Random access in communication system

1. A first device, comprising:

at least one processor; and

at least one memory including computer program code;

wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to:

determine whether a contention-free random access resource is allocated from a second device for a two-step random access type,

in accordance with a determination that the contention-free random access resource is allocated, compare a target transport block size corresponding to the contention-free random access resource with a first transport block size configured for a first group of random access preambles available for the two-step random access type, and

perform random access to the second device based on a result of the comparison.

2. The device of claim 1 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to perform the random access by:

in accordance with a determination that the target transport block size matches with the first transport block size, performing the random access based on the first group of random access preambles.

3. The device of claim 2 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to perform the random access based on the first group of random access preambles by:

in accordance with a determination that the two-step random access type is used, performing a contention-based random access attempt with the two-step random access type by transmitting a preamble selected from the first group of random access preambles and payload of the first transport block size.

4. The device of claim 2 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to perform the random access based on the first group of random access preambles by:

in accordance with a trigger of switching from the two-step random access type to a four-step random access type, selecting a second group of random access preambles for the four-step random access type corresponding to the first group of random access preambles; and

in accordance with a selection of the second group of random access preambles, performing a further contention-based random access attempt with the four-step random access type by transmitting a preamble selected from the second group of random access preambles.

5. The device of claim 4 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to select the second group of random access preambles by:

in accordance with a trigger of switching from the two-step random access type to a four-step random access type, determining whether the second group of random access preambles is available for the first device; and

in accordance with a determination that the second group of random access preambles is available, selecting the second group of random access preambles, and

wherein the at least one memory and the computer program code are configured to, with the at least one processor, further cause the first device to:

in accordance with a determination that the second group of random access preambles is unavailable, determine that the random access fails, without performing a contention-based random access attempt with the four-step random access type.

6. The device of claim 1 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to perform the random access by:

in accordance with a determination that the target transport block size mismatches with the first transport block size, performing the random access based on a third group of random access preambles available for the two-step random access type.

7. The device of claim 6 , wherein the third group of random access preambles comprises one of 3GPP New Radio Random Access Preambles group A and 3GPP New Radio Random Access Preambles group B for the two-step random access type.

8. The device of claim 1 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to compare the target transport block size with the first transport block size by:

in accordance with a determination that a previous contention-based random access attempt using a preamble selected from the first group of random access preambles fails, determining whether transmission on the contention-free random access resource is available, and

in accordance with a determination that the transmission on the contention-free random access resource is available, comparing the target transport block size with the first transport block size.

9. The device of claim 8 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to perform the random access by:

in accordance with a determination that the target transport block size mismatches with the first transport block size, preventing a contention-free random access attempt from being performed.

10. The device of claim 8 , wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the first device to perform the random access by:

in accordance with a determination that the target transport block size mismatches with the first transport block size, performing a further contention-based random access attempt with the two-step random access type based on the first group of random access preambles and the first transport block size.

11. A method comprising:

determining, at a first device, whether a contention-free random access resource is allocated from a second device for a two-step random access type;

in accordance with a determination that the contention-free random access resource is allocated, comparing a target transport block size corresponding to the contention-free random access resource with a first transport block size configured for a first group of random access preambles available for the two-step random access type; and

performing random access to the second device based on a result of the comparison.

12. The method of claim 11 , wherein performing the random access comprises:

in accordance with a determination that the target transport block size matches with the first transport block size, performing the random access attempt based on the first group of random access preambles.

13. The method of claim 12 , wherein performing the random access based on the first group of random access preambles comprises:

in accordance with a determination that the two-step random access type is used, performing a contention-based random access attempt with the two-step random access type by transmitting a preamble selected from the first group of random access preambles and payload of the first transport block size.

14. The method of claim 12 , wherein performing the random access based on the first group of random access preambles comprises:

in accordance with a trigger of switching from the two-step random access type to a four-step random access type, selecting a second group of random access preambles for the four-step random access type corresponding to the first group of random access preambles; and

in accordance with a selection of the second group of random access preambles, performing a further contention-based random access attempt with the four-step random access type by transmitting a preamble selected from the second group of random access preambles.

15. The method of claim 14 , wherein selecting the second group of random access preambles comprises:

in accordance with a trigger of switching from the two-step random access type to a four-step random access type, determining whether the second group of random access preambles is available for the first device; and

in accordance with a determination that the second group of random access preambles is available, selecting the second group of random access preambles, and

wherein the method further comprises:

in accordance with a determination that the second group of random access preambles is unavailable, determine that the random access fails, without performing a contention-based random access attempt with the four-step random access type.

16. The method of claim 11 , wherein performing the random access comprises:

in accordance with a determination that the target transport block size mismatches with the first transport block size, performing the random access based on a third group of random access preambles available for the two-step random access type.

17. The method of claim 11 , wherein comparing the target transport block size with the first transport block size comprises:

in accordance with a determination that a previous contention-based random access attempt using a preamble selected from the first group of random access preambles fails, determining whether transmission on the contention-free random access resource is available; and

in accordance with a determination that the transmission on the contention-free random access resource is available, comparing the target transport block size with the first transport block size.

18. The method of claim 17 , wherein performing the random access comprises:

in accordance with a determination that the target transport block size mismatches with the first transport block size, preventing a contention-free random access attempt from being performed.

19. The method of claim 17 , wherein performing the random access comprises:

in accordance with a determination that the target transport block size mismatches with the first transport block size, performing a further contention-based random access attempt with the two-step random access type based on the first group of random access preambles and the first transport block size.

20. A non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least:

determining, at a first device, whether a contention-free random access resource is allocated from a second device for a two-step random access type;

in accordance with a determination that the contention-free random access resource is allocated, comparing a target transport block size corresponding to the contention-free random access resource with a first transport block size configured for a first group of random access preambles available for the two-step random access type; and

performing random access to the second device based on a result of the comparison.