microwave definitions & terms

8/7/2019 Microwave definitions & terms

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Diversity scheme

In telecommunications, a diversity scheme refers to a method for improving the

reliability of a message signal by utilizing two or more communication channels withdifferent characteristics. Diversity plays an important role in combatting fading and co-

channel interference and avoiding error bursts. It is based on the fact that individualchannels experience different levels of fading and interference. Multiple versions of thesame signal may be transmitted and/or received and combined in the receiver.

Alternatively, a redundantforward error correction code may be added and different parts

of the message transmitted over different channels. Diversity techniques may exploit the

multipath propagation, resulting in a diversity gain, often measured in decibel.

The following classes of diversity schemes can be identified:

Time diversity: Multiple versions of the same signal are transmitted at different

time instants. Alternatively, a redundantforward error correction codeis added

and the message is spread in time by means ofbit-interleavingbefore it istransmitted. Thus, error bursts are avoided, which simplifies the error correction.

Or

Time Diversity is used in digital communication systems to combat that the

transmissions channel may suffer fromerror bursts due to time-varying channelconditions. The error bursts may be caused by fading in combination with a moving

receiver, transmitter or obstacle, or by intermittent electromagnetic interference, for

example from crosstalkin a cable, orco-channel interference from radio transmitters.

Time diversity implies that the same data is transmitted multiple times, or a redundanterror code is added. By means of bit-interleaving, the error bursts may be spread in time.

Frequency diversity: The signal is transferred using several frequency channels

or spread over a wide spectrum that is affected by frequency-selective fading.Examples are:

o OFDM modulation in combination with subcarrierinterleavingand

forward error correctiono Spread spectrum, for example frequency hoppingorDS-CDMA.

Space diversity: The signal is transferred over several different propagation

paths. In the case of wired transmission, this can be achieved by transmitting via

multiple wires. In the case of wireless transmission, it can be achieved byantennadiversity using multiple transmitter antennas (transmit diversity) and/or multiple

receiving antennas (diversity reception). In the latter case, adiversity combining
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experienced differences inattenuation, delay andphase shift while travelling from the

source to the receiver. It may also be caused by attenuation of a single signal.

The most common types of fading, known as "slow fading" and "fast fading", as theyapply to a mobile radio environment, are explained below.

Beforehand, it might be necessary to remind ourselves of a definition of fading:

Fading refers to the time variation of the received signal power caused by changes in the

transmission medium or path.

Slow Fading: Shadowing or Large-Scale fading is a kind of fading caused bylarger movements of a mobile or obstructions within the propagation

environment. This is often modelled as log-normal distribution with a standard

deviation according to the Log Distance Path Loss Model.

Fast Fading: Multipath fading or Small-Scale fading is a kind of fading occurringwith small movements of a mobile or obstacle.

For example, consider the common experience of stopping at traffic lights and hearing a

lot of static on your FM broadcast radio, which is immediately corrected if you move less

than a metre. Cellular phones also exhibit similar momentary fades. The reason for theselosses of signal is the destructive interference that multiple reflected copies of the signal

makes with itself. To understand how a signal can destructively interfere with itself,

consider the sum of two sinusoidal waveforms (which are similar to modulated carriersignals) with different phases.

The best way to combat fading is to ensure that multiple versions of the same signal aretransmitted, received, and coherently combined. This is usually termed diversity, and is

sometimes acquired through multiple antennas. Mathematically, the simplest model forthe fading phenomenon is multiplication of the signal waveform with a time-dependent

coefficient which is often modeled as a random variable, making the received signal-to-

noise ratio a random quantity.

Fading channel models are often used to model electromagnetic transmission ofinformation over wireless media such as with cellular phones, and in broadcast

communications. However, even for underwater acoustic communications the notion of

fading is useful in understanding the distortion caused by the medium.

Small-scale fading is usually divided into fading based on multipath time delay spreadand that based on Doppler spread.

There are two types of fading based on multipath time delay spread:

Flat fading, where the bandwidth of the signal is less than thecoherence

bandwidth of the channel or the delay spread is less than the symbol period.
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Frequency selective fading, where the bandwidth of the signal is greater than the

coherence bandwidth of the channel or the delay spread is greater than the symbol

period.

There are two types of fading based on dopplerspread:

Fast fading, which has a high doppler spread, and the coherence time is less than

the symbol period, and the channel variations are faster thanbaseband signal

variations.

