Rank considerations when determining a joint precoding matrix for use in a distributed antenna MIMO system

摘要

The present invention is concerned with transmissions from multiple distributed transmission points to a user terminal. The transmit points and the user terminal each have a multiple-input-multiple-output (MIMO) capability. For example, the present invention could be applied to coordinated multi-point (CoMP) transmissions. The user terminal determines a transmission rank and a precoding matrix for each of at least two transmission points of a plurality of transmission points. A joint transmission rank for a joint transmission involving each of the at least two transmission points is determined. Next, the user terminal selects a joint precoding matrix which provides the joint transmission rank and is based on the precoding matrices of respective transmission points. The user terminals transmits channel state information (CSI) including a description of the selected joint precoding matrix. The pre-coding matrix may be a diagonal matrix or a semi-diagonal matrix. It may include an intra-transmission point precoder and an inter-transmission point combiner. Also disclosed is a transmit point which uses received CSI reports to determine whether to operate in a MU-MIMO or SU-MIMO mode.