1

TGn SyncAn IEEE 802.11n Protocol Standard Proposal Alliance

PHY Overview

Agere Systems, Inc.Atheros Communications, Inc.

Cisco Systems, Inc.Intel CorporationNokia Corporation

Royal Philips ElectronicsSony Corporation

Toshiba Corporation

June 2004

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Fundamental Philosophies

MIMO Evolutions of 802.11 OFDM PHY Reuse of legacy blocks

FEC coding, interleaving, QAM mapping, etc. Self Defining Packets

PPDU decoding with NO a priori knowledge of transmission Mode

Seamless Legacy Interoperability 802.11a & g with no performance penalty

Minimize PLCP Overhead Support for new 802.11n MAC features Support for Advanced Features

SVD MIMO, Advanced coding

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Key Elements

20 and 40 MHz Channels – both Mandatory Baseline 2x2 – 40 MHz

Robust & low cost PAR solution MIMO Requirements

Minimum 2 spatial streams required Maximum 4 spatial streams

5x Peak Data Rate 2 Mbps 11 Mbps 54 Mbps

11 11b 11a 243 Mbps

TGn Sync 2x2-40 MHz

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Over-The-Air Throughput - Model D

0

20

40

60

80

100

120

140

160

180

200

220

240

0 5 10 15 20 25 30 35

Post Detection SNR (dB)

OT

A T

hro

ug

hp

ut

(Mb

ps)

140 Mbps 100 Mpbs Top-of-MAC

@ 70% MAC efficiency

2x2 - 40 MHz

2x2 - 20 MHz

2x3 - 20 MHz

Dashed: Includes 7/8 rate codeand 0.4 mu sec Guard Interval

Robust 2x2 40 MHz Solution

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Terminology & Notation

“Legacy” If not otherwise specified, refers to 11a or

11g Spatial Stream

An encoded and modulated stream of data MIMO SDM (spatial division multiplexing) maps

multiple spatial streams onto the antenna array

Some Parameters NSS = number of Spatial Streams NTx = number of transmitting antennas NRx = number of receiving antennas NSS min{NTx,NRx}

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Terminology & Notation

Short Training STF = Short Training Field

STS = Short Training Symbol Long Training

LTF = Long Training Field LTS = Long Training Symbol

L-LTF = Legacy LTF MIMO-LTF = additional LTF for MIMO

Signal Fields L-SIG – Legacy Signal Field (SIGNAL in 11a) HT-SIG – High Throughput Signal Field

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PPDU Format

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PPDU Fields

Legacy Compatible Preamble STF, L-LTF, L-SIG This is the key to PHY support for

seamless legacy interoperability HT Signal Field MIMO Training Fields MIMO Data

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PPDU Format

“Legacy compatible” means that a legacy 802.11a/g device can acquire, demodulate and decode through the legacy Signal Field (L-SIG).

HT-SIG onward is NOT legacy compatible.

L-STF L-LTFL-

SIGHT-SIG HT LTF HT LTF Data

Legacy CompatiblePreamble

HT Part

HTSTF

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Spoofing

RATE and LENGTH PPDU length in OFDM symbols

RATE modulation & code rate not compatible with the HT MIMO part

Spoofing Spoofing means that the legacy RATE and LENGTH fields

are falsely encoded in order to determine a specified length

L-SIG RATE = 6 Mbps spoofing duration up to ~5 msec

L-STF L-LTFL-

SIGHT-SIG HT LTF HT LTF Data

Legacy RATE and LENGTH fields => Packet Length in OFDM Symbols

HTSTF

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HT-SIG Contents

MCS = Modulation Coding Scheme modulation(s), code rate(s) & NSS

Length – up to 262 k bytes PPDU Option Flags (see part II) Scrambler Initialization

Single point of failure requires robust coding provided by HT-SIG

Strong 8 bit CRC protection CRC also covers L-SIG

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HT PPDU Detection

No a priori indication of HT vs. legacy packet type First point of differentiation is HT-SIG vs. Legacy

Data Cannot use reserve bit in L-SIG

Used by many legacy devices for additional parity

STF LTFL-

SIGHT-SIG

STF LTFLegacy

Data

or

Legacy Preamble

L-SIG

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HT PPDU Detection Solution: HT-SIG is modulated using Q-BPSK

