Data transmission over a reduced number of physical antennas

1. A method, performed in a multi-antenna radio access node comprising four logical antenna ports mapped to corresponding physical antennas, of transmitting data using three physical antennas, the method comprising:

determining loss of data transmission from one physical antenna of the four physical antennas;

determining a required multi-antenna transmission scheme;

selecting an antenna mapping matrix prestored in the radio access node, the selected antenna mapping matrix adapted to the required multi-antenna transmission scheme;

remapping data on the four logical antenna ports to the remaining three physical antennas by using the selected antenna mapping matrix;

transmitting the data from the remaining three physical antennas;

transmitting a set of reference signals on the remaining three physical antennas, each reference signal representing a respective antenna port of the four logical antenna ports; and

scaling transmission power for each physical antenna based on the number of reference signals transmitted on the respective physical antenna.

2. The method of claim 1 , wherein the loss of data transmission from one physical antenna results from an antenna failure.

3. The method of claim 1 , wherein the remapping is performed in a baseband/logical interface of the multi-antenna radio access node.

4. The method of claim 1 , wherein the remapping is performed in a radio/physical interface of the multi-antenna radio access node.

5. The method of claim 1 , wherein the transmission power is reduced by half, when two reference signals are transmitted from one physical antenna.

6. The method of claim 1 , further including remapping the at least four logical antenna ports to a remaining three physical antennas using a precoding matrix including a power scaling factor corresponding to the number of reference signals transmitted on a physical antenna.

7. The method of claim 1 :

wherein the remapping comprise remapping of a first logical antenna port to a first physical antenna, a second logical antenna port to a second physical antenna, and a third and fourth logical antenna port to a third physical antenna;

wherein the first and second physical antennas are co-polarized antennas and the third physical antenna element has an orthogonal polarization to the first and second physical antennas.

8. The method of claim 1 , wherein the determining the required multi-antenna transmission scheme comprises:

determining one or more types of antenna transmission scheme used in current transmission; and

selecting a remapping matrix from a group of prestored remapping matrices based on current antenna transmission scheme, each remapping matrix representing a remapping adapted to a specified antenna transmission scheme.

9. The method of claim 1 :

further comprising using an antenna transmission scheme with a transmit diversity scheme for at least one of: a Physical Broadcast Channel; a Physical Control Format Indicator Channel; a Physical Downlink Control Channel; a Physical Hybrid ARQ Indicator Channel; and a Physical Downlink Shared Channel;

wherein a first and second physical antenna represent main branches in the transmit diversity scheme and a third physical antenna represents a diversity branch; and

wherein power of transmissions on the third physical antenna element is scaled by a power scaling factor.

10. The method of claim 9 , wherein the power scaling factor is selected to align transmission power on the diversity branch to transmission power on the main branches.

11. A multi-antenna radio access node in a wireless network, the radio access node configured for enabling downlink data transmission over a reduced number of physical antennas, the radio access node comprising:

four logical antenna ports and four physical antennas;

a radio communications interface connecting to the four physical antennas;

a memory;

a processor comprising the four logical antenna ports and configured to:

determine loss of data transmission from one physical antenna of the four physical antennas;

determine a required multi-antenna transmission scheme;

select an antenna mapping matrix prestored in the memory, the selected antenna mapping matrix adapted to the required multi-antenna transmission scheme;

control remapping of data on the four logical antenna ports to remaining three physical antennas by using the selected antenna mapping matrix; and

initiate transmission of data from the remaining three physical antennas over the radio communication interface;

initiate transmission of a set of reference signals on the remaining three physical antennas, each reference signal representing a respective antenna port of the four logical antenna ports; and

scale transmission power for each physical antenna based on the number of reference signals transmitted on the respective physical antenna.

12. The multi-antenna radio access node of claim 11 , further comprising a physical antenna failure determination entity.

13. The multi-antenna radio access node of claim 11 , wherein the processor is configured to execute the remapping.

14. The multi-antenna radio access node of claim 11 , wherein the radio communications interface is configured to execute the remapping.

15. A computer program product stored in a non-transitory computer readable medium for controlling a multi-antenna radio access node comprising four logical antenna ports mapped to corresponding physical antennas so as to transmit data using three physical antennas, the computer program product comprising software instructions which, when run on a processor of the access node, causes the access node to:

determine loss of data transmission from one physical antenna of the four physical antennas;

determine a required multi-antenna transmission scheme;

select an antenna mapping matrix prestored in the radio access node, the selected antenna mapping matrix adapted to the required multi-antenna transmission scheme;

remap data on the four logical antenna ports to the remaining three physical antennas by using the selected antenna mapping matrix;

transmit the data from the remaining three physical antennas;

transmit a set of reference signals on the remaining three physical antennas, each reference signal representing a respective antenna port of the four logical antenna ports; and

scale transmission power for each physical antenna based on the number of reference signals transmitted on the respective physical antenna.