Tema04-ConceptosDeRadiofrecuencia

8/13/2019 Tema04-ConceptosDeRadiofrecuencia

1/58

Semana 05:

Conceptos de Radiofrecuencia


2/58

Wireless Radio Technology

1. Overview of waves 2.Bandwidth

3. Electromagnetic Spectrum 4. Size of a wave 5. Basics of EM waves 6. Wireles Propagation a. Attenuation b. free-Space Waves c. reflected waves d. diffraction e. refraction d. multipath reflection


3/58

Rick Graziani [email protected] 3

1. Overview of Waves

Waveis a disturbance or variation that travels through a medium. The medium through which the wave travels may experience somelocal oscillations as the wave passes, but the particles in the medium

do not travel with the wave.

Just like none of the individual people in the stadium are carried

around when they do the wave, they all remain at their seats.


4/58


5/58


Waves

The distance between 2 peaks (or 2 troughs (canal/valle))is called a wavelength

The deepest part of a trough or the highest part of a peakis called the amplitude

The frequencyis the number of wavelengths that pass byin 1 second

www.ewart.org.uk


6/58


a. Ondas Longitudinales

Longitudinal sound wavesin the air behave in much the same way. As the sound wave passes through, the particles in the air oscillate back and

forth from their equilibrium positions but it is the disturbance that travels, not

the individual particles in the medium.

Rick talks in a loud voice.

When he talks he causes the air near his mouth to compress. A compression wave then passes through the air to the ears of the people

around him.

A longitudinal sound wave has to travel through something - it cannot passthrough a vacuum because there aren't any particles to compress together.

It has a wavelength; a frequency and an amplitude.

www.ewart.org.uk


7/58Rick Graziani [email protected] 7

b. Transverse Waves

Transverse waveson a string are another example. The string is displaced up and down, as the wave travels from left to right, but

the string itself does not experience any net motion.

A light waveis a transverse wave. If you look at the waves on the sea they seem to move in one direction ....

towards you.

However, the part ic les that make up the wave only mo ve up and down. Look at the animation, on the right, al thou gh the wave seems to be moving

from lef t to r ight the blue part ic le is only moving u p and down.

interactive

activity 3.1.1



c. Sine waves

The sine waveis unique in that it represents energy entirelyconcentrated at a single frequency.

An ideal wireless signalhas a sine waveform With a frequency usually measured in cycles per second or Hertz

(Hz).

A million cycles per second is represented by megahertz (MHz). A billion cycles per second represented by gigahertz (GHz).



Sine waves

Amplitude :The distance from zero to the maximum value of eachalternation is called the amplitude.

The amplitude of the positive alternation and the amplitude of the

negative alternation are the same. Period :The time it takes for a sine wave to complete one cycle is

defined as the period of the waveform.

The distance traveled by the sine wave during this period isreferred to as its wavelength.

Wavelength:Indicated by the Greek lambda symbol . It is the distance between one value to the same value on the next

cycle.

Frequency :The number of repetitions or cycles per unit time is thefrequency, typically expressed in cycles per second, or Hertz(Hz).



Relationship between time and frequency

The inverse relationshipbetween time (t), the period in seconds, and

frequency (f), in Hz, is indicated by the following formulas:t= 1/f (time = 1 / frequency)

f= 1/t (frequency = 1 / time)

Examples:

1 second

t = 1/f 1 second = 1 / 1 Hz (1 cycle per second) f = 1/t 1 Hz = 1 / 1 second

second

t = 1/f second = 1 / 2 Hz (2 cycles per second)

f = 1/t 2 Hz = 1 / second

1/10,000,000thof a second

t = 1/f 1/10,000,000thof a second = 1 / 10,000,000 Hz (cycles/sec) = 1 / 10 MHz f = 1/t 10 MHz = 1 / 1/10,000,000thof sec



Sine waves

One full period or cycleof a sine wave is said to cover 360 degrees(360).

It is possible for one sine wave to lead or lag another sine wave by anynumber of degrees, except zero or 360.

When two sine waves differ by exactly zeroor 360,the two wavesare said to be in phase.

Two sine waves that differ in phaseby any other value are out ofphase, with respect to each other.

180Phase Shift



Analog to digital conversion

1. Analog wave amplitudes are sampled at specific instances in time.2. Each sample is assigned a discrete value.3. Each discrete value is converted to a stream of bits.



2. Bandwidth

There are two common ways of looking at bandwidth:1. Analog bandwidth2. Digital bandwidth

1. Analog bandwidth

Analog bandwidth can refer to the range of frequencies. Analog bandwidth is described in units of frequency, or cycles per

second, which is measured in Hz.

There is a direct correlation between the analog bandwidthofany medium and the data rate in bits per secondthat the medium

can support.



