Post on 18-Jan-2016
Submission
doc.: IEEE 802.11-12/0890r0July 2012
Fei Tong, CSRSlide 1
802.11ah Multi-User Aggregation PDU
Date: 2012-July-16
Name Affiliations Address Phone email Fei Tong CSR Churchill House,
Cambridge business park, CB4 0WZ
0044-1223692160
fei.tong@csr.com
Alex Thoukydides
CSR Churchill House, Cambridge business park, CB4 0WZ
Wei Shi CSR Churchill House, Cambridge business park, CB4 0WZ
Mark Rison CSR Churchill House, Cambridge business park, CB4 0WZ
Huai Rong Shao Samsung hr.shao@samsung.com
Authors:
Submission
doc.: IEEE 802.11-12/0890r0July 2012
Fei Tong, CSRSlide 2
Abstract
This presentation proposes to support aggregation of MPDUs for multiple receivers into a single PDU only used for transmission from AP to multiple stations. It also proposes a new ACK format, which carries multiple ACKs, destined to different stations. Both of these proposals can improve spectral efficiency for short PDU transmission significantly.
Submission
doc.: IEEE 802.11-12/0890r0
Low efficiency in short frame
• The spectral efficiency, measured in bit/Hz/Sec, is significantly lower for short frame as the relative transmission overhead increases
• Transmission overhead includes• Transmission gaps: SIFS/DIFS/Backoff
• Preamble transmission
• Example of short frames• Small-sized data PDU (for some applications)
• ACK frame
July 2012
Fei Tong, CSRSlide 3
Submission
doc.: IEEE 802.11-12/0890r0
Improving efficiency for short PDU
• For some applications, such as sensor network and health care etc, traffic duty cycle (per station) is low; existing aggregation of MPDUs (to/from the same station) will cause increased transmission latency
• For these applications, typically, the number of stations in the BSS is high. Aggregation of multiple PDUs addressing different stations (multi-user aggregation PDU, from AP to stations) can improve efficiency without increasing latency
• Requires a new frame format for Multi-User Aggregation PDU
July 2012
Fei Tong, CSRSlide 4
Submission
doc.: IEEE 802.11-12/0890r0
Multi-User Aggregation PDU frame
• A MUA indicator in SIG; A MUA-Header (2 bytes) per MPDU indicating the length of the MPDU
• Each MPDU separately coded and CRC protected
• May add a bitmap in the start of MUA-MPDU indicating a group of STAs, whose MPDUs are aggregated
July 2012
Fei Tong, CSRSlide 5
L-S
TF
L-LT
F
SIG
MUA-MPDU
DA
TA
MU
A in
dica
tor
MUA-Header MPDU1
2 Bytes
MUA-Header MPDU2
2 Bytes
MUA-Header MPDU3
2 BytesFor STA1 For STA2 For STA3
Aggregation size may vary
Submission
doc.: IEEE 802.11-12/0890r0
Selecting stations for aggregation
• As the aggregation frame is targeting multiple stations, the lowest MCS (among the MCSs selected for stations to be aggregated) is chosen for the aggregation frame
• In implementation, AP will select stations with similar channel condition (i.e. with similar selected MCS) for aggregation to achieve optimal spectrum efficiency
• As the number of stations is high for sensor network, of finding from two to eight stations with PDUs to transmit and similar MCS should be difficult
July 2012
Fei Tong, CSRSlide 6
Submission
doc.: IEEE 802.11-12/0890r0
Acknowledgement rule for MUA-PDU
• Medium is reserved for transmission of all ACK frames corresponding to aggregated unicast-only PDUs; the DUR field in each MAC header is set to reserve the transmission time for the following ACKs.
