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.
But, 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
There exists a mismatching problem for MU-MIMO transmission for ac due to mismatching the lengths of co-transmission STA’s PSDU.
An example is given here:
STA1’s transmission rate is 500Mbps and 400Mbps for SU-MIMO and MU-MIMO transmission, respectively
STA2’s transmission rate is assumed as same as that of STA1 for simple.
PSDU data length of STA1 is 2000 kb, therefore 5ms for MU-MIMO transmission
PSDU data length of STA2 is 800 kb, therefore 2ms for MU-MIMO transmission
STA 1
STA1-PSDU
STA 2
STA2-PSDU

4. 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
Direct transmission utilizing SU-MIMO for each STA in turn
The transmission duration for the example is 5.6ms without accounting for the overheads of preamble, SIFS, etc.
STA1-PSDU
STA2-PSDU

5. An efficient transmission scheme based on a new frame format
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 the whole frame.
Each VHT-segment consists of VHT-SIG-A, VHT-STF, VHT-LTFs, VHT-SIG-B and VHT data

6. An efficient transmission scheme based on a new frame format (con’t)
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

7. An efficient transmission scheme based on a 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

8. An efficient transmission scheme based on a 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.

9. Gain and Conclusion
The transmission duration for the former example is 4.4ms, therefore a gain of more than 10% can be obtained.
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 (for example more than 5.4ms).

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