doc.: ieee 802.11-04/0873r2 submission august 2004 john ketchum, et al, qualcommslide 1...

August 2004

John Ketchum, et al, QUALCOMM

Slide 1

doc.: IEEE 802.11-04/0873r2

Submission

High-Throughput Enhancements for 802.11: Features and Performance of QUALCOMM’s

Proposal

John Ketchum, Sanjiv Nanda, Rod Walton, Steve Howard, Mark Wallace, Bjorn Bjerke, Irina Medvedev, Santosh Abraham, Arnaud Meylan, Shravan

Surineni

QUALCOMM, Incorporated9 Damonmill Square, Suite 2A

Concord, MA 01742Phone: 781-276-0915Fax: [email protected]

August 2004


Slide 2

doc.: IEEE 802.11-04/0873r2

Submission

Guide to Qualcomm’s Proposal

Four proposal documents:• 11-04-870 High Throughput System Description and

Operating Principles. • 11-04-871 High Throughput Proposal Compliance

Statement (this document.) • 11-04-872 Link Level and System Performance Results for

High Throughput Enhancements.• 11-04-873 High Throughput Enhancements for 802.11:

Features and Performance.

August 2004


Slide 3

doc.: IEEE 802.11-04/0873r2

Submission

Agenda• Introductory remarks• MAC Features• System Performance• PHY Features• Link Performance

August 2004


Slide 4

doc.: IEEE 802.11-04/0873r2

Submission

Qualcomm’s Status Assessment• Submitted proposals contain the basis for an excellent solution to HT

requirements• Substantial commonality in proposed approaches

– MIMO OFDM– Advanced coding– Frame aggregation– Elimination of IFS

• Qualcomm is committed to working with TGn to establish rapid convergence to a draft standard

– Future proof– Optimized performance/complexity tradeoff

• Critical Issues– Informed transmitter operation

• Low-overhead feedback– Flexible rates– Minimal feature set for support of low-latency operation

August 2004


Slide 5

doc.: IEEE 802.11-04/0873r2

Submission

Main Points• 20 MHz operation• Maximum PHY data rates:

– 202 Mbps for 2 Tx; 404 Mbps for 4 Tx• Backward compatible modulation, coding and interleaving• Highly reliable, high-performance operation with existing 802.11

convolutional codes used in combination with Eigenvector Steering spatial multiplexing techniques

• Fall-back to robust Spatial Spreading waveform for uninformed transmitter

• Backward compatible preamble and PLCP with extended SIGNAL field.

• Adaptation of rates and spatial multiplexing mode through low overhead asynchronous feedback. Works with TXOPs obtained through EDCA, HCF or ACF.

• PHY techniques proven in FPGA-based prototype

August 2004


Slide 6

doc.: IEEE 802.11-04/0873r2

Submission

MAC Design Objectives

• Objectives– Preserve the simplicity and robustness of distributed coordination– Backward compatible– Enhancements for high throughput, low latency operation– Build on 802.11e, 802.11h feature set:

• TXOPs, • Block Ack, Delayed Block Ack, • Direct Link Protocol• Dynamic Frequency Selection• Transmit Power Control

August 2004


Slide 7

doc.: IEEE 802.11-04/0873r2

Submission

MAC Feature Summary• Low overhead Rate Feedback• Frame aggregation• Eliminate Immediate ACK for MIMO transmissions• PPDU Aggregation from AP to multiple STAs using SCHED and

SCAP– Reduced IFS

• Managed Peer-to-Peer• Adaptive Coordination Function (ACF)• Compressed Block Ack• QoS-capable IBSS with round-robin scheduling

August 2004


Slide 8

doc.: IEEE 802.11-04/0873r2

Submission

Extended Backward Compatible PLCP Header

• Set legacy RATE field to one of eight unused values. – Legacy STAs revert to CCA on seeing unrecognized RATE field.

• PPDU Size (number OFDM symbols). HT STAs can determine medium time occupied by the PPDU.

• Rate vector (DRV) and Training Type included in SIGNAL2 field.• Rate and mode feedback (DRVF) included in FEEDBACK field (extension of

SERVICE field).• MIMO OFDM Training symbols inserted as necessary.

PLCP Preamble16 us

(if present)

SIGNAL11 OFDMSymbol

0/2/3/4 TrainingSymbols

DATAVariable Number of OFDM Symbols

RATE/Type4 bits

Resv’d1 bit

DRV13 bits

PPDU Size/Request12 bits

Tail6 bits

Parity1 bit

PPDU Control SegmentRate and Format

SERVICE16 bits PSDU

Tail6 bits/Mode

PadVariable

PPDU Data Segment Rate andFormat

FEEDBACK16 bits

TrainingType3 bits

SIGNAL21 OFDMSymbol

Tail6 bits

Parity1 bit

Resv’d1 bit


PPDU Type 0000

August 2004


Slide 9

doc.: IEEE 802.11-04/0873r2

Submission

Flexible Frame Aggregation

• Frame aggregation– Length field per encapsulated frame– Maximum aggregate size can be negotiated per

flow– Second and subsequent MAC headers in the

aggregated frame can be compressed• Compressed Header Formats: Eliminate, TA,

RA, Duration/ID fields– Aggregation Header is always included when a

frame is transmitted in a MIMO OFDM PPDU.

AggregationHeader Type LengthReserved

Bits 2 2 12

FrameControl

Duration/ID Address 1 Address 2 Address 3 Sequence QoS

ControlAddress 4 Frame Body

Octets 2 2 2 6 6 6 2 6 2 n 4

FCSAggregationHeader

Aggregate MAC Frame(One or More Encapsulated MAC Frames)

EncapsulatedMAC Frame

(or Fragment)


(or Fragment)


(or Fragment)


(or Fragment)

PSDU

August 2004


Slide 10

doc.: IEEE 802.11-04/0873r2

Submission

Eliminate Immediate ACK, Reduced IFS

• MIMO OFDM transmissions impose greater burden on the receiver– Compared to 802.11a/g– Advanced decoders make matters worse

• Inefficient solution– Longer signal extension

• Efficient solution– Exploit 802.11e Block ACK and Delayed Block ACK mechanisms

• Eliminate Immediate ACK for MIMO OFDM transmissions• Scheduled transmissions permit Reduced IFS

– TXOP Bursting with zero IFS (AP transmissions) – TXOP Bursting with BIFS (STA transmissions)– Consecutive scheduled STA TXOPs separated by GIFS (800 ns Guard IFS)

• Block Ack (Window-based ARQ) offers a simple way to relieve PHY receiver complexity

August 2004


Slide 11

doc.: IEEE 802.11-04/0873r2

Submission

Scheduled Operation – SCHED Message

• SCHED message and Scheduled Access Period (SCAP) are enhancements of HCCA CAP

– 802.11n AP acquires the medium after PIFS (as in the HCCA CAP) and transmits a SCHED message (instead of Poll).

– The SCHED message defines the schedule of TXOPs for the SCAP. – Each TXOP assignment element in SCHED message indicates Tx and Rx STA,

start offset and duration. Complete information permits optimum power-save at STAs.

• No CCA required for scheduled STA transmissions during SCAP– Permits reduced IFS

Scheduled AccessPeriod

Scheduled Transmissions(AP-STA, STA-AP, STA-STA)

MIMO OFDMEDCA

FRACHPeriod

APto STA B

STA C toAP

STA Eto STA F

SCHED

APto STA D

APto STA G

STA Gto AP

STA Eto AP

CTSto

Self

August 2004


Slide 12

doc.: IEEE 802.11-04/0873r2

Submission

Scheduled Operation – Protection and Recovery

• Protection of SCAP– Mandatory DFS to avoid overlapping BSS.– CTS-to-Self to clear out NAV for SCAP. For 802.11n STAs NAV is set through

Duration field in SCHED frame.– Keep SCAP short (< 4 ms) to minimize impact of collisions with legacy STAs

during SCAP.– Can use RTS/CTS within scheduled TXOPs.

Scheduled AccessPeriod

Scheduled Transmissions(AP-STA, STA-AP, STA-STA)

MIMO OFDMEDCA

FRACHPeriod

APto STA B

STA C toAP

STA Eto STA F

SCHED

APto STA D

APto STA G

STA Gto AP

STA Eto AP

CTSto

Self

August 2004


Slide 13

doc.: IEEE 802.11-04/0873r2

Submission

Scheduled Operation – Managed Peer-to-Peer

• Managed Peer-to-Peer Operation is an enhancement of DLP• In Scheduled STA-STA TXOPs

– PPDU Size in SIGNAL1 is replaced by Request.– AP promiscuously decodes Request field in STA-STA transmissions. – STAs indicate SCHED Rate, QoS and requested length for subsequent TXOP.

• STAs do closed loop rate control• AP does scheduling

PLCP Preamble16 us

(if present)

SIGNAL11 OFDMSymbol

0/2/3/4 TrainingSymbols

DATAVariable Number of OFDM Symbols

RATE/Type4 bits

Resv’d1 bit

DRV13 bits

PPDU Size/Request12 bits

Tail6 bits

Parity1 bit


TrainingType3 bits

SIGNAL21 OFDMSymbol

Tail6 bits

Parity1 bit

Resv’d1 bit


PPDU Type 0000

August 2004


Slide 14

doc.: IEEE 802.11-04/0873r2

Submission

Operation of Adaptive Coordination Function (ACF)

• SCAP is an enhancement of the HCCA CAP• Setting NAV

– The Duration field in the SIGNAL field of the SCHED frame sets the NAV for the SCAP at all 802.11n STAs.

– To set the NAV for the SCAP at legacy STAs, the AP may transmit a CTS-to-Self prior to the transmission of the SCHED frame.

• SCAP Timing– 802.11n STAs respect the SCAP interval so that their transmissions terminate when the SCAP

expires. – The AP may schedule back-to-back SCAPs.

Beacon

SCAP

Beacon

Beacon

Scheduled AccessPeriodSCHEDHCCA

TXOPPoll

CAP

CAP

SCAP SCAP

EDCA

EDCA

EDCA

CAP

SCAP

EDCA

CAP

EDCA

SCAP SCAP SCAPCAP

SCAP SCAP

EDCA

CAP

EDCATXOP

EDCATXOP

EDCATXOP

HCCATXOPPoll

CTSto

Self

SCAP

APTXOP

August 2004


Slide 15

doc.: IEEE 802.11-04/0873r2

Submission

Compressed Block Ack

• Compressed Block Ack offers significant reduction in overhead.– Compressed format 1: Do not indicate status of fragments. Shrink BlockAck Frame from 152

to 32 octets. – Compressed format 2: Indicate status of fragments only if there are missing fragments– Compressed format 3: Remove trailing zeroes from Bitmap.