• Transmission order of ACK frame (from station) is determined by the order of the unicast PDUs in the aggregation frame
• ACK timeout timer needs to be increased depending the order the ACK transmitted
• No time slot is reserved for ACKs corresponding to multicast/broadcast PDUs aggregated in the MUA-PDUs
July 2012
Fei Tong, CSRSlide 7
Submission
doc.: IEEE 802.11-12/0890r0
Example for ACK frame timing
July 2012
Fei Tong, CSRSlide 8
MPDU1 MPDU2 MPDU3Preamble
STA1 ACK frame
SIFS
STA2 ACK frame
MPDU1 MPDU2 MPDU3Preamble
2xSIFS + T_ACK
T_ACKDifference in time of arrival
STA3
MPDU1 MPDU2 MPDU3Preamble
3xSIFS + 2xT_ACK
Difference in time of arrival
ACK frame
Three stations aggregated as example, not limited to three in real operation
Submission
doc.: IEEE 802.11-12/0890r0July 2012
Fei Tong, CSRSlide 9
Quantitative efficiency study (1)
• Transmit total 8 PDUs, 2MHz channel
• 11ah frame structure (normal CP)
• Data rate (DR) : MCS (0..7); same MCS for all frames
• SIFS = 160 us, Back off count = 7 (average of Cwin=15)
• Success transmission on the first attempt
• Aggregation level : 1, 2, 4, 8
Submission
doc.: IEEE 802.11-12/0890r0
Spectral efficiency gain (percentage) for transmitting 8 unicast PDUs (same MCS)
July 2012
Fei Tong, CSRSlide 10
MCS DR (Mb/s)
MCS 0 0.65
MCS 1 1.3
MCS 2 1.95
MCS 3 2.6
MCS 4 3.9
MCS 5 5.2
MCS 6 5.85
MCS 7 6.5
Submission
doc.: IEEE 802.11-12/0890r0July 2012
Fei Tong, CSRSlide 11
Quantitative efficiency study (2)
• Transmit total 8 PDUs for stations
• Random MCSs for 8 stations; minimum MCS is used for when stations are aggregated in one frame;
• Within the 8 stations, careful grouping is taken so that loss of data rate due to the aggregation is minimised.
• Other parameters are the same as previous study
Submission
doc.: IEEE 802.11-12/0890r0
CDF of Spectral efficiency for transmitting 8 unicast PDUs (random MCS)
July 2012
Fei Tong, CSRSlide 12
Submission
doc.: IEEE 802.11-12/0890r0
Aggregate multi-users’ ACK
• ACK frame is very inefficient when the PDU is short; existing block ACK causes high latency for low duty cycle traffic
• Multi-users’ ACK (from AP) can be aggregated in one frame (aggregation size may vary);
• ACK frame overhead can be further reduced by introducing a new PDU
July 2012
Fei Tong, CSRSlide 13
FCDur/ID
RA1 FCSRA2 RA3
Submission
doc.: IEEE 802.11-12/0890r0
Timeout rules when using MUA-ACK
• Station configured using MUA-ACK will need to set a longer timeout value
• Arrival of a new beacon frame will also trigger ACK time out
July 2012
Fei Tong, CSRSlide 14
STA1
STA2
PPDU
STA8
PPDU
PPDU
APMUA-ACK
ACK timeout length
PPDU
Contains ACKs for STA1 and STA2
ACK timeout length
Beacon Frame
STAn
Time-out for STA1
Whichever comes frist
Submission
doc.: IEEE 802.11-12/0890r0July 2012
Fei Tong, CSRSlide 15
Quantitative efficiency study
• Transmit 8 ACK back-to-back, 2MHz channel
• Compared to normal ACK and short ACK frame
• Data rate (DR) : MCS (0..7), SIFS = 160 us
• Aggregation level : 1, 2, 4, 8
Submission
doc.: IEEE 802.11-12/0890r0
Transmission time saving using MUA-ACK
• Total transmission of eight ACK frames back to back to focus on the saving of ACK transmission
July 2012
Fei Tong, CSRSlide 16
MCS DR (Mb/s)
MCS 0 0.65
MCS 1 1.3
MCS 2 1.95
MCS 3 2.6
MCS 4 3.9
MCS 5 5.2
MCS 6 5.85
MCS 7 6.5
Submission
doc.: IEEE 802.11-12/0890r0
Transmission time saving using MUA-ACK
• Comparison to short ACK frame (frame with preamble only)
July 2012
Fei Tong, CSRSlide 17
MCS DR (Mb/s)
MCS 0 0.65
MCS 1 1.3
MCS 2 1.95
MCS 3 2.6
MCS 4 3.9
MCS 5 5.2
MCS 6 5.85
MCS 7 6.5
Submission
doc.: IEEE 802.11-12/0890r0
Summaries
• For small PDU and low duty cycle traffic, MUA-PDU and MUA-ACK (from AP to stations) can improve spectrum efficiency without causing latency increasing
• Moderate aggregation level (such as 4 or 8) can achieve 50% spectrum efficiency improvement (w.r.t. to single PDU transmission) for MUA-PDU
• Moderate aggregation level (such as 4 or 8) can save more than 60% of the time used for ACK transmission
July 2012
Fei Tong, CSRSlide 18
Submission
doc.: IEEE 802.11-12/0890r0
References
• 802.11-12-0324r2, George Calcev et al, Considerations on short packet transmission overhead
• 802.11-11-1254r0, Yong Liu et al, Short Ack
July 2012
Fei Tong, CSRSlide 19