Apparatus, method and computer program product for selecting beam group and subset of beams in communication system

A computer program product comprising a program code stored in a non-transitory computer readable medium configured to:

• measure channel state information on a downlink from a base station;
• identify a selected beam group out of a set of beam groups according to a wideband property of said channel state information, wherein a characteristic of said set of beam groups depends on a transmission rank;
• and
• identify a selected subset of beams in said selected beam group according to at least one subband, wherein a number of beams in said selected subset of beams is equal to said transmission rank;
• wherein an angular range from a direction of maximum array gain of a first beam to a direction of maximum array gain of a last beam in said selected beam group overlaps in angular domain with an angular range from a direction of maximum array gain of a first beam to a direction of maximum array gain of a last beam in an adjacent beam group.

A method, comprising:

• measuring channel state information on a downlink from a base station;
• identifying a selected beam group out of a set of beam groups according to a wideband property of said channel state information, wherein a characteristic of said set of beam groups depends on a transmission rank;
• and
• identifying a selected subset of beams in said selected beam group according to at least one subband, wherein a number of beams in said selected subset of beams is equal to said transmission rank;
• wherein an angular range from a direction of maximum array gain of a first beam to a direction of maximum array gain of a last beam in said selected beam group overlaps in angular domain with an angular range from a direction of maximum array gain of a first beam to a direction of maximum array gain of a last beam in an adjacent beam group.

An apparatus, comprising:

• a processor;
• and
• memory including computer program code;
• said memory and said computer program code configured to, with said processor, cause said apparatus to perform at least the following:
• receive encoded feedback information from a user equipment identifying a selected beam group and a selected subset of beams for at least one subband in a double codebook format, wherein said selected beam group represents one of a set of beam groups according to a wideband property of channel state information measured by said user equipment and a characteristic of said set of beam groups based on a transmission rank, and wherein said selected subset of beams in said selected beam group is selected according to said at least one subband and a number of beams in said selected subset of beams is equal to said transmission rank, and
• form a precoder for transmission of a signal to said user equipment using said double codebook format;
• wherein an angular range from a direction of maximum array gain of a first beam to a direction of maximum array gain of a last beam in said selected beam group overlaps in angular domain with an angular range from a direction of maximum array gain of a first beam to a direction of maximum array gain of a last beam in an adjacent beam group.