FrameControl

Octets 2 2 6 6 2 2 8 4

Duration RA TA BAControl

Block Ack StartingSequence Control

No Fragments BlockAck Bitmap FCS

FrameControl

Octets 2 2 6 6 2 2 m 4



Mixed Block AckBitmap FCS

FrameControl

Octets 2 2 6 6 2 2 n 4



Shortened BlockAck Bitmap FCS

August 2004


Slide 16

doc.: IEEE 802.11-04/0873r2

Submission

RRBSS – QoS capable IBSS operation

• Provide QoS capability without AP– May also be used by low-end AP – Applicable to usage scenarios with CE devices with high throughput, high QoS needs– Exploit the large PHY data rates of MIMO OFDM to simplify scheduling and QoS

management.– Designed for up to 15 STAs– Distributed admission control. Self identification of QoS flows– Distributed Round-Robin Scheduling– Short Beacon Period for low latency– Robust operation: Explicit token passing, No single STA is designated “master”

Beacon Beacon

TBTT

RR TXOPRRID = X

RR TXOPRRID = Y

RR TXOPRRID = Z

RRP CP

Frame Frame

RR Schedule for Beacon Period: RRID X, RRID Y, RRID Z

ATIMWindow

August 2004


Slide 17

doc.: IEEE 802.11-04/0873r2

Submission

Low Latency Operation• Low latency operation is critical

– To operate with small buffers. This is critical at high data rates. – For MIMO operation with EDCA, HCCA or ACF

• Rate and Mode Feedback• Eigenvector Steering

– To meet low delay guarantees for multimedia applications in all operating regimes

• Different access methods can provide low latency in different operating environments

• EDCA/HCCA with lightly loaded system• RRBSS (with or without AP)• Scheduled operation for heavily loaded system

August 2004


Slide 18

doc.: IEEE 802.11-04/0873r2

Submission

System Simulation Methodology

• The simulator is based on ns2• Includes physical layer features

– TGn Channel Models– PHY Abstraction determines frame loss events

• MAC features– EDCA– HCCA– Adaptive Coordination Function (ACF): SCHED and SCAP– Frame Aggregation– ARQ with Block Ack. No compressed Block Ack– Closed Loop Rate Control (DRVF and DRV)– MIMO Modes (ES and SS)

• Scheduler– Based on delay requirement and buffer status of flows. Similar to 802.11e Annex H

• Transport– File Transfer mapped to TCP– QoS Flows mapped to UDP

August 2004


Slide 19

doc.: IEEE 802.11-04/0873r2

Submission

Simulation Conditions – Fixed • The following parameters are fixed for all system simulation results.

– Bandwidth: 20 MHz.– Frame Aggregation– Fragmentation Threshold: 100 kB– Delayed Block Ack– Adaptive Rate Control– Adaptive Mode Control between ES and SS

AC CW min CW max AIFS

0 127 1023 2 BlockAck/VoIP

1 127 1023 4 Video HDTV

2 127 1023 8 Other QoS

3 127 1023 10 Best effort

August 2004


Slide 20

doc.: IEEE 802.11-04/0873r2

Submission

Simulation Conditions – Varied• Bands:

– 2.4 GHz – 5.25 GHz

• MIMO: – 2x2: All STAs with 2 antennas– 4x4: All STAs with 4 antennas– Mixed:

• Scenario 1: the AP and the HDTV/SDTV displays are assumed to have 4 antennas; all other STAs have 2 antennas.

• Scenario 6: AP and all STAs, except VoIP terminals have 4 antennas; VoIP terminals have 2 antennas.

• OFDM symbols– Standard: 0.8 μs Guard Interval, 48 data subcarriers– SGI-EXP: 0.4 μs Shortened Guard Interval, 52 data subcarriers

• Access Mechanisms– ACF (SCHED/SCAP)– HCF (Poll/CAP)– EDCA with additional AC for Block Ack

August 2004


Slide 21

doc.: IEEE 802.11-04/0873r2

Submission

Additional Scenarios• Scenario 1 HT is an extension of Scenario 1:

– Additional FTP flow of up to 130 Mbps at 15.6 m from the AP. • Scenario 1 EXT is an extension of Scenario 1:

– Additional FTP flow of up to 130 Mbps at 15.6 m from the AP. – Maximum delay requirement for all video/audio streaming flows is

decreased from 100/200 ms to 50 ms.– Two HDTV flows are moved from 5 m from the AP, to 25 m from the

AP.• Scenario 6 EXT is an extension of Scenario 6:

– One FTP flow of 2 Mbps at 31.1 m from the AP is increased up to 80 Mbps for 4x4.

August 2004


Slide 22

doc.: IEEE 802.11-04/0873r2

Submission

Summary of Total Throughput Results

• 100 Mbps BSS throughput in realistic scenarios with 20 MHz– Scenario 1 EXT (Residential Extended)

• HDTV flows with 50 ms delay requirement; at 25 m from AP.• AP and HDTV (4 antennas); all other STAs (2 antennas)

– Scenario 4 Enterprise 2x2– Scenario 6 EXT Hot Spot

• VoIP STAs with 2 antennas; all other STAs with 4 antennas

Metric 1 Metric 2 Metric 3 Metric 1 Metric 2 Metric 3 Metric 1 Metric 2 Metric 3Scenario 1 - 2.4GHz standard symbols 84.029 84.029 84.029 84.035 84.035 84.035 NA NA NAScenario 1 - 2.4GHz SGI-EXP symbols 84.029 84.029 84.029 84.036 84.036 84.036 NA NA NAScenario 1 - 2.4GHz SGI-EXP HCF 58.885 58.138 53.201 NA NA NA NA NA NAScenario 1 - 2.4GHz SGI-EXP EDCA 54.532 54.389 51.673 NA NA NA NA NA NAScenario 1 - 5.25GHz SGI-EXP HCF 58.813 57.904 53.003 83.136 83.133 81.657 NA NA NAScenario 1 - 5.25GHz SGI-EXP EDCA 53.007 52.891 50.112 53.941 53.879 51.208 NA NA NAScenario 1 - 5.25GHz standard symbols 77.449 77.442 75.947 84.007 84.007 84.007 NA NA NAScenario 1 - 5.25GHz SGI-EXP symbols 84.018 84.018 84.018 84.032 84.032 84.032 NA NA NAScenario 1 HT - 2.4GHz SGI-EXP symbols 103.111 103.111 103.111 185.841 185.841 185.841 NA NA NAScenario 1 HT - 5.25GHz SGI-EXP symbol 95.069 95.069 95.069 164.750 164.750 164.750 NA NA NAScenario 1 EXT - 2.4GHz SGI-EXP symbols 86.152 86.152 82.165 164.706 164.705 164.705 121.204 121.204 121.204Scenario 1 EXT - 5.25GHz SGI-EXP symbols 68.137 68.087 64.211 130.365 130.363 130.363 105.213 105.213 104.716

Scenario 4 - 2.4GHz SGI-EXP symbols 104.980 104.980 104.980 199.995 199.995 199.995 NA NA NAScenario 4 - 5.25GHz SGI-EXP symbols 100.296 100.296 100.296 191.566 191.566 191.566 NA NA NA

Scenario 6 - 5.25GHz standard symbols 60.228 60.228 60.138 66.138 66.137 66.137 66.119 66.119 66.029Scenario 6 - 5.25GHz standard symbols HCF 44.825 44.689 32.967 NA NA NA NA NA NAScenario 6 - 5.25GHz standard symbols EDCA 45.608 45.167 7.029 NA NA NA NA NA NAScenario 6 EXT- 5.25GHz standard symbols 67.434 67.434 67.256 100.308 100.308 100.308 105.174 105.174 105.085

2x2 4x4 Mixed

August 2004


Slide 23

doc.: IEEE 802.11-04/0873r2

Submission

Summary of Total Throughput ResultsMetric Scenario ACF 2x2 ACF 4x4

CC3 Aggregate goodput (Metric 2) [Mbps]

Scenario 1 HT 95.1 164.8

Scenario 4 100.3 191.6

Scenario 6 EXT 67.4 100.3

CC18 Aggregate non-QoS throughput [Mbps]

Scenario 1 HT 16.0 78.0

Scenario 4 88.9 178.9

Scenario 6 EXT 21.4 52.1

CC19 Number of QoS flows supported

Scenario 1 HT 17 / 17 17 / 17

Scenario 4 18 / 18 18 / 18

Scenario 6 EXT 37 / 39 39 / 39

CC58 HT spectral efficiency [bps/Hz] 5.85

• Parameters– 5.25 GHz– SGI-EXP OFDM symbols, except for Scenario 6

• Significantly higher throughput compared to other proposals.

August 2004


Slide 24

doc.: IEEE 802.11-04/0873r2

Submission

Summary of QoS Flows SatisfiedNumber of Qos Flows

2x2 4x4 MixedScenario 1 - 2.4GHz standard symbols 17 17 17 NAScenario 1 - 2.4GHz SGI-EXP symbols 17 17 17 NAScenario 1 - 2.4GHz SGI-EXP HCF 17 13 NA NAScenario 1 - 2.4GHz SGI-EXP EDCA 17 7 NA NAScenario 1 - 5.25GHz SGI-EXP HCF 17 13 15 NAScenario 1 - 5.25GHz SGI-EXP EDCA 17 7 10 NAScenario 1 - 5.25GHz standard symbols 17 15 17 NAScenario 1 - 5.25GHz SGI-EXP symbols 17 17 17 NAScenario 1 HT - 2.4GHz SGI-EXP symbols 17 17 17 NAScenario 1 HT - 5.25GHz SGI-EXP symbol 17 17 17 NAScenario 1 EXT - 2.4GHz SGI-EXP symbols 17 16 17 17Scenario 1 EXT - 5.25GHz SGI-EXP symbols 17 15 17 16

