Mobile Radio Propagation
Large Scale Path Loss
Free Space Propagation Model
What
are reasons why
wireless signals are hard to
send and receive?
The
mobile radio channel places fundamental
limitations on the performance of wireless
communication systems
Wireless
transmission paths may be:
* Line-of-Sight [LOS]
* Non Line-of-Sight [NLOS] : Obstructed
by buildings, mountains, and foliage
Radio
channels are extremely
difficult to analyze (time varying)
Modeling
random
and
radio channels have been one of the
difficult paths of mobile radio system design.
Propagation Mechanism..
The
radio, microwave, infra-red
and visible light portions of the
electromagnetic spectrum can all
be use to transmit information.
Information can be sent by
modulating the Amplitude,
Frequency or Phase of the wave
Properties of Radio Waves
Are
easy to generate
Can travel long distances
Can penetrate buildings
May be used for indoor and outdoor
communication
Are omni-directional-can travel in all
directions
Can be narrowly focused at high
frequencies (greater than 100 MHz) using
parabolic antenna (like satellite dishes)
Properties of Radio
Waves
Frequency
dependence
Behave more like light at higher frequencies
Difficult to passing obstacles
More direct path
Absorbed by rain
Behave
more like radio wave at lower
frequencies
Can pass obstacles
Power falls off sharply with distance from sources
Subject
to interference from other radio
wave sources
Problems Unique to Wireless
systems
Interference
from other service
providers
Interference from other users
(same network)
CCI due to frequency reuse
ACI due to Tx/Rx design limitations &
large number users sharing finite BW
Shadowing
Obstructions to line-of-sight paths weak received signal strength
Fading
When no clear line-of-sight path exists,
signals are received that are reflections off
obstructions and diffractions around
obstructions
Multipath signals can be received that
interfere with each other
Fixed Wireless Channel random &
unpredictable
must be characterized in a statistical
fashion
field measurements often needed to
characterize radio channel performance
Propagation Models
Propagation
models Focused on
predicting the average received signal
strength at a given distance from the
transmitter.
Signal strength in close spatial proximity
to a particular location.
Propagation models that predict the
mean signal strength for an arbitrary
transmitter receiver [T-R] separation
distance are useful in estimating the
radio coverage are of a transmitter.
Propagation
models that
characterize the rapid
fluctuations of the received signal
strength over very short travel
distances (or) short time duration
are called- Small Scale fading.
Free Space Propagation
Model
Free
space propagation model
transmitter and Receiver have a
clear line of sight (LoS) path
between them.
* Satellite Communication
* Microwave Link
Free space propagation Received
power decays as function of the T-R
separation distance.
The
free space power received by a
receiver antenna which is separated
from a radiating transmitter antenna
by a distance d, given by the Friis free
space equation
Pr(d) = (PtGtGr2)/((4)2d2L)
Pt
- Transmitted Power
Pr - Received Power
Gt
- Transmitter Antenna Gain
Gr
Receiver Antenna Gain
d T-R Separation distance
System loss factor not related
to propagation (L1)
Wavelength meters
Antenna Gain
Gain
of an antenna is related to
its effective aperture Ae (i.e)
G = 4Ae/ 2
Ae Physical size of
antenna
is related to carrier frequency
= c/f = 2c/wc
f is the carrier frequency
Wc is the carrier frequency in
radians per second
C speed of light meters/sec
The
miscellaneous losses L (L1) are
usually due to transmission line
attenuation, filter losses, and antenna
losses in the communication systems.
Where L =1 indicates no loss in the
system hardware.
Friis free space equation shows that the
received power falls off as the square of
T-R separation distance.
Received power decays with distance at
a rate of 20dB/decay.
An
isotropic radiator is an ideal
antenna which radiates power
with unit gain uniformly in all
direction.
The effective isotropic radiated
power (EIRP) is define as
EIRP = PtGt
In
practice effective radiated
power (ERP) is used instead of
EIRP.
Path loss
The
path loss Difference (dB) between
the effective transmitted power and the
received power may or may not include
the effect of the antenna gains.
PL(dB)
= 10log (Pt /Pr )= -10 log [(GtGr2)/
((4)2d2)]
When
antenna gains are excluded then
PL(dB) = 10log (Pt /Pr )= -10 log [2/
((4)2d2)]
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.
The far-field (or) Fraunhofer
region of a transmitting antenna
is defined as the region beyond
the far-field distance df
df
D-
=2D2/
Largest physical dimension of
Far-field
region df must satisfy
df>>D
and df >>
Frirs
free space model equation does
not hold for d=0.
Large scale propagation model use a
close in distance d0 received power
reference point.
The received power Pr (d), at any
distance d>d0
The
received power in free space at a
distance greater than d0 is given by
Pr (d) = Pr (d0 )(d0 /d)2 dd0 df
Received power level in dBm or dBW
Pr (d)dBm = 10log[Pr (d0)/0.001W]
+20log(d0/d )
The reference d0 for practical system
1m in indoor environments
100m to 1Km in outdoor environments
