1. Fix the Issue on Number Of HE-SIG-B Symbols
Month Year
Doc Title
Fix the Issue on Number Of HE-SIG-B Symbols
Date:
Authors:
John Doe, Some Company

2. Comments CID Clause Page No. Comment Proposed Change 12060 28.3.10.7.2
410.20
In table 28-17, we only have 4 bits (B18-B21) in HE-SIG-A1 field to indicate the total number of SIGB symbols. However, the required SIG-B symbols will beyond 16 if it is modulated with low data rate, such MSC0 with DCM. So B18-B21 will bring false info to receiver.
Modify HE-SIG-A to allow the number of SIG-B can be greater than 16.
13046
410.21
In table 28-17, B18-B21 in HE-SIG-A1 field only provides 4 bits to indicate the number of SIGB OFDM symbols. That means the maximum of SIGB symbols is 16. However, the maximum required SIG B symbols is about 78 symbols if SIGB using MCS0 & DCM , 39 symbols if using MCS0 or MCS1 & DCM, 20 symbols if using MCS1 or MCS2 & DCM in 160MHz bandwidth. On the other hand, the maximun number STA scheduled over 80MHz or 160MHz if using lowest data rate for SIGB is only 12 under the condition of 16 the maximum SIGB symbols, significantly reducing the multi-user diversity introduced by OFDMA
Please correct the bug to allow more STAs to be scheduled in DL OFDMA or make the receiver know the HE-SIG-B boundary when lots of STAs are scheduled.

3. Issue Discussion
The maximum number of SIG B symbols now is 16 due to the 4 bits indication in SIGA
No enough space in SIG A to indicate more SIG B symbols
Only up to 12 users can be scheduled in one OFDMA PPDU no matter how large the bandwidth of OFDMA PPDU is if SIG B uses the lowest MCS (MCS 0+DCM)
16 SIG B symbols can only accommodate 6 user fields in each SIG B content channel, please refer to the CR document 18/0162r1 for the detail calculation
Less than 12 users can be scheduled if the dummy user field exists in SIG B content channel
The number of users can be doubled if the SIGB uses second lowest MCS (MCS 0)
However, 12 users in one OFDMA PPDU can not satisfy the requirement of high-end AP in dense network
In theory, up to 138 users can be scheduled based on the 11ax tone plan. 12 users seems too weak. Only MCS 4 and 5 used by SIG B can accommodate this number
74 26-tone RUs in 160 MHz Bandwidth, up to 8 MU-MIMO users on 106 tone or large RU
In LTE network, it does not have this kind of limitation due to such ugly design, i.e., it can schedule up to 200 users in PHICH in 20 MHz bandwidth.

4. Resolution
Propose to only change the meaning of value 15 in B18-B21 as shown in the Table 28-18—HE-SIG-A field of an HE MU PPDU
The value 15 in B18-B21 field when the HE-SIG-B compression field is 0 indicates that the number of OFDM symbols in the HE-SIG-B is greater than or equal to 16
It leaves at least 16 SIGB symbol transmission time to prepare the AGC setting for the STF field following SIG B field in the case of value 15
Add the capability to the STA which supports new encode of Number of HE-SIG-B field (B18-B21)
Address the coexistence issue with legacy 11ax devices
Two Parts of HE-SIG-A
Bit
Field
Number of bits
Description
HE-SIG-A1 …
B18-B21
Number Of HE-SIG-B Symbols Or MU-MIMO Users 4
If the SIGB Compression field is 0, indicates the number of OFDM symbols in the HE-SIG-B field minus 1.
If the SIGB Compression field is 1, indicates the number of MU-MIMO users minus 1.
Indicates the number of OFDM symbols in the HE-SIG-B field when the HE-SIG-B compression field is 0
Set to the number of OFDM symbols in the HE-SIG-B field minus 1 if the number of OFDM symbols in the HE-SIG-B field is less than 16;
Set to 15 to indicate that the number of OFDM symbols in the HE-SIG-B field is equal to 16 if More Scheduled STAs in DL HE MU PPDU Support subfield of the HE Capabilities element transmitted by at least one recipient STA is 0;
Set to 15 to indicate that the number of OFDM symbols in the HE-SIG-B field is greater than or equal to 16 if More Scheduled STAs in DL HE MU PPDU Support subfield of the HE Capabilities element transmitted by all the recipient STAs are 1. The exact number of OFDM symbols in the HE-SIG-B field is calculated based on the number of User fields within the HE-SIG-B content channel which is indicated by HE-SIG-B common field in this case.
Indicates the number of MU-MIMO users minus 1 when the HE-SIG-B compression field is 1 (#CID )

