Selecting between normal and virtual dual layer ACK/NACK

An apparatus comprising:

• at least one processor;
• and
• at least one memory including computer program instructions, the at least one memory and the computer program instructions configured, with the at least one processor, to cause the apparatus to perform the following:
• receive data over a bundling window comprising n downlink subframes spanning over l layers for data, in which l is an integer, and the n downlink subframes are associated with one uplink subframe;
• generate a resource-specific bit for each of the n downlink subframes on each of the l layers resulting in n*l resource-specific bits;
• based on a configuration, select from among at least
• a first mode that bundles the n*l resource-specific bits into a first set of groups wherein the bundling is across the n downlink subframes on each of the l layers, and
• a second mode that bundles the n*l resource-specific bits into a second set of a plurality of groups wherein the bundling is across the l layers;
• aggregate the n*l resource-specific bits into groups according to the selected mode;
• generate a reply for each of the groups aggregated according to the selected mode wherein the reply is representative of the aggregation;
• and
• transmit the replies for each of the groups aggregated according to the selected mode in the one uplink subframe.

A method comprising:

• sending an allocation of downlink resources and transmitting data on the allocated downlink resources;
• responsive to receiving a reply bit that is a negative acknowledgement of the transmitted data, selecting from among at least a first mode that bundles n downlink subframes on each of l layers resulting in n*l resource-specific bits into a first set of groups wherein the bundling is across the n downlink subframes, and
• a second mode that bundles the n*l resource-specific bits into a second set of groups wherein the bundling is across the l layers;
• determining a bundling window and layer combination from the selected mode;
• and
• retransmitting the data that is the subject of the negative acknowledgement in the allocated downlink resources which are indicated by the bundling window and layer combination.