What I have read so far, indicates that commercial multi-antenna wifi access points use selection combining i.e. uses only one antenna at a time (even if there are 4 antennas on the access point). 

I was wondering why not maximal ratio combining ?  Or why not other schemes which maximize the SINR(signal to Interference Noise Ratio). 

Regards

Sumit 

Any wifi product that is compliant with 802.11n or better will have MIMO.   In other words, the multiple antennas will be used for a variety of combining and MIMO techniques, and certainly not just selection diversity.

The early wifi standards, e.g., 802.11a/b/g, used selection diversity, but 802.11n added MIMO and subsequent improved amendments have built on that.