MIMO in HSDPA and Legacy-MIMO Co-Existence

Multiple Input Multiple Output (MIMO) transmission improves downlink spectral efficiency and is a key feature of Release 7 High Speed Downlink Packet Access (HSDPA). When the Node-B is configured in the MIMO mode with secondary common control pilot channel (S-CPICH), the power amplifiers (PAs) for the two transmit antennas are not balanced. A common pre-coder is required to balance the PAs for legacy and dual-stream MIMO transmissions. In addition, codebook restriction is needed to balance the PAs for single-stream MIMO transmissions. Despite codebook restriction, MIMO performance degradation is small. Legacy user performance in a MIMO cell suffers due to power and interference penalties associated with the Secondary Common Pilot Channel (S-CPICH). By reducing the S-CPICH transmit power, this degradation can be reduced, without significantly compromising the channel estimation quality for MIMO users in the cell.