Transmit power is directly translated to energy of the wave, so higher the transmit power, you will see wave goes further. This sounds like increase Tx power is a good idea.
Actually it is not a good idea though. You have to always remember other end is client devices (phones, tablet, pc) which usually does not have that much Tx power (due to handheld devices and battery life time is so important)
As you can see below, if AP Tx power is too high, you will get a good signal strength for client, then client try to transmit higher data rate. But as clients has low Tx power, AP cannot hear it well. So communication problem occurs. So you have to balance AP Tx power to match clients to get better results.
As example, your AP may be capable of Tx 23dBm, but phone 10-14dBm, in that case better to limit your AP Tx power 8-17dBm range rather increase it to max 20 or 23dBm
Pls check this Ciscolive presentation for better understanding above concepts.
HTH
Rasika
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