Slow fading, which has a low doppler spread. The coherence time is greater than

the symbol period and the channel variations are slower than the baseband signal

variations.

In addition to the small scale fading that is described above, for which the change in thesignal strength occurs, for mobile phone frequencies, on the order of a fraction of a meter,

the signal can also undergo shadow fading, orshadowing. This is due to the presence of

obstacles between the transmitter and the receiver, and the scale of distance required toexperience shadowing is about an order of magnitude larger than that of multipath fading.

Interference

is the superpositionof two or more waves resulting in a new wave pattern. As most

commonly used, the term usually refers to the interference of waves which are correlatedorcoherentwith each other, either because they come from the same source or because

they have the same or nearly the same frequency. Two non-monochromatic waves are

only fully coherent with each other if they both have exactly the same range of

wavelengths and the samephase differences at each of the constituent wavelengths.

The principle of superposition of waves states that the resultant displacement at a point is

equal to the sum of the displacements of different waves at that point. If a crest of a wave

meets a crest of another wave at the same point then the crests interfere constructively

and the resultant waveamplitude is greater. If a crest of a wave meets a trough of another

wave then they interfere destructively, and the overall amplitude is decreased.

Interference is involved in Thomas Young'sdouble-slit experiment where two beams of

light which are coherent with each other interfere to produce an interference pattern (thebeams of light both have the same wavelength range and at the center of the interference

pattern they have the samephases at each wavelength, as they both come from the same

source). More generally, this form of interference can occur whenever a wave canpropagate from a source to a destination by two or more paths of different length. Two ormore sources can only be used to produce interference when there is a fixed phase

relation between them, but in this case the interference generated is the same as with a

single source; see Huygens' principle.

Total phase difference is derived from the sum of both the path difference and the initial

phase difference (if the waves are generated from 2 or more different sources). Hence, we
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can then conclude whether the waves reaching a point are in phase(constructive

interference) or out of phase (destructive interference).

Light from any source can be used to obtain interference patterns, for example,Newton'srings can be produced with sunlight. However, in general white light is less suited for

producing clear interference patterns, as it is a mix of a full spectrum of colours, that eachhave different spacing of the interference fringes. Sodium lightis close to monochromatic

and is thus more suitable for producing interference patterns. The most suitable islaserlight because it is almost perfectly monochromatic

Constructive and destructive interference

Interference pattern produced with aMichelson interferometer. Bright bands are the

result ofconstructive interference while the dark bands are the result ofdestructive

interference.

When two sinusoidal waves superimpose, the resulting waveform depends on thefrequency (or wavelength) amplitude and relative phase of the two waves. If the twowaves have the same amplitudeA and wavelength the resultant waveform will have an

amplitude between 0 and 2A depending on whether the two waves are in phase orout of

phase.

combined

waveform

wave 1

wave 2

Two waves in phaseTwo waves 180 out

of phase

Consider two waves that are in phase,with amplitudesA1 andA2. Their troughs and peaks

line up and the resultant wave will have amplitude A =A1 +A2. This is known as

constructive interference.

If the two waves arepiradians, or 180, out of phase, then one wave's crests will coincidewith another wave's troughs and so will tend to cancel out. The resultant amplitude isA =

|A1 A2 | . IfA1 =A2, the resultant amplitude will be zero. This is known as destructive

interference.
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Wavelength

The wavelength is the distance between repeating units of a wavepattern. It iscommonly designated by the Greekletterlambda().

In a sine wave, the wavelength is the distance between the midpoints of the wave:

Thex axis represents distance, andIwould be some varying quantity at a given point in

time as a function ofx, for instance sound pressure (air pressure for a sound wave) or

strength of the electric ormagnetic field forlight.

Relationship with frequency

Wavelength has an inverse relationship to frequencyf, the number of peaks to pass a

point in a given time. The wavelength is equal to the speed of a wave type divided by thefrequency of the wave. When dealing with electromagnetic radiation in a vacuum, this

speed is the speed of lightc. For sound waves in air, this is the speed of sound in air. The

relationship is given by:

where

= wavelength of asound wave orelectromagnetic wave

vw is the speed of propagation of the wave, andf= frequency of the wave in 1/s = Hz.
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For radio waves this relationship is approximated with the formula: wavelength (in

metres) equals 3108 m/s divided by frequency (in hertz).

For sound waves in air, this relationship is approximated with the formula: wavelength

(in metres) = 343 m/s divided by frequency (in hertz). .

