Antennas focus the radio signal in a specific direction and in a narrow beam.
The increase in the signal power (compared to an omnidirectional antenna) when
it is focused in the desired direction is called gain.
Antennas are tuned to operate on a specific group of frequencies. Other
specific attributes such as beamwidth and gain are also fixed by the
manufacturer. Antennas should be selected and placed according to your site and
your application.
In general, the larger the antenna, the higher the gain and the larger the
mast required. It is best to use the smallest antenna that will provide
sufficient protection from interference and enough signal at the far end of the
link to provide good reception even with fading.
Other considerations include antenna beamwidth, front-to-side ratios,
front-to-back ratios, and cross-polarization rejection. Where interference from
other licensees on the same channel or adjacent channels is an issue, narrow
beamwidths, high front-to-back and front-to-side ratios, and high
cross-polarization rejection are likely to be required. Even when other
licensees are not an issue, if you are using a network deployment using the
"cell" approach, all these considerations are still important to reduce
interference between your own adjacent installations.
Antennas are an important part of a wireless
system because it directs where a wireless signal is transmitted and it
determines the direction that signals and noise are received from.
A short list of different types of antennas
- Corner Reflector Antenna - A directional antenna that is made up of a dipole driven element mounted in front of a 60-degree or 90-degree corner-shaped reflecting element.
- Dipole Antenna - A two-piece (di = two; pole=“pole” or “piece”) antenna that is the basic “building block” antenna element. A dipole is normally used as the “driven element” in most antenna systems. A dipole is made up of two ¼ wavelength-long antenna pieces arranged in a straight line. A coax transmission line feeds power to the middle of the dipole.
- Directional Antennas - An antenna with a radiation pattern that concentrates both the transmitting and receiving signal power into one favored direction. The power gain (the increase in signal power in the favored direction) is measured in dbi or dbd.
- Isotropic Antenna - An isotropic antenna is a theoretical antenna. If it existed in the real world, it would radiate a wireless signal equally in all directions (front, back, left, right, up, and down). The signal strength from a theoretical isotropic antenna is used as a reference level to measure the gain (focusing power) of real-world antennas.
- Omnidirectional Antenna - An antenna with a radiation pattern that, when viewed from above, is equally strong in all directions.
- Panel Antenna - A directional antenna made up of several phased driven elements mounted in front of a flat reflecting element. Panel antennas usually have a plastic or fiberglass cover that gives the antenna a panel-like appearance.
- Parabolic Antenna - A directional antenna made up of a dipole driven element mounted in front of a parabolic-shaped reflector. The reflector may be either a solid metal “dish” or a dish-shaped screen made of metallic rods or mesh.
- Patch Antenna - A smaller version of a directional panel antenna often used indoors.
- Yagi Antenna - A directional antenna made up of one “driven element” that is connected to the transmission line and one or more “reflectors” (signal reflecting elements) and/or “directors” (signal directing elements).
Atmospheric Absorption
A relatively small effect on the link is from oxygen and water vapor. It is
usually significant only on longer paths and particular frequencies. Attenuation
in the 2 to 14 GHz frequency range is approximately 0.01 dB/mile, which is not
significant.
Antenna Gain
See Gain
Antenna Lobe
Antenna lobes refer to the area around a wireless
antenna that have either directional or secondardy radiation patterns.
They do not radiate power equally in all directions. Therefore, antenna
radiation patterns or plots are a very important tool to both the antenna
designer and the end user. These plots show a quick picture of the overall
antenna response.
However, radiation patterns can be
confusing. Each antenna supplier/user has different standards as well as
plotting formats. Each format has its own pluses and minuses. Hopefully this
technical note will shed some light on understanding and using antenna radiation
patterns.
Antenna Radiation
Patterns: Antenna radiation plots can be quite complex because in
the real world they are three-dimensional. However, to simplify them a
Cartesian coordinate system (a two-dimensional system which refers to points in
free space) is often used. Radiation plots are most often shown in either the
plane of the axis of the antenna or the plane perpendicular to the axis and are
referred to as the azimuth or "E-plane" and the elevation or "H-plane"
respectively.
Diagram 1. This figure shows a rectangular
azimuth ("E" plane) plot presentation of a typical 10 element Yagi.
The detail is good but the pattern shape is not always apparent.
Many plotting formats or grids are in use.
Rectangular grids (Diagram 1) as well as polar coordinate systems (Diagram 2)
are in wide use. The principal objective is to show a radiation plot that is
representative of a complete 360 degrees in either the azimuth or the elevation
plane. In the case of highly directional antennas, the radiation pattern is
similar to a flashlight beam.
Diagram 2. This is a polar plot of the same
10 element Yagi and is similar to a compass rose. Therefore it is
more compatible with maps and directions. Note that it shows the sidelobes
of the antenna relative to the main beam in decibels. This type of
plot is preferred when the exact level of the
sidelobes is important.
In the VHF/UHF and microwave region, the
antenna radiation plot shows the relative field intensity in the far field (at
least 100 feet or 30 meters distant from typical antennas) in free space at a
distant point. Ground reflections are usually not a factor at these frequencies
so they are often ignored. The antenna supplier either measures the radiation
pattern by rotating the antenna on its axis or calculates the signal strength
around the points of the compass with respect to the main beam peak. This
provides a quick reference to the response of the antenna in any direction.
Note that the antenna radiation pattern is reciprocal so it receives and
transmits signals in the same direction.
For ease in use, clarity and maximum
versatility, radiation plots are usually normalized to the outer edge of the
coordinate system. Furthermore, most of us are not accustomed to thinking in
terms of signal strength in volts, microvolts etc. so radiation plots are
usually shown in relative dB (decibels).
For those not familiar with decibels, they
are used to express differences in power in a logarithmic fashion. A drop of 1
dB means that the power is decreased to about 80% of the original value while a
3 dB drop is a power decrease of 50% or one-half the power. The beamwidth
specified on most data sheets is usually the 3 dB or half-power beamwidth. A 10
dB drop is considered a large drop, a decrease to 10% of the original power
level.
Another reason for using dB is that
successive dB can be easily added or subtracted. A doubling of power is 3 dB
while a quadrupling is 6 dB. Therefore, if the antenna gain is doubled (3 dB)
and the transmitter power is quadrupled (6 dB), the overall improvement is 9 dB.
Likewise, dB can also be subtracted.
Antenna Matching
It is important to always match polarity with the sending and receiving attennas
for good reception. If the basestation antenna is set up with horizontal polarization,
then the receiving antenna also needs to be set up on a horizontal plane.
Attenuation
The loss of signal strength that occurs as a wireless signal travels through a transmission line, through the air and past (or through) obstructions.
Backbone
Backbone refers to the type of Internet backbone connectivity
a given wireless system has. Small wireless networks can be fed with a T1 circuit,
which provides 1.5 Mbps of bandwidth. Larger systems tend to start with a DS3, which
provides 45 Mbps of bandwidth. A T1 can usually support up to 100 customers where a DS3
can support thousands of customers.
Bandwidth
Bandwidth is often misused. Used correctly, the bandwidth of a channel is the "raw" data rate of a wireless link. For example, the raw data rate of an 802.11b access point is 11 million bits per second (11 Mbps). The amount of actual user data (the payload) that passes through a link (the throughput) is always less than the bandwidth of the link. (Half-duplex, "overhead“, interference, etc).
Beamwidth
Every directional antenna focuses its power into one main beam (main lobe) that travels in a favored direction. When viewed from above, the width, in degrees (around the vertical axis of the antenna) of this main lobe is the horizontal beamwidth. When viewed from the side, the thickness of this main lobe (up and down), in degrees, is the vertical beamwidth. Every antenna also has side lobes, smaller amounts of power that travel out the back and out the sides of the antenna.
Billing Gateway
A billing gateway sits somewhere in the network, either locally or further back in the
network, that collects all IP traffic statistics. It works the same way as call detail records
in the phone world, which collects all traffic for a given phone number. The difference is that
it uses an IP address instead of a phone number. The billing gateway can prevent access for
unauthorized users, manage bandwidth plans, and shut off service if a customer does't pay
their bill on time or if the customers credit card doesn't accept a new monthly charge for service.
Calculating a Link Budget
A link budget is a rough calculation of all known elements of the link to
determine if the signal will have the proper strength when it reaches the other
end of the link. To make this calculation, the following information should be
available:
- Frequency of the link
- Free space path loss
- Power of the transmitter
- Antenna gain
- Total length of transmission cable and loss per unit length at the specified
frequency
- Number of connectors used
- Loss of each connector at the specified frequency
- Path length
Sample Link Budget Calculation
The example below is based on the following assumptions:
|
Frequency |
5.0 GHz (U-NII) |
|
Length of Path |
7 miles |
|
Free Space Path Loss |
131.9 dB |
|
Transmitter Power |
23.8 dBm (limited by FCC EIRP guidelines) |
|
Cable Length
Power Feed Panel to Wireless
Transverter
Wireless Transverter to Antenna |
50 feet-LMR 400 (~2.6 dB loss per 100 feet at 420 MHz)
10 feet-LMR 400
(~10.7 dB loss per 100 feet at 5.7 GHz)
|
|
Number of Connectors Used |
4 (~ 0.5 dB loss per connector) |
|
Antenna Gain |
29 dBi transmit, 29 dBi receive |
|
Receiver Threshold |
-76 dBm |
|
Required Fade Margin |
20 dB (minimum) |
The following formulas can be used to determine if the fade margin meets the
requirement:
|
fade margin = received signal - receiver
threshold |
The received signal can be calculated with the formula:
|
received signal = |
transmitter power - transmitter cable and connector loss + transmitter
antenna gain - free space path loss + receiver antenna gain - receiver cable and
connector loss |
Based on the assumptions in the example, the formula becomes:
|
received signal = |
23.8 dBm - 2 dB (10 ft) + 29.0 dB - 131.9 dB + 29.0 dB - 2 dB (50 ft) = -54.1
dBm |
The fade margin is then calculated as follows:
|
fade margin = -54.1 dBm - (-76.2 dBm) = 22.1
dBm |
A fade margin of 22.1 dBm is above the required fade margin minimum (20 dB)
specified for this example.
Carrier Frequency
Cell Site
Central Office
CDMA
Cable Loss
There will always be some loss of signal strength through the cables and
connectors used to connect to the antenna. This loss is directly proportional to
the length of the cable and generally inversely proportional to the diameter of
the cable. Additional loss occurs for each connector used and must be considered
in planning. Your cable vendor can provide a chart indicating the loss for
various types and lengths of cable.
Coaxial Cable (Coax)
A type of transmission line made to carry microwave signals, with minimal attenuation. Coax consists of a wire center conductor surrounded by foam or air and one or two metallic shields.
Co-Channel and Adjacent Channel Interference
Co-channel interference results when another RF link is using the same
channel frequency. Adjacent-channel interference results when another RF link is
using an adjacent channel frequency. In selecting a site, a spectrum analyzer
can be used to determine if any strong signals are present at the site and, if
they are, to determine how close they are to the desired frequency. The further
away from your proposed frequency, the less likely they are to cause a problem.
Antenna placement and polarization, as well as the use of high-gain,
low-sidelobe antennas, is the most effective method of reducing this type of
interference.
Connector Loss
There will always be some loss of signal strength through the cables and
connectors used to connect to the antenna. This loss is directly proportional to
the length of the cable and generally inversely proportional to the diameter of
the cable. Additional loss occurs for each connector used and must be considered
in planning. Your cable vendor can provide a chart indicating the loss for
various types and lengths of cable.
Coverage Area
Customer Premises Equipment (CPE)
dB see Decibel for better explanation
Decibel. A dB value is one number that is a ratio of two power levels. A + dB value represents a power gain. A – dB value represents a power loss. Standard db reference levels include:
- dB - One single number (a ratio) that compares two power levels to each other.
- dBi – A ratio that compares the gain of an antenna to the gain of an isotropic antenna.
- dBd – A ratio that compares the gain of an antenna to the gain of a dipole antenna.
- dBm – A ratio that compares a power level to one milliwatt of power.
- dB “x” – A ratio that compares some power level to any reference standard that you choose.
Diversity and OFDM
When transmitted signals follow several paths between the transmitter and the
receiver, a condition called multipath occurs. Signals reflect off buildings,
water, and other objects, creating multiple paths to the receiver. On long
point-to-point radio links, stratification of the atmosphere can create multiple
paths by refracting the signals. Because of their longer path lengths, these
reflected or refracted signals take longer to arrive at the receiver, where they
can interfere with the main signal.
The diversity feature requires the installation of two antennas separated
vertically or horizontally (vertical separation works well for longer free-space
line-of-sight links, while horizontal separation works best for partially
obstructed or non-line-of-sight links). The signals received by both antennas
are combined to greatly enhance the quality of the signal where multipath
exists.
As a rule of thumb, the separation between antennas using this feature should
be a minimum of 100 to 200 times the wavelength of the frequency. The greater
distances are preferable. Table 2-1 shows a sample antenna separation
calculation.
DSLAM
Direct Sequence Spread Spectrum (DSSS)
Direct Sequence Spread Spectrum (DSSS). Direct sequence spreading is very
different from frequency hopping. Instead of splitting a data signal into
pieces, direct sequencing encodes each data bit into a longer bit string, called
a chip. Usually, 11 to 20 bits are used for the chip, depending on the
application. Because the military requires a much higher degree of security, it
generally uses much longer chips—even a long as 1,000 to 10,000 bits! An
eleven-bit chip is illustrated below.