Bandwidth

2. Digital bandwidth

Digital bandwidth is a measure of how much information can flow

from one place to another, in a given amount of time.

Digital bandwidth is measured in bits per second.

When dealing with data communications, the term bandwidth most

often signifies digital bandwidth.


15/58

3. EM (Electromagnetic) Spectrum



Basics of EM waves

EM (Electromagnetic)spectruma set of all types of radiation whendiscussed as a group.

Radiationis energy that travels in waves and spreads out over

distance. The visible light that comes from a lamp in a house and radio waves

that come from a radio station are two types of electromagnetic waves.

Other examples are microwaves, infrared light, ultraviolet light, X-rays,and gamma rays.



Basics of EM waves

All EM waves travel at the speed of light in a vacuum and have acharacteristic wavelength ()and frequency (f), which can bedetermined by using the following equation:

c = x f, where c = the speed of light (3 x 108m/s) Wavelength x Frequency = Speed of light Speed of light = 180,000 miles/sec or

300,000 kilometers/sec or

300,000,000 meters/sec



Basics of EM waves

wavelength (), frequency (f), speed of light (c) A wave of 1 cycle per second, has a wavelength of 300,000,000

meters or 300,000 kilometers or 180,000 miles.

Speed of a bit doesnt go beyond the speed of light, Dr. Einstein sayswe all go poof (my words, not his)

Speed is a function of increasing the number of waves, bits, in thesame amount of s ace, I.e. bits er second

300,000 kilometers

or 180,000 miles

150,000 km 150,000 km


19/58

4. Size of a Wave



Size of a Wave

Its important to visualize the physical size of a wireless

signal because the physical size determines:1. How that signal interacts with its environment

2. How well it is propagated from antenna to antenna

3. The physical size of the antenna (the smaller the signal

size, the smaller the antenna)



Speed of light = 186,000 miles/sec or 300,000,000 meters/sec (approx.)

1 second

186,000 miles

Mile: 0 Mile: 186,000

Speed of Light

Start here End here

1 mile

5,280 feet per mile; so 186,000 miles = 982,080,000 feet 63,360 inches per mile; so 186,000 miles = 11,784,960,000 inches



Wavelengthhttp://eosweb.larc.nasa.gov/EDDOCS/wavelength.html

Speed of the wave = Frequency x Wavelength

Wavelength= Speed of the wave or speed of light/ Frequency

Speed of light = 186,000 miles/sec or

982,080,000 feet/sec or

11,784,960,000 inches/sec

Wavelength= Speed of the waveor speed of light/ Frequency

10.93 feet = 982,080,000 feet per sec / 90,000,000 cycles per sec

All About Wavelength
http://eosweb.larc.nasa.gov/EDDOCS/wavelength.htmlhttp://eosweb.larc.nasa.gov/EDDOCS/wavelength.html



Speed of light = 186,000 miles/sec

Length of rope 186,000 miles long

1 second

Mile: 0,

beginning of

rope

0 seconds

After 1/2 second

After 1 second

Mile:

186,000,

end of rope

Length of rope (cuerda/cordel) 186,000 miles long traveling at thespeed of light, 186,000 miles/second

In 1 second we would see the entire length of rope go by.

0 second

Speed of Light

1 second


24/58

5. Basics of EM Waves


25/58


Basics of EM waves

EM waves exhibit the following properties:1. Reflection or bouncing

2. Refraction or bending3. Diffraction or spreading around obstacles

4. Scattering or being redirected by particles

This will be discussed in greater detail laterin this module. Also, the frequency and the wavelength of an EM wave are inversely

proportionally to one another.


26/58


Basics of EM waves

There are a number ofproperties that apply toall EM waves,

including:

1. Direction

2. Frequency3. Wavelength

4. Power

5. Polarization

6. Phase.


27/58


EM Spectrum Chart

One of the most important diagrams in both science and engineering is thechart of the EM spectrum .

The typical EM spectrum diagram summarizes the ranges of frequencies, orbands that are important to understanding many things in nature andtechnology.

EM waves can be classified according to their frequency in Hz or theirwavelength in meters.

The most important range for this course is the RF (Radio Frequency)spectrum.


28/58


EM Spectrum Chart

The RF spectrum includes several frequency bands including:1. Microwave

2. Ultra High Frequencies (UHF)

3. Very High Frequencies (VHF)

This is also where WLANs operate. The RF spectrumranges from 9 kHz to 300 GHz. Consists of two major sections of the EM spectrum: (RF Spectrum)

Radio Waves

Microwaves.

The RF frequencies, which cover a significant portion of the EM radiationspectrum, are used heavily for communications.

Most of the RF ranges are licensed, though a few key ranges are unlicensed.


