2 + 𝑑2
2 − 𝑑1 + 𝑑2
Assuming ℎ << 𝑑1 , 𝑑2 and ℎ >> 𝜆, then by substitution and Taylor Series Approximation
Δ ≈ℎ2
2
𝑑1 + 𝑑2𝑑1𝑑2
Phase difference, ϕ
𝜙 =2𝜋Δ
𝜆≈2𝜋
𝜆
ℎ2
2
𝑑1 + 𝑑2𝑑1𝑑2
=𝜋
2ℎ2
2(𝑑1 + 𝑑2)
𝜆𝑑1𝑑2
DiffractionFresnel zones Fresnel-Kirchoff Diffraction Parameter 𝑣 (dimensionless) characterizes phase difference between the two propagation paths is defined as
𝑣 = ℎ2(𝑑1 + 𝑑2)
𝜆𝑑1𝑑2= 𝛼
2𝑑1𝑑2𝜆(𝑑1 + 𝑑2)
where ℎ refers to the height of the obstruction and 𝛼 is in radians.
Phase difference, 𝜙 is given as
𝜙 =𝜋
2𝑣2
The phase difference between LOS and diffracted path is function of o obstruction’s height & position o transmitters and receivers height and position
DiffractionFresnel zones (cont.) Fresnel Zones explains the concept of diffraction loss as a function of path difference.
Secondary waves in successive regions have a path length 𝑛/2greater than LOS path.
o nth region is the region where path length of secondary waves is n/2greater than that of LOS path length
Regions form a series of ellipsoids with foci at Tx & Rx antennas
Source:
https://upload.wikimedia.org/wikipedia/commons/4/4b/
1st_Fresnel_Zone_Avoidance.png
)(
2
)(2
21
21
21
21
dd
dd
dd
ddh
v =
If h = 0, then and v are 0
TX RXd2
d1 d2d1
If and v are negative, then h
is negative
h
TX RX
DiffractionFresnel zones (cont.)
DiffractionFresnel zones (cont.)
On slicing a specific ellipsoid along the plane perpendicular to LOS yields a circle with radius rn given as
ℎ = 𝑟𝑛 =𝑛𝜆𝑑1𝑑2𝑑1 + 𝑑2
then Kirchoff diffraction parameter is given as
𝑣 = ℎ2(𝑑1 + 𝑑2)
𝜆𝑑1𝑑2=
𝑛𝜆𝑑1𝑑2𝑑1 + 𝑑2
2(𝑑1 + 𝑑2)
𝜆𝑑1𝑑2= 2𝑛
Thus, for given 𝑣 defines an ellipsoid with constant excess path = 𝑛/2 .
T
R
DiffractionFresnel zones (cont.)1st Fresnel Zone is volume enclosed
by the first ellipsoid.
2𝑛𝑑 Fresnel Zone is volume
enclosed between first and the second
ellipsoid.
At receiver, the contribution to the
electric field from the successive
Fresnel Zones will be in phase
opposition and therefore will interfere
destructively rather than constructively.
21
21
2
22 dd
ddhn
Phase Difference, Δ pertaining to 𝑛th Fresnel Zone is
DiffractionFresnel zones (cont.)
Source: http://www.cdt21.com/resources/Java_file/Applet2/pk_FresnelZoneE/fresnelzone01e.gif
DiffractionDiffraction Loss
Diffraction Loss is caused by blockage of secondary (diffracted) waves Partial energy from secondary waves is diffracted around an obstacleoobstruction blocks energy from some of the Fresnel zones and only a portion
of transmitted energy reaches receiver Received energy is vector sum of contributions from all unobstructed Fresnel zoneso depends on geometry of obstructiono phase of secondary (diffracted) E-field is indicated by the Fresnel Zones Obstacles may block transmission paths causing diffraction lossoconstruct a family of ellipsoids between TX & RX to represent Fresnel zones
ojoin all points for which excess path delay is multiple of 𝜆 2
ocompare geometry of obstacle with Fresnel zones to determine diffraction loss (or gain)
DiffractionDiffraction Loss (cont.)
