For a proper WLAN performance, the transmitting power + propagation loss +
receiving sensitivity must be greater than 0 . The remain gives the margin of the
system. A good WLAN
link has 6 to 10 dB margin.
Note: As transmitting and receiving properties are not always identical at both
sides, a link budget calculation must be performed for BOTH directions!
This page can be used to perform all calculations that are needed to
get an idea of the Link Budget of the radio Link that is investigated. In
other words: using this page you can predict if a projected radio link will be
possible or not
Please note: These calculations are theoretical. and represent only the maximum achievable
performance for a system. In real life interferences can occur from
other radio sources (such as other WLAN networks, bluetooth, microwaves). Also atmospheric losses (air
moisture, scattering, refraction), badly pointed antenna, reflections, can affect
the link performance. .
The Link Budget Calculation
1) To achieve a very reliable link, a margin of at least 10 dB is needed. This accommodates
for local fading (= variations of signal strength caused by refelections). A 4 to 6 dB margin is needed if the link
2) check if Fresnell and/or
diffraction limitations apply. Add extra losses to the margin that is needed
3) Polarisation errors: add 3 dB to the required margin when helical antennas
to horizontal or vertical antennas are used. add 30 dB in the case of polarity
mismatch between antennas. (Hor/vert antenne or left/right rotating antennas).
Some remarks regarding optimizing Link Power Within Legal Limits.
For achieving legal long range links you must strive to get always an
EiRP of 20 dBi, being the legal limit (in Europe).
If you are going to use a high gain antenna (> 5dBi), (for ranges > 1km),
you MUST REDUCE the output power in order
to stay within the legal power limit. This must be done without affecting
the receivers' sensitivity, so it can ONLY be done inside the WLAN equipment,
so BEFORE to the RF send/receive switch. You'll have to find WLAN equipment that is able to reduce
its power internally.
Note that receiver sensitivity varies much more over equipment
manufacturers than output power. sensitivity can vary over 10 dB(!) Since
output power is limited by legal limits, you MUST find yourself the most
sensitive receiver that's available. It's NOT a highest transmitter's power
that does the job for legal limit links, it's the best receiver's sensitivity!
Example 1: legal range of standard 15 dBm Wlan equipment , 3 dB cable
loss and an 8 dBi antenna is roughly 1 km.
Example 2: Equipment op Breezecom can reduce power to 4 mW (6dBm), which corresponds
theoretical reliable link of 2.7 km with a 10 dB fade
Example 3: With a 24 dB dish the output power must be reduced to -4 dBm
(yes, only 0.4 mW (!) to stay within the legal limits of 20dBm. However, the
maximum range for a reliable link will be 8,5 km, thanks to the highly
increased antenna gain in the receiver path. Yes, this is true!
The output power of the BreezeNET DS.11 can be
set at a level of -4, -2, 4, 6, 12 or 14 dBm. (Info from Kees, PA3HAN). So
the breezeNET DS.11 is ideal for Long Range Legal Limit Link
In the case you have knowledge of other WLAN equipment having the feature of
setting the output power at a very low level, please let me know at ' pa0hoo at qsl.net'.
An interesting thought for those who really do want to
perform Long Range Legal Limit Link experiments:
A potential alternative solution could
be in using the 'second' receiving only antenna, that most Wlan equipment
have. This 'receiving only' antenna is used for diversity reception. Despite
that such an antenna has been designed for receiving only, it has usually
has a -20dB leakage during transmitting. That means that a +15 dBm output
level at the 'first' transmitting antenna generates a -5 dBm output
level transmitting level at the 'second' receiving antenna. And that is exactly the
level that's needed for the 8,5 km legal limit link of example nr 3. So,
basically you can connect a 24 dBi antenna to the receiving antenna
connector in order to achieve a legal limit long range link. Extra advantage
is, that you still can use the existing local transmitting/receiving
antenna in order to connect to local wlan equipment. Interesting
thought, isn't it?
Have a look at http://seattlewireless.net/index.cgi/HardwareComparison
and find yourself the equipment with the best receiver sensitivity and
Power is expressed in watts or milliwatts. Power can be expressed on a
logarithmic scale relative to 1 mW, in dBm. ('deci-Bell
relative to one milliwat) . In that case, the output is compared to
(1 dBm= 10*log10(P/ 0.001))
(P in Watts)
Usually, WLAN equipment has an output power of 15 dBm (about 30 mW)
Connector loss depends on the quality of the connectors that are used. at
2,45 GHz, connector loss can vary between 0.1 and 0,5 dB. Avoid extra losses
by using as few as possible connectors of good quality. N connectors, SMA
connectors can be used. BNC connectors could be used, only in the case they
are of an extraordinary quality
Pigtails can have very high losses. Our 30 cm piggy tail had a cable
loss of 1.5 dB! Avoid using them. Use converters instead.