29/58


EM Spectrum Chart

Nasa.gov


30/58


Nasa.gov


31/58

Rick Graziani [email protected] 31www.britishlibrary.net


32/58


Licensed Frequencies

Frequency bands have a limited number of useable differentfrequencies, or communications channels.

Many parts of the EM spectrum are not useable for communicationsand many parts of the spectrum are already used extensively for thispurpose.

The electromagnetic spectrum is a finite resource. One way to allocate this limited, shared resource is to have

international and national institutions that set standards and laws as tohow the spectrum can be used.

In the US, it is the FCCthat regulates spectrum use. In Europe, the European Telecommunications Standards Institute

(ETSI) regulates the spectrum usage.

Frequency bands that require a license to operate within are called thelicensed spectrum.

Examples include amplitude modulation (AM) and frequencymodulation (FM) radio, ham or short wave radio, cell phones,broadcast television, aviation bands, and many others.

In order to operate a device in a licensed band, the user must firstapplyfor and be granted the appropriate license.


33/58


ISM (Industrial, Scientific, and Medical) &

U-NII (Unlicensed National Information Infrastructure)

Some areas of the spectrum have been left unlicensed.

This is favorable for certain applications, such as WLANs. An important area of the unlicensed spectrum is known as the industrial,

scientific, and medical (ISM) bands and the U-NII(Unlicensed NationalInformation Infrastructure)

ISM802.11b, 802.11g

U-NII802.11a

These bands are unlicensed in most countries of the world. The following are some examples of the regulated items that are related to

WLANs:

The FCC has defined eleven 802.11b DSSS channels and theircorresponding center frequencies. ETSI has defined 13.

The FCC requires that all antennas that are sold by a spread spectrumvendor be certified with the radio with which it is sold.

Unlicensed bands are generally license-free, provided that devices are lowpower.

After all, you dont need to license your microwave oven or portable phone.


34/58

6. Wireless Propagation



35/58


Wireless Propagation

There are several important simplifications which can be made. In a vacuum, 2.4 GHz microwaves travel at the speed of light. Once started, these microwaves will continue in the direction they were emitted

forever, unless they interact with some form of matter.

In the atmosphere, the microwaves are traveling in air, not in a vacuum. This does not significantly change their speed. Similar to light, when RF travels through transparent matter, some of the waves

are altered.

2.4 & 5 GHz microwaves also change, as they travel through matter.

Amount of alteration depends heavily on the frequency of the waves and thematter.

Wireless propagationis the total of everything that happens to a

wireless signal as the signal travels from Point A to Point B. The study of how EM waves travel and interact with matter can

become extremely complex.


36/58


Wireless Propagation

Mental picture

Wave is not a spot or a line, but a moving wave.

Like dropping a rock into a pond. Wireless waves spread out from the antenna. Wireless waves pass through air, space, people, objects,


37/58


a. Attenuation

Attenuationis the loss in amplitude that occurs whenever a signaltravels through wire, free space, or an obstruction. At times, after colliding with an object the signal strength remaining is

too small to make a reliable wireless link.

Same wavelength(frequency),less amplitude.


38/58


Attenuation and Obstructions

Longer the wavelength(lower frequency) of the wireless signal, the

lessthe signal is attenuated.

Same wavelength

(frequency), less

amplitude.

Shorter the wavelength(higher frequency) of the wireless signal, the

morethe signal it is attenuated.

O


39/58


Attenuation and Obstructions

The wavelength for the AM (810 kHz) channel is 1,214 feet The larger the wavelength of the signal relative to the size of theobstruction, the less the signal is attenuated.

The shorter the wavelength of the signal relative to the size of theobstruction, the more the signal is attenuated.

b F S W


40/58


b. Free-Space Waves

Free-space waveis a signal that propagates from Point Ato Point B without encountering or coming near an

obstruction.

The only amplitude reduction is due to free space loss(coming).

This is the ideal wireless scenario.

R fl t d W


41/58


c. Reflected Waves

When a wireless signal encounters an obstruction, normally two

things happen:1. AttenuationThe shorter the wavelengthof the signal relative to

the size of the obstruction, the more the signal is attenuated.

2. ReflectionThe shorter the wavelengthof the signal relative to thesize of the obstruction, the more likelyit is that some of the signal will

be reflectedoff the obstruction.

Mi R fl ti


42/58


Microwave Reflections

Microwave signals: Frequencies between 1 GHz30 GHz (this can vary among

experts). Wavelength between 12 inches down to less than 1 inch.

Microwave signals reflect off objects that are larger than theirwavelength, such as buildings, cars, flat stretches of ground, and

bodes of water.

Each time the signal is reflected, the amplitude is reduced.

R fl ti


43/58


Reflection

Reflection is the light bouncing back in the general direction from whichit came.

Consider a smooth metallic surface as an interface. As waves hit this surface, much of their energy will be bounced or

reflected.