Scenario 4 - 2.4GHz SGI-EXP symbols 18 18 18 NAScenario 4 - 5.25GHz SGI-EXP symbols 18 18 18 NA

Scenario 6 - 5.25GHz standard symbols 39 38 39 38Scenario 6 - 5.25GHz standard symbols HCF 39 36 NA NAScenario 6 - 5.25GHz standard symbols EDCA 39 13 NA NAScenario 6 EXT- 5.25GHz standard symbols 39 37 39 38

Number of flows that meet their QoS requrement

August 2004


Slide 25

doc.: IEEE 802.11-04/0873r2

Submission

Throughput versus Range for Channel Model DThroughput vs Range in 20MHz, channel model D

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

0 20 40 60 80 100 120 140 160 180 200

Distance [m]

Thro

ughp

ut [M

bps]

2x2 2.4Ghz

2x2 5.25Ghz

4x4 2.4Ghz

4x4 5.25Ghz

August 2004


Slide 26

doc.: IEEE 802.11-04/0873r2

Submission

Observations• MAC Efficiency with Frame Aggregation

– ACF: between 0.65-0.7 (2x2) reduces to 0.6 (4x4)– HCF: around 0.5 (2x2) reduces to 0.4 (4x4)– EDCA: around 0.45 (2x2) reduces to 0.23 (4x4). No increase in

throughput of EDCA with 4x4.• More QoS flows are satisfied with HCF than with EDCA. However,

ACF is required to address stringent QoS requirements.• Frame Aggregation is not enough, need ACF for a future-proof MAC• Throughput versus Range

– Throughput above the MAC of 100 Mbps is achieved at:• 29 m for 2x2, 5.25 GHz• 40 m for 2x2, 2.4 GHz• 47 m for 4x4, 5.25 GHz• 75 m for 4x4, 2.4 GHz• The plots for Channel Model B and Channel Model D are roughly similar.

– Significantly higher range compared to other proposals.

August 2004


Slide 27

doc.: IEEE 802.11-04/0873r2

Submission

Qualcomm 802.11n PHY Design• Fully backward compatible with 802.11a/b/g

– 20 MHz bandwidth with 802.11a/b/g spectral mask – OFDM based on 802.11a waveform

• Optional expanded OFDM symbol (4 add’l data subcarriers) and shortened guard interval• Modulation, coding, interleaving based on 802.11a

– Expanded rate set• Scalable MIMO architecture

– Supports a maximum of 4 wideband spatial streams• Two forms of spatial processing

– Eigenvector Steering (ES): via wideband spatial modes/SVD per subcarrier• Tx and Rx steering• Over the air calibration procedure required

– Spatial Spreading (SS): modulation and coding per wideband spatial channel• No calibration required• SNR per wideband spatial stream known at Tx

• Use of Eigenvector steering extends the life of low-complexity 802.11 BCC• Sustained high rate operation possible via rate adaptation

– low overhead asynchronous feedback.• PHY techniques proven in FPGA-based prototype

August 2004


Slide 28

doc.: IEEE 802.11-04/0873r2

Submission

Code Rates and ModulationBits/subcarrier Bit/s/spatial chan1 Bit/s/spatial chan2 Code Rate Modulation

0 0 0 0 N/A

0.50 6 Mbit/s 7.2 Mbit/s r=1/2 BPSK

0.75 9 10.8 r=3/4 BPSK

1.00 12 14.4 r=1/2 QPSK

1.50 18 21.7 r=3/4 QPSK

2.00 24 28.9 r=1/2 16 QAM

2.50 30 36.1 r=5/8 16 QAM

3.00 36 43.3 r=3/4 16 QAM

3.50 42 50.6 r=7/12 64QAM

4.00 48 57.8 r=2/3 64QAM

4.50 54 65 r=3/4 64QAM

5.00 60 72.2 r=5/6 64QAM

5.00 60 72.2 r=5/8 256 QAM

6.00 72 86.7 r=3/4 256 QAM

7.00 84 101.1 r=7/8 256 QAMNotes: 1) short OFDM symbols; 2) expanded OFDM symbols with short guard interval

August 2004


Slide 29

doc.: IEEE 802.11-04/0873r2

Submission

Spatial Processing• Two forms of Spatial Processing for data transmission

– Eigenvector Steering (ES): Tx attempts to steer optimally to intended Rx– Spatial Spreading (SS): Tx does not attempt to steer optimally to specific Rx

• ES operating modes take advantage of channel reciprocity inherent in TDD systems– Full MIMO channel characterization required at Tx– Calibration procedure required– Tx steering using per-bin channel eigenvectors from SVD– Rx steering renders multiple Tx streams orthogonal at receiver, allowing transmission of

multiple independent spatial streams– This approach maximizes data rate and range

• SS Operation for partially informed transmitter– No explicit knowledge of channel or channel eigenvectors at Tx– Tx has only data rate per wideband spatial channel– Transmit full power regardless of the number of streams Tx’d

• Requirement for robust CSMA operation– Maximize diversity per transmitted data stream

• Minimize outage probability maximize throughput– Backwards compatible operation– Spatial spreading of data with simple unitary matrices– Cyclic diversity transmission per Tx antenna to introduce additional diversity

