The default TX-Power of wireless is set to 20 dBm but you can increase it with a little trick to 30 dBm but let me warn you first that It might be illegal in your country, so use it at your own risk. Moreover Some models will not support these settings or wireless chip may state that it "can" transmit with higher power, but the device's manufacturer probably did not place the appropriate heat sink in order to accomplish this.

There are also many ways to do it but i will share two simple methods.

1st Method.
Code:
iw reg set BO
iwconfig wlan0 txpower 30
If above method gives the following error then go for 2nd method.
Error for wireless request "Set Tx Power" (8B26) :
SET failed on device wlan0 ; Invalid argument.

2nd Method.

Code:
ifconfig wlan0 down
iw reg set BO
ifconfig wlan0 up
iwconfig wlan0 channel 13
iwconfig wlan0 txpower 30

For your reading visit the below links
http://en.wikipedia.org/wiki/Wi-Fi
http://git.kernel.org/cgit/linux/ker...db.txt?id=HEAD