wireless communications eee: 464 · –satellite communication –microwave line-of-sight radio...

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.


• 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.


• 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.

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


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


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


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


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


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

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.

wireless communications eee: 464 · –satellite communication –microwave line-of-sight radio...

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