0=10010010110
1=01101101001
Notice that the binary string encoding a 0 has the opposite
form as the string encoding a 1—where a "1" is used in one chip, a zero is used
in the other.
The chip is then used to modulate (change) the signal
generated by the radio transmitter, spreading the signal out over a wide band of
frequencies. The receiver uses the same code and so listens for the unique
signature across the frequency spectrum. It then decodes the signal back to the
original data.
This is a simplified explanation of a very technical
subject, but hopefully it gives you an idea of how spread spectrum works. The
gist is that spread spectrum technology allows multiple radio signals to operate
in an open, unlicensed band with a minimum of interference. It also provides
security for the transmission.
Direct sequence spread spectrum. A transmission method where a wireless signal is modulated by a pre-selected sequential train of pulses. After modulation, the signal has a very wide bandwidth and a very low amplitude. On a non-DSSS receiver, the signal appears to be just noise.
E-1
The European equivalent of the North American 1.544 Mbps T-1, except that a E-1
carries information at the rate of 2.048 Mbps.
ERP (and EIRP)
Effective Radiated Power - The “effective” power transmitted in the favored direction by a particular antenna system. The effective radiated power equals the transmitter power fed into the antenna plus the power gained from the antenna’s directivity.
Ethernet
A local area network used for connecting computers, printers, workstations,
terminals, etc. within the same building or campus. Ethernet operates over twisted wire and over coaxial cable
at speeds up to 10 Mbps.
Fade Margin
The difference (in db) between the actual signal level at a receiver input and the minimum signal level needed for the receiver to begin operating. A fade margin of at least +10 db is usually needed to overcome the effects of fading. Longer links require more than +10 db of fade margin to compensate for the additional fading over the longer path. Fade margin is sometimes called system operating margin (SOM).
Fixed Wireless Antenna
Free-Space Path Loss
A signal degrades as it moves through space. The longer the path, the more
loss it experiences. This free-space path loss is a factor in calculating the
link viability.
Frequency Hopping Spread Spectrum (FHSS)
The United States military
developed a radio technology called spread spectrum during the 1950s and
1960s. Obviously, the first concern was ensuring that radio transmissions were
not intercepted. The second concern was to ensure that guided missile
communications were not jammed by enemy radio transmissions.
Though developed and implemented by the U.S. military, the
problem was first addressed by Hedy Lamarr, a famous actress of Austrian descent
in the 1930s and 1940s. She and a music composer, George Antheil, patented the
idea in 1940. She was so far ahead of her time in conceptualizing the idea that
she never received any monetary rewards for her patent. The patent license
expired before government and commercial implementation of the concepts
occurred.
Here's how the idea works. A communications signal (voice or
data) is split into separate parts. Instead of transmitting a signal
continuously over one narrow frequency band, the several parts are transmitted
separately over a wide spectrum of radio frequencies. A defined, but
random-appearing pattern of non-sequential bands is used, with successive parts
being transmitted over the next frequency band in the pattern. On the other end,
a receiver is configured to receive the signals in the same pattern. The radio
receiver then reassembles the pieces into the original signal. Since many
distinct patterns can be developed, it is possible to have multiple radios
transmitting at the same time, but never at the same frequency at the same
time.
The process of jumping quickly from one
frequency to another is called frequency hopping.
For example, an FHSS signal is the 2.4 GHz band can hop through the band
in 78 different patterns or hopping sequences. These 78 hopping
sequences are divided into 3 hopping-sequence sets.
Frequency hopping has two benefits. Electrical noise—random
electromagnetic signals which are not part of any communications signal—will
only affect a small part of the signal. Also, the effects of any other forms of
radio communications operating in narrow bands of the spectrum will be
minimized. Any such interference that occurs will result in only a slightly
reduced quality of voice transmission, or a small loss of data. Since data
networks acknowledge successful receipt of data, any missing pieces will trigger
a request to transmit the lost data.
Frequency
Frequency is measured in a unit called a hertz, after Heinrich Hertz, one
of the early experimenters with radio waves (originally, radio waves were called
Hertzian waves). The hertz is usually defined as one cycle per second, or
one wave per second. So, the spectrum of electromagnetic waves is described in
hertz, normally abbreviated Hz.
