4 Section 3

Advanced Digital Communications (EE5511)

MSc Module of Wireless Communication System

Dr Qiang Ni Brunel University 1/44Dr Qiang Ni Brunel University 1/45

MSc Module of Wireless Communication System

Dr. Qiang Ni

ECE, School of Eng & Design, Brunel UniversityE-mail: [email protected]

Homepage: http://people.brunel.ac.uk/~eestqqn/

Office: Howell Building H237

Section 3Section 3::

Wireless Channels and Channel

Dr Qiang Ni Brunel University 2/44

Wireless Channels and Channel Models (1)

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:

Dr Qiang Ni Brunel University 5/44Dr Qiang Ni Brunel University

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.

Marconi Antenna (2)


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

practical reasons.

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 far field region begins when the distance


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

λ

22DR >

λ

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 f


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 3x108m/s.

mf

v3

101

1038

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.

m025.01012

1039

8

=×

×=λ

mR 1620025.0

)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

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)


(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



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



effects which result in an overall attenuation of the

transmitted signal:

a(t)=aPL(t)*aFA(t)

Where aPL(t): attenuation of Large-scale Path Loss;

aFA(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).

Note 3: Path loss is usually measureded in dB (decibel).

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:

,GGd4

PP rt

2

tR

π

λ=

[ ] )log(10)log(104

log20log10 rt

t

R

t

R GGdP

PdB

P

P++

π

λ==

Or in dB:



where is the received power, is the transmitted

power, is the wavelength, Gt is the gain of the

transmitter antenna and Gr 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.

λRP

tP

Further notes

d = the distance between the transmitter antenna and the receiver antenna (m)

Pr = power received (W)

Pt = power transmitted (W)



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)λ

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.



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.

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 >> λ , where

D is the largest linear dimension of the antenna

λ is the carrier wavelength



λ 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 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

Gt = Gr = 10(2.15/10) = 1.64

( )W

W

d

GGPdP rtt

r

10

232

2

6

8

22

2

1096.2

105016

10144

10364.164.110

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

propagation over open fields.

Direct waveReflected wave


In the two-ray model, two propagation paths between the

transmitter/receiver are considered: the direct wave (LOS)

path, and the reflected wave path. (hTX, hRx and d are known.)

hTx


2222212 )( dhhddd

xRxT ++=+=

22

1 )( dhhdxRxT +−=

d

hhxx RT −

= arctanα

hTx

hRx

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:

hh2



,d

hhGGPP

2

2

RT

rtt1Rxx

=

Power falls off proportional to d4 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



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 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

path loss over irregular terrain. These models are empirical.

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 models.

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


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

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



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:


= ΩΩ 0

010 )()(log10

0a

f

f

d

d c ββµµ

0Ωµ is the power at 1 mile β is path loss exponent.

These parameters are determined from empirical measurements

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

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


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.


Log-distance path loss model.

Ericsson Multiple Breakpoint model.

Attenuation Factor model.

More Details see the referencing book:

Wireless Communications: Principles & Practice (2nd Ed)


Documents

4 Section 3