An extract from 3GPP TR 25.872.
HSPA based mobile internet offerings are becoming very popular and data usage is increasing rapidly.
Consequently, HSPA has begun to be deployed on more than one transmit antenna or more than one carrier. As an example, the single cell downlink MIMO (MIMO-Physical layer) feature was introduced in Release 7.
This feature allowed a NodeB to transmit two transport blocks to a single UE from the same cell on a pair of transmit antennas thus improving data rates at high geometries and providing a beamforming advantage to the UE in low geometry conditions. Subsequently, in Release-8 and Release-9, the dual cell HSDPA (DC-HSDPA) and dual band DC-HSDPA features were introduced.
Both these features allow the NodeB to serve one or more users by simultaneous operation of HSDPA on two different carrier frequencies in two geographically overlapping cells, thus improving the user experience across the entire cell coverage area. In Release 10 these concepts were extended so that simultaneous transmissions to a single UE could occur from four cells (4C-HSDPA).