NR-U Physical Frame structure (Candidates)
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.
Physical layer aspects
NR-U supports both Type-A and Type-B mapping already supported in NR.
Initial active DL/UL BWP is approximately
20MHz for 5GHz band, though the final value will be quantized to number of
PRBs. Initial active DL/UL BWP is approximately 20MHz for 6GHz band if similar
channelization as 5GHz band is used for 6GHz band.
Frame structure
Single and multiple DL to UL and UL to DL switching points within a shared gNB COT is identified to be beneficial and can be supported.
For NR-U DL operation, it is identified
that being able to operate all DL signal/channels with the same numerology for
a carrier and at least for intra-band CA on serving cells on unlicensed bands
has at least the following benefits (at least for standalone operation)
- Lower
implementation complexity (e.g., a single FFT, no switching gaps)
- Lower
specification impact
- No need
for gaps for measurements on frequencies with a configured serving cell in
unlicensed bands
For NR-U UL operation, it is identified
that being able to operate all UL signal/channels (except PRACH) with the same
numerology for a carrier and at least for intra-band CA on serving cells on
unlicensed bands has at least the following benefits:
- Lower
implementation complexity (e.g., a single FFT, no switching gaps)
- Lower
specification impact
- Common
interlace structure
- No need
for gaps for transmission of SRS on a configured serving cell in unlicensed
bands
For unlicensed PCell, the UE assumes single
SSB numerology per band.
It has
been identified to be beneficial for the NR-U design to not require the gNB to
change a pre-determined TBS for a PDSCH transmission depending on the LBT
outcome, at least when the PDSCH is transmitted at the beginning of the gNB's
COT.
The
following options have been identified as possible candidates for PDSCH
transmission in the partial slot at least for the first PDSCH(s) transmitted in
the DL transmission burst. The options are not mutually exclusive.
- Option
1: PDSCH(s) as in Rel-15 NR
- Option
2: Punctured PDSCH depending on LBT outcome
- Option
3: PDSCH mapping type B with durations other than 2/4/7 symbols
- Option
4: PDSCH across slot boundary
In addition to the functionalities provided
by DCI format 2_0 in Rel-15 NR, indication of the COT structure in the time
domain has been identified as being beneficial.
It has
been identified to be beneficial for the NR-U design to not require the UE to
change a granted TBS for a PUSCH transmission depending on the LBT outcome.
The
following options have been identified as possible candidate at least for the
first PUSCH(s) transmitted in the UL transmission burst.
- Option 1:
PUSCH(s) as in Rel-15 NR
- Option
2: Multiple starting positions in one or multiple slot(s) are allowed for
PUSCH(s) scheduled by a single UL grant (i.e., not a configured grant) and one
of the multiple PUSCH starting positions can be decided depending on LBT
outcome.
For above options, the ending position of the PUSCH is fixed as indicated by the UL grant.
It has been identified that FBE operation for the scenario where it is
guaranteed that LBE nodes are absent on a long term basis (e.g., by level of
regulation) and FBE gNBs are synchronized can achieve the following: Ability to use frequency reuse factor 1; Lower complexity for
channel access due to lack of necessity to perform random backoff.
It is noted that this does not imply that
LBE does not have benefits in similar scenarios although there are differences
between the two modes of operation. It is also noted that FBE may also have some
disadvantages compared to other modes of operation such as LBE, e.g., a fixed
overhead for idle time during a frame.
For wideband operation for both DL and UL,
- Bandwidth larger than 20 MHz can be
supported with multiple serving cells.
- NR-U should support that a serving cell can
be configured with bandwidth larger than 20 MHz.
For DL operation, the following options for BWP-based
operation within a carrier with bandwidth larger than 20 MHz can be considered.
- Option 1a: Multiple BWPs configured, multiple
BWPs activated, transmission of PDSCH on one or more BWPs
- Option 1b: Multiple BWPs configured,
multiple BWPs activated, transmission of PDSCH on single BWP
- Option 2: Multiple BWPs can be configured,
single BWP activated, gNB transmits PDSCH on a single BWP if CCA is successful
at gNB for the whole BWP
- Option 3: Multiple BWPs can be configured,
single BWP activated, gNB transmits PDSCH on parts or whole of single BWP where
CCA is successful at gNB
For UL
operation, the following options for BWP-based operation within a carrier with
bandwidth larger than 20 MHz can be considered.
- Option 1a: Multiple BWPs configured,
multiple BWPs activated, transmission of PUSCH on one or more BWPs
- Option 1b: Multiple BWPs configured,
multiple BWPs activated, transmission of PUSCH on single BWP
- Option 2: Multiple BWPs can be configured,
single BWP activated, UE transmits PUSCH on a single BWP if CCA is successful
at UE for the whole BWP
- Option 3: Multiple BWPs can be configured,
single BWP activated, UE transmits PUSCH on parts or whole of single BWP where
CCA is successful at UE
CCA is declared to be successful or
not in multiples of 20 MHz.
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