Add connector loss to cable loss before calculating the Link Budget
Receiver has a minimum received power threshold (on the card connector) that
the signal must have to achieve a certain bitrate. If the signal power is lower
the maximum achievable bitrate will be decreased or performance will decrease.
So we have better use receiver with low threshold value, here are some typical
receiver sensitivity values:
Receiver sensitivity is not the only parameter for the receiver, we have also
to take into account the signal to noise power ratio. It's the minimum power
difference to achieve between the wanted received signal and the noise (thermal
noise, industrial noise due for example to microwave ovens, interering noise due
to other WLAN on the same frequency band). It is defined as:
Signal/Noise Ratio [dB] = 10 * Log10 (Signal Power [W] / Noise Power [W])
If the signal is more powerful than the noise, signal/noise ratio (also
called S/N ratio) will be positive. If the signal is buried in the noise, the
ratio will be negative. In order to be able to work at a certain data rate the
system needs a minimum S/N ratio:
Orinoco PCMCIA Silver/Gold: 11Mbps => 16 dB ; 5.5 Mbps => 11 dB ; 2
Mbps => 7 dB ; 1 Mbps => 4 dB.
If the noise level is very low then the system will be limited more by the
receiver sensitivity than by the S/N ratio. If the noise level is high then it
will be the Signal/Noise ratio that will count to achieve a given data rate. If
the noise level is high we will need more received power. In normal conditions
whithout any other WLAN on the frequency and whithout industrial noise the noise
level will be around -100dBm. For example to achieve a 11 Mbps data rate with an
Orinoco 802.11b card we would need a received power 16dB higher (S/N ratio) so a
level of -100+16=-84 dBm but in fact the minimum receiver sensitivity is at -82
dBm...higher than -84. It means in that case the minimum receiver sensitivity is
the limiting factor for the system.
A simple and quick explanation of Fresnel ellispsoid role in radio
propagation is to see the thing like a virtual "pipe" where most of the energy
travels between a transmitting and receiving site. So in order to avoid losses
there should be NO obstacles inside this zone (forbidden region) because an
obstacle will disturb "the energy flow". (the explanation is really simplified
For example, if half of the forbidden zone is masked (antenna at the limit of
line of sight), there will be a signal power loss of 6 dB (power loss of 75 %).
These values are only valid for a frequency of 2.45 GHz !
(The radius of forbidden region here is 0.6 x Radius of first Fresnel
When an obstacle is located between the transmitter and the receiver some
energy still pass through thanks to the diffraction phenomenon on the top edge
of the obstacle. The higher the frequency of the transmission the higher the
loss will be.
These calculation are valid in the case of D1 and D2 far greater than h.
This loss is to add to the free space propagation loss.
The loss is the same in a transmission in the opposite direction
(transmitter replaced by receiver and vice versa).
Reference: S. Saunders: Antenna and propagation for wireless communication
Wave polarisation is introduced by the type of your antenna and its orientation
(radiating element) to the ground. Yagi antennas can be used vertically or
horizontally polarised. Helical antennas produce circular polarisation. Circular polarisation can turn either right or
left. Use always the same type op polarisation for both stations.
A transmission system with circular polarisation antennas is a good way to
attenuate the effect of reflections (principle used for GPS).
Radio waves reflect on the obstacles they meet. At the receiver side we
catch then at the same time the direct wave (if in line of sight) and the
reflected waves. This leads to cancelled power at certain frequencies and also a
time difference between the different received components that makes the
received signal spread in the time domain. Consequence on the system is harmful
and lead to decreased performances (transmission errors). Probably you have seen
this effect at bad television reception (ghost images)
In order to reduce the effects of this phenomena the receiver has what
is called an equaliser that counteract these delay spread faults. Anyway this has a limited capacity and manufacturers give delay spread
limit in order to achieve minimum error rate at a certain data rate:
Orinoco PCMCIA 802.11b card, delay spread values for a frame error rate
(FER) lower than 1%: 11Mbps => 65 ns ; 5.5 Mbps => 225ns ; 2 Mbps =>
400ns ; 1Mbps => 500 ns.
We see that for higher bit rate we have better not the long
reflections. The time difference for a reflection can be easily calculated as
radio wave travel at the speed of light (300.000.000 m/s):
Time difference [s] = Length difference between direct path and reflected
path [m] / 300.000.000
So a time difference of 50 nanoseconds corresponds to a path length
difference of 15 meters. In order to minimise the reflection rate it is better
using directive antennas, even if you are at short distance, and being in line of sight. Another possibility is also to
use circular wave polarisation antennas (helical antenna) that cancel quite well
the first reflexions. (that is because the reflected signal has the opposite
circulation direction (left becomes right), so the receiver is insensitive to
this refelected signal) The helical would be ideal.
Reflections also exists in the ensemble coaxial cable-connectors-antennas if
these are not well adapted and designed (bad impedance, badly tuned antenna
=> standing waves, bad SWR) and so may lead to transmission errors. So use
good cable and connectors.