Codebook design and structure for multi-granular feedback

An apparatus comprising:

• at least one processor;
• and
• at least one memory storing computer program code configured to, with the at least one processor, cause the apparatus at least to perform:
• select a first precoding matrix W1 from a first codebook comprising sets of rank specific precoding matrices, the first codebook characterized by there being equal or fewer precoding matrices when the ranks increase, and characterized by precoding matrices associated with ranks above a certain rank all being diagonal matrices;
• use the selected first precoding matrix W1 to select a rank-specific second precoding matrix W2 from a second codebook such that the selected first and second precoding matrices form a joint precoder specific to a desired rank, in which the second codebook comprises differently sized precoding matrices associated with N total ranks, in which N is an integer greater than one;
• and
• report information on the joint precoder to a network node over an uplink transmission channel.

A computer program embodied on a non-transitory computer readable storage medium, the computer program configured to control a processor to perform operations, comprising:

• selecting a first precoding matrix W1 from a first codebook comprising sets of rank specific precoding matrices, the first codebook characterized by there being equal or fewer precoding matrices when the ranks increase, and characterized by precoding matrices associated with ranks above a certain rank all being diagonal matrices;
• using the selected first precoding matrix W1 to select a rank-specific second precoding matrix W2 from a second codebook such that the selected first and second precoding matrices form a joint precoder specific to a desired rank, in which the second codebook comprises differently sized precoding matrices associated with N total ranks, in which N is an integer greater than one;
• and
• reporting information on the joint precoder to a network node over an uplink transmission channel.

A method comprising:

• receiving from a user equipment an indication of a joint precoder;
• determining from the received indication a first precoding matrix W1 by accessing a first codebook stored in a computer readable memory, the first codebook comprising sets of rank specific precoding matrices and characterized by there being equal or fewer precoding matrices when the ranks increase, and characterized by precoding matrices associated with ranks above a certain rank all being diagonal matrices;
• determining from the received indication a second precoding matrix W2 by accessing a second codebook stored in the computer readable memory, the second codebook comprises differently sized precoding matrices associated with N total ranks, in which N is an integer greater than one;
• and
• using at least the determined first and second precoding matrices to spatially encode a transmission to at least the user equipment.

An apparatus comprising:

• at least one processor;
• and
• at least one memory storing computer program code configured to, with the at least one processor, cause the apparatus at least to perform:
• determine, from a received indication of a joint precoder, a first precoding matrix W1 by accessing a first codebook stored in the memory, the first codebook comprising sets of rank specific precoding matrices and characterized by there being equal or fewer precoding matrices when the rank increase, and characterized by precoding matrices associated with ranks above a certain rank all being diagonal matrices;
• determine from the received indication a second precoding matrix W2 by accessing a second codebook stored in the memory, the second codebook comprises differently sized precoding matrices associated with N total ranks, in which N is an integer greater than one;
• and
• use at least the determined first and second precoding matrices to spatially encode a transmission to at least one user equipment.