Note that the speed of light in a vacuum, and thus in a vacuum

or

In non-vacuum mediums

When light waves (and other electromagnetic waves) enter a medium, their wavelength is

reduced by a factor equal to the refractive indexn of the medium but the frequency of the

wave is unchanged. The wavelength of the wave in the medium, ' is given by:

where:

0 is the vacuum wavelength of the wave

Wavelengths of electromagnetic radiation, no matter what medium they are traveling

through, are usually quoted in terms of the vacuum wavelength, although this is not

always explicitly stated.

Quantum wavelength of particles

Louis de Broglie discovered that all particles withmomentumhave a wavelength

associated with theirquantum mechanicalwavefunction, called the de Broglie

wavelength:

where

h is Planck's constant, and

p is the momentumof the object.

In general, large particles have much smaller wavelengths than photons. For particles andobjects, their wavelength depends on their speed, as well as their mass.
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Received signal level

In telecommunications, and particularly inradio,signal strength is the measure of how

strongly a transmitted signalis being received, measured, or predicted, at a referencepoint that is a significant distance from the transmitting antenna. It may also be referred

to as received signal level orfield strength. Typically, this is measured as signalelectricfield strength ofvoltage perlengthorsignal powerreceived by a reference antenna.Higher powered transmissions such asbroadcastinguse units ofdB-millivolts permetre

(dBmV/m). Very low-power uses such as mobile phones are most often expressed in dB-

microvolts per metre (dBV/m) or indecibels above a reference level of one milliwatt

(eg -80 dBm).

In broadcasting terminology 1 mV/m is 0 dBm (a shortened dB(mV/m)), or 60 dB

(often written dBu) and has no reference to the dB milliwatt, the more common use of

dBm.

Some examples

100 dB or 100 mV/m:blanketinginterference occurs

60 dB or 1 mV/m: the edge of aradio station's protected area

40 dB or 100 V/m: the minimum strength a station can be received

Cell Phone Signals

Although there are cell phone base station tower networks across many nations globally,

there are still many areas within those nations that do not have good reception. Some

rural areas are unlikely ever to be effectively covered since the cost of erecting a cell

tower is too high for only a few customers. Even in high reception areas it is often foundthat basements and the interiors of large buildings have poor reception.

Weak signal strength can also be caused by destructive interferenceof the signals from

local towers in urban areas, or by the construction materials used in some buildings

causing rapid attenuation of signal strength. Large buildings such as warehouses,hospitals and factories often have no useable signal further than a few metres from the

outside walls.

This is particularly true for the networks which operate at higherfrequency since theseare attenuated more rapidly by intervening obstacles, although they are able to use

reflection anddiffraction to circumvent obstacles.

Cell phones in the U.S. operate at around 800MHz andPCS phones at 1900MHz:

classified as UHFand low energy microwavesrespectively. This has lead to the rapidgrowth in the home cellular repeatermarket. The more advanced models now typically

include an external directional antenna and anamplifier(usually operating at 55db gain) -

which is generally enough to turn a very weak signal into a clear one over the local area

(from around a thousand square feet to over twenty thousand).
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Fade margin

In telecommunication, the term fade margin (fading margin) has the following

meanings:

A design allowance that provides for sufficient systemgainorsensitivity toaccommodate expected fading, for the purpose of ensuring that the required

quality of serviceis maintained.

The amount by which a received signal level may be reduced without causingsystem performance to fall below a specified threshold value.

Link budget

A link budget is the accounting of all of the gains and losses from the transmitter,

through the medium (free space, cable, waveguide, fiber, etc.) to the receiver in atelecommunication system. It takes into account the attenuation of the transmitted signal

due to propagation, as well as the loss, or gain, due to the antenna. Random attenuations

such as fading are not taken into account in link budget calculations with the assumption

that fading will be handled with diversity techniques.

