1. WF on UL Transmission for Difficult Band Combinations in LTE-NR DC
RAN4#84bis, Dubrovnik Oct
Agenda Item:
R4-17xxxxx
WF on UL Transmission for Difficult Band Combinations in LTE-NR DC
AT&T, DT, Softbank, Sprint, Vodafone, Verizon

2. Motivation
Dual connectivity between LTE and NR is a key feature used by the Option 3X NSA architecture. This requires separate UL transmission on the MCG and SCG to deliver the UCI independently to each one of them.
UL PUCCH on SCG and MCG delivers the HARQ ACK/NACK, CSI measurements
UL PUSCH on SCG and MCG allows for the UL bearer to be split.
In some band combinations such as NR on 3.5GHz and LTE on 2GHz there is potential issue of supporting simultaneous UL on both bands due to MSD issues that the UE experiences while transmitting on both the UL frequencies.
Even for these band combinations the problem only happens when the transmit power in both or one of these bands is high. When the transmit power in both of these bands is low then the problem is mitigated significantly and it is possible to sustain simultaneous UL transmission on both the bands
This contribution highlights potential techniques and their pros and cons for addressing this issue.

3. Dual UL is the Baseline for Dual Connectivity (for good reasons)
Dual UL Tx allows the UCI carrying HARQ ACK/NACK to be sent on both the PCell and PSCell simultaneously. This allows the system to use all the HARQ processes on both MCG and SCG resulting on peak data rate on both LTE and NR
When simultaneous UL is not possible peak data rate is impacted as ALL HARQ processes are not usable on both MCG and SCG as shown below.
Dual UL Tx allows for independent timing advance on PCell and PSCell which allows for additional deployment flexibility such as MCG on macro and SCG on pico
When simultaneous UL is not possible having independent timing advanced on both MCG and SCG will not be possible leading to some constraints on the deployment. P1 P2 P3 P4
PCell
In this TDM example only half the HARQ processes are usable on each CG
PSCell
Overlapped transmission
PCell
PSCell

4. Key Observations
Observation 1: Dual UL Tx for LTE-NR DC allows for aggregation of peak data rate on both LTE and NR simultaneously.
Observation 2: Dual UL Tx for LTE-NR DC allows for deployment flexibility such as MCG and SCG on different tiers (macro/pico)
Observation 3: When single UL Tx is used for LTE-NR DC both of these observations are impacted.
Observation 4: Due to IMD dual UL Tx in certain combinations of the two UL frequencies lead to performance and sensitivity degradation at the UE (MSD)
Observation 5: This MSD occurs only in certain parts of the band and not across the entire band.
Observation 6: This MSD is mitigated to a large extent when the transmit power level on both the UL are reduced.
Observation 7: Defining a “difficult band combination” based soleyly on peak Tx power on both UL and across the entire band seems like an overkill and overstating the magnitude of the potential issue

5. RAN Plenary Agreement and Liaison Letter (RP-172085, RP-172100)
Define in RAN4 specs for which band combinations and channel allocations (to the extent specified by RAN4) within that band combination the UE is allowed to indicate that it does not support 2 simultaneous UL tx (this is not implying the granularity of the signalling)
Signalling to be defined to support the 'red text' from RAN4 part of the single tx discussion (i.e. as in RP ):
UE capability indicates that the UE does not allow 2 simultaneous UL transmission for the RAN4 specified channel allocations in a given band combination. If the network chooses to operate the UE in a way that is not consistent with this capability indication then the UE behavior is not specified and the UE might not meet the performance criteria
To RAN WG4 and RAN WG2 groups.
ACTION: RAN asks RAN WG2 and RAN WG4 groups to take the above into account and complete the work. The approval of single UL support in LTE – NR DC and related specification text in text proposals, draft TS proposals (presented for approval) and CRs (e.g. LTE specifications) will be done as a package in RAN#78.

6. RAN2 Agreement (this week)
Current Agreement in RAN2:
RAN2 will define capability signaling per problematic case (as defined in RAN4) to indicate whether the UE supports 2 simultaneous UL transmission for the problematic case . FFS how this is structured in RAN2 (e.g. per UE bitmap, or per BC bits etc.)

7. Proposals for RAN4 Work
Proposal 1: While defining difficult combination RAN4 will take the following into consideration
Difficult combination based ONLY on the impact to Pcell of MCG and SCG
RF architecture and the isolation between the various Tx and Rx paths in determining a difficult combination
Proposal 2: RAN4 defines a difficult combination based on ‘channel allocation’ instead of the entire bands.
A ‘channel allocation’ is defined as X MHz of contiguous spectrum within a band.
Different ‘channel allocations’ DO NOT overlap
The value of X is band/operator specific
The exact value of X is FFS.
Proposal 3: RAN4 defines a power threshold PTh for a given difficult ‘channel allocation’ combinations:
PTh is defined such that when power in the 2 UL transmission becomes greater than this, MSD issues become pertinent.
PTh depends on the band combinations and the exact value and granularity of PTh is FFS