Detection algorithm:

2 2

HT

Q

subcarriers subcarriersLegacy

I

I

Q

+1-1 I

Q+1

-1

BPSK Q-BPSK

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Tx Data Path Architecture

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Basic Tx Data Path

channelencoder

frequencyinterleaver

QAMmap

IFFT

spatialstreamparser

GI &RF chain

puncture

frequencyinterleaver

QAMmap

IFFTGI &

RF chain

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Basic Tx Data Path

FEC Coding Conventional K = 7 Convolutional Code

Rates: 1/2, 2/3 and ¾ Needed to support legacy operations

Optional LDPC Spatial stream parsing

Spatially interleaves bits Frequency Interleaving

Block interleaver w/ QAM bit rotation (like 11a) 20 MHz 16 columns freq. sep. = 3 subcarriers 40 MHz 18 columns freq. sep. = 6 subcarriers

QAM Modulation BPSK, QPSK, 16 QAM and 64 QAM BICM (bit-interleave coding/modulation)

Same bit mapping as 11a

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Training Fields

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Training Fields

These space-time diagrams apply to both 20 and 40 MHz channels.

LTS12GI L SIGLTS2 HT SIG 1 HT SIG 2 LTS1 LTS2 DATA

DATA

DATA

DATA

DATA

DATA

DATA

DATA

DATA

STF L LTF L SIG HT SIG HT LTF Data

Antenna

1

2

3

LTS12GI LTS2

LTS12GI LTS2

L SIG HT SIG 1 HT SIG 2

L SIG HT SIG 1 HT SIG 2

HTSTS

HTSTS

HTSTS

LTS1 LTS2

LTS1 LTS2

LTS1 LTS2

LTS1 LTS2

LTS1 LTS2

HT LTFHT STF

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Legacy Compatible PreambleCDD

LTS12GI L SIGLTS2 HT SIG 1 HT SIG 2

L-STF L-LTF L-SIG HT-SIG

Antenna

1

2

3

LTS12GI LTS2

LTS12GI LTS2

L SIG HT SIG 1 HT SIG 2

L SIG HT SIG 1 HT SIG 2

HTSTS

HTSTS

HTSTS

LTS12GI L SIGLTS2 HT SIG 1 HT SIG 2

L-STF L-LTF L-SIG HT-SIG

Antenna

1

2

3

HTSTS

HTSTS

HTSTS

or single antenna

The L-STF, L-LTF, L-SIG and HT-SIG is transmitted as a single spatial stream. This may be either transmitted on all Tx antennas via a method such as Cyclic Delay Diversity, or on a single antenna. These are implementation options.

Requirement: These fields must be transmitted in an omni-directional mode that can be demodulated by legacy receivers.

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HT Training Fields

HT-STS Used for 2nd AGC

HT-LTF Used for channel estimation Additional frequency or time alignment

Tone interleaving of spatial streams

HT SIG 2 LTS1 LTS2 DATA

DATA

DATA

HT LTF

HT SIG 2

HT SIG 2

HTSTS

HTSTS

HTSTS

LTS1 LTS2

LTS1 LTS2

LTS1 LTS2

LTS1 LTS2

LTS1 LTS2

HT LTFHTSTF

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Spatial Stream Tone Interleaving

1 SpatialStream

2 SpatialStreams

3 SpatialStreams

4 SpatialStreams

• Color indicate spatial stream• Each training symbol has equal representation from each spatial stream• For HT-STS, symbols are selected to control beam forming• For HT-LTS, symbols are selected to control PAPR• => Distinct symbol designs for different Nss

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Why Tone Interleaving?

Rx Power STS Error

STF LTF 1 LTF 2 Data

Even if all spatial streams are transmitted with equal power, the can create power differences at the receiver. For Model B (15 n sec delay spread) this can result frequent power differentials of ~6dB between spatial streams.

Tone interleaving of spatial streams results in averaging power levels across all spatial streams on each training symbol. The result is essentially no Rx power fluctuation of the STS and LTS with respect to the data symbols.

Clipping in Rx ADC?