Place ideal, perfectly straight screen between Tx and Rx If top of screen is well below LOS path then screen will have little effect o the Electric field at Rx = 𝐸𝐿𝑂𝑆 (free
space value) As screen height increases, Electric field will vary as screen blocks more Fresnel zones The amplitude of oscillation increases until the screen is just in line with Tx and Rxofield strength = ½ of unobstructed
field strength
If (55 to 60)% of 1st Fresnel zone is clear than further Fresnel zone clearing does not significantly alter diffraction loss
For free-space transmission conditions,1st Fresnel Zone is kept unblocked
Diffraction Knife Edge Diffraction Model
Diffraction Losseso estimating attenuation caused by diffraction over obstacles is essential for predicting field strength in a given service areao not possible to estimate losses preciselyo theoretical approximations typically corrected with empirical measurements
Computing Diffraction Losseso for simple terrain: expressions have been obtained o for complex terrain: computing diffraction losses is complex
Diffraction Knife Edge Diffraction Model (cont.)
Knife-edge model is the simplest model that
provides insight about magnitude of diffraction loss
o Diffraction losses are estimated using the classical Fresnel solution for field behind a knife edge
o Useful for shadowing caused by 1 knife edge object
Considers receiver R is located in shadowed region
E-field strength at R is vector sum of all fields due to secondary Huygens’ sources in the plane above the knife edge
Huygens
secondary
source
Diffraction Knife Edge Diffraction Model (cont.)
The diffraction gain due to the presence of knife edge, as compared to the free space E-field
Diffraction Knife Edge Diffraction Model (cont.)
DiffractionMultiple Knife-Edge Diffraction Model
Bullington's model
o with more than one obstruction: compute total diffraction loss
o replace multiple obstacles with one equivalent obstacle
o use single knife edge model
Disadvantage:
o oversimplifies problem
o often produces overly optimistic estimates of received signal strength
Scattering Scattering occurs when obstacle size is less than or of the order of the wavelength of propagating wave Causes the transmitter energy to be radiated in many directions Occur due to small objects, rough surfaces, and other irregularities of the channel. For example: Lamp posts and street, etc. Number of obstacles are quite large Scattering follows same principles as diffraction
Scattering (cont.) Received signal strength is often stronger than that predicted by reflection/diffraction models alone
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 receiver
o flat surface → EM reflection (one direction)
o rough surface → EM scattering (many directions)
Scattering (cont.) Critical height for surface protuberances ℎ𝑐 for given incident angle 𝜃𝑖
ℎ𝑐 =𝜆
8 sin 𝜃𝑖
Let ℎ be the maximum protuberances, then surface is considered
o smooth if ℎ < ℎ𝑐o rough if ℎ > ℎ𝑐
Reflection, Diffraction, and Scattering As a mobile moves through a region, these mechanisms have an impact on the instantaneous received signal strength
o In case LOS path exists between the devices, diffraction and scattering will not dominate the propagation.
oIf device is at a street level without LOS path, then diffraction and scattering will probably dominate the propagation.
Path Loss Models Radio Propagation models are derived using a combination of empirical and analytical methods.
These methods implicitly take into account all the propagation factors both known and unknown through the actual measurements.
Path loss models are used to estimate the received signal level as a function of distance.
With the help of this model we can predict SNR for a mobile communication system.
Path loss estimation techniqueso Log - Distance Path Loss Model
o Log - Normal Shadowing
Path Loss ModelsLog-distance path loss model
Average large scale path loss is
is ensemble average of all possible path loss values for given value of d
On log-log scale path loss is a straight line with slope equal to 10 n dB/decade
0
0 log10)()(d
dndPLdBPL
PL
Path Loss Models (cont.)
Path loss exponent for different environments
Path Loss Models (cont.)
Path Loss ModelsLog-Normal Shadowing Model
Log normal
o If 𝑌 is Gaussian RV and 𝑍 is defined such that 𝑌 = log𝑍, then 𝑍 is log-normal RV
Shadowing
oAlso called slow-fading
oAccounts for random variations in received power observed over distances comparable to the widths of buildings
o Extra transmit power (a fading margin) must be provided to compensate for these fades
Surrounding environment clutter may be vastly different at two different locations having same T-R separation
Path Loss ModelsLog-Normal Shadowing Model (cont.)