Like data transfer rates, frequencies can be very large, so
the standard large units are used to note them: kilo (K), mega (M), and giga
(G). Radio waves have frequencies from about 150kHz (kilohertz) through 300GHz
(gigahertz). In contrast, light waves are much shorter and have much higher
frequencies. Light wave frequencies are in the area of about 100 trillion hertz,
or 100THz (terahertz).
Radio-frequency energy in an antenna rapidly
changes back and forth between a peak positive value and a peak negative value.
The number of times that the signal completes this positive-negative-positive
cycle each second is the frequency. One complete positive-negative-positive
cycle is called one Hertz (Hz). At a frequency of 2.4 GHz (2.4 gigahertz)
the energy completes 2,400,000,000 cycles (2400 million or 2.4 billion Hertz)
each second. (See also - “Wavelength”).
Frequency Band Division
Each broadband fixed wireless system is a full-duplex system. Two frequency
bands are used to achieve this two-way operation, with the higher frequency band
considered the "high" band in the link, and the lower frequency considered the
"low" band. The transmitter at one end of the link will use the high band; the
transmitter at the other end will use the low band.
Fresnel Reflection
In optics, the reflection of a portion of incident light at a discrete interface
between two media having different refractive indices. 1: Fresnel reflection
occurs at the air-glass interfaces at the entrance and exit ends of an optical
fiber. Resultant transmission losses, on the order of 4% per interface, can be
reduced considerably by the use of index-matching materials. 2: The coefficient
of reflection depends upon the refractive index difference, the angle of incidence,
and the polarization of the incident radiation. For a normal ray, the fraction of
reflected incident power is given by:
...where R is the reflection coefficient
and n1 and n2 are the respective refractive
indices of the two media. In general,
the greater the angle of incidence with respect to the normal, the greater the
Fresnel reflection coefficient, but for radiation that is linearly polarized in
the plane of incidence, there is zero reflection at Brewster’s angle. 3: Macroscopic
optical elements may be given antireflection coatings consisting of one or
more dielectric thin-film layers having specific refractive indices and
thicknesses. Antireflection coatings reduce overall Fresnel reflection
by mutual interference of individual Fresnel reflections at
the boundaries of the individual layers.
Fresnel Zone
Wireless signals do not travel in laser-like beams; they
spread out as they leave an antenna and travel through free space. The Fresnel
(pronounced “fre nel”) zone is an extra clearance zone around and in addition to
the the visual line-of-sight path. To avoid attenuation, at least 60 percent of
the Fresnel zone must be free of obstructions.
In radio communications, one of a (theoretically infinite) number of a
concentric ellipsoids of revolution which define volumes in the radiation
pattern of a (usually) circular aperture. 1: The cross section of the first Fresnel
zone is circular. Subsequent Fresnel zones are annular in cross section, and
concentric with the first. 2: Odd-numbered Fresnel zones have
relatively intense field strengths, whereas even numbered Fresnel zones are
nulls. 3: Fresnel zones result from diffraction by the circular aperture.
The characteristics of a radio signal cause it to occupy a broad
cross-section of space, called the Fresnel Zone, between the antennas.
Figure 2-1 shows the area occupied by the strongest radio signal, called the
First Fresnel Zone, which surrounds the direct line between the
antennas.
Because of the shape of the First Fresnel Zone, what appears to be a clear
line-of-sight path may not be. As long as 60 percent of the First Fresnel Zone
is clear of obstructions, the link behaves essentially the same as a clear
free-space path.
Front-to-Back (f/b) Ratio
No directional antenna is perfectly directional - all antennas have side lobes -
that is all antennas “spill” some energy out the sides and out the back. The
ratio of the power radiated out the front of the antenna to the power radiated out the
back is the front-to-back ratio of the antenna. Because this is a ratio of two powers,
we can express it in dB. A good very directional antenna will have a f/b ratio of 30
dB or more.
1. Of an antenna, the gain in a specified
direction, i.e., azimuth, usually that of maximum gain, compared to the
gain in a direction 180o from the specified azimuth. Front-to-back ratio
is usually expressed in dB. 2. A ratio of parameters used to characterize
rectifiers or other devices, in which electrical current, signal strength,
resistance, or other parameters, in one direction is compared with that in the opposite
direction.
Full Duplex
Gain
Antenna gain is an indicator of how well an antenna focuses RF energy in a
preferred direction. Antenna gain is expressed in dBi (the ratio of the power
radiated by the antenna in a specific direction to the power radiated in that
direction by an isotropic antenna fed by the same transmitter). Antenna
manufacturers normally specify the antenna gain for each antenna they
manufacture.
The focusing power of an antenna (in dBi or dBd) when compared to either an isotropic antenna or a dipole antenna.
