characteristics of radiowaves

8/12/2019 Characteristics of Radiowaves

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CHARACTERISTICS OF RADIO

WAVES


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Contents

Multipath characterstics of radio waves

Fading

LCR fading statistics

Doppler spread

Diversity techniques


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Radio Propagation

What is Radio?

Radio Xmitter induces E&M fields

Electrostatic field components 1/d3

Induction field components 1/d2

Radiation field components 1/d

Radiation field has E and B componentField strength at distance d = EB 1/d2

Surface area of sphere centered at transmitter


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General Intuition

Two main factors affecting signal at receiver

Distance (or delay) Path attenuation

MultipathPhase differences

Greensignal travels 1/2farther thanYellow to reach receiver, who seesblue.

For 2.4 GHz, (wavelength) =12.5cm.


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Objective

Invent models to predict what the field

looks like at the receiver.

Attenuation, absorption, reflection, diffraction...

Motion of receiver and environment

Natural and man-made radio interference...

What does the field look like at the receiver?


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Fading

Fading refers to the distortion that a carrier-

modulated telecommunication signal

experiences over certain propagation media. A fading channel is a communication

channel that experiences fading. In wireless

systems, fading is due to multipathpropagation and is sometimes referred to as

multipath induced fading.


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In wireless communications, the presence of reflectors in the environmentsurrounding a transmitter and receiver create multiple paths that atransmitted signal can traverse. As a result, the receiver sees thesuperposition of multiple copies of the transmitted signal, each traversing adifferent path.

Each signal copy will experienced differences in attenuation, delay andphase shift while travelling from the source to the receiver. This can resultin either constructive or destructive interference, amplifying orattenuating the signal power seen at the receiver.

Strong destructive interference is frequently referred to as a deep fadeandmay result in temporary failure of communication due to a severe drop in

the channel signal-to-noise ratio


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Fading channel models are often used to model theeffects of electromagnetic transmission ofinformation over the air in cellular networks and

broadcast communication. Fading channel models are also used in underwater

acoustic communications to model the distortioncaused by the water.

Mathematically, fading is usually modeled as a time-varying random change in the amplitude and phase ofthe transmitted signal.


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Types of Fading

Slow Fading

Fast Fading

The terms slowand fastfading refer to the rate atwhich the magnitude and phase change imposed by

the channel on the signal changes.

The coherence timeis a measure of the minimum

time required for the magnitude change of the

channel to become decorrelated from its previous

value.


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Slow fading

Slow fadingarises when the coherence time of the channel islarge relative to the delay constraint of the channel. In thisregime, the amplitude and phase change imposed by the

channel can be considered roughly constant over the period ofuse.

Slow fading can be caused by events such as shadowing,where a large obstruction such as a hill or large buildingobscures the main signal path between the transmitter and the

receiver. The amplitude change caused by shadowing is often modeled

using a log-normal distribution with a standard deviationaccording to the Log Distance Path Loss Model


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Flat vs. Frequency-selective

Fading

In flat fading, the coherence bandwidth of the

channel is larger than the bandwidth of the signal.

Therefore, all frequency components of the signal

will experience the same magnitude of fading.

In frequency-selective fading, the coherence

bandwidth of the channel is smaller than the

bandwidth of the signal. Different frequencycomponents of the signal therefore experience

decorrelated fading.


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In a frequency-selective fading channel, sincedifferent frequency components of the signal areaffected independently, it is highly unlikely that all

parts of the signal will be simultaneously affected bya deep fade.

Certain modulation schemes such as OFDM andCDMA are well-suited to employing frequencydiversity to provide robustness to fading.

Frequency-selective fading channels are alsodispersive, in that the signal energy associated witheach symbol is spread out in time.


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Mitigation

Fundamentally, fading causes poor performance intraditional communication systems because thequality of the communications link depends on a

single path or channel, and due to fading there is asignificant probability that the channel willexperience a deep fade.

The probability of experiencing a fade (and

associated bit errors as the signal-to-noise ratio drops)on the channel becomes the limiting factor in thelink's performance.


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The effects of fading can be combated by using

diversity to transmit the signal over multiple channels

that experience independent fading and coherently

combining them at the receiver.

The probability of experiencing a fade in this

composite channel is then proportional to the

probability that all the component channelssimultaneously experience a fade, a much more

unlikely event.


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How to overcome fading?

Common techniques used to overcome signal

fading include:

Diversity reception and transmission

OFDM

Rake receivers

Spacetime codes

MIMO


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Diversity Techniques:
http://localhost/var/www/apps/conversion/tmp/scratch_8/wiki/Image:Space_diversity.gif


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In telecommunications, a diversity scheme refers to a method forimproving the reliability of a message signal by utilizing two or morecommunication channels with different characteristics.

Diversity plays an important role in combating fading and co-channelinterference and avoiding error bursts.

It is based on the fact that individual channels experience different levels offading and interference.

Multiple versions of the same signal may be transmitted and/or receivedand combined in the receiver. Alternatively, a redundant forward errorcorrection code may be added and different parts of the messagetransmitted over different channels.

Diversity techniques may exploit the multipath propagation, resulting in adiversity gain, often measured in decibels.