PL(d) is random and log-normally distributed about the mean distance-dependent value
𝑋𝜎: zero-mean Gaussian distributed random variable (in dB) with standard deviation 𝜎
The probability that received signal level exceed a certain value 𝛾 is
The probability that received signal level is below 𝛾 is
Xd
dndPLXdPLdPL
0
0 log10)()()(
)(])(Pr[
dPQdP r
r
)(])(Pr[
dPQdP r
r
Path Loss Models Outdoor Propagation
We will look to the propagation characteristics of the three outdoor environments
o Propagation in macrocells
o Propagation in microcells
o Propagation in street microcells
Outdoor PropagationMacrocells
Base stations at high-points
Coverage of several kilometers
The average path loss in dB has normal distribution
oAverage path loss is a result of forward scattering over a large number of obstacles each contributing a random multiplicative factor. On changing to dB, it is a sum of random variables
o Sum is normally distributed because of central limit theorem
Outdoor PropagationLongley-Rice Propagation Prediction Model Point-to-point communication in frequency range 40 MHZ to 100GHz
Also referred as irregular terrain model (ITM)
Predicts median transmission loss, takes terrain into account, uses path geometry, calculates diffraction losses
Inputs of computer program of Longley-Rice model :o Frequency
o Path length
o Polarization and antenna heights
o Surface refractivity
o Effective radius of earth
oGround conductivity
oGround dielectric constant
oClimate
Outdoor PropagationLongley-Rice Propagation Prediction Model (cont.)
Computer program operates on path specific parameters
Disadvantages o Does not take into account details of terrain near the receiver
o Does not consider Buildings, Foliage, Multipath
Original model modified by Okamura for urban terrain (include extra term called urban factor)
Outdoor Propagation Okumura Model
In early days, the models were based on emprical studies
Okumura did comprehesive measurements in 1968 and came up with a model.
oDiscovered that a good model for path loss was a simple power law where the exponent 𝑛 is a function of the frequency, antenna heights, etc.
o It is one of the most widely used models for signal prediction in urban areas,
oValid for frequencies in: 150 MHz – 1920 MHz for distances: 1km –100km
Outdoor Propagation Okumura Model (cont.)
o 𝐿50: 50th percentile (i.e. median) of path loss
o 𝐿𝐹(𝑑): free space propagation pathloss
o 𝐴𝑚𝑢(𝑓, 𝑑): median attenuation relative to free space
o 𝐺(ℎ𝑡𝑒): base station antenna heightgain factor
o 𝐺(ℎ𝑟𝑒): mobile antenna height gain factor
o 𝐺𝐴𝑅𝐸𝐴: gain due to different type of environment
o ℎ𝑡𝑒: transmitter antenna height
o ℎ𝑟𝑒: receiver antenna height
𝐺 ℎ𝑡𝑒 and 𝐺 ℎ𝑟𝑒 are determined for different antenna height
𝐿50(𝑑)(𝑑𝐵)= 𝐿𝐹(𝑑) + 𝐴𝑚𝑢(𝑓, 𝑑) – 𝐺(ℎ𝑡𝑒) – 𝐺(ℎ𝑟𝑒) – 𝐺𝐴𝑅𝐸𝐴
Outdoor Propagation Okumura Model (cont.)
Outdoor Propagation Okumura Model (cont.)
Advantage
o Okumuras’ model is considered to be among the simplestand best in terms of accuracy in path loss prediction formature cellular and land mobile system in a clutteredenvironment.
Disadvantage
o Low response to rapid changes in terrain
Outdoor Propagation Hata Model
Empirical formulation of the graphical path loss data provided by Okumura
Valid from 150 MHz to 1500 MHz
For urban areas the formula is
𝐿 𝑢𝑟𝑏𝑎𝑛, 𝑑 𝑑𝐵 = 69.55 + 26.16 log 𝑓𝑐 − 13.82 log ℎ𝑡𝑒– 𝑎 ℎ𝑟𝑒 + 44.9 – 6.55 log ℎ𝑡𝑒 log 𝑑
o d is T-R separation in km
o a(hre) is the correction factor for effective
mobile antenna height which is a
function of coverage area (different for
large and medium city)
o fc is the ferquency in MHz
o hte is effective transmitter antenna
height in meters (30-200m)
o hre is effective receiver antenna
height in meters (1-10 m)
Outdoor Propagation Hata Model (cont.)
For small to medium sized city:
For large city:
In sub urban areas, path loss is:
In open rural areas, path loss is:
Outdoor Propagation Hata Model (cont.)