GHz
Gigahertz. A unit used to measure the frequency of wireless signals. One GHz is one thousand million (one billion) hertz (cycles per second). For example: 2,400,000,000 cycles per second is 2.4 GHz.
Infrared Communications
Computer technology that uses the infrared spectrum is
becoming common. For example, wireless keyboards and receivers are commonly
distributed with computers that serve as a base for home entertainment systems.
A receiver is attached to the keyboard connector on the back of a computer case.
An infrared transmitter operating at a proprietary frequency (each wireless
keyboard manufacturer typically uses a different frequency) translates the
keystroke coding into an infrared signal and sends it to the receiver. Also,
some computers now come with an infrared port which allows information from a
hand-held or pocket computer to be transmitted to the desktop
computer.
Interference
An important part of planning your broadband fixed wireless system is the
avoidance of interference. Interference can be caused by effects within the
system or outside the system. Good planning for frequencies and antennas can
overcome most interference challenges.
kHz
Kilohertz. A unit used to measure the frequency of wireless signals. One kHz is one thousand hertz (cycles per second).
Line-of-Sight
The presence of an unobstructed visual or unobstructed wireless path between two points or between two antennas. Both a visual line-of-sight path AND a clear wireless Fresnel Zone are needed to have a clear wireless line-of-sight path.
LMDS
Local Multipoint Distribution Service. LMDS is a licensed wireless service that has the capability to provide broadband access. It operates in the 29-32 GHz frequency range.
MHz
Megahertz. A unit used to measure the frequency of wireless signals. One MHz is one million hertz (cycles per second).
MMDS
Multichannel Multipoint Distribution Service. MMDS is a licensed wireless service that has the capability to provide broadband access. It operates just below and just above the 2.4 GHz license-free ISM band. (ITFS shares these bands).
Multipath
Multipath is the almost-simultaneous reception of a direct signal and one or more reflected echoes of the direct signal. Multipath may occasionally increase the total received signal strength but typically, it causes fading and a reduction in the total received signal strength.
mW
MilliWatt. A unit of power equal to one thousandth (1/1000) of a watt.
Lightning Arrestor
For any Wireless ISP, external antennas are
involved, the threat of a lightning strike can be very real. Make sure that
proper measures are
implemented to minimize the risk of lightning strikes. Most manufacturers of
wireless bridges sell an optional device called a lightning arrestor. It is
normally installed between the antenna and the bridge. Also make sure the
antenna is properly grounded.
Link Budget Calculators
Mbps
Megabits per second
Microwave Communications
The complete electromagnetic spectrum includes many types of
wavelengths we've become very familiar with, at least in name. First among these
is visible light. Two other types of wavelengths, just at either end of the
visible spectrum, are infrared and ultraviolet light. These are the wavelengths
that bring us "night vision" technology and tanning booths, respectively.
Another portion of the electromagnetic spectrum we're becoming familiar with are
frequencies called microwaves. These exist below infrared frequencies,
but above normal radio frequencies.
Many of the data communications services offered by major
telecommunications companies are supported by microwave technology. While it is
a viable alternative even in private communications, it has two drawbacks.
First, microwave communication requires FCC licensing. Second, the cost of
implementing microwave technology (tower/dish infrastructure) is higher than
other options. On the other hand, microwave communication is extremely resistant
to interference. But, because of its cost, it will not be an adequate
alternative for many rural community networks.
Milliwatt (mW)
“Milli” means 1/1000 so a milliwatt is one thousandth (1/1000) of 1 Watt (W). One mW is also a standard wireless reference level for dB measurement. A power level of 0 dBm is defined as equal to 1 mW. All “+” dBm values are greater than 1 mW. All “-” dBm values are less than 1 mW.
Ohm’s Law
The basic equation that explains the constant relationship between DC voltage, current, and resistance.
Path Loss (or Free-Space Path Loss)
The attenuation (in dB) of a wireless signal as it travels in “free space” between antennas without the benefit of a wire to conduct it. Additional path loss occurs when the signal encounters obstructions such as buildings, trees, terrain, and weather.
Path Planning
To get the most value from a wireless system, path planning is essential. In
addition to the fact that radio signals dissipate as they travel, many other
factors operate on a microwave signal as it moves through space. All of these
must be taken into account, because any obstructions in the path will attenuate
the signal.
Point-to-Point Network
Point-to-Multipoint Network
Polarization
A wireless signal consists of two expanding energy fields
- a magnetic field and an electric field. The polarization is the orientation, relative
to the earth, of the signal's electric field. Polarization can be vertical, horizontal,
circular, or some combination. The polarization of a signal shifts when the signal
is reflected off of an object.
