Beam selection and refinement during a random access channel (RACH) procedure

1. A method for wireless communication by a node B (NB), comprising:

transmitting, by the NB, a first one or more directional beams to enable a user equipment (UE) to select a first beam;

receiving, by the NB, a message via the first beam from the UE as part of a random access channel (RACH) procedure; and

transmitting, by the NB, a second one or more directional beams for beam refinement during the RACH procedure, wherein the second one or more directional beams enable the UE to switch to a second beam, wherein the second beam is a narrower beam within the first beam.

2. The method of claim 1 , wherein the second one or more directional beams for beam refinement comprises a new radio reference signal (NR-SS).

3. The method of claim 1 , wherein the second one or more directional beams for beam refinement are transmitted after receiving at least one of a Physical Random Access Channel (PRACH) preamble or Radio Resource Connection (RRC) request associated with the RACH procedure from the UE.

4. The method of claim 1 , wherein:

transmitting the first one or more directional beams comprises transmitting a beam reference signal (BRS) during the RACH procedure; and further comprising:

receiving an indication that a signal strength associated with the BRS is below a threshold value, wherein

in response to the indication, transmitting the second one or more beams for beam refinement.

5. The method of claim 1 , further comprising:

receiving an indication from the UE of more than one beam of similar strength from a cell associated with the NB during the RACH procedure; and

in response to the indication, transmitting the second one or more beams for beam refinement.

6. The method of claim 1 , further comprising:

determining an uplink signal strength associated with the received message of the RACH procedure is below a threshold value; and

in response to the determination, transmitting the second one or more beams for beam refinement.

7. The method of claim 1 , further comprising:

receiving the message as part of the RACH procedure from multiple receive beams associated with a similar signal strength; and

in response to receiving the message, transmitting the second one or more beams for beam refinement.

8. The method of claim 1 , wherein the message received via the first beam from the UE as part of the RACH procedure comprises a request for the second one or more directional beams for beam refinement.

9. The method of claim 1 ,

wherein transmitting the second one or more directional beams signal for beam refinement comprises transmitting multiple measurement reference signal (MRS)-based beams; and further comprising:

receiving a report, from the UE, comprising at least one of a MRS reference signal receive power (RSRP) or MRS ID associated with each of the MRS-based beams.

10. The method of claim 9 , further comprising:

instructing the UE to switch to one of the MRS-based beams for the RACH procedure based, at least in part, on the received report.

11. The method of claim 1 , further comprising:

after transmitting the second one or more directional beams for beam refinement, receiving a message as part of the RACH procedure via the second beam.

12. The method of claim 1 , wherein the second one or more directional beams for beam refinement comprises a directional reference signal beam.

13. A method for wireless communication by a user equipment (UE), comprising:

receiving, by the UE, a first one or more directional beams to enable the UE to select a first beam;

transmitting, by the UE to a node B (NB), a message via the first beam as part of a random access channel (RACH) procedure; and

receiving, by the UE from the NB, a second one or more directional beams for beam refinement during the RACH procedure, wherein the second one or more directional beams enable the UE to switch to a second beam, wherein the second beam is a narrower beam within the first beam.

14. The method of claim 13 , wherein the second one or more directional beams for beam refinement comprises a new radio reference signal (NR-SS).

15. The method of claim 13 , wherein the second one or more directional beams for beam refinement are received after one of:

transmitting at least one of a Physical Random Access Channel (PRACH) preamble or Radio Resource Connection (RRC) request to the NB as part of the RACH procedure.

16. The method of claim 13 , wherein:

receiving the first one or more directional beams comprises receiving a beam reference signal (BRS) during the RACH procedure; and further comprising:

transmitting an indication that a signal strength associated with the BRS is below a threshold value, wherein

in response to the indication, receiving the second one or more directional beams for beam refinement.

17. The method of claim 13 , further comprising:

transmitting an indication of more than one beam of similar strength from a cell associated with the NB during the RACH procedure; and

in response to the indication, receiving the second one or more directional beams for beam refinement.

18. The method of claim 13 , wherein a signal strength of the transmitted message via the first beam is below a threshold value.

19. The method of claim 13 , further comprising:

transmitting the message as part of the RACH procedure from multiple transmit beams associated with a similar signal strength; and

in response to the transmitted message, receiving the second one or more directional beams for beam refinement.

20. The method of claim 13 , wherein the message transmitted via the first beam as part of the RACH procedure comprises a request for the second one or more directional beams for beam refinement.

21. The method of claim 13 ,

wherein receiving the second one or more directional beams for beam refinement comprises receiving multiple measurement reference signal (MRS)-based beams; and further comprising:

measuring the multiple MRS-based beams; and

transmitting a report, to the NB, comprising at least one of a MRS reference signal receive power (RSRP) or MRS ID associated with each of the MRS-based beams.

22. The method of claim 21 , further comprising:

receiving, from the NB, an instruction to switch to one of the MRS-based beams for the RACH procedure based, at least in part, on the transmitted report.

23. The method of claim 13 , further comprising:

after receiving the least second one or more directional beams for beam refinement, transmitting a message as part of the RACH procedure via second beam.

24. The method of claim 13 , wherein the second one or more directional beams for beam refinement comprises a directional reference signal beam.

25. An apparatus for wireless communication by a node B (NB), comprising:

means for transmitting, by the NB, a first one or more directional beams to enable a user equipment (UE) to select a first beam;

means for receiving, by the NB, a message via the first beam from the UE as part of a random access channel (RACH) procedure; and

means for transmitting, by the NB, a second or one more directional beams for beam refinement during the RACH procedure, wherein the second one or more directional beams enable the UE to switch to a second beam, wherein the second beam is a narrower beam within the first beam.

26. The apparatus of claim 25 , wherein the second one or more directional beams for beam refinement comprises a new radio reference signal (NR-SS).

27. The apparatus of claim 25 , wherein the second one or more directional beams for beam refinement are transmitted after receiving at least one of a Physical Random Access Channel (PRACH) preamble or Radio Resource Connection (RRC) request associated with the RACH procedure from the UE.

28. An apparatus for wireless communication by a user equipment (UE), comprising:

means for receiving, by the UE, a first one or more directional beams to enable the UE to select a first beam;

means for transmitting, by the UE to a node B (NB), a message via the first beam as part of a random access channel (RACH) procedure; and

means for receiving, by the UE from the NB, s second one or more directional beams for beam refinement during the RACH procedure, wherein the second one or more directional beams enable the UE to switch to a second beam, wherein the second beam is a narrower beam within the first beam.

29. The apparatus of claim 28 , wherein the second one or more directional beams comprises a new radio reference signal (NR-SS).

30. The apparatus of claim 28 , further comprising:

means for transmitting an indication of more than one beam of similar strength from a cell associated with the NB during the RACH procedure; and

in response to the indication, means for receiving the second one or more directional beams for beam refinement.