5. Complexity Discussion
Hardware and software for the proposed change are already implemented in 11ax chips
Parsing SIG B common field to obtain the number of user filed is there
It is used to locate the CRC field for User Block field in each SIG B content channel
It is used to decide whether the last User Block field in each SIG B content channel contains one or two user fields
Calculating the number of SIG B symbols based on the number of user field is there
There are two schemes to obtain the number of SIG B symbols now
(1) One is signaled by the Number Of HE-SIG-B Symbols in the HE-SIG-A field for OFDMA case
(2) The other is signaled by the MU-MIMO Users field in the HE-SIG-A field for full bandwidth MU-MIMO case (the same philosophy as proposed change)
No additional requirement for Low-end AP when transmitting OFDMA PPDU
Do not need to consider the capabilities of each device, just schedule the OFDMA transmission as now
All non-AP STAs can obtain the number of SIG B symbol correctly as long as it does not exceed 16.

6. Use case
Before the discussion on use case and benefits, just one question
Conceptually, why would the standard place any limitation on the total number of the scheduled users in one OFDMA transmission without extra complexity? It is a product decision.
11ax is targeted to dense network, such as enterprise network
It requires to improve the number of connections
For example, more than 100 active IoT devices in the network
Feasibility for serving more than 12 IoT devices based on the existing 11ax tone plan
Scheduling up to 74 IOT devices in 160 MHz bandwidth is feasible even if without MU-MIMO
MAC frame has around 38 bytes by assuming IoT packet has 10 bytes payload
The MAC frame transmission time is about 405 us by using MCS 0 on 26 tone RU, much less than a typical TXOP time i.e., 3 ms. It also work even if it adopts MCS0+DCM
Moreover, the small packets account for about 70% of the real traffic according to the 11ax simulation document.

7. Benefits Overhead reduction and latency improvement
If without proposed change, the following transmission procedure is required to repeat at least twice
If repeating it twice, the proposed solution will save at least 156 us,
including Legacy preamble transmission time (20 us) + HE preamble transmission time (without HE-SIG User specific field, 48us) + 2*SIFS (32 us)+ UL OFDMA BA preamble transmission time (MCS 0, 56us preamble portion) with assumption of only 1 stream, 16 us 4x OFDM symbol and 6 us HE-SIG-SIG common field.
Overhead can be more if considering UL OFDMA BA transmission rule
If repeating it three times, the proposed solution will save at least 312 us

8. Benefits Avoid asymmetric issue for DL OFDMA and UL OFDMA transmission
No any restriction on the number of scheduled users in UL OFDMA transmission, but it has for DL OFDMA
It brings the issue that AP can not respond the UL transmission with DL OFDMA BA when the number of UL scheduled user goes beyond the limitation of that in DL OFDMA
Note that UL OFDMA PPDU shall solicit the immediate response from AP
Support dummy user fields in HE-SIG-B for unused RUs
To obtain frequency selective gain of OFDMA, some deep fading or interfered RU will be discarded, leading to dummy user fields
Dummy user fields may break the limitation of 16 SIG B symbols although the number of scheduled users is small

9. Benefits
Make multiplexing on spatial and frequency (OFDMA+MU-MIMO) feasible
One of the most important motivations of introducing Partial bandwidth MU-MIMO is to increase the number of connections
8 users are supported over a 106 tone-RU instead of each user over 26-tone RU. It almost doubles the number of connections in a single transmission.
If without proposed change, it restricts the application of partial bandwidth MU-MIMO
For example, if an AP wants to schedule 8 MU-MIMO users over each 20MHz within an 160 MHz bandwidth (64 users in total), the AP will meet difficulty in generating this DL OFDMA PPDU .

10. Reference
[1] ax Draft 2.0