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 here:
PSDU data length of STA1 is 2000 kb
STA1’s transmission rate is 500Mbps by using 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. set the set of the (i+1)-th segment is a subset of the i-th set

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 current VHT-segment.

12. Remarks
Although the transmission scheme based on the proposed frame format is somewhat like MU-TXOP, 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