Think of common experiences, such as looking at a mirror or watchingsunlight reflect off a metallic surface or water.

When waves travel from one medium to another, a certain percentageof the light is reflected.

This is called a Fresnel reflection (Fresnel coming later).

R fl ti


44/58


Reflection

Radio waves can bounce off of different layers of the atmosphere. The reflecting properties of the area where the WLAN is to be installed

are extremely important and can determine whether a WLAN works orfails.

Furthermore, the connectorsat both ends of the transmission linegoing to the antenna should be properly designed and installed, so

that no reflection of radio waves takes place.

R fl ti


45/58


Reflections

Mi R fl ti


46/58


Microwave Reflections

Advantage: Can use reflection to go around obstruction. Disadvantage: Mult ipath ref lect ionoccurs when reflections causemore than one copy of the same transmission to arrive at the receiver

at slightly different times.

Multipath Reflection

d Diff ti


47/58


d. Diffraction

Diffractionof a wireless signal occurs when the signal is partiallyblocked or obstructed by a large object in the signals path. A diffracted signal is usually attenuated so much it is too weak to

provide a reliable microwave connection.

Do not plan to use a diffracted signal, and always try to obtain an

unobstructed path between microwave antennas.

Diffracted

Signal

W th P i it ti


48/58


Weather - Precipitation

Precipitation: Rain, snow, hail, fog, and sleet.

Rain, Snow and Hail

Wavelength of 2.4 GHz802.11b/g signal is 4.8 inches Wavelength of 5.7 GHz802.11a signal is 2 inches

Much larger than rain drops and snow, thus do not significantly

attenuate these signals.

At frequencies 10 GHz and above, partially melted snow and hail do

start to cause significant attenuation.

Weather Precipitation


49/58


Weather - Precipitation

Rain can have other effects: Get inside tiny holes in antenna systems, degrading the

performance.

Cause surfaces (roads, buildings, leaves) to become more

reflective, increasing multipath fading.

Tip: Use unobstructed paths between antennas, and do not try to blast

through trees, or will have problems.

Weather Ice Collapsed tower


50/58


Weather - Ice

Ice buildup on antenna systems can: Reduce system performance

Physically damage the antenna system

Collapsed tower

Weather Wind


51/58


Weather - Wind

The affect of wind: Antenna on the the mast or tower can turn, decreasing the aim ofthe antenna.

The mast or tower can sway or twist, changing the aim.

The antenna, mast or tower could fall potentially injuring someone

or something.

e Refraction


52/58


e. Refraction

Refraction(or bending) of signals is due to temperature, pressure, andwater vapor content in the atmosphere.

Amount of refractivity depends on the height above ground. Refractivity is usually largest at low elevations. The refractivity gradient (k-factor) usually causes microwave signals to

curve slightly downward toward the earth, making the radio horizon

father away than the visual horizon.

This can increase the microwave path by about 15%,

Normal

Refraction

Refraction (straight line)

Sub-Refraction

Earth

Refraction


53/58


Refraction

Radio waves also bend when entering different materials. This can be very important when analyzing propagation in the

atmosphere.

It is not very significant in WLANs, but it is included here, as part of ageneral background for the behavior of electromagnetic waves.

f Multipath Reflection


54/58


Reflected signals 1 and 2 take slightly longer paths than direct signal,arriving slightly later.

These reflected signals sometimes cause problems at the receiver bypartially canceling the direct signal, effectively reducing the amplitude.

The link throughput slows down because the receiver needs more timeto either separate the real signal from the reflected echoes or to wait

for missed frames to be retransmitted.

Solution discussed later.

f. Multipath Reflection

Free Space Path Loss


55/58

Free Space Path Loss

It regards to the distance that RF signals can successfullytravel and be received properly.

Itsthe result of the normal attenuation that happens, asthe signal gradually weakens over the distance it travels.

Factors that determines the effects of FSPL:

A. Active gain: AP can amplify the signal B. Passive gain : It comes from the particular shape of the

antenna pattern itself.


FSPL: Formulas


56/58

FSPL: Formulas



57/58

En el transmisor:

La antena transforma la seal elctrica en una ondaelectromagntica mediante la excitacin de campos

elctricos o magnticos en su medio circundante

inmediato.

En el receptor:La antena captura energa del campo

electromagntico y la transforma en corriente y voltaje

en el circuito elctrico.



58/58

El campo elctrico o magntico oscilatorio genera una

onda electromagntica que se propaga con la velocidad dela luz c .

La velocidad de la luz en el espacio vaco c es299.792.458 m/s.

Para efectos prcticos ser : 3x10^8 m/s

Tema04-ConceptosDeRadiofrecuencia

Documents

Transcript of Tema04-ConceptosDeRadiofrecuencia