1. Efficient Frame Format for MU Transmission
January 2010
doc.: IEEE /0084r3
Efficient Frame Format for MU Transmission
Date:
Authors:
Brian Hart, Cisco Systems

2. Abstract
MU-MIMO is a key technique to attain at least 1 Gbps throughput at MAC SAP for Multi-STA.
However, the gain of MU-MIMO maybe shrink significantly due to mismatching of the lengths of co-transmission STA’s PSDU.
A sustainable frame format is proposed here to exploit the gain of MU-MIMO against the mismatching problem for co-transmission STAs

3. The mismatching problem
The gain derived from MU-MIMO in actual scenario not only depends on the composite channel HMU-MIMO=[H1, H2,…Hn], but also on the buffer situation of each co-transmission STA
In ac, there must exist a mismatching problem for MU-MIMO transmission due to mismatching the lengths of co-transmission STA’s PSDU.

4. An example for the mismatching problem
Related parameters are assumed as following:
PSDU data length of STA1 is 2000 kb
STA1’s transmission rate is 500Mbps by using SU-MIMO, therefore 4ms is needed for STA1’s PSDU transmission
PSDU data length of STA2 is 800 kb
STA2’s transmission rate is the same as that of STA1, i.e. 500Mbps for SU-MIMO transmission, therefore 1.6ms is needed for STA2’s PSDU transmission
If MU-MIMO is used for STA1 and STA2, and their data rates are assumed identical, equal to 400Mbps, therefore 5ms and 2ms are needed respectively
The mismatching problem appear.
STA 1
STA1-PSDU
STA 2
STA2-PSDU

5. Transmission schemes for Multi-STA with mismatching lengths of their PSDU
Direct transmission by using SU-MIMO and MU-TXOP for each STA in turn
The transmission duration for the example is 5.6ms without accounting for the overheads of preamble, SIFS, etc.
SIFS
header
STA1-PSDU
header
STA2-PSDU

6. Transmission schemes for Multi-STA with mismatching lengths of their PSDU
Direct transmission utilizing MU-MIMO
The LENGTH field in L-SIG is set according the maximum among the lengths of these multi-STA’s PSDU
The LENGTH field in each STA’s VHT-SIG-B is set according the actual length of the STA’s PSDU
The transmission duration for the example is 5ms

7. A new frame format for efficient MU-MIMO transmission in 802.11ac
A new frame format with sustainable VHT-segment proposed here:
A series of VHT-segments with the maximum of 4 can be concatenated together to form a whole frame.
Each VHT-segment consists of VHT-SIG-A, VHT-STF, VHT-LTFs, VHT-SIG-B and VHT data
The set of co-transmission STA {STA1, … STAn} should be nested, i.e. the STA set of the (i+1)-th segment is a subset of the STA set of the i-th segment.

8. The transmission scheme based on the proposed frame format
The transmission duration for the former example is 4.4ms, therefore a gain of more than 10% can be obtained.

9. Details on the new frame format
A new frame format with sustainable VHT-segment proposed here:
The LENGTH field in L-SIG is set according to the total length of the whole frame consisted of all VHT-segments

10. Details on the new frame format (con’t)
A new frame format with sustainable VHT-segment proposed here:
Each STA’s NSTS field in VHT-SIG-A implies whether its PSDU has been ended or not

11. Details on the new frame format (con’t)
A new frame format with sustainable VHT-segment proposed here:
The length field in VHT-SIG-B of each VHT-segment indicates the actual length of VHT-data in the current VHT-segment.

12. Remarks
Although the transmission scheme based on the proposed frame format is somewhat like MU-TXOP in role, but it is superior to MU-TXOP due to omitting SIFS, L-STF and L-LTF.
Nevertheless, TXOP/MU-TXOP isn’t substitutable in providing continuous transmission with longer duration.

13. References
Specification Framework for TGac, IEEE /0992r15, September 2010