Link Planning for Wireless LAN (WLAN)

This page may be helpful in predicting the performance of (future) WLAN radio links.  

Link power budget

The link power budget is the computation of the whole transmission chain. 

A radio Link consists of three basic elements: 

• Effective transmitting power:  transmitter power [dBm] {minus} (cable +connector) loss [dB] {plus} antenna gain [dBi]
• Propagation loss [dB]: Free space loss [dB].
• Effective receiving sensibility:  antenna gain[dBi]- cable loss [dB]- receiver sensitivity [dBm]

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

Remarks: 
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 reliability is moderate. 
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 with a theoretical reliable link  of 2.7 km with a 10 dB fade margin. 
• 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 experiments 
  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 variable power. 

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Power

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 one milliwatt. 

(1 dBm= 10*log10(P/ 0.001))
(P in Watts) 

• Usually, WLAN equipment has an output power of 15 dBm (about 30 mW) 
• some equipment, have an output power of 100mW. Note: this is a DISadvantage if you want to make a long distance link within legal limits. See: Optimizing Link Power Within Legal Limits

Conversion calculator  from Watts (W) to decibels "milliwatts" (dBm)  or visa versa. :

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Loss in a coaxial cable at 2.45 GHz

Here are some loss value for common coaxial cables:

• RG 58 (quite common, used for Ethernet): 1 dB per meter.
• RG 213 ("big black", quite common): 0.6 dB per meter.
• RG 174 (thin, seems to be the one used for pigtail adapter cables): 2 dB per meter.
• Aircom : 0.21 dB/m.
• Aircell : 0.38 dB/m.
• LMR-400: 0.22 dB/m
• IEEE 802.3 (thick 'yellow' Ethernet coax) 0.3 dB/m

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Antenna

• Antenna gain is normally given in decibels over an isotropic antenne [dBi]. It's the power gain in comparison to a hypothetical   isotropic (all directions equal) antenna.
• Some antennas have their gain expressed in [dBd], it's the gain compared to a dipole antenna. In this case you should add 2.14 to obtain the corresponding gain in [dBi].
• The more gain an antenna has, the more it is directive (energy sent in a specific direction), the less local (noise) signals are picked up. This improves the signal to noise ratio. 

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Connector losses

• 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

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Radiated power

Radiated power (power sent by the antenna at its direction of  maximum gain) can be easily computed (in dBm):

Radiated power [dBm] = Transmitter power [dBm] - cable loss [dB] + antenna gain[dBi]

• Legal limit for radiated power (EiRP) for WLAN in Europe (except France) is 100mW (= +20dBm). 
  France is only 7 dBm (5 mW) .

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Free space loss at 2.45 GHz

It is the power loss of wave travelling in free space (without obstacles).

(Friis formula)

Lp(dB)= 92,45 + 20log10 F+20 LOG10d 
Lp= Path loss
F= frequency in GHz
dB= decibels
d= Distance in kilometres
Example:  A distance of 6 kilometre provides a free space loss of –116 dB.

Correspondance between free space gain loss in dB and distance in kilometer (km) :

The calculation below can be used for any desired frequency, so also for the 5 GHz 802.11a band.

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Receiver sensitivity

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:

• Orinocco cards PCMCIA Silver/Gold : 11Mbps => -82 dBm ; 5.5Mbps => -87 dBm; 2Mbps=> -91 dBm; 1Mbps=> -94 dBm.
• CISCO cards Aironet 350: 11Mbps => -85 dBm ; 5.5 Mbps => -89 dBm; 2 Mbps => -91 dBm; 1 Mbps => -94 dBm.
• Edimax USB client: 11Mbps => -81 dBm; 
• Belkin router/AP: 11 Mbps =>-78 dBm

(These are values given by the manufacturer).

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Signal to Noise Ratio

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.

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Propagation: Fresnel ellipsoid

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 ellipsoid)

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Propagation: Diffraction

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 systems.

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Propagation: Polarisation

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).

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Reflections and delay spread

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.  

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References, documentation

• voortplanting van radiogolven door VE3JF (zendamateur): VHF/UHF/Microwave Radio Propagation: A Primer for Digital Experimenters
• S.R. Saunders: Antennas and propagation for wireless communication systems, Wiley 1999
• http://www.s3.kth.se/radio/COURSES/RKBASIC_2E1511_2001/lecture_3.pdf
• www.wirelessleiden.nl
• www.wirelessnederland.nl 

Please send any errors or omission to pa0hoo at qsl.net. Thanks!