Selecting between normal and virtual dual layer ACK/NACK

1. A method comprising:

receiving data over a bundling window comprising n downlink subframes spanning over l layers, in which l is an integer, and n downlink subframes are associated with one uplink subframe;

generating 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, selecting 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;

aggregating the n*l resource-specific bits into groups according to the selected mode;

generating a reply for each of the groups aggregated according to the selected mode wherein the reply is representative of the aggregation; and

transmitting the replies for each of the groups aggregated according to the selected mode in the one uplink subframe.

2. The method of claim 1 , in which the second mode bundles downlink resources in sub-bundles of resources across all of the l layers such that each per-bundle reply bit is specific to resources within a group of subframes that spans all of the l layers.

3. The method of claim 1 , wherein l equals 2.

4. The method of claim 1 ,

wherein each reply and each of the resource-specific bits is an acknowledgement or negative acknowledgement of data received or not received, and

wherein the first mode comprises a logical AND operation of all resource-specific acknowledgement and negative acknowledgment bits for all allocated resources per l th layer, and the second mode comprises a logical AND operation of all resource-specific acknowledgement and negative acknowledgment bits for all of the allocated resources across all of the l layers.

5. The method of claim 1 , wherein the first mode aggregates for each layer the resource-specific bits of all n downlink subframes to generate one reply bit for each of the l layers.

6. The method of claim 1 , wherein the second mode aggregates for all downlink resources in a group of the plurality of groups, all of the resource-specific bits associated with each of the downlink resources in that group and across the l layers.

7. The method of claim 1 , wherein the first mode and the second mode aggregate using a logical AND operation.

8. The method of claim 1 , where l equals 1.

9. The method of claim 1 , wherein the configuration is generated at a higher layer, the higher layer being higher than the physical layer.

10. 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.

11. The apparatus of claim 10 , in which the second mode bundles downlink resources in sub-bundles of resources across all of the l layers such that each per-bundle reply bit is specific to resources within a group of subframes that spans all of the l layers.

12. The apparatus of claim 10 , wherein l equals 2.

13. The apparatus of claim 10 , wherein each reply and each of the resource-specific bits is an acknowledgement or negative acknowledgement of data received or not received, and wherein the first mode comprises a logical AND operation of all resource-specific acknowledgement and negative acknowledgment bits for all allocated resources per l th layer, and the second mode comprises a logical AND operation of all resource-specific acknowledgement and negative acknowledgment bits for all of the allocated resources across all of the l layers.

14. The apparatus of claim 10 , wherein the first mode aggregates for each layer the resource-specific bits of all n downlink subframes to generate one reply bit for each of the l layers.

15. The apparatus of claim 10 , wherein the second mode aggregates for all downlink resources in a group of the plurality of groups, all of the resource-specific bits associated with each of the downlink resources in that group and across the l layers.

16. The apparatus of claim 10 , wherein the first mode and the second mode aggregate using a logical AND operation.

17. The apparatus of claim 10 , where l equals 1.

18. The apparatus of claim 10 , wherein the configuration is generated at a higher layer, the higher layer being higher than the physical layer.

19. 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.

20. The method of claim 19 , wherein the selecting is from among only the first mode and the second mode, and is further based on a change criterion indicative of how correlated in time are the allocated downlink resources.

21. The method of claim 19 , executed by an access node of a wireless communication network.