WLAN Design Considerations

 
In order to create a design for a wireless system, you must consider common
WLAN transmission and reception impairments such as attenuation, RF interference,
and application and structural considerations. Many environmental factors
can also affect your WLAN design.This section explains various common types
of impairments and considerations that you may face in your wireless design and
testing efforts.
Designing & Planning…
Indoor vs. Outdoor Implementation
A lot of the issues covered here as WLAN design considerations are based
on factors that can occur both indoors and outdoors. However, as a general
rule, if you are looking at implementing a WLAN in a building that has
been built within the last ten years and does not have any special structural
considerations (i.e. concrete shielding for radiation labs), then implementing
a WLAN should be pretty straight forward. There are a lot of
additional factors when extending a WLAN over long distances outdoors,
but in a standard office implementation these are usually not an issue.
Attenuation
Attenuation is the decrease in strength of a radio wave between a transmitter and
a receiver; the strength decreases as the distance from the antenna increases. It can
be caused by natural conductivity or by resistance of all sorts of physical matter,
but the greatest resistor to radio waves is the Earth. Radiated energy from the
Earth and interference from trees and buildings will cause attenuation of a signal’s
ground waves, just as radiated energy and interference from water and dust particles
in the atmosphere will affect a signal’s sky waves.You must plan your design
and equipment use based on affecting ground and sky wave propagation such as
transmitter height, distance between transmitters, and solar radiation factors.  Attenuation Due to Antenna Cabling
Loss due to antenna cable length must always be considered when designing a
wireless system. Cisco cabling produces 6.7 decibels (dBi, also referred to as dB)
of loss per 100 feet of cabling.The reason for this is that the radio wave actually
starts at the radio device.The radiated energy traveling through the cabling from
the radio device to the antenna induces a voltage in the cabling, decreasing the
strength of the wave as the distance from the radio device to the antenna
becomes greater.
Attenuation Due to Exterior Considerations
If you plan on coverage outdoors that is point-to-point or point-to-multipoint,
you will need to pay particular attention to considerations that are distancerelated.
For example, Earth bulge will come into play only if you are implementing
a point-to-point or point-to-multipoint WLAN, whereas weather is a
consideration for any outdoor implementation.
All matter produces attenuation (loss) to some degree. Because weather can
produce rain, snow, or fog, all of which are matter, weather must be considered in
a WLAN design.
Researching any unusual weather conditions that are common to the site
location is important.These conditions can include excessive amounts of rain or
fog, wind velocity, or extreme temperature ranges. If extreme conditions exist
that may affect the integrity of the radio link, you should take these conditions
into consideration early in the planning process.
Rain, Snow, and Fog
Except in extreme conditions, attenuation (weakening of the signal) due to rain
does not require serious consideration for frequencies up to the range of 6 or 8
GHz. When microwave frequencies are at 11 or 12 GHz or above, attenuation
due to rain becomes much more of a concern, especially in areas where rainfall is
of high density and long duration.
The attenuation rate for snow is generally higher, due in large part to the size
of the particles of snow or for that matter rain and fog as well, in compared to
the wavelength of the signal. For example, a 2.4 GHz signal will have a wavelength
of approximately 125 millimeters, or 4.9 inches. A 23 GHz signal will
have a wavelength of approximately 0.5 inches. A raindrop approaches 0.25 of an
inch. At 2.4 GHz, heavy rain or snow should not have much of an impact on the wireless system; however, in a 23 GHz system, the wavelength is reduced to half
by this rain. At this size, the rain or snow becomes a reflective surface and disperses
the 23 GHz signal.
In most cases, the effects of fog are considered to be much the same as rain.
However, fog can adversely affect the radio link when it is accompanied by
atmospheric conditions such as temperature inversion, or very still air accompanied
by stratification (layers of significantly differing air temperatures).
Temperature inversion can negate clearances, and still air along with stratification
can cause severe refractive or reflective conditions, with unpredictable results.
Temperature inversions and stratification can also cause ducting, which may
increase the potential for interference between systems that do not normally
interfere with each other. Where these conditions exist, use shorter paths and
adequate clearances.
Atmospheric Absorption
A relatively small effect on the wireless link is from gases and moisture in the
atmosphere. It is usually significant only on longer paths and particular frequencies.