August 2004


Slide 30

doc.: IEEE 802.11-04/0873r2

Submission

Spatial Channels and Spatial Streams• ES and SS approaches result in synthesis of spatial channels, or

wideband spatial channels.– Also referred to as eigenmodes, or wideband eigenmodes

• On MIMO channel between a transmitting STA with NTx antennas and a receiving STA with NRx antennas, maximum of

wideband spatial channels available.• Each resulting spatial channel may carry a payload, referred to as a

spatial stream.• Number of spatial streams, NS, may not be greater than the Nm

min ,m Tx RxN N N

August 2004


Slide 31

doc.: IEEE 802.11-04/0873r2

Submission

Over-the-Air Calibration

• ES approach requires over-the-air calibration procedure – Compensates for amplitude and phase differences in Tx

and Rx chains– Calibration required infrequently– typically on

association only– Simple exchange of calibration symbols and

measurement information requires little overhead and background processing• Total of ~1000 bytes of calibration data exchanged for 2x2 link• ~2800 bytes for 4x4 link

August 2004


Slide 32

doc.: IEEE 802.11-04/0873r2

Submission

Legacy and MIMO Training for 2, 3, and 4 Tx

• STS: 802.11a STS• LTS: 802.11a LTS• SIG1: 802.11a SIGNAL• SIG2: Extended SIGNAL• MTSn: MIMO training symbol

for Tx antenna n• CDx: x ns cyclic delay

STS LTS SIG 1 SIG 2 MTS 1 MTS 1

STS CD50 LTS CD50

SIG 1 CD50

SIG 2 CD50

MTS 2 CD50

-MTS 2 CD50


STS CD50 LTS CD50

SIG 1 CD50

SIG 2 CD50

MTS 2 CD50

-MTS 2 CD50

STS CD100

LTS CD100

SIG 1 CD100

SIG 2 CD100

MTS 3 CD100

MTS 3 CD100

STS CD150

LTS CD150

SIG 1 CD150

SIG 2 CD150

MTS 4 CD150

-MTS 4 CD150

MTS 1 MTS 1

MTS 2 CD50

-MTS 2 CD50

-MTS 3 CD100

-MTS 3 CD100

-MTS 4 CD150

MTS 4 CD150

Tx 1

Tx 2

Tx 1

Tx 2

Tx 3

Tx 4


STS CD50 LTS CD50

SIG 1 CD50

SIG 2 CD50

MTS 2 CD50

W·MTS 2 CD50

STS CD100

LTS CD100

SIG 1 CD100

SIG 2 CD100

MTS 3 CD100

W2·MTS 3 CD100

MTS 1

W2·MTS 2 CD50

W4·MTS 3 CD100

Tx 1

Tx 2

Tx 3

W = exp(j2π/3)

8 µs 8 µs 4 µs 4 µs 4 µs 4 µs 4 µs 4 µs

August 2004


Slide 33

doc.: IEEE 802.11-04/0873r2

Submission

Preamble and Training Sequences• Use Standard 802.11a preamble with enhancements

– Time and Frequency acquisition and AGC– Last short preamble symbol is inverted to provide improved timing

reference– Cyclic delay is applied across Tx antennas

• Cyclic delay applied to entire 8 µs short preamble• Cyclic delay applied to entire 8 µs long preamble • Delay increment Tcd=50 ns

• Extended SIGNAL field

August 2004


Slide 34

doc.: IEEE 802.11-04/0873r2

Submission

Preamble and Training Sequences• MIMO Training Sequence

– Orthonormal (in time) cover sequence• Walsh for 2 Tx and 4 Tx• Fourier for 3 Tx

– Cyclic shift k*50 ns on Tx antenna k– Combination of orthonormal cover and cyclic shift ensures equal Rx

power on all preamble symbols• Length of MIMO training sequence is always equal to the number of

transmit antennas– This ensures that the receiver can always track the full channel state, and

thus make fully informed rate decisions– Also ensures stable received power estimate

• Two forms of MIMO Training– Steered MIMO Training Sequence supports Eigensteered operation– Direct MIMO Training Sequence supports direct channel estimation

August 2004


Slide 35

doc.: IEEE 802.11-04/0873r2

Submission

Feedback for ES and SS Modes• Rate adaptation

– Receiving STA determines preferred rates on each of up to four wideband spatial channels

• One rate per wideband spatial channel – NO adaptive bit loading– Sends one 4-bit value per spatial channel, differentially encoded, (13 bits

total) to inform corresponding STA/AP of rate selections• Corresponding STA/AP uses this info to select Tx rates• Piggy-backed on out-going PPDUs

– SS Mode can use single rate across all spatial streams• Channel state information

– For ES operation, Tx must have full channel state information– This is obtained through exchange of transmitted training sequences that

are part of PLCP header• Very low overhead.

