NR-U Physical layer channel designs(Candidates)
For physical layer channel design, NR design will be used as baseline, and the following potential design changes are to be studied to support the following channels/signals in NR-U.
- PDCCH/PDSCH
- PUCCH/PUSCH
- PSS/SSS/PBCH
- PRACH
- DL and
UL reference signals applicable to the operational frequency range
For SS/PBCH block transmission, extended CP
is not supported for NR-U operation.
For PSS/SSS/PBCH transmission, NR-U should have a signal that
contains at least SS/PBCH block burst set transmission. The design of this
signal should consider the following characteristics specific to unlicensed
band operation:
- There
are no gaps within the time span the signal is transmitted at least within a
beam
- The
occupied channel bandwidth is satisfied (although this may not be a
requirement)
- Strive
to minimize the channel occupancy time of the signal
- Characteristics
that may facilitate fast channel access
Inclusion of the CSI-RS and
RMSI-CORESET(s)+PDSCH(s) (carrying RMSI) associated with SS/PBCH block(s) in
addition to the SS/PBCH burst set in one contiguous burst (referred to as the
NR-U DRS) can be beneficial for
- Meeting
OCB requirement
- Compacting
signals in time domain to limit the required number of channel access and for
short channel occupancy
- Support
of stand-alone NR-U deployments
- Support
of automatic neighbour relations (ANR) functionality in an NR-U deployment
- Resolution
of PCI confusion in an NR-U deployment
The transmission of additional signals such
as OSI and paging within the NR-U DRS is allowed and can be beneficial.
Support of Pattern 1 is recommended for
multiplexing of SS/PBCH block(s) and CORESET(s)#0 in NR-U, where Pattern 1 is
understood as CORESET#0 and an SS/PBCH block occuring in different time
instances, and the CORESET#0 bandwidth overlaping with the transmission
bandwidth of the SS/PBCH block.
As one element to facilitate a NR-U DRS
design without gaps in the time domain, the CORESET#0 configuration(s) and/or
Type0-PDCCH common search space configuration(s) may need enhancements compared
to NR Rel-15, such as additional time domain configurations of the common
search space(s).
The detection of a gNB's transmission burst
by the UE has been studied, and concerns on the UE power consumption required
for Tx burst detection e.g. if the UE needs to frequently detect/monitor the
PDCCH have been raised. The proposals that have been made by contributions
regarding these topics include existing NR signal(s) with potential
enhancement(s), a channel such as PDCCH with potential enhancement(s), and the
802.11a/802.11ax preamble with potential enhancement(s); consensus was not
achieved on any of these proposals. The detection/decoding reliability of each
of the proposals has not been sufficiently evaluated for a complete evaluation
of the proposals against each other. The power consumption and
detection/decoding complexity of each of the proposals have not been
sufficiently evaluated for a complete evaluation of the proposals against each
other. The relation of a proposal with C-DRX and/or measurement gap(s) may need
further consideration when specifications are being developed.
Compared to NR Rel-15, it has been
identified to be beneficial if the time domain instances in which the UE is
expected to receive PDCCH can change dynamically, e.g. by implicit
determination related to the gNB's COT, or explicitly signalled by the gNB.
For UL waveform for PUSCH, PUCCH, and PRACH,
it has been identified that an interlaced waveform can have benefits in some scenarios including link budget limited cases with given PSD constraint, and as one
option to efficiently meet the occupied channel bandwidth requirement.
On the other hand, it is RAN1's understanding that
the temporal allowance of not meeting occupied channel bandwidth by regulation
can be exploited if the minimum bandwidth requirement, e.g., 2 MHz, is
satisfied. Therefore, a waveform contiguous in frequency may be
adequate in some scenarios, which implies that Release 15 NR contiguous allocation designs can be used for NR-U as well.
Support for Rel-15 NR PUSCH can be considered.
However, it has been identified that block-interlaced based PUSCH can be
beneficial.
