1. 3GPP RAN1 #90 meeting summary on LAA Enhancements
Month Year
doc.: IEEE yy/xxxxr0
September 2017
3GPP RAN1 #90 meeting summary on LAA Enhancements
Date:
Authors:
Shubhodeep Adhikari, Broadcom
John Doe, Some Company

2. September 2017
Overview
In the 3GPP RAN Plenary #75, a new Work Item (WI) on Enhancements to LTE operation in unlicensed spectrum was approved with following objectives [1]:
Specify support for multiple starting and ending positions in a subframe for UL and DL on SCell with Frame structure type 3.
Study, and specify if needed, support for autonomous uplink access with Frame Structure type 3 considering solutions from the L2 latency reduction work item
This presentation covers the status of the following topics related to this WI in the recently concluded RAN1 #90 meeting (21-25 Aug 2017)
Channel access design for LAA Autonomous Uplink
Additional starting/ending positions for an LAA DL subframe
Additional starting/ending positions for an LAA UL subframe
Shubhodeep Adhikari, Broadcom

3. Channel Access for LAA Autonomous UL
September 2017
Channel Access for LAA Autonomous UL
Most of the time allotted to this WI in RAN1#90 was spent on Autonomous UpLink (AUL), with a significant part on channel access. This topic has strong interest in 3GPP and was prioritized above other topics in the WI, since it can reduce the uplink latency (which is relatively high in Rel. 14 eLAA compared to uplink)
Various proposals were made on AUL [2] [3] [4] [5] [6] [7]. Within these contributions, some proposals were made that could result in LAA unfairly gaining a larger share of the unlicensed channel relative to :
Sharing the TXOP acquired by a UE with the eNB so that the eNB does only 25us LBT and transmits to multiple UEs.
No relation between the LBT priority class used by the UE for contending for the channel and the priority class of the data that it transmits.
Use of 25us LBT for AUL transmissions if they lie within a TXOP acquired by the eNB without the corresponding restrictions that have been specified for Scheduled LAA UL
The UE to assume that an AUL transmission is correct unless there is feedback from the eNB that it is in error.
However, after various discussions, the WF agreement for channel access design for AUL [8] did not include these proposals
Shubhodeep Adhikari, Broadcom

4. Additional starting symbol positions for LAA DL
September 2017
Additional starting symbol positions for LAA DL
Additional starting positions in LAA DL have a twofold benefit:
Reduce the duration of reservation signals which enables better coexistence with
Increase spectral efficiency of LAA by reducing TXOP wastage.
Several companies proposed additional DL starting positions at symbols 4 and 11 of a subframe [9] [10] [15].
Symbols 4 and 11 were proposed since LTE reference signals are transmitted in these symbols, and so subframes starting at these positions can be more easily detected by an LTE UE.
Another proposal would have allowed DL starting positions at every symbol in an LTE subframe [11]
In principle, this would be the most efficient scheme possible in the current LTE design and also have the best coexistence with
However, the introduction of even a small number of additional starting positions (i.e. at symbols 4 and 11) was opposed by several companies
Shubhodeep Adhikari, Broadcom

5. Additional starting symbol positions for LAA DL [2]
September 2017
Additional starting symbol positions for LAA DL [2]
In [12], it was claimed that any additional starting positions will degrade the subframe detection reliability of the UE and increase UE complexity
However, it is noted that LTE DL has reference signals in two symbols 4 and 11 of every subframe to enable reliable detection at speeds up to 300Km/hr; however an LAA cell is expected to operate at speeds lower than 30 Km/hr. So, if a UE satisfies the LTE design requirements, halving of the number of reference symbols per subframe from symbols 4 and 11 to either symbol 4 or symbol 11, should have no impact on performance at 30 Km/hr.
Some other companies claimed that eNB implementation may not be able to meet the shorter transport block preparation times.
However these views contradicted contributions in previous meetings which proposed introduction of additional starting positions [13] [14]
Given the opposing viewpoints and that most of the meeting time being spent on LAA Autonomous UL, no decision was made on introducing addition starting positions in LAA DL. This topic will be raised again in the next RAN1 meeting (09 – 13 Oct 2017)
Shubhodeep Adhikari, Broadcom

6. Additional starting and ending symbol positions for LAA UL
September 2017
Additional starting and ending symbol positions for LAA UL
Additional starting symbol positions:
The previous 3GPP RAN1 meeting (RAN1#89) had decided to introduce 1 additional starting position at the slot boundary (symbol #7) for LAA UL [16].
The low number of starting positions in LAA UL does not harm coexistence with as LAA UE will not transmit reservation signals.
Additional ending symbol positions:
Currently an LAA/LTE transmission can only end at subframe (1ms) boundaries.
So, an LAA UE may have to transmit lower priority data or padding to align the end of transmission to a subframe boundary. This may harm coexistence with
It also lowers spectral efficiency of LAA due to TXOP wastage (by 0.5ms on average), since LAA can win the channel at any time but transmit only up to a 1ms boundary.
A proposed was made for additional UL ending positions at symbols 3 and 10 [17] [10].
Symbols 3 and 10 were proposed since LTE UL reference signals are transmitted in these symbols, and so subframes ending at these positions can be more easily detected by an LTE eNB.
However, this topic was not discussed in RAN1#90 due to lack of time. It will be discussed again at the next meeting.
Shubhodeep Adhikari, Broadcom

7. September 2017
Next steps
IEEE should continue to monitor this 3GPP WI, since its outcomes may significantly impact coexistence fairness between unlicensed LTE and
Note the starting and ending position topic directly relates to the recent LS exchanges between and 3GPP, most recently the LS from 3GPP RAN1 in R
“Furthermore, RAN1 notes that RAN plenary has approved a new work item (RP ) on LAA in Rel-15 with the specific objective of specifying multiple start positions for initial and end partial subframes for both DL and UL to enable more efficient medium occupancy and reduce the overhead.”
RAN1 notes that all the specified initial and end DL partial subframes in Rel-13 are optional for UE implementation. Rel-14 does not specify any UL end partial subframes. The discussion of mandatory and optional features for Rel-15 (and for any release in general) happens at the end of each release at which time further information could be provided on which start and end positions for DL and UL transmissions are mandatory for Rel-15 LAA capable UEs.
Shubhodeep Adhikari, Broadcom

8. September 2017
References
[1] RP , Work Item on Enhancements to LTE operation in unlicensed spectrum [2] R , Channel access mechanism for autonomous UL access, Qualcomm [3] R , On Channel Access for AUL, Ericsson [4] R , Consideration on channel access mechanism for autonomous uplink access, Intel [5] R , Channel access for autonomous UL access on unlicensed Scell, Huawei [6] R , On channel access for autonomous UL access, Nokia [7] R , Channel access procedure for autonomous UL access, LG [8] R , WF on channel access for Autonomous UL in FeLAA [9] R , Discussion on additional starting positions for DL in LAA, Broadcom [10] R , WF on additional DL and UL start and end positions in FeLAA, Broadcom, CableLabs, Brocade, BlackBerry, Comcast, Sequans [11] R , Multiple starting and ending positions in a subframe in UL, Qualcomm [12] R , On the additional downlink starting positions in a subframe for FS3, Intel [13] R , Multiple starting positions for LAA in DL, Ericsson [14] R , On multiple DL starting positions in a subframe for feLAA, Nokia [15] R , Discussion on multiple starting positions for LAA DL, LG [16] RAN1 89 Chairman’s notes [17] R , Discussion on additional starting and ending positions for UL in LAA, Broadcom
Nihar Jindal, Broadcom