Transcript Asenkron Motorlar

WIRELESS COMMUNICATIONS
Assist.Prof.Dr. Nuray At
2
The mobile radio channel places fundamental limitations on the performance of
wireless communication systems.
 Radio channels are extremely random and do not offer easy analysis
 The speed of motion impacts how rapidly the signal level fades as a mobile
terminal moves.
 Modeling issues
Radio Wave Propagation
The mechanisms behind electromagnetic wave propagation are diverse:
 Reflection at large obstacles
 Diffraction at edges
 Scattering at small objects
 …
3
Reflection
Diffraction
Scattering
Due to multiple reflections from various objects in urban areas, the
electromagnetic waves travel along different paths of varying lengths.
 The interaction between these waves causes multipath fading at a specific
location
 The strengths of the waves decrease as the distance between the transmitter
and receiver increases, path loss
4
Small-scale or fading models characterize the rapid fluctuations of the received
signal strength over very short travel distances or short time durations
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. Since the phases are random, the sum of the contributions varies
widely.
In small-scale fading, the received signal power may vary by as much as three or
four orders of magnitude (30 or 40dB) when the receiver is moved by only a
fraction of a wavelength.
5
Large-scale propagation models predict the average received signal strength for
an arbitrary transmitter-receiver separation distance.
 Useful in estimating the radio coverage area of a transmitter.
As the mobile moves away from the transmitter, the local average received signal
will gradually decrease, and it is this local average signal level that is predicted by
large-scale propagation models.
Typically, the local average received power is computed by averaging signal
measurements over a measurement track of 5𝜆 to 40 𝜆, where 𝜆= c/f.
6
Small-scale and Large-scale Fading
7
Free Space Propagation Model
Used to predict received signal strength when the transmitter and receiver have a
clear, unobstructed line-of-sight (LOS) between them.
The free space power received by a receiver antenna which is separated from a
radiating transmitter antenna by a distance d:
Friis free space equation
Pt: transmitted power
Gt: transmitter antenna gain
Gr: receiver antenna gain
d: T-R separation distance in meters
L: system loss factor not related to propagation (𝐿 ≥ 1)
𝜆: wavelength in meters
8
 The Friis free space equation shows that the received power falls off as the
square of the T-R separation distance.
 The gain of an antenna is related to its effective aperture, Ae , by
The effective aperture is related to the physical size of the antenna.
 An isotropic radiator is an ideal antenna which radiates power with unit gain
uniformly in all directions. The effective isotropic radiated power (EIRP):
represents the maximum radiated power available from a transmitter in the
direction of max. antenna gain, as compared to an isotropic radiator
9
In practice, effective radiated power (ERP) is used instead of EIRP to denote the
maximum radiated power as compared to a half-wave dipole antenna. Since a
dipole antenna has a gain of 1.64 (2.15 dB above an isotropic), the ERP will be 2.15
dB smaller than the EIRP for the same transmission system.
The Path Loss represents signal attenuation and 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 the antennas are assumed to have unity gain,
10
 The Friis free space model is valid for values of d which are 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. The Fraunhofer distance df is given by
where D is the largest physical linear dimension of the antenna.
 Additionally, to be in the far-field region, df must satisfy
 Furthermore, large scale propagation models use a close-in distance d0, as a
known received power reference point. Hence,
The Three Basic Propagation Mechanisms
11
Reflection
Occurs when a propagating EM wave impinges upon an object which has very
large dimensions when compared to the wavelength of the propagating wave.
 Reflections occur from the surface of the earth and from buildings and walls.
When a radio wave propagating in one medium impinges upon another medium
having different electrical properties, the wave is partially reflected and partially
transmitted.
 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.
 If the plane wave is incident on a perfect conductor, then all incident energy is
reflected back into the first medium without loss of energy.
The Three Basic Propagation Mechanisms
12
 The electric field intensity of the reflected and transmitted waves may be
related to the incident wave in the medium of origin through the Fresnel
reflection coefficient (Γ).
Reflection from Dielectrics: The nature of reflection varies with the direction of
polarization of the E-field.