Support for Rel-15 NR PUCCH formats can be
considered, however, not necessarily all Release 15 NR PUCCH formats are
applicable to NR-U. It has been identified that legacy PUCCH formats PF2 and
PF3 are beneficial for NR-U for the scenario of contiguous allocations due to
the fact that they may be configured with bandwidth that meets the minimum
temporal allowance of 2 MHz (12/6/3 PRBs for 15/30/60 kHz SCS). It has been identified
that legacy PUCCH formats PF0/1/4 are not well-suited for NR-U for the scenario
of contiguous allocations since they support only single PRB.
When new block
interlace waveform for PUCCH is to be defined, it is beneficial to use the same
block interlace structure for PUCCH and PUSCH.
It has been identified that enhancement of one or more legacy PUCCH
formats is feasible to support block interlaced PUCCH transmission. There is
consensus that enhanced PUCCH with both short and long duration is beneficial
for NR-U; however, no consensus has been achieved about which legacy PUCCH
format(s) should be the starting point for an enhanced PUCCH design. Some
sources suggest introducing just one or two new enhanced PUCCH formats, while
other sources suggest enhancing all or almost all legacy PUCCH formats
(PF0,1,2,3,4). Regardless of which format(s) is(are) chosen as a starting point
for enhancement, the following common aspects have been identified as important
to consider in the detailed design of the enhanced PUCCH format(s) when
specifications are developed:
- Flexible
number of OFDM symbols
- Short
duration, e.g., 1 or 2 OFDM symbols
- Long
duration, e.g., 4 – 14 OFDM symbols
- Flexible
UCI payload
- Small
payload, e.g., 1 or 2 bit
- Larger
payloads, e.g., > 2 bits
- Coding
of UCI payload, e.g.,
- Extend
legacy (NR Rel-15) PUCCH encoder to handle small payloads
- Repetition
of coded UCI bits across PRBs of an interlace
- UCI
Codebits over all PRBs, i.e. no repetition coding.
- Number
of supported PUCCH formats
- Support
for user multiplexing of both UCI payload and DMRS on an interlace, e.g.,
- OCCs
- Cyclic
shifts
- FDM
within an interlace
- Multiplexing
method of UCI payload and DMRS, e.g,
- TDM
- FDM
- Mechanism
to control PAPR, e.g.,
- OCC
cycling
- Bit
level processing
- PRB
level processing
- Sequence
hopping
- PUCCH
waveform, e.g.,
- CP-OFDM
- DFT-s-OFDM
- Performance,
e.g.,
- Required
SNR to achieve a target BLER
- Required
SNR to achieve target ACK to NACK rate, NACK to ACK rate and DTX to ACK rate
- Coverage
considering CM/PAPR
Support for Rel-15 NR PRACH formats can be
considered, however, not necessarily all Release 15 NR PRACH formats are
applicable to NR-U. It is RAN1's understanding that certain formats do not meet
the minimum bandwidth requirement by regulation. Exclusion of the support of
certain formats is to be identified.
It is identified that interlaced based
PRACH can be beneficial.
It has been identified that enhancement of one or more legacy PRACH
formats is feasible for NR-U. Four potential design alternatives, including no
interlacing, have been identified for the frequency mapping of PRACH sequences
for NR-U, where consensus on which one(s) to support for NR-U has not yet been
achieved:
- Alt-1:
Uniform PRB-level interlace mapping
- In this
approach a PRACH sequence for a particular PRACH occasion is mapped to all
of the PRBs of one or more of the
interlaces in the PRB-based block interlace structure. Within a PRB, either all
or a subset of REs are used. Different PRACH occasions are defined using an
orthogonal set of PRBs, or an orthogonal set of REs within the PRBs, from one
or more same/different interlaces.
- It has
been identified that a uniform mapping (equal spacing of PRBs) in the frequency
domain produces a zero-autocorrelation zone, of which the duration is inversely
proportional to the frequency spacing between the PRBs.
- Alt-2:
Non-uniform PRB-level interlace mapping
- In this
approach a PRACH sequence for a particular PRACH occasion is mapped to some or
all of the PRBs of one or more of the
interlaces in the same PRB-based block interlace structure used for
PUSCH/PUCCH. Within a PRB, either all or a subset of REs are used. Different
PRACH occasions are defined using an orthogonal set of PRBs, or an orthogonal
set of REs within the PRBs, from one or more same/different interlaces.
- It has
been identified that an irregular mapping (non-equal spacing of PRBs/REs) in
the frequency domain reduces the false peaks in the PRACH preamble
auto-correlation function.
- Alt-3: Uniform
RE-level interlace mapping
- In this
approach, a PRACH sequence for a particular PRACH occasion consists of a "comb-like"
mapping in the frequency domain with equal spacing between all used REs.
Different PRACH occasions are defined by way of different comb offsets.
- Since
this approach does not fit with the common PUSCH/PUCCH interlace structure, one
source suggests that only TDM multiplexing of PUSCH/PUCCH and PRACH should be
supported. Another source suggests that puncturing/rate matching PUSCH/PUCCH
around the used PRACH REs may be used.
- Alt-4:
Non-interlaced mapping
- In this
approach, a PRACH sequence for a particular PRACH occasion is mapped to a
number of contiguous PRBs, same or similar to NR Rel-15.
- Some
sources propose that to fulfill the minimum OCB requirement, that the PRACH
sequence is mapped to a set of contiguous PRBs, and the PRACH sequence mapping
is repeated across the frequency domain, potentially with guard RE(s)/PRB(s)
between repetitions. For each repetition, a different cyclic shift or different
base sequence may or may not be applied.
It has been identified that the long PRACH
sequence length defined in NR Rel-15 (L = 839) is not beneficial for NR-U,
since PRACH formats based on this length are tailored toward large cells not
expected in an NR-U deployment. However, when it comes to shorter sequence
lengths, some sources propose reusing the short sequence length (L = 139)
defined in NR-Rel-15, whereas other sources propose defining new sequence
lengths depending on which of the 4 alternatives above is supported.
It has been identified that the following
common design attributes need to be considered in the detailed design of an
interlaced PRACH waveform for 4-step random access for NR-U when specifications
are developed:
- Multiplexing
of PRACH and PUSCH/PUCCH, considering block interlaced structure used for
PUSCH/PUCCH, e.g.,
- FDM
- TDM
- Supported
PRACH sequence and PRACH sequence length(s)
- PRACH
capacity
- Number
of PRACH preambles per cell
- Number
of root sequences
- Number
of cyclic shifts
- Number
of PRACH occasions
- Maximum
supported Tx power
- PAPR/CM
- Number
of PRACH formats
- Simulation
assumptions for evaluation of performance, e.g.,
- Single
vs. multi-cell assumptions
- Performance
metrics
- Timing
estimation error
- Miss-detection
probability
- False-detection
probability
- False-alarm
probability
For
scenarios in which a block-interlaced waveform is used for PUCCH/PUSCH, it has
been identified that from FDM-based user-multiplexing standpoint it can be
beneficial to have UL channels on a common interlace structure, at least for
PUSCH, PUCCH, associated DMRS, and potentially PRACH
On the
other hand, for scenarios in which a contiguous allocation for PUSCH and PUCCH
is used, it is beneficial to use contiguous resource allocation for PRACH
For
scenarios in which a block-interlaced waveform is used for UL transmission, a
PRB-based block-interlace design has been identified as beneficial at least for
15 and 30 kHz SCS, and potentially for 60 kHz SCS. One identified benefit is
better link budget with given PSD constraint. However, it has been observed
that power boosting gains decrease with increasing SCS. Another identified
benefit is as one option to efficiently meet the occupied channel bandwidth
requirement. Compared with sub-PRB interlace design, the PRB-based
block-interlace design has comparatively less specification impact.