The orientation of the antenna will change the orientation of the signal. The
transmitting and receiving antennas should be both polarized either horizontally
or vertically. Adjacent antennas on different frequencies can be cross-polarized
to help reduce interference between the two, if your operating license permits
this.
Radio Bands
In order to keep people in the United States from
interfering with each other's use of radio signals, the Federal Communications
Commission (FCC) is in charge of assigning small sections of the radio
frequencies to specific uses. These are called licensed frequencies. In
order to broadcast radio signals at these frequencies you must apply to the FCC
for a license.
However, to allow use of some of the radio spectrum for
small applications that would not require a license, the FCC has allocated three
separate bands of radio frequencies as public bands. No license is
required to use equipment transmitting at these frequencies. These are called
the ISM bands, short for Industrial, Scientific, and Medical bands. Table
10 shows the frequencies reserved for these bands.
|
Table 10. ISM radio frequency
bands. |
|
Frequency Range |
Band Description |
Bandwidth Available
|
|
902-928MHz |
Industrial Band |
26.0MHz |
|
2.40-2.4835GHz |
Scientific Band |
83.5MHz |
|
5.725-5.850GHz |
Medical Band |
125.0MHz |
|
Notice that the bandwidth available increases in the higher
frequency ranges. These higher frequencies will support higher data transfer
rates. Therefore, many wireless bridge products being sold today operate in the
2.4GHz and 5.7GHz frequencies. As throughput increases, computer networking
becomes more of a real possibility. And, with more companies producing RF
wireless networking products, prices are continuing to fall, making wireless
networking a viable alternative to land-based lines in many local
areas.
RF
Radio Frequency (RF). An alternating current in an antenna that changes direction fast enough to create electric and magnetic waves. The waves leave the antenna and travel away through free space, wirelessly.
Radio Waves
When radio waves are described as the technology used to
broadcast radio and TV programs, some people assume radio waves are a little
like sound waves. Thus the term "airwaves." A sound is made when something
causes the air to vibrate. This vibration is transferred to our eardrums when
the sound wave arrives. The vibration is then translated into a signal
transmitted to our brains, where we perceive the sound. But radio stations
broadcast their signals through the air, so why don't we hear them without a
radio receiver?
This is a trick question. Radio waves are really not at all
like sound waves. They do not create vibrations in our ears. They do not rely on
vibrations in the air; in fact, they do not need air for transmission. Instead
of being a vibration, they are a form of energy. They are part of what is called
the electromagnetic spectrum. This energy spectrum includes the full
range of radiation created by the interaction of electrons and magnetic fields.
As we mentioned in the sidebar "Airwaves," these types of radiation include
radio waves, microwaves, infrared light, visible light, ultraviolet light, and
x-rays.
Satellite Communications
Normally, satellite communications are unavailable to small
entities for network connectivity. The cost of leasing a transponder is
prohibitive. However, one viable exception is the use of satellites to connect
end-users to the Internet. A couple of companies currently offer high-speed
Internet access to home and business customers through the installation of a
small parabolic antenna (satellite dish).
While this type of connectivity provides over 400Kbps
download connectivity, it is strictly a one-way medium. In order to make use of
these services, end users must also maintain a land-based, physical connection
(lower speed) to the Internet. In this scenario, requests are sent to web sites
over the land-based connection and are received through the satellite
connection. This provides quick response times from the Internet but is
unsuitable for community network connectivity.
On the other hand, new satellite technology is being
explored. Low-Earth-orbiting (LEO) satellites are being deployed as this manual
is being written. With hundreds of these satellites available in the next few
years, the cost of transmit-and-receive capability through satellite technology
may drop. Projects such as Iridium and Teledesic bear watching
over the next few years. Iridium is being deployed as a wireless phone service
alternative. Teledesic, with the moniker "Internet-in-the-Sky," is set to
provide wireless Internet service to businesses, schools, and end
users.
Signal to Noise Ratio
Sectorize
To design and deploy a wireless system that divides a coverage area into more that one (typically two or three) sections. Each sector is served by its own dedicated directional antenna system. Normally, each sector has its own access point but sometimes one radio is shared between two or more sectors.
Sensitivity
The ability of a wireless receiver to detect and decode an incoming wireless signal.
Selectivity
The ability of a wireless receiver to discriminate between one desired incoming signal and all of the other (undesired) incoming wireless signals.
