4 Section 3

Advanced Digital Communications

(EE5511)

MSc Module of Wireless Communication System

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/

Section 3

Section 3::

Wireless Channels and Channel

Wireless Channels and Channel

Antenna and Radio Propagation

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). 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:  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 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.

The half-wave dipole antenna

(or called Hertz Antenna) becomes quite common where space

permits. It can be erected

Hertz Antenna (1)

permits. It can be erected

vertically but it is more often than not erected horizontally for

You will note that the

up- and down hand halves are merely

quarter wave sections.

Hertz Antenna (2)

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 doesn’t collapse its field back into the antenna

ִ Near field and far field are two designators for antenna fields

ִ The The far fieldfar field region begins when the distance 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

λ

2

2 D 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

=

λ

v is the velocity of the wave in meters per second and is f

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 3x108_m/s.

m f v 3 10 1 10 3 8 8 = × × = = λ 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

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 = × × = λ 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  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

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

Overall Damaging Effects of

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)

(denoted by a(t)), and additive impact damaging the signal – known as noise (denoted by n(t)) and interference

Overall Channel Damaging Effects (2)

s(t): transmitted signal

a(t): radio channel attenuation j(t): interfering signal

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)

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

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)

 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)

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).

Large-scale Propagation Models

Large-scale Propagation Models

Large-scale Propagation Models

Two Simplified Outdoor models:



Free-Space Propagation model



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). receiver).

It’s 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

_{t r} 2 t R













π

λ

=

[ ]

10log( ) 10log( ) 4 log 20 log 10 _{t r} t R t R _{G G} d P P dB P P + +       π λ = = Or in dB:

Free-Space Propagation (2)

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.

λ

R

P

t

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)

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)

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)

direction and the wave can be seen as a sphere of increasing radius.

2. Since energy can’t 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.

Assumes far-field (d - distance)

ִ

d >> D and d >> λ , where

 D is the largest linear dimension of the antenna

 λ is the carrier wavelength

Free-Space Propagation (5)

 λ is the carrier wavelength

No interference, no obstructions

Path Loss is a measure of attenuation based

only on the distance to the transmitter

Example:

Two λ/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 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 ) ( − × = × π         × × × × × = π λ =

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)

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

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 2 2 ) (h h d d d d = + = + + 2 2 1 (h h ) d d = _{T x} − _Rx +

d

h

h

x x R T

−

= arctan

α

h_Tx h_Rx

path length of direct wave:

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)

,

d

h

h

G

G

P

P

2 2 R T r t t 1 R x x













=

Power falls off proportional to d4 and is independent of

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)

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.

The above 2 simplified outdoor propagation models are

attempt to predict path loss close to the Earth’s 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

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

Empirical Outdoor models

Empirical path loss models based on extensive

measurements.

First, we’ll show the 2 most commonly used

empirical outdoor models in conjunction with 900

empirical outdoor models in conjunction with 900

MHz (macro) cellular systems:

Hata’s mode

and

Lee’s model

.

 By

macro-cell

we mean a cell typically on the order of

tens of kilometers.

Then, we’ll list some other empirical outdoor

Okumura

-

Hata’s 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 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

-

Hata’s 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

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

When applies to small to medium cities,

a(hm) = [1.1 log10(fc) – 0.7]hm – 1.56 log10(fc) – 0.8

Okumura

-

Hata’s models (3)

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

Lee’s models

Lee’s path loss model is used to model a flat terrain.

Lee’s model has been known to be more of a “North American model”

than that of Hata.

Received signal power in dBm is given by:













=

_Ω Ω 0 0 10

(

)

(

)

log

10

0

a

f

f

d

d

_{β c β}

µ

µ

0 Ω

µ

_{is the power at 1 mile}

β

_{is path loss exponent.}

Other Empirical models (1)

Okumura’s 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

Sakagmi and Kuboi model

- extend Okumura’s 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 Hata’s 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.

less than 20 m.

Longley-Rice model

- point-to-point communication system in the frequency range from 40MHz to 100 GHz.

Durkin’s model

Walfisch and Bertoni’s model

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 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

Indoor Propagation Models (2)

Partition losses (same floors)

.

Partition losses between floors

.

Log-distance path loss model

.

Log-distance path loss model

.

Ericsson Multiple Breakpoint model

.

Attenuation Factor model

.

More Details see the referencing book: