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NR-Unlicensed Standardization Updates for Fair Coexistence with 802.11

This presentation provides updates on the standardization of NR-Unlicensed with a focus on fair coexistence with 802.11 (Wi-Fi). Topics include 25us LBT, response from 3GPP RAN1, difference between NR-U and LAA DRS, and simulation results evaluating the impact on Wi-Fi messages.

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NR-Unlicensed Standardization Updates for Fair Coexistence with 802.11

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  1. Authors: 3GPP RAN1 status on NR-Unlicensed Date:2019-03-13

  2. Abstract This presentation provides updates from the latest 3GPP RAN1 meeting (RAN1#96) on the standardization of NR-Unlicensed, with a focus on fair coexistence with 802.11.

  3. Outline The presentation discusses status of the following topics: 25us LBT for transmission of control messages LS from ETSI-BRAN to 3GPP RAN1 and response from 3GPP RAN1 Difference between NR-U DRS and LAA DRS Simulation results evaluating the impact of NR-U DRS on equivalent Wi-Fi messages (beacons) and on Wi-Fi voice. Discussion on a common preamble between NR-U and Wi-Fi in 5 GHz and 6 GHz. A Proposal for IEEE 802.11 to send an LS to the 3GPP RAN Plenary requesting it to continue the discussions on a common preamble

  4. Use of Short-LBT: LS from ETSI-BRAN to 3GPP RAN1 (1) The LS (R1-1900157) covered the following salient topics Conditions to be satisfied by a Responding Device when sharing a COT won by an Initiating Device Gap between successive transmissions from a single responding device shall be <= 16us In a Paused COT, the minimum gap before the first transmission of a responding device shall be >= 100us. Proposal received by ETSI-BRAN to remove the use of no-LBT and reduce the use of short LBT from 5% to 1% airtime per device, when such transmissions happen outside of a COT. Note that (H)ARQ feedbacks within SIFS of the data transmission are considered to be inside a COT by ETSI-BRAN and are hence, are not a part of this discussion. There was a long discussion on topic 1) while topic 2) was discussed over email. On 1): It is known that the 3GPP specs do not specify the restrictions/conditions specified by ETSI-BRAN in 1 a) and b). However, the 3GPP spec doesn’t prevent a base station vendor to implement these restrictions in a region specific manner. So, instead of directly stating that the 3GPP specs do not specify any of the conditions in 1 a) and b), the LS response from RAN1 states that “it is RAN1’s understanding that for the case where more than one grant is provided to a responding device, 3GPP Rel-14/15 LTE-LAA specifications allow behaviour that is consistent with the provided clarifications and this is also expected to be the case for NR-U specifications currently being developed”.

  5. Use of Short-LBT: LS from ETSI-BRAN to 3GPP RAN1 (2) On 2) i.e. the proposal to remove the use of no-LBT and reduce the use of short LBT from 5% to 1% airtime per device, when such transmissions happen outside of a COT, the email discussions showed the following: There seemed to be consensus on removing no-LBT. There was no consensus that the short-LBT clause reduce the airtime from 5% to 1% per device. There seemed to be consensus that such a change, if any, should not be applied retrospectively to LAA. However, the LS response from 3GPP RAN1 (which was decided in the final hours of the NR-U online meeting) states that there is no consensus even on removing no-LBT “There is no consensus on the proposal to (1) ban the use of no LBT transmissions and (2) to restrict the use of short LBT transmissions so that it can only be used 1% of time rather than 5% as currently defined in the clause on Short Control Signalling Transmissions. There is also no consensus on limiting NR-U DRS transmissions to a 1% limit.”

  6. Use of Short-LBT: Difference between NR-U DRS and LAA DRS (3) Note: Even though it is being claimed by 3GPP RAN1 that the Short-Control-Signaling clause in ETSI-BRAN will be used only for DRS transmissions by LAA and NR-U, this is not a fully accurate representation of the situation as there are significant points of difference between LAA and NR-U DRSs. The LAA “DRS” and the NR-U “DRS” are the same only in name. This is because, the LAA DRS consists only PSS/SSS/CRS signals, while the NR-U DRS consists of many more signals/messages. The name “DRS” for such messages in NR-U conveys the impression that they are the same as LAA DRS and hence, NR-U “DRS” should also be allowed to use the Short-Control-Signaling clause in ETSI-BRAN. However NR-U “DRS” is actually a new set of signals/messages unlike the LAA DRS In the agreements, the set of signals were “tentatively referred to as the NR-U DRS” with the note that “it can be called something else in the future” and that it “can include signals and channels that are required for cell acquisition etc. and is not limited only to reference signals” Hence, these signals can include PBCH in addition to PSS/SSS These can also include CSI-RS, RMSI-CORESET(s),RMSI-PDSCH(s) These may also include OSI and paging. Also, Short Control Signaling clause will be used by NR-U not only for transmission of such a “DRS” message but also for some other messages that are time/frequency multiplexed with the “DRS”. This was not the case for LAA DRS. The RAN1 agreement for utilizing the Short Control Signaling clause covers “DRS alone or multiplexed with non-unicast data (e.g. OSI, paging, RAR)” where proponents interpret “non-unicast data” to mean any message or data, whether broadcast or directed to a single UE, that does not have HARQ feedback.

  7. Use of Short-LBTby NR-U: Other developments (4) Broadcom submitted a contribution in RAN1 (R1-1903370) that evaluates the impact of short-LBT by NR-U on latency sensitive Wi-Fi messages such as beacons and on Wi-Fi voice. The evaluations show that, even in a simple simulation scenario, the percentile delays of Wi-Fi beacon/voice increase by 22% to 150% when NR-U uses 25us short-LBT for transmission of DRS with 5% duty cycle compared to when NR-U DRS uses LBT based on exponential backoff with the highest channel access priority class (AC_VO). These are the only evaluations that specifically consider the impact of short-LBT by NR-U on equivalent control messages in Wi-Fi (such as beacons) and on Wi-Fi voice. LBT based on exponential backoff with the highest access priority class is used by Wi-Fi for transmission of all control messages that are equivalent to NR-U DRS. The only exceptions are messages such as Channel Switch Assignment which are not equivalent to NR-U DRS, and it is not yet clear how similar indications will be transmitted by NR-U. It is possible that such messages will be transmitted over the licensed carrier wherever available. An agreement was made in RAN1 to not use short-LBT for transmission of any other messages in the DL or UL, even though there have been proposals from some companies to use short-LBT and even no-LBT for some other messages.

  8. On the use of a common preamble between NR-U and Wi-Fi (1) The common preamble proposal is as follows: In 5 GHz: The preamble is proposed to consist of a prefix of 20us duration consisting ofthe 802.11a preamble followed by a suffix with information/signals specific to NR-U. In 6 GHz: There hasn’t been much discussion for 6 GHz. However, some companies have proposed either the 802.11ax preamble or variations thereof or an alternate design. The proposal is to use such a common preamble as an optional feature in addition to the mandatory Energy Detection. Status: Many companies in 3GPP RAN1 agree that a common preamble between NR-U and Wi-Fi can have significant benefits in terms of improved coexistence between NR-U and Wi-Fi, improved operator control in trading fairness and spectral efficiency in a mixed NR-U/Wi-Fi network, improved power saving etc. There are some other companies, who believe that such a common preamble will be too difficult to implement, even though they accept the in-principle advantages of using such a common preamble. There are also some companies who believe that a common preamble does not have any advantages and that a common Energy Detection (ED) scheme should be used instead. Broadcom presented a contribution (R1-1903371) that discussed the advantages of using a common preamble in 5 GHz along with responses to the salient concerns raised in RAN1 on the use of such a preamble. It also presented schemes to implement such a preamble in NR-U.

  9. On the use of a common preamble between NR-U and Wi-Fi (2) Status (continued): It has been agreed in RAN1 to allow (or to not preclude) the use of 802.11 preambles by implementation. However, there is no consensus on implementing such a common preamble either in 5 GHz or 6 GHz in a standardized manner. The status is captured in the RAN1 meeting notes: A common preamble and a common energy detection threshold between NR-U and other technologies for 5 and 6 GHz was discussed. There is no consensus on these aspects at this stage. There is a risk that the upcoming RAN Plenary next week may decide to close the discussion on a common preamble. Some companies (including AT&T, Orange, Comcast, Cisco, CableLabs and Broadcom) have submitted a contribution to the RAN Plenary requesting that the discussions on a common preamble be allowed to continue for both 5 GHz and 6 GHz. It will be very helpful if IEEE 802.11 too sends an LS to the RAN Plenary requesting the continuance of the discussion on a common preamble in RAN1 and also in the upcoming IEEE – 3GPP coexistence workshop. The LS need not discuss the merits or demerits of the use of a common preamble, but only a request to let the discussion continue in RAN1 and in the coexistence workshop; especially since this is an optional feature for NR-U and NR-U standardization is slated to complete in RAN1 only by Dec/2019.

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