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Showing posts with label 2-step RACH. Show all posts
Showing posts with label 2-step RACH. Show all posts

Wednesday, November 25, 2020

NR-U Layer 2 (Candidates)


 Note: NR-Unlicensed is still in the study phase with barely any work done on the TS. It is expected that by 2022 (so Release 18) we may see some specification for the 6GHz band in Europe. As of now, only potential candidates for NR-U implementation can be guessed. 

RACH (4-step, 2-step)

Both 4-step and 2-step RACH will be supported for NR-U. Here 2-step RACH refers to the procedure which can complete contention-based RACH (CBRA) in two steps as explained below. One additional benefit of 2-step RACH is due to less LBT impact with the reduced number of messages. However, in order to alleviate the impact of LBT failures further, additional opportunities for the RACH messages may be introduced, e.g. in time or frequency domain, for both 4-step and 2-step RACH.  The additional opportunities for 4-step RACH will be applicable to both msg1 and msg3.

NR-U will support contention-free RACH (CFRA) and CBRA for both 2-step and 4-step RACH. On SCells, CFRA is supported as a baseline while both CBRA and CFRA are supported on SpCells.

For 4-step RACH, the messages in time order are named as msg1, msg2, msg3, msg4 and for 2-step RACH, they are named msgA and msgB.

A single RACH procedure will be used and thus multiple RACH procedures in parallel will not be supported for NR-U.As a baseline, the random-access response to msg1 will be on SpCell and msg3 is assumed to use a predetermined HARQ ID.

In legacy RACH, the counters for preamble transmission and power ramping are increased with every attempt. In NR-U, power ramping is not applied when preamble is not transmitted due to LBT failure. This will require an indication from the physical layer to the MAC. In addition, ra-ResponseWindow is not started when the preamble is not transmitted due to LBT failure.

It is assumed that ra-ContentionResolutionTimer may need to be extended with larger values to overcome the LBT impact.

For 2-step RACH, the msgA is a signal to detect the UE and a payload while the second message is for contention resolution for CBRA with a possible payload. msgA will at least include the equivalent information which is transmitted in msg3 for 4-step RACH.

As a baseline, all the triggers for 4-step RACH are also applicable to 2-step RACH; however further analysis is needed on SI request and BFR as well as how timing advance and grants can be obtained for msgA.

The contention resolution in 2-step RACH will be performed by including a UE identifier in the first message which is echoed in the second message. The type of UE identifier(s) is FFS.

Fall-back from 2-step RACH to 4-step RACH will be supported. The fallback after msgA transmission is feasible only if detection of the UE without the decoding of the payload is possible and thus relies on such support at the physical layer.

If 2-step RACH is used for initial access, the parameters for 2-step RACH procedure including resources for msgA will be broadcasted.

NOTE: 2-step RACH if applied to licensed operation would not take into account LBT.

 MAC (except RACH)

For scheduling request (SR), a prohibit timer as in NR licensed can be used. However, this should not prevent the UE from attempting to transmit an SR again if the triggered SR was not transmitted due to LBT failure.

 Other

For channel access and transmissions in NR-U the mechanisms and associated signaling adopted by LTE LAA (e.g. standardized QCI to access priority mapping for DL and UL, how access priority per logical channel is determined for scheduled UL and AUL transmissions etc) are used as the baseline. Any changes due to new physical layer design and channel access mechanisms for NR-U (e.g. introduction of PRACH, support of FBE) can also be introduced.

In addition, access priority for control signaling (transmissions over SRBs) over unlicensed carriers should be introduced for stand-alone and DC NR-U. In this case, it is assumed that control signaling will have the highest access priority.

Related:

  1. NR-U Inactive and Idle procedures (Candidates)
  2. NR-U Control plane (Candidates)

  3. NR-U Physical layer channel designs(Candidates)

  4. NR-U Physical Frame structure (Candidates)

  5. NR-U Channel Access Schemes (Candidates)

Saturday, November 7, 2020

2-step RACH for NR


The Rel-16 2-step RACH for NR achieves the following objectives:

- A simplified random access procedure was developed. This reduces the number of interactions between the UE and network during the connection setup and connection resume, thereby enabling a lower control plane latency. In case of connected mode, a small amount of data can be sent via 2-step RACH procedure thus also enabling a lower latency for UL UP data for connected mode UEs.

- Channel structure of transmitting PRACH and PUSCH in one step (i.e. without an intermediate message from the network) was developed. The PRACH and PUSCH are separated by a pre-configured guard period.

-  The above enhancements are applicable to both licensed spectrum and shared spectrum (i.e. NR-U).


The general procedure of 4-step RACH and 2-step RACH are depicted in Figure 1. The first step of 2-step RACH comprises an UL MSGA transmission which includes the equivalent contents of msg1 and msg3 of 4-step RACH. The second step of 2-step RACH is a DL MSGB reception which includes the equivalent content of msg2 and/or msg4 of 4-step RACH, depending on the detection of UL MSGA.

  

4-step and 2-step RACH process

RA type selection

For contention based random access (CBRA), all the triggers for Rel-15 NR 4-step RACH are also applicable to 2-step RACH except when CA is configured, the 2-step RACH is only applicable on PCell. Contention free random access (CFRA) procedure with 2-step RACH is only supported for handover.

The UE selects the type of random access at initiation of the random access procedure based on network configuration:

-   when CFRA resources are not configured, an RSRP threshold is used by the UE to select between 2-step RA type and 4-step RA type;

- when CFRA resources for 4-step RA type are configured, UE performs random access with 4-step RA type;

- when CFRA resources for 2-step RA type are configured, UE performs random access with 2-step RA type.

In case of random access in a cell configured with SUL, UE performs carrier selection (between SUL and NUL) before selecting between 2-step and 4-step RA type.

MSGA structure: PRACH

The MSGA in 2-step RACH comprise a PRACH and a PUSCH. The PRACH resources for 2-step RACH in time/frequency domain can be either shared with 4-step RACH or can be configured to be separate. All the preamble formats and the PRACH configuration indexes defined in Rel-15 and in Rel-16 NR-U and TEI can be used. In case of shared time domain PRACH resources between 4-step RACH and 2-step RACH, different preambles are allocated to differentiate the RA types.  The mapping between SSB and PRACH occasion reuses that for 4-step RACH.

MSGA structure: PUSCH

2-step RACH uses a specified mapping rule to determine the PUSCH resource of MSGA that is associated with the selected PRACH resource. Each PRACH slot is mapped to a number of PUSCH occasions with associated DMRS resource, once the UE selects a preamble in a PRACH occasion, the corresponding PUSCH occasion and DMRS resource can be determined by a predefined mapping order.

MSGB

After MSGA transmission, the UE monitors the downlink for a response from the network within a configured window. This response from the network is called the MSGB. The contents of MSGB depend on whether or not the gNB is able to successfully detect both the PRACH and the PUSCH parts.

-              If the PRACH is detected but the decoding of PUSCH fails, network will include a fallback indication in MSGB and the subsequent UE procedure will be similar to that for a UE monitoring msg2 in the 4 step RACH.

-              If both preamble and PUSCH are decoded, network will include a successRAR and reception of this at the UE completes the contention resolution. HARQ feedback is enabled for the successful reception of the successRAR.

 

References

[1]                        RP-200085, Revised WID on 2 step RACH for NR

[2]                        RP-200622, Status report for WI - NR 2-step RACH