A simple link budget equation looks like this:

Received Power (dB) = Transmitted Power (dBm) + Gains (dB) - Losses (dB)

Link budget for radio systems

For a line of sightradio system, a link budget equation might look like this:

RxP = TxP + TxG - TxL - FSL - ML + RxG - RxL

where:

RxP = received power (dBm)

TxP = transmitter output power (dBm)

TxG = transmitter antenna gain (dBi)

TxL = transmitter losses (coax, connectors...) (dB)

FSL = free space loss or path loss (dB)

ML = miscellaneous losses (fading, body loss, polarization

mismatch, other losses...) (dB)

RxG = receiver antenna gain (dBi)

RxL = receiver losses (coax, connectors...) (dB)

Line of sight deployments for example will have path losses that are related to the inversesquareof the distance. The Free Space Loss equation can be written in several ways

depending on the units of measure. Here are three examples:
http://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Systemhttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Sensitivity_(electronics)http://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Signal_strengthhttp://en.wikipedia.org/wiki/Thresholdhttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Decibelhttp://en.wikipedia.org/wiki/Line_of_sighthttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Free_space_losshttp://en.wikipedia.org/wiki/Path_losshttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Inverse_squarehttp://en.wikipedia.org/wiki/Inverse_squarehttp://en.wikipedia.org/wiki/Inverse_squarehttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Systemhttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Sensitivity_(electronics)http://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Quality_of_servicehttp://en.wikipedia.org/wiki/Signal_strengthhttp://en.wikipedia.org/wiki/Thresholdhttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Decibelhttp://en.wikipedia.org/wiki/Line_of_sighthttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Free_space_losshttp://en.wikipedia.org/wiki/Path_losshttp://en.wikipedia.org/wiki/Fadinghttp://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Inverse_squarehttp://en.wikipedia.org/wiki/Inverse_square


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Fordin meters ,fc in GHzand meters / second,

(3)

By taking of both sides of equation to obtain the dB version

L0 =

=

=

Free-space loss

In telecommunication, free-space loss is the loss in signal strength (see discussion) that

would result if all absorbing, diffracting, obstructing, refracting,scattering, and reflecting

influences were sufficiently removed having no effect on itspropagation.

As the name implies, free space loss assumes the transmitter and receiver are both locatedin free space and does not consider other sources of loss such as reflections, cable,

connectors etc. A discussion of these losses is included in the article on Link budget.

Similarly it does not take account of gains from particularantennas.

Free space power loss is proportional to the square of the distance between the transmitter

and receiver and also proportional to the square of the frequency of the radio signal.
http://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Signal_strengthhttp://en.wikipedia.org/wiki/Scatteringhttp://en.wikipedia.org/wiki/Scatteringhttp://en.wikipedia.org/wiki/Scatteringhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Free_spacehttp://en.wikipedia.org/wiki/Link_budgethttp://en.wikipedia.org/wiki/Antenna_(radio)http://en.wikipedia.org/wiki/Antenna_(radio)http://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Signal_strengthhttp://en.wikipedia.org/wiki/Scatteringhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Free_spacehttp://en.wikipedia.org/wiki/Link_budgethttp://en.wikipedia.org/wiki/Antenna_(radio)http://en.wikipedia.org/wiki/Frequency


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A particularly convenient way to express free space loss is in terms ofdB. The loss can

be expressed as:

FSL(dB) = 20log10(d) + 20log10(f) + K

where d is the distance, f is the frequency, and K is a constant that depends on the unitsused and details of the radio link.

If d is measured in meters, f in Hz, and the link uses isotropic antennas, the expression

becomes:

FSL(dB) = 20log10(d) + 20log10(f) 147.5

As an example, the FSL(dB) of a 1000 meter link operating at 1GHz using isotropicantennas is 92.5 dB.

Very useful for fast calculation is expression where d is measures in km and f in MHz(link usesisotropic antennas):

FSL(dB) = 32.45 + 20log10(d) + 20log10(f)

Note: Free-space loss is primarily caused bybeam divergence,i.e., signal energyspreading over larger areas at increased distances from the source, and by the inverse

square law ofelectromagnetic radiation.

The equation for free-space loss is below where is the signal wavelength, fis the signal

frequency, Ris the distance or radius of the signal from the transmitter, and c is the speed

of light in the signal transmission medium. Note that the units used should be consistent,e.g., and R in meters, and c in meters per second.

Path loss

Path loss (orpath attenuation) is the reduction in power density (attenuation) of an

electromagnetic wave as it propagates through space. Path loss is a major component inthe analysis and design of thelink budget of a telecommunication system.