– Distributed computation of steering vectors (SVD calculation)• STAs do SVD, send resulting training sequence to AP

– For SS operation, unsteered training sequences transmitted in PLCP header to support channel estimation at receiver

• Feedback operates with asynchronous MAC transmissions

August 2004


Slide 36

doc.: IEEE 802.11-04/0873r2

Submission

Wideband Eigenmodes and OFDM

• OFDM chosen so that subcarrier spacing << coherence bandwidth

• Find ranked eigenmodes/eigenvalues in each OFDM subcarrier:

• Ensemble of eigenmodes of a given rank across OFDM symbol comprise a wideband eigenmode

• Highest ranked wideband eigenmodes exhibit very little frequency selectivity

• Smallest ranked wideband eigenmode exhibits frequency selectivity of underlying channel

1 2( ) ( ) ( )Nk k k

August 2004


Slide 37

doc.: IEEE 802.11-04/0873r2

Submission

Wideband Eigenmodes TGn Channel B

Power is relative to average total receive power at a single antenna

August 2004


Slide 38

doc.: IEEE 802.11-04/0873r2

Submission

Use of Reference for Eigensteering• STAs must be calibrated to use Tx steering• MIMO training sequence part of PLCP preamble for all PPDUs• STA can compute Tx and Rx steering vectors from either steered MIMO training

sequence or direct MIMO training sequence– If unsteered MIMO training sequence is used, SVD or similar is required to compute

steering vectors from direct channel estimate– One STA in a corresponding pair must compute SVD from direct channel estimate– STA that does SVD sends steered MIMO training sequence in PLCP preamble of

PPDU with steered data. Receiving STA uses steered MIMO training sequence to compute Rx and Tx steering

– STA not computing SVD must send direct MIMO training sequence to STA computing SVD

• Can be part of broadcast message such as SCHED at AP• Can be MIMO training sequence in PLCP preamble

• Support of bi-directional steering with SVD calculation distributed to client STAs– Off-loads SVD from AP– Minimal complexity hit to STA

August 2004


Slide 39

doc.: IEEE 802.11-04/0873r2

Submission

Simulation of Spatial Multiplexing Using Tx & Rx Eigensteering

• Common MIMO Training Sequence broadcast by AP once every SCAP (Scheduled Access Period) (…,t0,t3,…). Forward link (FL) channel coefficients estimated by STA receiver

• FL Dedicated MIMO Training Sequence (steered) transmitted by AP at t1=0.5 ms, immediately followed by FL data PPDU

• Reverse link (RL) Dedicated MIMO Training Sequence transmitted by STA at t2=1.5 ms, immediately followed by RL data PPDU

• Transmit and receive steering vectors derived from most recent channel estimates

• Closed-loop rate adaptation: FL and RL data rates determined based on receive SNRs observed in previous frames

FLdata

SCAP (2.048 ms)

FL data RL data

t1=0.5 mst0=0 ms t2=1.5 ms t3=2.048 ms

August 2004


Slide 40

doc.: IEEE 802.11-04/0873r2

Submission

Simulation Parameters• 2x2, 4x2, and 4x4 system configurations• IEEE 802.11n channel models B, D and E• IEEE 802.11n impairment models:

– Time-domain channel simulator with 5x oversampling rate (Ts=10 ns)– Rapp nonlinear power amplifier model (IM1):

• Total Tx power = 17 dBm; Psat = 25 dBm• 2x2 backoff = 11 dB per PA; 4x4 backoff = 14 dB per PA

– Carrier frequency offset : -13.675 PPM (IM2)– Sampling clock frequency offset: -13.675 PPM (IM2)– Phase noise at both transmitter and receiver (IM4)

• 100 channel realizations generated for each SNR point • In each channel realization the Doppler process evolves over three

SCAPs to allow simulation of channel estimation, closed-loop rate adaptation and FL/RL data transmission in fading conditions

• Stopping criterion: 10 packet errors or 400 packets transmitted per channel realization

• Targeted packet error rate performance: mean PER <= 1%

August 2004


Slide 41

doc.: IEEE 802.11-04/0873r2

Submission

PHY Simulation Results• What we simulated

– Standard OFDM symbols • Eigenvector Steering• Spatial Spreading

– Expanded OFDM symbols (52 data tones/400ns guard interval: SGI-52)• Eigenvector Steering• Spatial Spreading

• PER vs SNR for Fourier channel 1×1, 2×2, 3×3, and 4×4 (CC59) – All above cases

• PHY throughput and PER vs SNR; CDFs of throughput and PER – Standard OFDM symbols, ES & SS

• 2×2, 4×4, and 4×2• Channels B, D, and E

– SGI-52 OFDM symbols, ES & SS• 2×2, 4×4, and 4×2• Channel B

August 2004


Slide 42

doc.: IEEE 802.11-04/0873r2

Submission

PHY Simulation Results (2)• Average PER vs SNR

– Standard OFDM symbols, ES & SS • 2×2, 4×4, and 4×2• Channels B, D, and E

August 2004


Slide 43

doc.: IEEE 802.11-04/0873r2

Submission

Highlights of PHY Simulation Results• Highest PHY throughputs achieved in Eigenvector Steering mode

– Eigenvector steering is very effective in combination with 802.11 convolutional codes

– 256-QAM contributes substantially to throughput in ES mode. ES array gain overcomes effects of receiver impairments in these cases

• Convolutional codes not as effective in Spatial Spreading mode– High SNR variance across subcarriers within an OFDM symbol on an SS

spatial channel degrades the performance of convolutional codes– This is particularly pronounced on channel B and on link with 4 Tx and 2

Rx.– Reducing number of streams (NS < min(NTx,NRx)) reduces variance and

improves overall performance.• Rate adaptation has clearly demonstrated benefits

– Many cases where a given fixed rate has poor average performance, but using rate adaptation, higher overall throughput is achieved with lower PER

– Part of rate adaptation is controlling the number of streams used

August 2004


Slide 44

doc.: IEEE 802.11-04/0873r2

Submission

Highlights of PHY Simulation Results• Use of shortened guard interval and extra data subcarriers contributes

to increased throughput– Increased vulnerability to delay spread and ACI.– Improved receiver design should help with this– Optional mode can be turned off in the presence of too much delay spread– Many environments where high rates will be required, such as residential

media distribution, have naturally low delay spread.

