Antenna port indication for sidelink

1. A method comprising:

receiving, by a first wireless device from a base station, one or more radio resource control (RRC) messages indicating a type of demodulation reference signal (DMRS);

determining a length of the DMRS;

determining a DMRS configuration of a physical sidelink shared channel (PSSCH) based on the type and the length of the DMRS; and

transmitting, to a second wireless device, sidelink control information (SCI) comprising the DMRS configuration of the PSSCH.

2. The method of claim 1 , wherein the DMRS configuration is one of a plurality of DMRS configurations.

3. The method of claim 1 , wherein the length of the DMRS indicates the length of a position of the DMRS in units of symbols.

4. The method of claim 1 , wherein the SCI comprises a field indicating one or more antenna ports of DMRS for the PSSCH, and wherein a size of the field is based on the type.

5. The method of claim 1 , wherein the DMRS configuration comprises an index.

6. The method of claim 1 , wherein the SCI further indicates a sidelink radio resource of the PSSCH.

7. The method of claim 1 , wherein the SCI further indicates one or more antenna ports of DMRS for the PSSCH.

8. The method of claim 1 , wherein the DMRS configuration is activated by a medium access control element (MAC CE) signaling.

9. The method of claim 8 , wherein an indication of the activated DMRS configuration is based on an identity of the activated DMRS configuration.

10. The method of claim 9 , wherein the indication of the activated DMRS configuration is based on an increasing order of identities of a plurality of activated DMRS configurations.

11. A wireless device comprising:

one or more processors; and

memory storing instructions that, when executed by the one or more processors, cause the wireless device to:

receive one or more radio resource control (RRC) messages indicating a type of demodulation reference signal (DMRS);

determine a length of the DMRS;

determine a DMRS configuration of a physical sidelink shared channel (PSSCH) based on the type and the length of the DMRS; and

transmit, to a second wireless device, sidelink control information (SCI) comprising the DMRS configuration of the PSSCH.

12. The wireless device of claim 11 , wherein the DMRS configuration is one of a plurality of DMRS configurations.

13. The wireless device of claim 11 , wherein the length of the DMRS indicates the length of a position of the DMRS in units of symbols.

14. The wireless device of claim 11 , wherein the SCI comprises a field indicating one or more antenna ports of DMRS for the PSSCH, and wherein a size of the field is based on the type.

15. The wireless device of claim 11 , wherein the DMRS configuration comprises an index.

16. The wireless device of claim 11 , wherein the SCI further indicates a sidelink radio resource of the PSSCH.

17. The wireless device of claim 11 , wherein the SCI further indicates one or more antenna ports of DMRS for the PSSCH.

18. The wireless device of claim 11 , wherein the DMRS configuration is activated by a medium access control element (MAC CE) signaling.

19. The wireless device of claim 18 , wherein an indication of the activated DMRS configuration is based on an identity of the activated DMRS configuration.

20. A system comprising:

a base station comprising:

one or more first processors; and

first memory storing first instructions that, when executed by the one or more first processors, cause the base station to:

transmit one or more radio resource control (RRC) messages indicating a type of demodulation reference signal (DMRS); and

a wireless device comprising:

one or more second processors; and

second memory storing instructions that, when executed by the one or more second processors, cause the wireless device to:

receive the one or more RRC messages;

determine a length of DMRS;

determine a DMRS configuration of a physical sidelink shared channel (PSSCH) based on the type and the length of DMRS; and

transmit, to the second wireless device, sidelink control information (SCI) comprising the DMRS configuration of the PSSCH.