wireless communications eee: 464 · –satellite communication –microwave line-of-sight radio...
TRANSCRIPT
-
Wireless Communications
EEE: 464
-
Introduction
Name : Taimur Shahzad
E.Mail : [email protected]
Department `: CAST (Academic Block I- 5th
Floor)
mailto:[email protected]
-
Radio Propagation Concepts
-
Radio Waves Propagation
• Propagation are how radio waves travel from
Transmitter to Receiver.
• Events occurred between Transmitter and Receiver
affect the communication.
• Propagation effects and change the amplitude,
phase or frequency of the transmission.
• Propagation minimizes/reduces received signal
strength.
-
Radio Link Model
-
Radio Transmission
-
Radio Waves Propagation• Propagation Effects:Reflection (from the ground or large objects)
Diffraction (from the edges and corners of terrain or buildings)
Scattering (from foliage or other small objects)
Attenuation (from rain or the atmosphere)
Doppler (from moving users)
• Wireless channels are very different then Wired
channels. They are Unreliable and take more
Bandwidth.
-
Radio Communication
Coding Modulation Antenna
De
modulationDecoding Antenna
-
Radio Wave Propagation
• Mechanism behind electromagnetic wave
propagation are diverse, but can generally be
attributed to reflection, diffraction and scattering.
• The transmission path between the transmitter (Tx)
and receiver (Rx) can vary from simple line-of-
sight (LOS) to one that is severely obstructed by
buildings, mountains, and foliage (NLOS).
• The radio channels are extremely random and can
cause severe loss in signal power.
-
Radio Wave Propagation
• The mobile radio channel places fundamental
limitations on the performance of wireless
communication systems.
• Mobile radio path is severely obstructed by
buildings, mountains, and foliage.
• 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 terminals moves in
the space.
-
Radio Wave Propagation
• Propagation models that predict the mean signal
strength for an arbitrary Tx-Rx separation distance
are useful in estimating the radio coverage area of a
transmitter and are called Large-Scale propagation
model.
• Propagation model that characterize the rapid
fluctuations of the received signal strength over very
travel distance are called Small Scale/Fading models.
-
Radio Wave Propagation
-
Propagation Mechanism
• Electromagnetic waves can generally be attributed
as: Reflection, Diffraction and Scattering.
• In urban areas, there is no direct line-of-sight path
between the transmitter and the receiver, and where
the presence of high- rise buildings causes severe
diffraction loss. Multiple reflections cause multi-
path fading.
-
Reflection, Diffraction and Scattering
-
Free Space Propagation Model
• Received Signal strength is determined by free space propagation model if there exist a clear and unobstructed LOS between Tx and Rx.
• In free space, the power density of an EM wave decreases with increase in distance between the Txand Rx.
• Simple Wireless Link:
LOS
-
Free Space Propagation Model
The free space propagation model is used to predict
received signal strength when the transmitter and
receiver have a clear line-of-sight path between
them.
– satellite communication
– microwave line-of-sight radio link
Friis free space equation
Ld
GGPdP rttr 22
2
)4()(
-
Free Space Propagation Model(cont)
: transmitted power : T-R separation distance (m)
: received power : system loss
: transmitter antenna gain : wave length in meters
: receiver antenna gain
The gain of the antenna
: effective aperture is related to the physical size of
the antenna
tP
)(dPr
tG
rG
d
L
2
4
eAG
eA
-
Free Space Propagation Model(cont)
The wave length is related to the carrier frequency by
: carrier frequency in Hertz
: carrier frequency in radians
: speed of light (meters/s)
The losses are usually due to transmission line
attenuation, filter losses, and antenna losses in the
communication system. A value of L=1 indicates
no loss in the system hardware.
c
c
f
c
2
f
c
c
-
Free Space Propagation Model(cont)
Isotropic radiator is an ideal antenna which radiates
power with unit gain.
Effective isotropic radiated power (EIRP) is defined
as:
and represents the maximum radiated power
available from transmitter in the direction of
maximum antenna gain as compared to an isotropic
radiator.
ttGPEIRP
-
Free Space Propagation Model(cont)
Path loss for the free space model with antenna gains
When antenna gains are excluded
The Friis free space model is only a valid predictor
for for values of d which is in the far-field
(Fraunhofer region) of the transmission antenna.
22
2
)4(log10log10)(
d
GG
P
PdBPL rt
r
t
22
2
)4(log10log10)(
dP
PdBPL
r
t
rP
-
Free Space Propagation Model(cont)
The far-field region of a transmitting antenna is
defined as the region beyond the far-field distance
where D is the largest physical linear dimension of
the antenna.
To be in the far-filed region the following equations
must be satisfied
22Dd f
Dd f fd
-
Free Space Propagation Model(cont)
Furthermore the following equation does not hold
for d=0.
Use close-in distance and a known received
power at that point
or
Ld
GGPdP rttr 22
2
)4()(
0d
)( 0dPr
2
00)()(
d
ddPdP rr fddd 0
d
ddPdP rr
00 log20W 001.0
)(log10dBm )( fddd 0
-
Problem
• Consider an indoor wireless LAN with fc = 900
MHz, cells of radius 10 m, and omni-directional
antennas (G = 1). Under the free-space path loss
model, what transmit power is required at the
access point such that all terminals within the cell
receive a minimum power of 10 μW. How does this
change if the system frequency is 5 GHz?
We must find the transmit power such that the
terminals at the cell boundary receive the minimum
required power.
-
Problem(cont.)
2 2
6
22
4 4 x1010x10 1.42W 1.52dBW
1 3t r
t r
dP P
G G
• At 5 GHz, λ changes only.
43.86W 16.42dBWtP
-
Multipath Propagation
-
Indoor Propagation
-
Outdoor Propagation
-
Reflection, Refraction and Diffraction
• These three properties are shared by light and radio
waves.
• For both reflection and refraction, it is assumed that
the surfaces involved are much larger than the
wavelength; if not, diffraction will occur.
-
Reflection
• Reflection of waves from a smooth surface
(specular reflection) results in the angle of
reflection being equal to the angle of incidence.
-
Other Types of Reflection
Corner reflector Parabolic reflector Diffuse Reflection
-
Refraction
• A transition from one medium to another results in
the bending of radio waves, just as it does with
light
• Snell’s Law governs the behavior of
electromagnetic waves being refracted:
n1 sin1 n2 sin2
-
Diffraction
• As a result of diffraction, electromagnetic waves
can appear to “go around corners”
• Diffraction is more apparent when the object has
sharp edges, that is when the dimensions are small
in comparison to the wavelength
-
End
-
Problem (Quiz)
If 50W is applied to a unity gain antenna with a
900MHz 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 receive antenna.