August 2004


Slide 45

doc.: IEEE 802.11-04/0873r2

Submission

PHY Throughput and PER Ch. B, 2×2: Eigenvector Steering

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

120

140

160

ES, 802.11n Ch. B, 2x2

Es/N0 (dB)

Ave

rage

Thr

ough

put (

Mbp

s)FLRL

0 5 10 15 20 25 30 35 40 45 5010

-3

10-2

10-1

Ave

rage

PER

August 2004


Slide 46

doc.: IEEE 802.11-04/0873r2

Submission

PHY Throughput and PER Ch. B, 2×2: Spatial Spreading

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

120

140

160

SS, 802.11n Ch. B, 2x2

Es/N0 (dB)

Ave

rage

Thr

ough

put (

Mbp

s)FLRL

0 5 10 15 20 25 30 35 40 45 50

10-2

10-1

Ave

rage

PER

August 2004


Slide 47

doc.: IEEE 802.11-04/0873r2

Submission

PHY Throughput and PER Ch. B, 2×2: Eigenvector Steering, SGI-52

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

120

140

160

180

200

ES, 802.11n Ch. B, 2x2

Es/N0 (dB)

Ave

rage

Thr

ough

put (

Mbp

s)

FLRL

0 5 10 15 20 25 30 35 40 45 50

10-3

10-2

10-1

Ave

rage

PER

August 2004


Slide 48

doc.: IEEE 802.11-04/0873r2

Submission

PHY Throughput and PER Ch. B, 4×4 : Eigenvector Steering

0 5 10 15 20 25 30 35 40 45 500

40

80

120

160

200

240

280

320

ES, 802.11n Ch. B, 4x4

Es/N0 (dB)

Ave

rage

Thr

ough

put (

Mbp

s)FLRL

0 5 10 15 20 25 30 35 40 45 50

10-4

10-3

10-2

10-1

Ave

rage

PER

August 2004


Slide 49

doc.: IEEE 802.11-04/0873r2

Submission

PHY Throughput and PER Ch. B, 4×4: Eigenvector Steering, SGI-52

0 5 10 15 20 25 30 35 40 45 500

40

80

120

160

200

240

280

320

360

400

ES, 802.11n Ch. B, 4x4

Es/N0 (dB)

Ave

rage

Thr

ough

put (

Mbp

s)

FLRL

0 5 10 15 20 25 30 35 40 45 5010

-3

10-2

10-1

Ave

rage

PER

August 2004


Slide 50

doc.: IEEE 802.11-04/0873r2

Submission

PHY Throughput and PER Ch. B, 4×2 : Eigenvector Steering

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

120

140

160

ES, 802.11n Ch. B, 4x2

Es/N0 (dB)

Ave

rage

Thr

ough

put (

Mbp

s)FLRL

0 5 10 15 20 25 30 35 40 45 50

10-4

10-3

10-2

10-1

Ave

rage

PER

August 2004


Slide 51

doc.: IEEE 802.11-04/0873r2

Submission

PHY Throughput and PER Ch. B, 4×2: Spatial Spreading

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

120

140

160

SS, 802.11n Ch. B, 4x2

Es/N0 (dB)

Ave

rage

Thr

ough

put (

Mbp

s)FLRL

0 5 10 15 20 25 30 35 40 45 50

10-3

10-2

10-1

Ave

rage

PER

August 2004


Slide 52

doc.: IEEE 802.11-04/0873r2

Submission

Effect of increased latency on Eigensteering: Average Throughput, 2x2, Channel B

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

120

140

160

ES, 802.11n Ch. B, 2x2, FL

Es/N0 (dB)

Aver

age

Thro

ughp

ut (M

bps)

1x5x10x20x

August 2004


Slide 53

doc.: IEEE 802.11-04/0873r2

Submission

Summary

• MIMO PHY design builds on existing 802.11a,g PHY design• Two operating modes provide highly robust operation under a wide range

of conditions– Eigenvector Steering provides best rate/range performance– Spatial Spreading

• Adaptive rate control through low-overhead rate feedback supports sustained high throughput operation

• Low-overhead training sequence exchange supports high-capacity Eigenvector Steered operation for best rate/range performance

• Spatial Spreading operation provides robust high throughput operation when Tx does not have sufficiently accurate channel state information

• MAC enhancements are required to take full advantage of HT PHY– Required for 100 Mbps throughput in realistic operating environments– QoS-sensitive applications are not satisfied with EDCA

doc.: ieee 802.11-04/0873r2 submission august 2004 john ketchum, et al, qualcommslide 1...

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