Doc.: IEEE 802.11-10/0324r0 Submission Slide 1Michelle Gong, Intel March 2010 DL MU MIMO Error Handling and Simulation Results Date: 2010-03-15 Authors:

doc.: IEEE 802.11-10/0324r0

Submission Slide 1 Michelle Gong, Intel

March 2010

DL MU MIMO Error Handling and Simulation Results

Date: 2010-03-15

Name Affiliations Address Phone email Michelle Gong Intel Corporation 2200 Mission College

Santa Clara, CA [email protected]

Robert Stacey Intel Corporation [email protected]

James Cho Atheros [email protected]

Authors:

doc.: IEEE 802.11-10/0324r0


March 2010

Outline• Motivation

• Overview of simulated schemes– DL MU MIMO with polled ack

– DL MU MIMO with scheduled ack

• Overview of MAC protection

• Overview of error handling at the AP– Medium access behavior

• Simulation scenarios and parameters

• Simulation results

• Summary

doc.: IEEE 802.11-10/0324r0


March 2010

Motivation

• Downlink Multi-user MIMO is identified as a key technology to improve the overall network performance– Two DL MU MIMO Ack mechanisms were proposed in 11-09-

1172/r0

• The goals of the simulation are– To study the behavior, including the error recovery behavior, of

the two DL MU MIMO ack mechanisms– To study the medium access behavior of the AP when transmitting

a DL MU MIMO burst– To evaluate the performance difference of the two DL MU MIMO

response mechanisms in different scenarios

doc.: IEEE 802.11-10/0324r0


March 2010

DL MU MIMO with scheduled Ack• The AP contends for the medium using EDCA

– Once a contention is won, the AP transmits a downlink MU MIMO burst to multiple STAs

– Each data packet defines an offset value such that each STA knows when to transmit back a BA

• Scheduled Ack introduces less overhead but a STA needs to schedule its BA transmission

Data (STA1)

Data (STA3)

BA

Data (STA2)

BA

BA

SIFSSIFS or

RIFS SIFS or RIFS

pad

pad

doc.: IEEE 802.11-10/0324r0


March 2010

DL SDMA scheduled ack schemes do not have an error recovery mechanism

• If a STA does not correctly receive an A-MPDU, it does not transmit a BA– There will be a gap in the scheduled response sequence

– When a BA is transmitted at certain data rates, there is not enough time for the AP to transmit even a NULL data frame to fill the gap

– Other STAs that have not correctly set the NAV may try to contend for the medium during the gap

Data (STA1)

Data (STA3)

Data (STA2)

BA

BA

SIFS SIFS SIFS

doc.: IEEE 802.11-10/0324r0


March 2010

DL MU MIMO with Polled ACKs• The AP contends for the medium using EDCA

– Once a contention is won, the AP transmits a downlink MU MIMO burst to multiple STAs

– One STA will transmit a BA immediately after receiving the data packet

– The AP sends BAR frames to poll the remaining STAs for BAs

• Polled Ack introduces more overhead yet STAs don’t need to schedule BA transmissions

Data (STA1)

Data (STA3)

BA

Data (STA2)

BA

BA

BAR BAR

SIFS SIFS SIFS SIFS SIFS

pad

pad

doc.: IEEE 802.11-10/0324r0


March 2010

Error recovery for the polled ack scheme

• If the AP senses the medium as idle PIFS after transmitting a BAR frame, it transmits a BAR frame to poll the next STA to which it has transmitted a packet in the DL SDMA burst

Data (STA1)

Data (STA3)

Data (STA2)

BA

BA

BAR BAR BAR

SIFS

PIFS

doc.: IEEE 802.11-10/0324r0


March 2010

Overview of MAC protection

• With MAC protection: • One RTS/CTS exchange at the beginning of a TXOP

– One RTS sent to a random STA, the STA replies with a CTS

• Without MAC protection

Data (STA1)

Data (STA3)

BA

Data (STA2)

BA

BA

BAR

SIFS

RTS

CTS

SIFSSIFS or

RIFS SIFS or RIFS

doc.: IEEE 802.11-10/0324r0


March 2010

Medium access behavior at the AP• As long as one BA is received by the AP, the AP treats the DL SDMA transmission as a

success– CW = CWmin

• If no BA is received from any STA, the AP treats the DL SDMA transmission as a failure– CW = (CW+1)*2-1

Data (STA1)

Data (STA3)

BA

Data (STA2)

BA

BA

SIFS

Backoff (CWmin)

Data (STA1)

BA

Data (STA2)

BA

RIFS RIFS

doc.: IEEE 802.11-10/0324r0


March 2010

Simulation scenario and Traffic pattern• One AP and multiple STAs in one BSS• Fully loaded network:

– Bi-directional video conferencing traffic (UDP)• Study the network capacity in terms of the network saturation

throughput

doc.: IEEE 802.11-10/0324r0


March 2010

Simulation parameters• One AP (4 antennas), three STAs (each with 2 antennas)

• TXOP limit: 3 ms• 20MHz: 52 data subcarriers, 4 pilot tones• SIFS=16 us, RIFS=2us, aSlotTime=9 us • Data packet size: 1500 bytes• CWmin=7, CWmax=63 for AC_VI; CWmin=15, CWmax=1023 for AC_BE