For
sub-PRB block interlace designs, in some scenarios, sub-PRB block interlacing
can be beneficial in terms of power boosting. However, the sub-PRB block
interlace design has at least the following specification impacts: Reference
signal design (e.g., DMRS); Channel estimation aspects; Resource allocation.
Both PRB and sub-PRB interlacing for 60 kHz have been studied. For sub-PRB
interlacing the following aspects have been considered:
- Power
boosting potential depending on resource allocation size
- PUSCH
DMRS configuration aspects
- Channel
estimation performance
- Number
of REs per interlace unit
It has
been identified as beneficial to support a block-interlaced structure in which
the number of interlaces (M) decreases with increasing SCS, and the nominal
number of PRBs per interlace (N) is similar for each SCS (in a given bandwidth)
at least for 15 and 30 kHz SCS, and potentially 60 kHz depending on supported
interlace design.
From a
RAN1 perspective it has been identified that supporting a non-uniform interlace
structure in which the number of PRBs per interlace is allowed to be different
for different interlaces is beneficial from a spectrum utilization point of
view. It is up to RAN4 to investigate whether or not the non-uniform interlace
structure has an impact on MPR/A-MPR requirements for PUSCH.
Within a 20 MHz bandwidth, the following candidate PRB-based interlace designs have been identified where M is the number of interlaces and N is the number of PRBs per interlace in a 20 MHz bandwidth. Where two values are listed for N, it means that some interlaces have one more PRB than others (non-uniform interlace design)
For
carriers with bandwidth larger than 20 MHz, two candidate interlace designs
have been identified:
- Alt-1:
Same interlace spacing for all interlaces regardless of carrier BW. This
alternative uses Point A as a reference for the interlace definition
- Alt-2:
Interlacing defined on a sub-band (20 MHz) basis. (Note: Possible interlace
spacing discontinuity at edges of sub-band).
Additional
candidates have been identified, but consensus has not been achieved, e.g., (1)
for carriers with bandwidth larger than 20 MHz, retain the same number of PRBs
per interlace (N) for all interlaces regardless of carrier BW; (2) Partial
interlace allocation. Detailed design can be further discussed when
specifications are developed taking RF aspects into account.
It has
been identified that support of different numerology candidates at least has the
following specification impacts:
- For
PRB-based block-interlace design for 15, 30, and 60 kHz SCS, the following spec
impacts have been identified: Number of interlaces and number of PRBs per interlace
need to be defined; the resource allocation mechanism needs to be defined;
channel estimation aspects need to be considered, such as impact on PRG. In
addition to the above impact, for sub-PRB-based block-interlace design for 60
kHz SCS, reference signal design (such as DMRS) needs to be revisited and
alternative resource allocation mechanism is needed.
- For NR-U
DRS design for 15 and 30 kHz SCS, the SS/PBCH block time domain pattern is
already supported in Rel-15. For 60 kHz SCS, there is no SS/PBCH block time
domain pattern defined in Rel-15. SS/PBCH block to CORESET configuration tables
(38.213 Section 13) need to be defined as well.
- For PRACH
design for 15, 30, and 60 kHz SCS, signalling mechanism of RACH configuration
indicating PRACH numerology may need modification to support more than two
numerologies for PRACH for NR-U.
It has been identified as beneficial for
NR-U to introduce additional flexibility in configuring/triggering SRS compared
to NR Rel-15. The following candidate enhancements have been discussed; design
details can be further discussed when specifications are developed:
- Additional
OFDM symbol locations for an SRS resource within a slot other than the last 6
symbols
- Interlaced
waveform
- Additional
flexibility in frequency domain configuration
It may be beneficial to apply restrictions
on the use of DFT-s-OFDM in NR-U to avoid significant design efforts specific
to operation in unlicensed spectrum.
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