Monday, 3 May 2010

CWNA Study Guide Chapter 2 Radio Frequency Fundamentals Key Learning Points - Part 2

RF BEHAVIOURS

As RF waves travel they encounter various media tha affect them in different ways, these medi include air, water, trees, walls etc. Anything that the RF waves encounter may affect the way the waves behave.

These RF propogation behaviours inclued absorption, reflection, refraction, scattering, diffraction, free space path loss, multipath, attenuation and gain.

The way RF waves move, propogate, as it moves away from the antenna can vary dramatically depending on what is in its path.

Different materials, combinations of materials or events can have significantly different effects on a signals charcteristics.

ABSORPTION

Absorption is a fundamental property of RF waves, different media will absorb more or less RF and attenuate a signal to a greater or lesser degree. For example concrete walls will generally have a highe absorption tan glass therefore greater attenuation.

REFLECTION

RF waves will bounce off, reflect from, surfaces that are smooth. If the object is larger than the wave itself the wave will reflect, therefore reflection is dependent on frequency. Different object reflect different wavelengths to different degrees. Filing cabinets, doors erc can reflect RF wave sin the Wi-Fi spectrum, the angle of reflection depends on the angle of incidence.

Reflection can degrade signal strength and cause signal attenuation and data corruption. Hardware strategies help to overcome overcome these problems with directional antennas or antenna diversity.

Multiple reflected signals cause an effect known as multipath. MIMO antenna technologies actually take advantage of multipah.

SCATTERING

Two types of scattering may occur the first type has a lesser effect on the RF wave and is caused when the wave travels through a medium containing minute particles like fog. The second type is when an RF wave hits an uneven surface and is reflected in many directions, foliage and rocky terain may cause this type of scattering.

REFRACTION

refraction occurs when an RF wave travels through mediums of different density that cause the direction of the wave to change. Generally this is encountered over long distance outdoor bridge links where water vapour and changes in air pressure and temmperature may cause this effect.

The k factor is a unit of refractivity index in long distance bridge links. A k factor of 1 means no bending, a k factor less tahn 1 represents a signal bending away from earth. Normal atmospheric conditions have a k factorof 4/3 wic is bending sligtly towards te earth.

DIFFRACTION

This is where an RF wave bends around an object and should not be confused with refraction. This would typically be some kind of obstruction. A dead zone may exist immediately behing the obstruction known as an RF shadow.

LOSS

Loss or attenuation is the decrease in amplitude or signal strength. Loss may occur on the wire due to impedance of the cable or in the air due to absorption and distance. Different materials have different absorption properties.

Loss and gain may be gauged by a relative measurement of the change in pwer called dB.

FREE SPACE PATH LOSS

Free space path loss (FSPL) is the attenuation of the signal due to the natural phenomenon of the signal spreading out over a larger area as the signal moves farther away from the antenna. Loss in signal strength is logarithmic and not linear therefore a signals strength does not decrease as muchin subsequent segments of equal distance as in the segment.

A 2.4 GHz signal will attenuate by approximately 80dB in the first 100m from the antenna source and only 6dB in the second 100m from the antenna source.

FSPL = 36.6 + (20log10(ƒ)) + (20log10(D))

where FSPL=free space path loss, ƒ=frequency and D= distance in miles.

FSPL = 32.44 + (20log10(ƒ)) + (20log10(D))

where FSPL=free space path loss, ƒ=frequency and D= distance in kilometres.

This can be described as the 6dB rule in that for every doubling of the distance te in loss of amp;itude will be 6dB.

MULTIPATH

Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipat are scattering, refraction, difraction and reflection. Typically reflection is the main cause of multipath.

The reflected signals will arrive at the receiving antenna nanoseconds after the priniciple signal due to travelling further, this time difference is knwn as delay spread.

The results of multipath may either be good or bad. generally the results are destructive due to the differences in phase of the multiple paths. The combined signal may attenuate, amplify or be corrupted.

The four possible results of multipath are

Downfade is decreased signal strength caused when the received signal arrives at the receiver at the same time as the principle signal but are out of phase by between 121 and 179 degrees.

Upfade is increased signal strength caused when the received signal arrives at the receiver at the same time as the principle signal and is in phase or nearly phase. The phase differences are between 0 and 120 degrees. The received signal can never be stronger than the transmitted signal due to free space path loss.

Nulling is the complete cancellation of the signal caused when the received signal arrives at the receiver at the same time as the principle signal but are out of phase by 180 degrees.

Data corruption can occur due to the differences in time between the reflected signals and the principle signal. The delay spread time differential may cause overlapping and the receiver may have problems demodulating the signal.

Multipath may be problematic causing layer 2 retransmissions and negatively impacting the WLAN throughput.

Direction antennas and antenna diversity may help reduce multipath problems.

GAIN

Gain is the increase in amplitude and there are two types of gain, active and passive.

Active gain is the use of an amplifier on the wire and generally requies an external power source.

Passive gain is the focussing of the RF signal by use of an antenna.

Note

Frequency Domain Tool - spectrum analyser

Time Domain Tool - oscilloscope

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