• Data rates: • With BF only:

– 802.11a, 16QAM, r=1/2 for control rate (BAR/BA), 802.11n MCS15 (64QAM, r=5/6, nSS=2) for data rate

• With DL SDMA:– 802.11a, 16QAM, r=1/2 for control rate (BAR/BA), 802.11n MCS7 (64QAM, r=5/6, nSS=1)

for downlink data rate – 802.11a, 16QAM, r=1/2 for control rate (BAR/BA), 802.11n MCS15 (64QAM, r=5/6, nSS=2)

for uplink data rate – Assumption: each STA needs 2 antennas to receive one spatial stream in DL SDMA (MMSE

precoding and MMSE receiver for resolvable LTFs)

• Training with implicit feedback• Comparison:

• DL SDMA with scheduled ACK (SIFS)• DL SDMA with scheduled ACK (RIFS)• DL SDMA with polled ACK

doc.: IEEE 802.11-10/0324r0


March 2010

Without MAC protection (AC_VI)

Throughput achieved by polled ack scheme is 7-9% higher than scheduled ack schemes

doc.: IEEE 802.11-10/0324r0


March 2010

Without MAC protection (AC_BE)

doc.: IEEE 802.11-10/0324r0


March 2010

With MAC protection (AC_VI and AC_BE)

doc.: IEEE 802.11-10/0324r0


March 2010

Simulation scenario (a hidden-node case)• Fully loaded network

– Bi-directional UDP traffic• The AP can transmit simultaneously to STA1, 2, and 3

– STA4 cannot decode DL SDMA packets that are intended for STA1, 2, and 3• STA2 has high PER

– This can be viewed as STA2 experiences frequent collisions from hidden nodes in another BSS

– STA2 is always the second one to respond a BA

AP1

STA2STA1

STA3

STA4

doc.: IEEE 802.11-10/0324r0


March 2010

In this scenario, a gap may occur in the response sequence for scheduled ack scheme

• STA4 cannot decode the DL SDMA packets and thus does not set the NAV– STA4 and STA1 are hidden nodes (STA4 cannot receive BA from STA1)– STA4 waits for EIFS after receiving the DL SDMA burst– If scheduled ack is used, because STA4 may transmit during the gap in

response sequence, the remaining BA will collide with STA4’s transmission

Data (STA1)

Data (STA3)

Data (STA2)

BAR

BA

BA

SIFS

EIFS = 94us

doc.: IEEE 802.11-10/0324r0


March 2010

Polled ack scheme performs better than scheduled ack schemes in this scenario

doc.: IEEE 802.11-10/0324r0


March 2010

Discussion of the hidden-node scenario• Polled ack scheme performs better than scheduled ack

schemes– STA4 cannot set NAV based on DL MU MIMO packets– After receiving a BAR frame transmitted from the AP, STA4 can

set the NAV correctly– Because STA4 won’t transmit during the gap in the response

sequence, the AP can successfully receive the remaining BA (no unnecessary retransmission needed)

Data (STA1)

Data (STA3)

Data (STA2)

BA

BA

SIFS

EIFS = 94us

Data (STA1)

Data (STA3)

Data (STA2)

BA

BA

BAR BAR BAR

SIFS

PIFS

doc.: IEEE 802.11-10/0324r0


March 2010

Summary• Three DL MU MIMO response schemes

• Error recovery – Polled response scheme implements an error recovery mechanism

– Scheduled response schemes do not have an error recovery mechanism

• MAC protection– One RTS sent from the AP to a randomly selected STA

• AP’s medium access behavior– The AP initiates exponential backoff only when no BA response is

received (i.e. treated as a traditional transmission failure)

– The AP initiates success backoff when at least one BA response is received (i.e. treated as a traditional transmission success)

doc.: IEEE 802.11-10/0324r0


March 2010

Conclusion• With MAC protection, the three response mechanisms have

similar performance – Transmitting RTS/CTS for every TXOP lowers MAC efficiency

– The overhead of RTS/CTS becomes more significant if the data rate is higher or the TXOP size is smaller

• Polled ack mechanism is more robust– In general, polled ack performs better than scheduled ack schemes when

there is no MAC protection

– Even for some STAs that are not able to decode DL MU MIMO packets, the BAR frame can still set the NAV at those STAs

• The polled ack mechanism is preferred over scheduled ack mechanisms– Lower complexity

– More robust (Consistent performance is required to support QoS traffic)

doc.: IEEE 802.11-10/0324r0


March 2010

Straw Poll #1

• Do you prefer the polled ack scheme over the scheduled ack scheme?

– Yes

– No

doc.: IEEE 802.11-10/0324r0


March 2010

Straw Poll #2

• Do you support the AP medium access behavior as described below?1) The AP shall consider the DL MU MIMO transmission as a

failure when no BA response to the DL MU-MIMO TX is received

2) The AP may consider the DL MU MIMO transmission as a success when at least one BA response to the DL MU-MIMO TX is received

– Yes

– No

Documents

Doc.: IEEE 802.11-10/0324r0 Submission Slide 1Michelle Gong, Intel March 2010 DL MU MIMO Error Handling and Simulation Results Date: 2010-03-15 Authors: