Advanced Digital Communications

(EE5511)
MSc Module of Wireless Communication System
Dr Qiang Ni Brunel University 1/44 Dr Qiang Ni Brunel University 1/45
MSc Module of Wireless Communication System
Dr. Qiang Ni
ECE, School of Eng & Design, Brunel University
E-mail: Qiang.Ni@brunel.ac.uk
Homepage: http://people.brunel.ac.uk/~eestqqn/
Office: Howell Building H237
Section 3 Section 3::
Wireless Channels and Channel
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Wireless Channels and Channel
Models (1)
Antenna and Radio Propagation
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Antenna and Radio Propagation
Functionality of Antenna
The functionality of an antenna is to transform
electromagnetic energy into electromagnetic waves
(transmission side) and to transform electromagnetic
waves back into electromagnetic energy (reception).
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waves back into electromagnetic energy (reception).
Question:
Should antenna preferably be erected as high and
be as long as is possible or desirable?
Antenna Basics
In the following we only present two basic
types of antennas used for radio
propagation.
More knowledge, Recommend 2 Books:
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Antennas and Propagation for Wireless
Communication Systems by Simon R. Saunders
Wiley, ISBN 10:0471986097(H/B)
PRACTICAL ANTENNA HANDBOOK -
By Joseph Carr
Marconi Antenna (1)
The most basic antenna is called "a quarter-wave
vertical (or called Marconi Antenna).
It is a quarter wavelength long and is a vertical
radiator. Typical examples would be seen installed on
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radiator. Typical examples would be seen installed on
motor vehicles for two way communications.
Technically Marconi antenna is an "isotropic
radiator". This is a mythical antenna which radiates in
all directions as does the light from a lamp bulb.
The quarter-wave vertical antenna is usually the
simplest to construct and erect.
Marconi Antenna (2)
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The half-wave dipole antenna
(or called Hertz Antenna) becomes
quite common where space
permits. It can be erected
Hertz Antenna (1)
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permits. It can be erected
vertically but it is more often than
not erected horizontally for
practical reasons.
You will note that the
up- and down hand
halves are merely
quarter wave sections.
Hertz Antenna (2)
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quarter wave sections.
The input impedance
of this half-wave dipole
example is nominally 75
ohm.
Antenna Radiation Field
It is defined as the radiation that surrounds an antenna but
doesnt collapse its field back into the antenna
Near field and far field are two designators for antenna
fields
The far field region begins when the distance
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The far field region begins when the distance
where R = distance from the antenna (m)
D = dimension of the antenna (m)
= wavelength of the transmitted signal (m)
The near field will be any distance less than R

2
2D
R >

How to calculate the wavelength

Definition: The distance travelled by the wave during a
period of once cycle
is the velocity of the wave in meters per second and is
f
v
=
v f
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is the velocity of the wave in meters per second and is
the frequency
Example: Calculate the wavelength of a 100MHz signal
travel in free space. Note that the velocity of
electromagnetic waves in free space is 3x10
8
m/s.
m
f
v
3
10 1
10 3
8
8
=

= =
v f
Example
Determine the distance from a parabolic reflector with
diameter (D) = 4.5m to the boundary of the far-field region
if the parabolic reflector is used for Ku-band transmission
of a 12-GHz signal.
Solution:
The wavelength for a 12-GHz signal is approximately
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D = 4.5m, therefore
Therefore, the boundary for the far field region for
this parabolic reflector is a distance greater than
1620 meters from the antenna.
m 025 . 0
10 12
10 3
9
8
=

=
m R 1620
025 . 0
) 5 . 4 ( 2
2
=

>
Antenna Radiation Pattern
Radiation pattern is an indication of radiated
field strength around the antenna
Omnidirectional: a spherical radiation pattern
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Omnidirectional: a spherical radiation pattern
Bidirectional: concentrates energy in certain
directions at the expense of lower energy in other
directions

Antenna Gain
Antenna Gain is a measure of how much more power in
dB an antenna will radiate in a certain direction with
respect to that which would be radiated by a reference
antenna
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antenna
Expressed as dBi, if the reference antenna is an
isotropic point source
Expressed as dBd, if the reference antenna is an
half wavelength dipole antenna
For example, the half-wave dipole antenna has a 2.15dB
gain as compared to an isotropic radiator
Overall Damaging Effects of
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Overall Damaging Effects of
Wireless Channel on Signal
The overall damaging effects of Wireless Channel have
both multiplicative impact damaging the signal - attenuation
(denoted by a(t)), and additive impact damaging the signal
Overall Channel Damaging Effects (1)
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(denoted by a(t)), and additive impact damaging the signal
known as noise (denoted by n(t)) and interference
(denoted by j(t)), as shown in the figure next slice
Overall Channel Damaging Effects (2)
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s(t): transmitted signal
a(t): radio channel attenuation
j(t): interfering signal
n(t): time-varying random noise
y(t): received signal y(t) = a(t) * s(t) + j(t) + n(t)
As shown in the last figure, the received signal may first
be influenced by a multiplicative factor, the attenuation
a(t). Actually there are two main different attenuation
effects which result in an overall attenuation of the
Overall Channel Damaging Effects (3)
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effects which result in an overall attenuation of the
transmitted signal:
a(t)=a
PL
(t)*a
FA
(t)
Where a
PL
(t): attenuation of Large-scale Path Loss;
a
FA
(t): attenuation of Small-scale Fading and Multipath.
Large-Scale Path Loss Effects
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Large-Scale Path Loss Effects
Path Loss is a type of deterministic effect
depending only on the distance between the
transmitter and receiver.
Path Loss (1)
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It plays an important role on larger time scales (e.g.
seconds or minutes), since the distance between
transmitter and receiver in most situations does not
change significantly on smaller time scales.
Definition: In a communication system, path loss is the
attenuation undergone by an electromagnetic wave in
transit between a transmitter and receiver.
Note 1: Path loss may be due to many effects such as
Path Loss (2)
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Note 1: Path loss may be due to many effects such as
free-space loss, refraction, reflection, diffraction,
scattering, aperture-medium, and absorption.
Note 2: Path loss usually refers to long-distance loss (km).
Note 3: Path loss is usually measureded in dB (decibel).
Large-scale Propagation Models
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Large-scale Propagation Models
Large-scale Propagation Models
Two Simplified Outdoor models:
Free-Space Propagation model
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Two-Ray Propagation model
Other Outdoor Propagation models
Some Indoor Propagation models
Free-Space Propagation (1)
In free space, a signal suffers from propagating over a
distance between two antennas assuming line of sight (LOS: no
objects obstructing the path between the transmitter and
receiver).
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receiver).
Its usually called a free-space path loss, which can be
calculated using the Maxwell equations and is given by:
, G G
d 4
P P
r t
2
t R
|

\
|

=
[ ] ) log( 10 ) log( 10
4
log 20 log 10
r t
t
R
t
R
G G
d P
P
dB
P
P
+ +
|

\
|

= =
Or in dB:
Free-Space Propagation (2)
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where is the received power, is the transmitted
power, is the wavelength, G
t
is the gain of the
transmitter antenna and G
r
is the gain of the receiver
antenna (both gains in the direction of the straight line that
connects the two antennas in space), d is the distance.

R
P
t
P
Further notes
d = the distance between the transmitter
antenna and the receiver antenna (m)
Pr = power received (W)
Pt = power transmitted (W)
Free-Space Propagation (3)
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Pt = power transmitted (W)
Gt = transmitting antenna gain compared to
isotropic radiator (not in dB). Normally a Unit
Gain is chosen in many cases, i.e. G =1
Gr = receiving antenna gain compared to isotropic
radiator (not in dB)
= wavelength (m)
The received power is inversely proportional to the square of
the distance and the square of the frequency.
Physical explanation:
1. In free space, the radiated energy propagates equally in every
direction and the wave can be seen as a sphere of increasing radius.
Free-Space Propagation (4)
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direction and the wave can be seen as a sphere of increasing radius.
2. Since energy cant be destroyed, it will be the same whatever the
distance from the radiating point is. So that the total energy over
the sphere is the same independent of the radius, the energy per
unit surface must decrease.
3. As the surface increases with the square of the radius, so does
energy per unit surface decrease at the inverse rate.
Assumes far-field (d - distance)
d >> D and d >> , where
D is the largest linear dimension of the antenna
is the carrier wavelength
Free-Space Propagation (5)
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is the carrier wavelength
No interference, no obstructions
Path Loss is a measure of attenuation based
only on the distance to the transmitter
Free space model only valid in far-field
Example:
Two /2 dipoles are separated by 50km. They are aligned
for optimum reception. The transmitter feeds its
antenna with 10W at 144MHz. Calculate the power
received.
Solution:
The two dipoles have a gain of 2.15dB. Therefore
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The two dipoles have a gain of 2.15dB. Therefore
G
t
= G
r
= 10
(2.15/10)
= 1.64
( )
W
W
d
G G P
d P
r t t
r
10
2
3 2
2
6
8
2 2
2
10 96 . 2
10 50 16
10 144
10 3
64 . 1 64 . 1 10
16
) (

=

|
|

\
|

=
Since most communications happen close to the earth
surface, the scenario for free-space loss is unrealistic.
The two-ray model is a simple model based on physical-
optics theory which takes into account the reflection on the
Two-Ray Propagation Model (1)
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optics theory which takes into account the reflection on the
earth surface. It also assumes LOS and no influence on
propagation besides the earth surface.
It is a useful starting point for the study of propagation for
personal communications. It is often used to describe
propagation over open fields.
Direct wave
Reflected wave
Two-Ray Propagation Model (2)
In the two-ray model, two propagation paths between the
transmitter/receiver are considered: the direct wave (LOS)
path, and the reflected wave path. (h
TX
, h
Rx
and d are known.)
h
Tx
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2 2
22 21 2
) ( d h h d d d
x
R
x
T
+ + = + =
2 2
1
) ( d h h d
x
R
x
T
+ =
d
h h
x x
R T

