I recently managed a successful 5km link across town, using just standard access points and antenna. While working out all of the signal loss calculations to see if the link was even possible, I started to understand how long range links work, and how manufacturers such as Cisco have been able to offer long range links while staying within the legal transmission power limits for Wi-Fi.
I had always assumed that the maximum range a wireless connection could be was about 1km (based on uk power limit) and this was in line with the practical range tests I had carried out. To get increased range, always required sticking a big antenna on the Access Point, pushing the overall signal well over the uk limit (20dbm).
While carrying out the 5km link test I found that the snr (signal to noise ratio) was not the really critical factor, but that it was the received power level that decided if a link was possible.
Transmit Power is independant of Receive SensitivityI had always assumed (wrongly) that an access point that transmitted at a higher power, would receive the signal at a higher power as well. This is not the case. Increasing the power that an access point (or any wireless device) transmits at, does not affect the level of signal it needs to receive in order to make a connection.
When you stick an antenna onto a wireless device, the antenna gain acts on both the transmit power and receive sensitivity, so you are not only sending your signal further (in a more focused direction), but your are also able to listen to weaker signals. The EU regulations only limit the transmit signal strength, not the receive sensitivity, so we need a way of increasing the receive sensitivity, but not increasing the transmit signal beyond the 20db limit.
Most access points and wireless devices in Europe transmit at 15 - 17.5 db. (Some may claim 20db , but this includes the gain of the small antenna they are using) This would limit the antenna gain to just 2.5 - 5db (or less), which is not going to affect the range significantly. In free space (ie outside, with clear line of sight  ) for every 6db of increased signal strength or receive sensitivity you can double the range between the devices.
Some wireless devices however, let you reduce the transmit power, which allows us to use a more powerful antenna. This does not increase the overall strength of the signal , but does increase the receive sensitivity. If we do this at both ends of the link, we can increase the range, while staying within legal limits.
The Maths Explained
TPowerA + AntGainA - SignalLossAtoB + AntGainB > ReceiveSensB
TPowerA = Transmit Power of A
In order to see how far we can get a signal, we need to calculate how much signal loss we can cope with, and still get a strong enough signal to B.
Maximum SignalLossAtoB = TPowerA + AntGainA + AntGainB - ReceiveSensB
As an example, if we base our figures on a 15db wireless device, 5db antenna at each end and -92 db receive sensitivity we get.
Maximum SignalLossAtoB = 15 + 5 + 5 - -92
In practice 117db should give you a range of 1-2 km (In theory it should be over 5km). If we now look at reducing the transmit power to 7db, and using 13db antenna on each end thus keeping the transmit signal strength to 20db, we get
Maximum SignalLossAtoB = 7 + 13 + 13 - -92
This gives us over twice the range of before, yet still keeps the signal strength legal.
 Device power is actually measured in dbm and antenna gain in dbi, but if you already know
this, then this article is probably not for you.
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