E-field in the plane of incidence
E-field normal to the plane of incidence
The Three Basic Propagation Mechanisms
Parameters 𝜀1 , 𝜇1 , 𝜎1 and 𝜀2 , 𝜇2 , 𝜎2 represent the permittivity, permeability, and
conductance of the two media, respectively.
 The dielectric constant of a perfect (lossless) dielectric is related to a relative
value of permittivity, 𝜀𝑟 , such that 𝜀 = 𝜀0 𝜀𝑟 where 𝜀0 = 8.85x10-12 F/m
 For the case when the first medium is free space and 𝜇1 = 𝜇2 , the reflection
coefficients for the two cases of vertical and horizontal polarization
13
The Three Basic Propagation Mechanisms
14
Example:
Demonstrate that if medium 1 is free space and medium 2 is a dielectric, both |Γ|| |
and |Γ⊥ | approach 1 as 𝜃𝑖 approaches 0o regardless of 𝜀𝑟 .
Substituting 𝜃𝑖 = 0 in the above equations
Γ|| = 1, |Γ|| | = 1
Γ⊥ = -1, Γ⊥ = 1
 This example illustrates that ground may be modeled as a perfect reflector
with a reflection coefficient of unit magnitude when an incident wave grazes
the earth, regardless of polarization or ground dielectric properties.
The Three Basic Propagation Mechanisms
Example: Some typical values
Surface
Relative dielectric
constant
Dry ground
4-7
Average ground
15
Wet ground
25-30
Sea water
81
Fresh water
81
15
The Three Basic Propagation Mechanisms
16
Brewster Angle
The angle at which no reflection occurs in the medium of origin. It occurs when
the incident angle 𝜃𝐵 is such that the reflection coefficient Γ|| = 0. The Brewster
angle is given by the value of 𝜃𝐵 which satisfies
 For the case when the first medium is free space and the second medium has a
relative permittivity 𝜀𝑟
 Note that the Brewster angle occurs only for vertical polarization.
The Three Basic Propagation Mechanisms
Reflection from Perfect Conductors
Since EM energy cannot pass through a perfect conductor, a plane wave incident
on a conductor has all of its energy reflected. For a perfect conductor,
Γ|| = 1 and Γ⊥ = -1
regardless of the incident angle.
17
The Three Basic Propagation Mechanisms
18
Ground Reflection (Two-Ray) Model (1/6)
Useful propagation model that is based on geometric optics, and considers both
the direct path and a ground reflected propagation path between transmitter and
receiver.
 In most mobile communication systems, the max. T-R separation distance is at
most only a few tens of kilometers, and the earth may be assumed to be flat.
The Three Basic Propagation Mechanisms
Ground Reflection (Two-Ray) Model (2/6)
If E0 is the free-space E-field at a reference distance d0 from the transmitter, then
for d > d0, the free space propagating E-field
Two propagating waves arrive at the receiver:
 The direct wave that travels a distance d’
 The reflected wave that travels a distance d’’
where Γ is the reflection coefficient for ground.
19
The Three Basic Propagation Mechanisms
20
Ground Reflection (Two-Ray) Model (3/6)
According to laws of reflection in dielectrics:
Assuming perfect horizontal E-field polarization and ground reflection (Γ⊥ = -1 and
Et = 0), the resultant E-field is the vector sum of ELOS and Eg :
The path difference, ∆, between the LOS and the ground reflected paths
The Three Basic Propagation Mechanisms
Ground Reflection (Two-Ray) Model (4/6)
When the T-R separation distance d is very large compared to ht + hr,
The phase difference between the two E-field components:
The time delay between the arrival of the two components:
If the received E-field is evaluated at some time, say at t = d’’/c
21
The Three Basic Propagation Mechanisms
Ground Reflection (Two-Ray) Model (5/6)
Note that as d becomes large, the difference between the distances d’ and d’’
becomes very small, and the amplitudes of ELOS and Eg are virtually identical and
differ only in phase.
ETOT(d) decays in an oscillatory fashion. When
As long as
22
The Three Basic Propagation Mechanisms
Ground Reflection (Two-Ray) Model (6/6)
The received power at a distance d from the transmitter:
 This is a much more rapid path loss than is experienced in free space.
23