Precoder feedback information scheme for downlink colocated multiple input multiple output (MIMO) or distributed MIMO

A method of determining a composite precoder for transmitting to user equipment (UE) using a group of one or more of radio units of a radio access system, the radio access system comprising a distributed unit and a plurality of radio units to wirelessly transmit and receive radio frequency signals to and from the UE using a wireless interface, each of the radio units associated with a respective set of antennas and wherein the distributed unit is communicatively coupled to the plurality of radio units, the method comprising:

• for each of a plurality of subsets of the radio units, transmitting separate reference signals from the different antennas of only the radio units included in that subset;
• receiving subset-specific preferred precoder index reports from the UE for at least each subset that includes one or more radio units included in the group;
• determining the composite precoder for the UE using at least some of the subset-specific preferred precoder index reports received from the UE;
• and
• using the composite precoder for transmitting to the UE using the one or more radio units included in the group;
• and
• wherein each of the subsets includes less than all of the radio units.