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Classes of Diversity Schemes

Time diversity: Multiple versions of the same signal aretransmitted at different time instants. Alternatively, aredundant forward error correction code is added and the

message is spread in time by means of bit-interleaving beforeit is transmitted. Thus, error bursts are avoided, whichsimplifies the error correction.

Frequency diversity: The signal is transferred using severalfrequency channels or spread over a wide spectrum that is

affected by frequency-selective fading. Examples are: OFDM modulation in combination with subcarrier interleaving and

forward error correction

Spread spectrum, for example frequency hopping or DS-CDMA.


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Space diversity: The signal is transferred over several differentpropagation paths. In the case of wired transmission, this can be achievedby transmitting via multiple wires. In the case of wireless transmission, itcan be achieved by antenna diversity using multiple transmitter antennas(transmit diversity) and/or multiple receiving antennas (diversity

reception). In the latter case, a diversity combining technique is appliedbefore further signal processing takes place. If the antennas are at fardistance, for example at different cellular base station sites or WLANaccess points, this is called macrodiversity). If the antennas are at a distancein the order of one wavelength, this is called microdiversity. A specialcase is phased antenna arrays, which also can be utilized for beamforming,

MIMO channels and Space

time coding (STC). Polarisation diversity: Multiple versions of a signal are transmitted andreceived via antennas with different polarization. A diversity combiningtechnique is applied on the receiver side.


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Multiuser diversity: Multiuser diversity is obtained byopportunistic user scheduling at either the transmitter or thereceiver. Opportunistic user scheduling is as follows that thetransmit selects the best user among candidate receivers

according to qualities of each channel between the transmitand each receiver. In FDD systems, a receiver must feed backthe channel quality information to the transmitter with thelimited level of resolution.

Antenna diversity: transmitted along different propagation

paths. Cooperative diversity: enables to achieve the Antenna

diversity gain by the use of the cooperation of distributedantennas belonging to each node.


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LCR fading Statistics:

Rayleigh fading is a statistical model for the effect of a propagationenvironment on a radio signal, such as that used by wireless devices.

It assumes that the magnitude of a signal that has passed through such atransmission medium (also called a communications channel) will varyrandomly, or fade, according to a Rayleigh distribution the radialcomponent of the sum of two uncorrelated Gaussian random variables.

It is a reasonable model for tropospheric and ionospheric signalpropagation as well as the effect of heavily built-up urban environments onradio signals.

Rayleigh fading is most applicable when there is no dominant propagationalong a line of sight between the transmitter and receiver.

If there is a dominant line of sight, Rician fading may be more applicable.


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The model

Rayleigh fading is a reasonable model when there are many objects in theenvironment that scatter the radio signal before it arrives at the receiver.The central limit theorem holds that, if there is sufficiently much scatter,the channel impulse response will be well-modelled as a Gaussian processirrespective of the distribution of the individual components.

If there is no dominant component to the scatter, then such a process willhave zero mean and phase evenly distributed between 0 and 2radians.

The envelope of the channel response will therefore be Rayleighdistributed. Calling this random variable R, it will have a probabilitydensity function where =E(R2).

Often, the gain and phase elements of a channel's distortion areconveniently represented as a complex number. In this case, Rayleighfading is exhibited by the assumption that the real and imaginary parts ofthe response are modelled by independent and identically distributed zero-mean Gaussian processes so that the amplitude of the response is the sumof two such processes.


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Applicability

Densely-built Manhattan has been shown to approach a Rayleigh fading environment.
http://localhost/var/www/apps/conversion/tmp/scratch_8/wiki/Image:LowerManhattan2005.jpg


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One second of Rayleigh fading with a maximum Doppler shiftof 10Hz.

The requirement that there be many scatterers present means

that Rayleigh fading can be a useful model in heavily built-upcity centres where there is no line of sight between thetransmitter and receiver and many buildings and other objectsattenuate, reflect, refract and diffract the signal.

Rayleigh fading is a small-scale effect. There will be bulk

properties of the environment such as path loss and shadowingupon which the fading is superimposed.


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Doppler power spectral density

The normalized Doppler power spectrum of Rayleigh fading with amaximum Doppler shift of 10Hz.

The Doppler power spectral density of a fading channel describes howmuch spectral broadening it causes. This shows how a pure frequency e.g. a

pure sinusoid, which is an impulse in the frequency domain is spread outacross frequency when it passes through the channel. It is the Fouriertransform of the time-autocorrelation function. For Rayleigh fading with avertical receive antenna with equal sensitivity in all directions, this has

been shown to be where is the frequency shift relative to the carrierfrequency. This equation is only valid for values of between ; the spectrumis zero outside this range.

This spectrum is shown in the figure for a maximum Doppler shift of 10Hz. The 'bowl shape' or 'bathtub shape' is the classic form of this dopplerspectrum.


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Fading Models

Nakagami fading

Rayleigh fading

Rician fading

Dispersive fading models, with several echoes, each exposedto different delay, gain and phase shift, often constant.Thisresults in frequency selective fading and inter-symbolinterference. The gains may be Rayleigh or Riciandistributed.The echoes may also be exposed to doppler-shift,

resulting in a time varying channel model.

Log-normal shadow fading

characteristics of radiowaves

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