I have slight disagreement on layer and stream definition. As an example , for rank 1 case - there can only be 1 layer but 1,2 or 4 streams. These 1,2 or 4 streams are in case of transmit diversity with 1,2 or 4 antenna support. Layers and streams will map 1 to 1 in case of spatial multiplexing,ex 2 layers = 2 streams.