10 Commandments of Wireless
1. Thou shall know thy dBm and recall thy high school
Radio Frequency (RF) power is measured in milli Watts
(mW) or, more usefully, in a logarithmic scale of decibels (dB), or
decibels referenced to 1 mW of power (dBm). Since RF power
attenuates as a logarithmic function, the dBm scale is most useful.
Here are some examples of how these scales relate:
A 2-fold increase in power yields
3dBm of signal.
A 10-fold increase in power yields
10dBm of signal.
A 100-fold increase in power yields
20dBm of signal.
2. Covet not high frequencies - as the lower the
frequency, the more forgiveth the laws of physics and propagation.
Industrial applications typically operate in
"unlicensed" frequency bands, also referred to as ISM (Industrial,
Scientific and Medical). The frequencies and power of these bands
varies from country to country. The most common frequencies
2.4 GHz - nearly worldwide
915 MHz band - North America,
South America, some other countries
868 MHz band - Europe
As frequency rises, available bandwidth typically
rises, but distance and ability to overcome obstacles is reduced.
For any given distance, a 2.4 GHz installation will have roughly 8.5
dB of additional path loss when compared to 900 MHz. However, lower
frequencies require larger antennas to achieve the same gain.
3. Honor thy receive sensitivity - as long-range
performance is not a function of transmit power alone.
The more sensitive the radio, the lower the power
signal it can successfully receive, stretching right down to the
noise floor. There is so much variety in "specsmanship" for radio
sensitivity, that it is difficult to make a meaningful comparison
between products. The most meaningful specification is expressed at
a particular bit error rate and will be given for an ideal
environment shielded from external noise. Unless you're in a high RF
noise environment (typically resulting from numerous similar
frequency radio transmitters located nearby), the odds are good that
the noise floor will be well below the receive sensitivity, so the
manufacturer's rated receive sensitivity will be a key factor in
your wireless system and range estimates.
You can often improve your receive sensitivity, and
therefore your range, by reducing data rates over the air. Receive
sensitivity is a function of the transmission baud rate so, as baud
rate goes down, the receive sensitivity goes up. Many radios give
the user the ability to reduce the baud rate to maximize range.
The receive sensitivity of a radio also improves at
lower frequencies, providing another significant range advantage of
900 MHz (vs. 2.4 GHz) - as much as six to twelve dB!
4. Thou shalt be wary of radio noise and recognize
situations whereth radio noise may hamper thine installation.
RF background noise comes from many sources, ranging
from solar activity to high frequency digital products to all forms
of other radio communications. That background noise establishes a
noise floor which is the point where the desired signals are lost in
the background ruckus. The noise floor will vary by frequency.
Typically the noise floor will be lower than the
receive sensitivity of your radio, so it will not be a factor in
your system design. If, however, you're in an environment where high
degrees of RF noise may exist in your frequency band, then use the
noise floor figures instead of radio receive sensitivity in your
calculations. If you suspect this is the case, a simple site survey
to determine the noise floor value can be a high payoff investment.
When in doubt, look about. Antennas are everywhere
nowadays - on the sides of buildings, water towers, billboards,
chimneys, even disguised as trees. Many sources of interference may
not be obvious.
5. Thou shalt always know thine fade margin - lest ye
have a wireless link that worketh not in rain, snow, or the presence
Fade margin is a term critical to wireless success.
Fade margin describes how many dB a received signal may be reduced
by without causing system performance to fall below an acceptable
value. Walking away from a newly commissioned wireless installation
without understanding how much fade margin exists is the number one
cause of wireless woes.
Establishing a fade margin of no less than 10dB in
good weather conditions will provide a high degree of assurance that
the system will continue to operate effectively in a variety of
weather, solar, and RF interference conditions.
There are a number of creative ways to estimate fade
margin of a system without investing in specialty gear. Pick one or
more of the following and use it to ensure you've got a robust
Some radios have programmable
output power. Reduce the power until performance degrades, then
dial the power back up a minimum of 10dB. Remember again,
doubling output power yields 3 dB, and an increase of 10dB
requires a ten-fold increase in transmit power.
Invest in a small 10dB
attenuator (pick the correct one for your radio frequency!). If
you lose communications when you install the attenuator
installed in-line with one of your antennas, you don't have
enough fade margin.
Antenna cable is lossy, more
so at higher frequencies. Specifications vary by type and
manufacturer so check them yourself but, at 900MHz, a coil of
RG58 in the range of 50 to 100 feet (15 to 30 m) will be 10dB.
At 2.4GHz, a cable length of 20-40 feet (6 to 12 m) will yield
10dB. If your system still operates reliably with the test
length of cable installed, you've got at least 10dB of fade
6. Thou shalt use thine given powers of mathematics
and logic when specifying wireless equipment.
Contrary to popular opinion, no black art is required
to make a reasonable prediction of the range of a given radio
signal. Several simple concepts must be understood first, and then
we can apply some simple rules of thumb.
