1. Discussion on TB-ack/CBG-Nack for initial transmissions
3GPP TSG RAN WG1 NR#3
Nagoya, Japan,
R1-171xxxx
Discussion on TB-ack/CBG-Nack for initial transmissions
ZTE, Sanechips, [MediaTek, AT&T, Sequans, Interdigital,KT, Wilus, HTC],…

2. Background (1/3) – Agreement from RAN1#90
In RAN#90 it has been agreed for a CBG based retransmisison, that CBG ack is reported when the same CBG has been decoded correctly
It remained FFS how to handle the case when TB CRC check is not passed while all CBG´CRC check have passed
It also remained unspecified how to handle the feedback for the initial transmission when CBGs are configured
Agreements:
For single CW case with CBG based retransmission for the semi-static codebook with HARQ-ACK multiplexing, at least following is supported for the HARQ-ACK composition and mapping per TB
HARQ-ACK codebook includes HARQ-ACK corresponding to all the CBGs (including the non- scheduled CBG(s))
At least followings are supported
HARQ-ACK payload size is the same with the configured number of CBGs
Each HARQ-ACK bit corresponds to each CBG
FFS payload size reduction
ACK is reported for a CBG if the same CBG has been successfully decoded
FFS how to handle the case if TB CRC check is not passed while CB CRC check is passed for all the CBs

3. Background (2/3)
In RAN1 adhoc#2, several options about the relationship between TB-ack/nack and CBG ack/nack were up for agreement until July 13th
From Chairman notes RAN1_NRAH2_final, for agreement
When CBG-based retransmission is configured, TB-level HARQ-A/N is supported and at least following options can be considered for down- selection in RAN1#90.
Option 1. Add 1 bit upon CBG-level HARQ-ACK bits
Option 2. Use all NACK of CBG-level HARQ-ACK bits
Option 3. Use different PUCCH format or PUCCH resource
No progress was made on the above topic, it needs to be discussed again

4. Background (3/3) Observations:
CBGs are an important tool to ensure the eMBB performance
RAN1 should ensure that CBG transmission can be used effiently
We need to specify the UE behavior when TB CRC has failed but all CB CRC checks have been correct
For assumed BLER with 10% it would be inefficient to report CBG-acks évery thime the initial transmission has succesful
A TB-ack is sufficient in these cases.
TB-level A/N can use 1-2 bits payload with low power, leading to UE power savings and uplink interference reduction

5. Proposals for discussion
When CBG-based retransmission is configured, the UE can be configured to support TB-level HARQ-A/N
When both TB and CBG level HARQ A/N are configured
In case TB CRC check failed but all CB CRC checks passed, the UE uses TB-level A/N for feedback (solves FFS from previous agreement)
In case TB CRC check passed but some CB CRC failed, the UE uses CBG-level HARQ A/N
In case TB CRC check failed and one/some CB CRC checks passed, the UE uses CBG- level A/N for feedback
In case of TB CRC check passed and all CB CRC checks passed, the UE uses TB- HARQ A/N
Decide upon how to resolve the TB and CBG-level HARQ A/N format and resource allocation.
E.g. down-select between options 1-3 from slid “Background 2/3”