= arctan
h
Tx
h
Rx
path length of direct wave:
path length of reflected wave:
Why?
After some approximation, the two-ray propagation model
is simplified as the known 4th-power-law form:
h h
2
| |
Two-Ray Propagation Model (3)
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,
d
h h
G G P P
2
2
R T
r t t 1 R
x x
|
|

\
|
=
Power falls off proportional to d
4
and is independent of
signal wavelength.
The Two-Ray Ground Reflection model has
been found to be reasonably accurate for
predicting large-scale signal strength over
distances of several kilometers for mobile
Two-Ray Propagation Model (4)
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distances of several kilometers for mobile
radio systems that use tall towers (heights
which exceed 50m), as well as for LOS
microcell channels in urban environments.
This model is not accurate for complicated indoor
environments.
The above 2 simplified outdoor propagation models are
attempt to predict path loss close to the Earths surface.
However, communication often takes place over irregular
terrain. Hence, the above assumptions are unrealistic:
The terrain profile of a particular area needs to be taken into
account for obtaining better estimates of path loss.
Other Outdoor Empirical Models
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account for obtaining better estimates of path loss.
Irregular terrain, like in cities, doesn't lend itself to simple
analytical path loss models.
For example, the terrain profile may vary from a simple curved
Earth profile to a highly mountainous profile.
A number of propagation models were proposed to predict
path loss over irregular terrain. These models are empirical.
Empirical Outdoor models
Empirical path loss models based on extensive
measurements.
First, well show the 2 most commonly used
empirical outdoor models in conjunction with 900
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empirical outdoor models in conjunction with 900
MHz (macro) cellular systems: Hatas mode and
Lees model.
By macro-cell we mean a cell typically on the order of
tens of kilometers.
Then, well list some other empirical outdoor
models.
Okumura-Hatas models (1)
The Hata model is an empirical formulation of the graphical
path loss data which was provided by Okumura.
Hata presented the urban propagation loss as a standard
formula and supplied correction Equations for Applications to
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formula and supplied correction Equations for Applications to
other situations
Carrier Frequency : 150 MHz fc 1500 MHz
Base Station Height : 30m hb 200m
Mobile Station Height: 1m hm 10m
T-R distance : 1km d 20km
Okumura-Hatas models (2)
Lp is the path loss:
for urban area Lp = A + B log10(d)
for suburban area Lp = A + B log10(d) - C
for open area Lp = A + B log10(d) - D
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for open area Lp = A + B log10(d) - D
A = 69.55 + 26.16 log10(fc) 13.82 log10(hb) a(hm)
B = 44.9 6.55 log10(hb)
C = 5.4 + 2[log10(fc/28)]2
D = 40.94 + 4.78 [log10(fc)]2 18.33 log10(fc)
When applies to small to medium cities,
a(hm) = [1.1 log10(fc) 0.7]hm 1.56 log10(fc) 0.8
Okumura-Hatas models (3)
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When large cities and for fc 400 MHz:
a(hm) = 8.28 [log10(1.54 hm)]2 1.1
When large cities and for fc 400 MHz.
a(hm) = 3.2 [log10(11.75 hm)]2 4.97
Lees models
Lees path loss model is used to model a flat terrain.
Lees model has been known to be more of a North American model
than that of Hata.
Received signal power in dBm is given by:
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(

=
0
0
10
) ( ) ( log 10
0
a
f
f
d
d
c


0

is the power at 1 mile

is path loss exponent.

These parameters are determined from empirical measurements
Other Empirical models (1)
Okumuras model - One of most widely used for Urban.
- based on free space path loss + correction factors for
urban, suburban and rural areas, irregular terrain, street
orientations
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Sakagmi and Kuboi model
- extend Okumuras model using regression analysis of
data.
Ibrahim and Parsons model
- equations developed to best fit data observed at
London. (freq. 168-900 MHz)
Other Empirical models (2)
COST231-HATA model
- the COST231-Hata model extends Hatas model for use in the
1500-2000 MHz frequency range, which does take into account
parameters such as roof heights, street widths and building separation.
Two Slope model
- transmission distances range up to 500 m and antenna heights are
less than 20 m.
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less than 20 m.
Longley-Rice model
- point-to-point communication system in the frequency range from
40MHz to 100 GHz.
Durkins model
Walfisch and Bertonis model
Wideband PCS Microcell model
More details read book: Wireless Com: Principles & Practice
Indoor Propagation Models (1)
Indoor propagation is also dominated by reflection,
diffraction and scattering as outdoor, but conditions are
much more variable.
Specialized models for indoor propagation also exist.
These factor losses within the same floor (partition losses
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These factor losses within the same floor (partition losses
due to walls and other materials, including furniture) or
losses for propagation across floors. Losses due to the latter
are adjusted by way of the floor attenuation factor (FAF).
Finally sophisticated ray-tracing and site-specific
modeling techniques also have been developed.
Indoor Propagation Models (2)
Partition losses (same floors).
Partition losses between floors.
Log-distance path loss model.
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Log-distance path loss model.
Ericsson Multiple Breakpoint model.
Attenuation Factor model.
More Details see the referencing book:
Wireless Communications: Principles & Practice (2
nd
Ed)
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