Can wifi cell superpostion double-triple a LAN-network bandwidth?
 

802.11a/b/g access points located close to each other, working in different channels, is it true that the bandwidth performance improves twice or more the maximum value? I suppose that because of the wave superpostion property, there shall be a better signal strength through a larger distance, but to have more bandwidth than 54 MBps? I´ve heard it is possible... why and where can I find more info. THANKS.

Not in a/b/g as far as I know (assuming you're asking about bandwidth available to individual clients rather than total network bandwidth) but there is a non-standard Super G extension from Atheros that uses channel bonding (along with frame-bursting and compression) to increase bandwidth. It was criticized for causing interference to existing b/g networks and other users of the 2.4 GHz ISM band.

There is another 802.11 non-standard extension that uses only frame-bursting and compression techniques within existing channeling to increase bandwidth: 125 High Speed Mode.

Current focus is on MIMO's Spatial Multiplexing technique that increases bandwidth and range by transmitting unique streams through individual antennas on the same channel and some clever code at the receiver to recover data from the individual streams using their spatial signatures. This will feature in 802.11n.