Covering range and link margin for WLAN
Summary "For how long distances is it possible to use a link for 2.4 GHz?" is a question that we get frequently. Here we show how to calculate this, and some examples for much spread equipment. For a more thorough answer we recommend our on line tools
WLAN Design Tools,
see link above under the support menu.Here is a summary for the ones who can not be bothered to read further.
WaveLAN black card, 11 Mbps
WaveLAN black card, 2 Mbps
WaveLAN red card, 11 Mbps
WaveLAN red card, 2 Mbps
Covering range in km with 5 dB link margin.
Linkbudget
The Linkbudget is calculated with the formula PRX = PEIRP - FSPL + GRX - LRX PRX received signal (dBm) PEIRP transmitted power, max 20 dBm at 2.4 GHz (dBm) FSPL free space path loss (dB) GRX gain for the receiving antenna (dBi) LRX attenuation for receiver cable (dB)
Free Space Path Loss
Radiated power from transmitting antenna is projected on an imaginary sphere taking in to consideration the antenna radiation pattern. This can be seen as attenuation between the transmitting and the receiving antenna, Free Space Path Loss (FSPL) which is calculated as FSPL = 20*log10(4*pi*R/wl) (dB) R distance between antennas (m) wl wavelength (m), ca 0.12 m at 2450 MHz 6 dB corresponds to a doubling or halving of the distance. Here are a few approximate values to memorise (for 2.4 GHz):
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R [km]
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FSPL [dB]
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0.5
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94
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1
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100
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2
|
106
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4
|
112
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Received signal Received signal (PRX) has to exceed a certain level for communication to be possible at a given data rate. Example
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11 Mbps
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5.5 Mbps
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2 Mbps
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1 Mbps
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Lucent WaveLAN
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-82 dBm
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-87 dBm
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-91 dBm
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-94 dBm
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Note: our measurements show that Lucent WaveLAN consequently has better sensitivity than stated at 11 Mbps.
Linkmargin Some equipment work at different data rates and consequently has different sensitivity for reception. If it has so called i>fallback, it automatically switches down to a lower speed when the signal strength is not sufficient.For equipment that has fallback we recommend 5-7 dB link margin at 2.4 GHz and distance up to 5 km. For equipment that does not have fallback it is sometimes good to have a larger margin 10-15 dB, especially with high demands on accessibility. Best is to study the variation in signal strength on the link for a longer period of time and see how it varies.Maximal output power
The maximal allowed output power can be produced in a number of ways, depending on type of antenna to be used. Here are a few examples on suitable kits who deliver close to allowed output power 20 dBm. Attenuation in pigtail is 0.7 dB.
Lucent black card (15 dBm) with
VP165/24 (9 dBi) patch + 10 m LowLoss (5 dB) = 18.3 dBm
VO6/24 (6 dBi) omni + 6 m C2FCP (1.2 dB) = 19.1 dBm
3VS9/24 (9 dBi) multisector + 15 m C2FCP (3 dB) = 20.3 dBm
CA27/24 (14 dBi) cavity + 14 m LowLoss (7 dB) = 21.3 dBm
SP45/24 (18 dBi) parabolic + 20 m LowLoss (10 dB) = 22.3 dBm
Lucent red card (8 dBm) with
VP470/24 (14 dBi) panel + 6 m C2FCP (1.2 dB) = 20.1 dBm
SP45/24 (18 dBi) parabolic + 22 m C2FCP (4.4 dB) = 20.9 dBm
Maximum range
When using the same type radio card in both ends of a link is it to be considered as symmetric. For Lucent black card is it therefore allowed to have 5 dB efficient gain, i.e. the sum of cable attenuation and antenna gain. This quantity is reciprocal, i.e. has the same power both for transmission and reception.So maximum gain is therefore only archived with 20 dBm EIRP and -77 dBm input signal (to the 5 dB gain antenna). This gives 5 dB margin at 11 Mbps data rate. . Maximum FSPL is therefore (77+20+5=)102 dB, or 1.3 km. 1 Mbps data rate and the same conditions gives FSPL=114 dB and maximum range 5 km.
Lower output power gives longer range
By sending with lower output power from the radio card is it possible to use an antenna with more gain. This gain is useful as "better ears" when receiving. Using the same way of thinking gives 12 dB efficient gain, i.e. FSPL @ 11 Mbps = 114 dB which gives a range of 3 km or 11 km @ 1 Mbps data rate.
Variation of noise levell
Besides variations in signal strength the amplitude of the noise floor can vary, which gives a lesser signal - noise ratio (SNR). It is therefore best to look at the signal and the noise amplitude separately, and not only SNR. It can be fully possible to use a link with very low SNR as long as the noise and the signal are on stable levels.
Guaranties
Values given here are to be considered as calculation examples that give a correct but not covering view on how to dimension WLAN. Free space path loss refers to free line of sight without interference from reflexes or Fresnelzon. Gains in antennas, attenuation in cables and output power from radio devices are to be considered as guide values. Every WLAN operator alone is responsible for archiving the required knowledge to follow the current regulations.
Sources
Data for the products are collected from the manufacturers manual or data on the Internet.
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