Spread Spectrum Technology
Most communication technologies we are
familiar with—radio, television, two-way radios—use what is called
narrowband communications.
Each station or channel operates over a very thin slice of the radio spectrum.
Because the station is assigned that particular band, and the FCC ensures that
no other broadcasters in the local area use that same band through licensing,
there is no interference. The range of each station is limited, so the same
frequency can be re-used a great distance away without interference.
Because many devices might use the ISM bands in a local
area, additional technology is required to keep the various signals from
interfering with each other. Fortunately, a technology has been developed over
the past fifty years which permits such bandwidth "sharing." This technology
provides a way to spread the radio signal over a wide "spectrum" of radio
frequencies, minimizing the impact of narrowband interference. In most cases,
only small parts of the transmission are corrupted by any interference, and
coding techniques allow that data to be recaptured. This technology is now
generally known as spread spectrum.
There are currently two different spreading
techniques used.
Both use a coded pattern of communication. A receiving unit is synchronized to
use the same pattern and successfully receive the transmission. Any other radio
unit hears the signal as noise because it is not programmed with the appropriate
coding. The two techniques are called frequency hopping spread spectrum
and direct sequence spread spectrum.
Towers
When it is not possible to obtain a clear line-of-sight between the two antennas
involved in a wireless connection, a mast or radio tower may provide additional
height for the antenna, clearing obstacles such as trees or buildings which lie
in the path of the radio signal.
Masts are generally mounted on the roof and may be 10-50 feet in height. If a
mast is used in your implementation, be sure it is tied down properly to
minimize the risk of wind damage.
Radio towers are generally independent structures erected to
raise antennas when extended distances are desired. They may also be required
when tall buildings (larger than three stories) or topographical features lie
directly in the path of the radio signal between two antennas. Towers can be
erected at heights of 50 feet and higher. Obviously, depending on the
application, the cost of erecting tall towers can be prohibitive.
When planning antenna placement, it might be necessary to build a
free-standing tower for the antenna. Regulations and limitations define the
height and location of these towers with respect to airports, runways, and
airplane approach paths. These regulations are controlled by the FAA. In some
circumstances, the tower installations must be approved by the FAA, registered
with the FCC, or both.
To ensure compliance, make sure you visit the FCC's Antenna Structure Registration
website to review regulations regarding antenna structures.
Transceiver
To create a computer network connection over radio waves, two puzzle pieces are
needed. First, a network device such as a bridge or a router is needed. The
network bridge/router handles the data traffic. It routes the appropriate data
signals bound from the computer network in one building to the network at the
other end of the radio connection. Second, a radio transmitter and receiver,
commonly called a transceiver, is required. The radio transceiver handles
the radio signal communications between locations.
The interesting part of this marriage of technologies is
that radios have always dealt with electrical signals. The radio transmitter
modulates, or changes, an electrical signal so that its frequency is
raised to one appropriate to radio communications. Then the signal is passed on
to a radio antenna. We'll discuss the work of antennas more in the section "How
the Antennas Work."
At the other end of the transmission, the receiving portion
of the radio transceiver takes the radio signal and de-modulates it back to its
normal frequency. Then the resulting electrical signal is passed to the
bridge/router side for processing by the network. While the actual process of
modulation/demodulation is technical, the concept of radio transmission is very
simple.
Likewise, when a response is sent back to the originating
site, the radio transceiver "flips" from reception mode to transmission mode.
The radio transceivers at each end have this characteristic. Transmit-receive,
transmit-receive. They change modes as many as thousands of times per second.
This characteristic leads to a delay in communications called latency. It
is idiosyncratic to radio communications and negatively affects data throughput.
See the section "Throughput vs. Data Rates" below for more
information.
Wind
Any system components mounted outdoors will be subject to the effect of wind.
It is important to know the direction and velocity of the wind common to the
site. Antennas and their supporting structures must be able to prevent these
forces from affecting the antenna or causing damage to the building or tower on
which the components are mounted.
Antenna designs react differently to wind forces, depending on the area
presented to the wind. This is known as wind loading. Most antenna manufacturers
will specify wind loading for each type of antenna manufactured.
Wi-Fi Hotspot
A Wi-Fi Hotspot usually refers to an indoor access
point that provides high-speed Internet connectivity for free or for a minimum
fee per day. A Wi-Fi Hotspot usually has a range of 300-500 feet and can provide access
for up to 20-50 users depending on how the access point is setup. Sometimes
Wi-Fi Hotspots are located outside and can be accessed by people as far away
as 1000 feet with clear line of sight and a 200 mW PCMCIA Card.
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