Two-step random access procedure in next generation wireless networks

1. A method for a user equipment (UE) for performing a random access (RA) procedure, the method comprising:

receiving, at the UE, at least one broadcast System Information Block (SIB) comprising a two-step RA configuration with a reference signal received power (RSRP) threshold, the at least one broadcast SIB further indicating whether switching from a two-step RA procedure to a four-step RA procedure is allowed;

receiving, at the UE, a four-step RA configuration;

determining whether a detected RSRP exceeds the RSRP threshold once the RA procedure is triggered; and

performing the two-step RA procedure after determining that the detected RSRP exceeds the RSRP threshold by:

transmitting a first message to a base station in a first step of the two-step RA procedure;

determining, after transmitting the first message, whether a second message from the base station in a second step of the two-step RA procedure is received successfully based on an RA response time threshold; and

in response to determining that the second message is received successfully and an attempt to perform a fallback four-step RA procedure in response to the second message is not successful, attempting to retransmit the first message to the base station, wherein the attempt to perform the fallback four-step RA procedure in response to the second message is determined not to be successful by:

transmitting a third message to the base station based on fallback information received in the second message, and

determining that a contention resolution associated with the fallback four-step RA procedure is not successful; and

wherein attempting to retransmit the first message to the base station comprises:

in a case that the at least one broadcast SIB indicates that switching from the two-step RA procedure to the four-step RA procedure is allowed and a first threshold number of retransmissions of the first message has been reached, switching from the two-step RA procedure to the four-step RA procedure, and

in a case that the at least one broadcast SIB indicates that switching from the two-step RA procedure to the four-step RA procedure is not allowed and a second threshold number of retransmissions of the first message has been reached, declaring, to an upper layer of the UE, a failure of the two-step RA procedure.

2. The method of claim 1 , further comprising:

in response to determining that the second message is not received successfully, attempting to retransmit the first message to the base station.

3. The method of claim 2 , wherein determining that the second message is not received successfully comprises determining that the RA response time threshold has expired while the second message is still not received.

4. The method of claim 1 , wherein:

the first message comprises a preamble and a payload, and

the second message comprises at least one response including at least one of contention resolution information associated with the two-step RA procedure and the fallback information.

5. The method of claim 4 , wherein the second message comprises a media access control (MAC) subheader of a MAC sub-Protocol Data Unit (subPDU) having at least one field, wherein the at least one field indicates whether the at least one response contains the contention resolution information associated with the two-step RA procedure for indicating a successful two-step RA procedure or the fallback information for transmitting the third message to the base station.

6. The method of claim 1 , wherein the second message comprises a radio resource control (RRC) message that is a response message to an RRC procedure request indicated in the first message.

7. The method of claim 1 , wherein transmitting the third message to the base station comprises transmitting the third message using an uplink grant indicated in the fallback information of the second message received from the base station.

8. A user equipment (UE) comprising:

one or more non-transitory computer-readable media storing one or more computer-executable instructions for performing a random access (RA) procedure; and

at least one processor coupled to the one or more non-transitory computer-readable media, and configured to execute the computer-executable instructions to cause the UE to:

receive, at the UE, at least one broadcast System Information Block (SIB) comprising a two-step RA configuration with a reference signal received power (RSRP) threshold, the at least one broadcast SIB further indicating whether switching from a two-step RA procedure to a four-step RA procedure is allowed;

receive, at the UE, a four-step RA configuration;

determine whether a detected RSRP exceeds the RSRP threshold once the RA procedure is triggered; and

perform the two-step RA procedure after determining that the detected RSRP exceeds the RSRP threshold by:

transmitting a first message to a base station in a first step of the two-step RA procedure;

determining, after transmitting the first message, whether a second message from the base station in a second step of the two-step RA procedure is received successfully based on an RA response time threshold; and

in response to determining that the second message is received successfully and an attempt to perform a fallback four-step RA procedure in response to the second message is not successful, attempting to retransmit the first message to the base station, wherein the attempt to perform the fallback four-step RA procedure in response to the second message is determined not to be successful by:

transmitting a third message to the base station based on fallback information received in the second message, and

determining that a contention resolution associated with the fallback four-step RA procedure is not successful; and

wherein attempting to retransmit the first message to the base station comprises:

in a case that the at least one broadcast SIB indicates that switching from the two-step RA procedure to the four-step RA procedure is allowed and a first threshold number of retransmissions of the first message has been reached, switching from the two-step RA procedure to the four-step RA procedure, and

in a case that the at least one broadcast SIB indicates that switching from the two-step RA procedure to the four-step RA procedure is not allowed and a second threshold number of retransmissions of the first message has been reached, declaring, to an upper layer of the UE, a failure of the two-step RA procedure.

9. The UE of claim 8 , wherein the at least one processor is further configured to execute the computer-executable instructions to cause the UE to:

in response to determining that the second message is not received successfully, attempt to retransmit the first message to the base station.

10. The UE of claim 9 , wherein determining that the second message is not received successfully comprises determining that the RA response time threshold has expired while the second message is still not received.

11. The UE of claim 8 , wherein:

the first message comprises a preamble and a payload, and

the second message comprises at least one response including at least one of contention resolution information associated with the two-step RA procedure and the fallback information.

12. The UE of claim 11 , wherein the second message comprises a media access control (MAC) subheader of a MAC sub-Protocol Data Unit (subPDU) having at least one field, wherein the at least one field indicates whether the at least one response contains the contention resolution information associated with the two-step RA procedure for indicating a successful two-step RA procedure or the fallback information for transmitting the third message to the base station.

13. The UE of claim 8 , wherein the second message comprises a radio resource control (RRC) message that is a response message to an RRC procedure request indicated in the first message.

14. The UE of claim 8 , wherein transmitting the third message to the base station comprises transmitting the third message using an uplink grant indicated in the fallback information of the second message received from the base station.