Random access procedure(s) for radio system

1. A user equipment (UE), comprising:

one or more non-transitory computer-readable media having computer-executable instructions embodied thereon; 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:

initiate a random access (RA) procedure by a medium access control (MAC) layer of the UE for a system information (SI) request triggered by an upper layer of the UE;

select a specific preamble index corresponding to the SI request from a group of RA preambles, the specific preamble index being associated with a specific Radio Network Temporary Identifier (RNTI) corresponding to the SI request;

transmit, to a base station (BS), an RA preamble based on the specific preamble index using the specific RNTI on a physical random access channel (PRACH) resource during the RA procedure;

receive, from the BS, a downlink assignment on a Physical Downlink Control Channel (PDCCH) for the specific RNTI, the downlink assignment scheduling a Physical Downlink Shared Channel (PDSCH);

receive, from the BS, a Random Access Response (RAR) on the PDSCH, the RAR including a MAC protocol data unit (PDU) during the RA procedure, the MAC PDU including a first MAC subheader;

consider the RAR reception successful in a case that the first MAC subheader includes a random access preamble identifier (RAPID) corresponding to the RA preamble;

indicate, by the MAC layer of the UE, successful reception of the RAR for the SI request to the upper layer of the UE after the RAR reception is considered successful; and

consider the RA procedure successfully completed in a case that the MAC PDU includes no MAC RAR corresponding to the first MAC subheader.

2. The UE of claim 1 , wherein the at least one processor is further configured to execute the computer-executable instructions to indicate, by the MAC layer of the UE, to the upper layer of the UE that the SI request is successful.

3. The UE of claim 1 , wherein the RA preamble is configured, by the BS, for the SI request.

4. The UE of claim 1 , wherein the PRACH resource is configured, by the BS, for the SI request.

5. The UE of claim 1 , wherein the SI request is a request for an on-demand system information block (SIB).

6. The UE of claim 1 , wherein the MAC PDU further includes a second MAC subheader that includes a backoff indicator.

7. The UE of claim 1 , wherein the MAC PDU further includes a second MAC subheader and a MAC RAR corresponding to the second MAC subheader.

8. The UE of claim 7 , wherein the MAC RAR includes at least one of a timing advance command, an uplink (UL) grant, and a temporary Cell Radio Network Temporary Identifier (C-RNTI).

9. A method for performing a system information (SI) request by a user equipment (UE), the method comprising:

initiating a random access (RA) procedure by a medium access control (MAC) layer of the UE for the SI request triggered by an upper layer of the UE;

selecting a specific preamble index corresponding to the SI request from a group of RA preambles, the specific preamble index being associated with a specific Radio Network Temporary Identifier (RNTI) corresponding to the SI request;

transmitting, to a base station (BS), an RA preamble based on the specific preamble index using the specific RNTI on a physical random access channel (PRACH) resource during the RA procedure;

receiving, from the BS, a downlink assignment on a Physical Downlink Control Channel (PDCCH) for the specific RNTI, the downlink assignment scheduling a Physical Downlink Shared Channel (PDSCH);

receiving, from the BS, a Random Access Response (RAR) on the PDSCH, the RAR including a MAC protocol data unit (PDU) during the RA procedure, the MAC PDU including a first MAC subheader;

considering the RAR reception successful in a case that the first MAC subheader includes a random access preamble identifier (RAPID) corresponding to the RA preamble;

indicating, by the MAC layer of the UE, successful reception of the RAR for the SI request to the upper layer of the UE after the RAR reception is considered successful; and

considering the RA procedure successfully completed in a case that the MAC PDU includes no MAC RAR corresponding to the first MAC subheader.

10. The method of claim 9 , further comprising indicating, by the MAC layer of the UE, to the upper layer of the UE that the SI request is successful.

11. The method of claim 9 , wherein the RA preamble is configured, by the BS, for the SI request.

12. The method of claim 9 , wherein the PRACH resource is configured, by the BS, for the SI request.

13. The method of claim 9 , wherein the SI request is a request for an on-demand system information block (SIB).

14. The method of claim 9 , wherein the MAC PDU further includes a second MAC subheader that includes a backoff indicator.

15. The method of claim 9 , wherein the MAC PDU further includes a second MAC subheader and a MAC RAR corresponding to the second MAC subheader.

16. The method of claim 15 , wherein the MAC RAR includes at least one of a timing advance command, an uplink (UL) grant, and a temporary Cell Radio Network Temporary Identifier (C-RNTI).