This term is commonly used in wireless communications and signalpropagation. Path

loss may be due to many effects, such as free-space loss, refraction,diffraction,

reflection, aperture-mediumcoupling loss, and absorption. Path loss is also influenced byterrain contours, environment (urban or rural, vegetation and foliage), propagation

medium (dry or moist air), the distance between the transmitter and the receiver, and the

height and location of antennas.
http://en.wikipedia.org/wiki/DBhttp://en.wikipedia.org/wiki/Isotropic_antennashttp://en.wikipedia.org/wiki/Isotropic_antennashttp://en.wikipedia.org/wiki/Isotropic_antennashttp://en.wikipedia.org/wiki/Beam_divergencehttp://en.wikipedia.org/wiki/Beam_divergencehttp://en.wikipedia.org/wiki/Beam_divergencehttp://en.wikipedia.org/wiki/Inverse_square_lawhttp://en.wikipedia.org/wiki/Inverse_square_lawhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Attenuation_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Electromagnetic_wavehttp://en.wikipedia.org/wiki/Link_budgethttp://en.wikipedia.org/wiki/Link_budgethttp://en.wiktionary.org/wiki/signalhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Free-space_losshttp://en.wikipedia.org/wiki/Refractionhttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Reflection_(physics)http://en.wikipedia.org/wiki/Aperture_(antenna)http://en.wikipedia.org/wiki/Transmission_mediumhttp://en.wikipedia.org/wiki/Coupling_losshttp://en.wikipedia.org/wiki/Coupling_losshttp://en.wikipedia.org/wiki/Absorption_(optics)http://en.wikipedia.org/wiki/Absorption_(optics)http://en.wikipedia.org/wiki/DBhttp://en.wikipedia.org/wiki/Isotropic_antennashttp://en.wikipedia.org/wiki/Isotropic_antennashttp://en.wikipedia.org/wiki/Beam_divergencehttp://en.wikipedia.org/wiki/Inverse_square_lawhttp://en.wikipedia.org/wiki/Inverse_square_lawhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Attenuation_(electromagnetic_radiation)http://en.wikipedia.org/wiki/Electromagnetic_wavehttp://en.wikipedia.org/wiki/Link_budgethttp://en.wiktionary.org/wiki/signalhttp://en.wikipedia.org/wiki/Wave_propagationhttp://en.wikipedia.org/wiki/Free-space_losshttp://en.wikipedia.org/wiki/Refractionhttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Reflection_(physics)http://en.wikipedia.org/wiki/Aperture_(antenna)http://en.wikipedia.org/wiki/Transmission_mediumhttp://en.wikipedia.org/wiki/Coupling_losshttp://en.wikipedia.org/wiki/Absorption_(optics)


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Causes

Path loss normally includespropagation losses caused by the natural expansion of the

radio wave front in free space (which usually takes the shape of an ever-increasingsphere), absorption losses (sometimes called penetration losses), when the signal passes

through media not transparent to electromagnetic waves,diffraction losses when part ofthe radiowave front is obstructed by an opaque obstacle, and losses caused by otherphenomena.

The signal radiated by a transmitter may also travel along many and different paths to a

receiver simultaneously; this effect is called multipath. Multipath can either increase or

decrease received signal strength, depending on whether the individual multipathwavefronts interfere constructively or destructively. The total power of interfering waves

in a Rayleigh fading scenario vary quickly as a function of space (which is known assmall scale fading), resulting infast fades which are very sensitive to receiver position

K-factor

In telecommunication, the term k-factor has the following meanings:

1. In tropospheric radiopropagation, the ratio of the effective Earth radius to the actual

Earth radius.

Note: The k-factor is approximately 4/3.

2. In ionospheric radio propagation, a correction factor that (a) is applied in calculations

related to curved layers, and (b) is a function of distance and the real height ofionospheric reflection.

3. In laser diodetechnology, it describes the exceed spontaneous emission noise in gain-

guided lasers.
http://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Multipathhttp://en.wikipedia.org/wiki/Rayleigh_fadinghttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radio_propagationhttp://en.wikipedia.org/wiki/Effective_Earth_radiushttp://en.wikipedia.org/wiki/Ionospheric_reflectionhttp://en.wikipedia.org/wiki/Laser_diodehttp://en.wikipedia.org/wiki/Laser_diodehttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Electromagnetic_waveshttp://en.wikipedia.org/wiki/Diffractionhttp://en.wikipedia.org/wiki/Multipathhttp://en.wikipedia.org/wiki/Rayleigh_fadinghttp://en.wikipedia.org/wiki/Telecommunicationhttp://en.wikipedia.org/wiki/Radiohttp://en.wikipedia.org/wiki/Radio_propagationhttp://en.wikipedia.org/wiki/Effective_Earth_radiushttp://en.wikipedia.org/wiki/Ionospheric_reflectionhttp://en.wikipedia.org/wiki/Laser_diode

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