Attenuation (loss) in the 2 to 14 GHz frequency range is approximately 0.01
dB/mile.You may have to include atmospheric absorption in your design consideration
if you are planning on implementing a wireless system above 10 GHz
where atmospheric absorption is prevalent.There are some wireless systems on
the market today licensed in the 23 GHz band, that are significantly impacted by
this type of loss. Antenna height has some impact on loss related to atmospheric
absorption, because the density of the air decreases as altitude increases.Thus, a
23 GHz system with an antenna significantly elevated over a similar implementation
at a lower elevation will suffer less from attenuation due to atmospheric
absorption.Table 1.2 depicts attenuation due to atmospheric absorption versus
path distance. Attenuation is listed as negative decibels, or –dB.
Table 1.2 Attenuation (Absorption) over Distance
Path
Distance
(In Miles) 2–6 GHz 8 GHz 10 GHz 12 GHz 14 GHz
20 –0.20 dB –0.26 dB –0.32 dB –0.38 dB –0.48 dB
40 –0.40 dB –0.52 dB –0.64 dB –0.76 dB –0.96 dB
60 –0.60 dB –0.78 dB –0.96 dB –1.14 dB –1.44 dB Table 1.2 Attenuation (Absorption) over Distance
Path
Distance
(In Miles) 2–6 GHz 8 GHz 10 GHz 12 GHz 14 GHz
80 –0.80 dB –1.04 dB –1.28 dB –1.52 dB –1.92 dB
100 –1.00 dB –1.30 dB –1.60 dB –1.90 dB –2.40 dB
Multipath Distortion
Multipath distortion is caused by the transmitted signal traveling to the receiver
via more than one path: A common cause of this is reflection of the signal from
bodies of water, hills, or tall buildings. Figure 1.5 shows an example of multipath
distortion caused by reflection.The antennas are the same height. In the worst
case, the reflected signal arrives at the receiving antenna at the same time as the
intended signal, but out of phase with the intended signal, both signals will
cancel each other out, resulting in complete loss of data. Best case, the reflected
signal arrives a moment later than the intended signal causing distortion and
therefore reduced performance. Examples of reflective surfaces include water,
asphalt, fields, metal roofs, or any smooth, relatively flat surface. Dispersing extraneous
radio waves is better than reflecting them. Examples of dispersal surfaces
include rough, rocky surfaces, shrubbery, trees, and so on. In a big city, more
people receive an echoed distortion of the wireless signal than receive the actual
signal, because the original signal bounces off buildings. The best way to reduce multipath distortion is to use a directional rooftop
antenna (for example, a directional antenna that will only pick up signals coming
from the direction of the transmitter and will reject reflections that arrive at its
sides or its back). A Yagi antenna is one example of a directional antenna that will
help reduce or eliminate multipath distortion (see Figure 1.6).
It is also sometimes possible to mount the antenna so that the mounting structure
screens it from the reflections but not from the wanted signal. By changing the
antenna height you can effectively reduce or eliminate the multipath signals by dispersing
the signals away from the receiving antenna (see Figure 1.7). Refraction
When a radio wave travels between two substances of different densities, the
wave will bend or refract because electromagnetic signals move slower through
substances of greater density.This phenomena impacts a radio wave as it travels
through the atmosphere.The density of the Earth’s atmosphere decreases as altitude
increases.Therefore, the bottom of the radio wave travels through a denser
atmosphere than the top of the wave.This means the bottom of the wave will
move slower than the top of the wave, causing the signal to bend towards the
Earth’s surface and follow the curvature of the Earth, but at an arc radius approximately
1.33 times greater than the Earth’s arc radius (see Figure 1.8). At night, the air cools and much of the moisture in the air moves closer to
the Earth’s surface.The cool, wet air near the Earth is denser than the air higher
in the atmosphere, so radio signals can bend farther than they do in the daylight
hours.This is known as super refraction.
Other refraction phenomena, such as ducting or bending, can also occur.
Ducting happens when radio waves are trapped in a high-density duct between
two areas of lower density.
Bending is similar to super refraction, but is not caused by atmospheric conditions
related to day or night. Instead, differences in air density in a horizontal
plane, like cooler air over a lake or field and warmer air over a shore or highway,
cause the radio waves to bend in the direction of the cooler, denser air over the
lake or field.
Refraction is one reason why radio line-of-sight is not necessarily the same
as optical line-of-sight. Refraction is minimal for paths under 10 miles, with the
exception of hot, humid areas like the southeastern section of the U.S.