The equation for successful radio reception is:
TX power + TX antenna gain - Path loss - Cabling loss
+ RX antenna gain - 10dB fade margin > RX Radio sensitivity or (less
commonly) RF noise floor
Note that most of the equation's parameters are
easily gleaned from the manufacturer's data. That leaves only path
loss and, in cases of heavy RF interference, RF noise floor as the
two parameters that you must established for your particular
In a perfect world, you will measure your path loss
and your RF noise conditions. For the majority of us that don't,
there are rules of thumb to follow to help ensure a reliable radio
7. Thou shalt not allow leafy greens or mounds of
earth between thine antennas; and thou shalt elevate thy antennas
towards the heavens; and thou shalt never, ever, attempt a system at
the manufacturer's maximum advertised distance.
In a clear path through the air, radio signals
attenuate with the square of distance. Doubling range requires a
four-fold increase in power, therefore:
When indoors, paths tend to be more complex, so use a
more aggressive rule of thumb, as follows:
Radio manufacturers advertise "line of sight" range
figures. Line of sight means that, from antenna A, you can see
antenna B. Being able to see the building that antenna B is in does
not count as line of sight. For every obstacle in the path, de-rate
the "line of sight" figure specified for each obstacle in the path.
The type of obstacle, the location of the obstacle, and the number
of obstacles will all play a role in path loss.
Visualize the connection between antennas, picturing
lines radiating in an elliptical path between the antennas in the
shape of a football. Directly in the center of the two antennas the
RF path is wide with many pathways. A single obstacle here will have
minimal impact on path loss. As you approach each antenna, the
meaningful RF field is concentrated on the antenna itself.
Obstructions located close to the antennas cause dramatic path loss.
Be sure you know the distance between antennas. This
is often underestimated. If it's a short-range application, pace it
off. If it's a long-range application, establish the actual distance
with a GPS or Google Maps.
The most effective way to reduce path loss is to
elevate the antennas. At approximately 6 feet high (2 m), line of
sight due to the Earth's curvature is about 3 miles (5 km), so
anything taller than a well-manicured lawn becomes an obstacle.
Weather conditions also play a large role. Increased
moisture in the air increases path loss. The higher the frequency,
the higher the path loss.
Beware leafy greens. While a few saplings mid-path
are tolerable, it's very difficult for RF to penetrate significant
woodlands. If you're crossing a wooded area you must elevate your
antennas over the treetops.
Industrial installations often include many
reflective obstacles leading to numerous paths between the antennas.
The received signal is the vector sum of each of these paths.
Depending on the phase of each signal, they can be added or
subtracted. In multiple path environments, simply moving the antenna
slightly can significantly change the signal strength.
Some obstacles are mobile. More than one wireless
application has been stymied by temporary obstacles such as a stack
of containers, a parked truck or material handling equipment.
Remember, metal is not your friend. An antenna will not transmit out
from inside a metal box or through a storage tank.
Path Loss Rules of Thumb:
To ensure basic fade margin
in a perfect line of sight application, never exceed 50% of the
manufacturer's rated line of site distance. This in itself
yields a theoretical 6dB fade margin - still short of the
De-rate more aggressively if
you have obstacles between the two antennas, but not near the
De-rate to 10% of the
manufacture's line of site ratings if you have multiple
obstacles, obstacles located near the antennas, or the antennas
are located indoors
Antennas increase the effective power by focusing the
radiated energy in the desired direction. Using the correct antenna
not only focuses power into the desired area but it also reduces the
amount of power broadcast into areas where it is not needed.
Wireless applications have exploded in popularity
with everyone seeking out the highest convenient point to mount
their antenna. It's not uncommon to arrive at a job site to find
other antennas sprouting from your installation point. Assuming
these systems are spread spectrum and potentially in other ISM or
licensed frequency bands, you still want to maximize the distance
from the antennas as much as possible. Most antennas broadcast in a
horizontal pattern, so vertical separation is more meaningful than
horizontal separation. Try to separate antennas with
like-polarization by a minimum of two wavelengths, which is about 26
inches (0.66 m) at 900 MHz, or 10 inches (0.25 m) at 2.4 GHz.
9. Cable loss
Those high frequencies you are piping to your
antennas don't propagate particularly well through cable and
connectors. Use high quality RF cable between the antenna connector
and your antenna and ensure that all connectors are high quality and
carefully installed. Factor in a 0.2 dB loss per coaxial connector
in addition to the cable attenuation itself. Typical attenuation
figures for two popular cable types are listed below.
Loss per 10 feet (3 meters) of cable
While long cable runs to an antenna create signal
loss, the benefit of elevating the antenna another 25 feet (7.6 m)
can more than compensate for those lost dB.
10. Thou shalt recognize the issues of latency and
packetization before thou issueth purchase orders.
Before you lift a finger towards the perfect wireless
installation, think about the impact of wireless communications on
your application. Acceptable bit error rates are many orders of
magnitude higher than wired communications. Most radios quietly
handle error detection and retries for you - at the expense of
throughput and variable latencies.
Software must be well designed and communication
protocols must be tolerant of variable latencies. Not every protocol
can tolerate simply replacing wires with radios. Protocols sensitive
to inter-byte delays may require special attention or specific
protocol support from the radio. Do your homework up front to
confirm that your software won't choke, that the intended radio is
friendly towards your protocol, and that your application software
can handle it as well.
If you have any specific questions, please do
not hesitate to