No path specific corrections
Suitable for large cell mobile system (d >1 km)
Not suitable for PCS
Outdoor Propagation PCS Extension of Hata Model
Higher frequencies: up to 2 GHz
Smaller cell sizes
Lower antenna heights
For 𝑑 = 1 to 20 km
𝐿 𝑑𝐵 = 46.3 + 33.9 log 𝑓𝑐 − 13.82 log ℎ𝑡𝑒– 𝑎 ℎ𝑟𝑒 + 44.9 – 6.55 log ℎ𝑡𝑒 log 𝑑 + 𝐶𝑚
where𝐶𝑚 = 0 and 3 dB for medium and metro city, respectively
Path Loss ModelsMicrocells
Propagation differs significantly
o Milder propagation characteristics
o Small multipath delay spread and shallow fading imply thefeasibility of higher data-rate transmission
o Mostly used in crowded urban areas
o If transmitter antenna is lower than the surrounding building thanthe signals propagate along the streets: Street Microcells
Item Macrocell Microcell
Cell Radius 1 to 20km 0.1 to 1km
Tx Power 1 to 10W 0.1 to 1W
Fading Rayleigh Nakgami-Rice
RMS Delay Spread 0.1 to 10s 10 to 100ns
Max. Bit Rate 0.3 Mbps 1 Mbps
Path Loss ModelsMacrocells versus Microcells
69
Path Loss Models Street Microcells
Most of the signal power propagates along the street
The signals may reach with LOS paths if the receiver is along the same street with the transmitter
The signals may reach via indirect propagation mechanisms if the receiver turns to another street
Path Loss Models: Street Microcells
BA
D
C
log (distance)
received power (dB) received power (dB)
A
C
BA
D
Breakpoint
Breakpoint
n=2
n=4
n=2
n=4~8
15~20dB
Building Blocks
Indoor Propagation Indoor channels are different from traditional mobile radio channels in two different ways:o The distances covered are much smaller
o The variablity of the environment is much greater for a much smaller range of T-R separation distances.
The propagation inside a building is influenced by:o Layout of the building
o Construction materials
o Building type: sports arena, residential home, factory, etc
Path Loss Models Indoor Propagation
Indoor path loss models are less generalized
o Environment comparatively more dynamic
Significant features are physically smaller
o Smaller propagation distances
Less assurance of Far-field for all receiver locations and antenna types.
o More clutter, scattering, less LOS
Path Loss Models Indoor Propagation (cont.)
Indoor propagation is domited by the same mechanisms as outdoor: reflection, scattering, diffraction.o However, conditions are much more variable Doors/windows open or not
The mounting place of antenna: desk, ceiling, etc.
The level of floors
Indoor channels are classified aso Line-of-sight (LOS)
o Obstructed (OBS) with varying degrees of clutter.
Path Loss Models Indoor Propagation (cont.) Buiding types
o Residential homes in suburban areas
o Residential homes in urban areas
o Traditional office buildings with fixed walls (hard partitions)
o Open plan buildings with movable wall panels (soft partitions)
o Factory buildings
o Grocery stores
o Retail stores
o Sport arenas
Indoor propagation Events and parameters Temporal fading for fixed and moving terminals
oMotion of people inside building causes Ricean Fading for the stationary receivers
oPortable receivers experience in general:
Rayleigh fading for obstructed propagation paths
Ricean fading for LOS paths.
Multipath Delay Spread
o Buildings with fewer metals and hard-partitions typically have small rms delay spreads: 30-60ns.
Can support data rates excess of several Mbps without equalization
o Larger buildings with great amount of metal and open aisles may have rms delay spreads as large as 300ns.
Can not support data rates more than a few hundred Kbps without equalization.
Indoor propagation Models Log-distance path loss model
Same floor partition losses
oHard partitions (cannot be moved) /soft partitions (can be moved)
oInternal walls & external walls
Partition loss between floors
oDetermined by the dimensions/materials used/surroundings, including number of windows) /floor attenuation
Ericsson multiple breakpoint model
oObtained by measurements in a multiple office building
oHas 4 breakpoints and has upper & lower bound on the PL
oModel assumes 30 db attenuation at do=1m, for f=900Mhz, unity gain antenna
References[1] T. S. Rappaport, Wireless Communications: Principles and Practice (2nd edition), Pearson Education, 2010.
[2] S. Haykin and M. Moher, Modern Wireless Communications, Pearson Education, 2005.
