Forum

Dan is definitely correct. In fact, you'll see many APs now that are 3x3, but only support 2 spatial streams. You can see from Quantenna's product certificate that this is a consumer AP and not an enterprise product. What cwnpchina is describing in the last sentence there is Space-Time Block Coding (STBC).

When there are fewer spatial streams than there are Tx radio chains, STBC is used (optionally) to turn N number of spatial streams into N+1 or N+2 space-time streams, which are transmitted by the otherwise unused transmit chains. This is an oversimplification, but the space-time streams are like redundant copies of one or both of the spatial streams. There's a lot of complex math with STBC that I don't understand, but the basic purpose of STBC is to add reliability (and thus improve channel efficiency, data rates, usable range, etc) with more robust transmission formats.