Power offset compensation schemes between CRS and DRS for downlink dual layer beamforming in LTE system

LTE Release 9 supports a new DL transmission mode which combines UE specific beamforming and spatial multiplexing. As one option this transmission mode can be operated based on UE (user equipment) feedback of channel rank. However in this case, the rank is in many cases not very accurate due the difference in precoding between the signal used for data demodulation (DRS - dedicated reference signal) and the signal used for estimating rank (CRS - common reference signal). To maintain good performance it is necessary to introduce a compensation factor to inform the UE about the difference between the channel experienced on DRS and CRS to obtain reasonable rank indication (RI). In this paper we study two compensation schemes, named UE-specific scheme and cell-specific scheme. System level simulations are used to validate the performance of the proposed schemes and support the view that cell-specific compensation is sufficient.