Selective retransmission of groupcast data

1. An apparatus comprising:

a transceiver that:

transmits groupcast data via sidelink communication to a set of user equipment devices (“UEs”) using a first cast-type; and

receives negative acknowledgement feedback via sidelink communication from at least one UE of the set, the negative acknowledgement feedback indicating unsuccessful reception of the groupcast data; and

a processor that:

determines a retransmission mode based on a number of UEs sending negative acknowledgement feedback, wherein the retransmission mode indicates a cast-type that is different from the first cast-type; and

controls the transceiver to send selectively beamformed retransmission of the groupcast data according to the determined retransmission mode.

2. The apparatus of claim 1 , wherein the cast-type of the retransmission mode comprises unicast retransmission with beamforming or groupcast with partial beam sweeping.

3. The apparatus of claim 1 , wherein determining the retransmission mode is further based on a Quality of Service (“QoS”) priority of the groupcast data.

4. The apparatus of claim 1 , wherein the transceiver receives a sidelink groupcast hybrid automatic repeat request (“HARQ”) retransmission profile from a base station in a mobile communication network, the sidelink communication not passing through the base station.

5. The apparatus of claim 1 , wherein the transceiver transmits sidelink control information (“SCI”) to the set of UEs.

6. The apparatus of claim 5 , wherein the sidelink control information includes a data transmission index and a transmission occasion.

7. The apparatus of claim 5 , wherein the sidelink control information indicates a physical sidelink feedback channel (“PSFCH”) format for feedback, wherein the apparatus receives the negative acknowledgement feedback from the at least one UE using the indicated PSFCH format.

8. The apparatus of claim 1 , wherein receiving the negative acknowledgement feedback comprises receiving best beam information from the at least one UE, wherein sending selectively beamformed retransmission of the groupcast data comprises retransmitting on one or more beams selected using the best beam information.

9. The apparatus of claim 1 , wherein receiving negative acknowledgement feedback comprises receiving location information for the at least one UE, wherein sending selectively beamformed retransmission of the groupcast data comprises retransmitting on one or more beams selected using the location information.

10. The apparatus of claim 1 , wherein the processor implicitly determines a best beam for retransmission based on beam information used by a UE of the set to transmit hybrid automatic repeat request (“HARQ”) feedback for the groupcast data.

11. The apparatus of claim 1 , further comprising a plurality of antenna panels, wherein transmitting the groupcast data comprises transmitting two or more data transmission blocks (“TBs”) simultaneously using different antenna panels.

12. A method of a transmitting remote unit comprising:

transmitting groupcast data via sidelink communication to a set of user equipment devices (“UEs”) using a first cast-type;

receiving negative acknowledgement feedback via sidelink communication from at least one UE of the set, the negative acknowledgement feedback indicating unsuccessful reception of the groupcast data;

determining a retransmission mode based on a number of UEs sending negative acknowledgement feedback, wherein the retransmission mode indicates a cast-type that is different from the first cast-type; and

sending selectively beamformed retransmission of the groupcast data according to the determined retransmission mode.

13. An apparatus comprising:

a transceiver that receives a groupcast signal from a transmitter user equipment device (“Tx UE”) using a first cast-type; and

a processor that:

detects an unsuccessful decoding the groupcast signal;

determines best beam information; and

sends feedback to the Tx UE via the transceiver, the feedback comprising a negative acknowledgement and the best beam information;

wherein:

the transceiver receives a retransmission from the Tx UE using a retransmission mode with a cast-type that is different from the first cast-type; and

the retransmission mode is based on the best beam information and selected from a unicast retransmission with beamforming and a groupcast with partial beam sweeping.

14. The apparatus of claim 13 , wherein the transceiver receives a sidelink reference signal associated with the groupcast signal, wherein the processor derives the best beam information using the sidelink reference signal.

15. The apparatus of claim 13 , wherein the feedback further comprises sidelink channel state information.

16. The apparatus of claim 13 , wherein the transceiver receives sidelink control information (“SCI”) prior to receiving the groupcast signal.

17. The apparatus of claim 16 , wherein the SCI is received together with a sidelink reference signal associated with the groupcast signal.

18. The apparatus of claim 13 , wherein sending feedback to the Tx UE comprises transmitting the feedback towards the Tx UE using beam sweeping.

19. A method of a receiving remote unit comprising:

receiving a groupcast signal from a transmitter user equipment device (“Tx UE”) using a first cast-type;

unsuccessfully decoding the groupcast signal;

determining best beam information; and

sending feedback to the Tx UE, the feedback comprising a negative acknowledgement and the best beam information,

receiving a retransmission from the Tx UE using a retransmission mode with a cast-type that is different from the first cast-type, wherein the retransmission mode is based on the best beam information and selected from a unicast retransmission with beamforming and a groupcast with partial beam sweeping.

20. The method of claim 19 , further comprising receiving a sidelink reference signal associated with the groupcast signal, wherein the best beam information is derived using the sidelink reference signal.