No toneinterleaving

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HT - Short Training Field

Precise specification of STS is TBD

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Long Training Fields

Guard Interval 1.6 sec for L-LTF 0.8 sec for HT-LTF

Other Details TBD

LTS1GI LTS2

3.2sec 3.2sec

0.8 sec

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40 MHz Channels

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40 MHz Channel

108 Data + 6 Pilot Subcarriers Duplicate Format on Legacy Part

Provides Interoperability with 20 MHz legacy clients within a 40 MHz BSS

0 32 64

16 30 442 58-44 -30 -16-58 -2

-32-64

Note: Actual pilot locations are TBD

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Duplicate Format for 40 MHzLegacy Preamble

40 MHz channel = two 20 MHz channels Duplicate Format for Legacy Preamble

Applied to the legacy STF, LTF and SIG 90 deg phase shift on upper sub-channel for

L-STF, L-LTF, MIMO-LTF and L-SIG Modulate Both 20 MHz subchannels exactly as

if for legacy 11a modulation Why Duplicate Format?

Why not preamble in one 20 MHz subchannel? Need full frequency observability for STF & LTF

Minimal PLCP overhead

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40 MHz PPDU Format

Duplicate Format Preamble Provides interoperability with 20 MHz legacy STAs Data, pilot and training tones in each 20 MHz subchannel are

identical to corresponding 20 MHz format. 90 deg phase shift on upper sub-channel controls PAPR

HT Pare 108 data tones + 6 pilots 3 center nulls (not shown)

DuplicateL-STF(90 deg)

DuplicateL-STF

DuplicateL-LTF(90 deg)

DuplicateL-LTF

Dup.

L-SIG(90 deg)

Dup.

L-SIG

DuplicateHT-SIG(90 deg)

DuplicateHT-SIG

HT

STS

HT-LTF1 HT-LTF2 Data Data

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L-SIG & HT-SIG Coding & Modulation

conv.encoder

rate = 1/2

11aInterleaver

BPSKmap

IFFT

uppersub-channel

48 modulation symbols

24 bitSF

lowersub-channel

BPSKmap

BPSKmap

BPSKmap

BPSKmap

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Duplicate Receiver

Combining Equalizer: Simple MRC combiningNote: If upper sub-channel is not present, combining weights are zero.

Comb.Eq.

Comb.Eq.

Comb.Eq.

Comb.Eq.

Comb.Eq.

FFT

uppersub-channel

lowersub-channel 11a

InterleaverViterbi

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20-40 MHz Interoperability

20 MHz PPDU 40 MHz receiver Combine modulation symbols from upper & lower sub-bands 20 MHz PPDU in lower sub-channel

zero combining weights in upper subchannel No loss in performance relative to a 20 MHz receiver

Use differential sub-channel energy to detect 20 v. 40 MHz signals

40 MHz PPDU 20 MHz receiver One sub-channel is sufficient to decode the L-SIG Detects only half of the 40 MHz signal

3 dB performance penalty for 20 MHz clients

See MAC slides for additional information on 20-40 inter-op

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Beam-Forming Modes

(optional)

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Beam Forming Modes

Channel Adaptive Beam-Forming SVD-MIMO Requires CSI (Channel State

Information) Un-trained Beam Forming

No CSI Why?

NSS < NTx Array Gain Tx Diversity via CDD

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channelencoder

frequencyinterleaver

QAMmap

BeamSteeringTrans-

formation

persubcarrier

IFFT

IFFT

IFFT

GI & analogRF

GI & analogRF

GI & analogRF

TxN transmit antennaSSN spatial streams

puncture

spatialstreamparser

SpatialStreamPowerRatios

frequencyinterleaver

QAMmap

puncture

SVD-MIMO Data Path

Per Spatial StreamPuncturing

Per Spatial StreamPower Settings

Per SubcarrierBeam SteeringTransformation

Per spatial stream puncturing & power settingare used only for SVD-MIMO.

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Per Spatial Stream LTS

LTF is applied prior to steering matrix One LTF per spatial stream Rx estimates combined channel

steering matrix x antenna-to-antenna channel

steeringmatrix

V

spatial streamQAM symbol

vectorX

ant.-to-ant.channel

H

receiverobservable

vectorY

combined channel H

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Per Spatial Stream Training

Self Defining Packets No priori knowledge of beam steering matrices MIMO equalizer requires combined channel

Not the antenna-to-antenna channel

Minimal PHY complexity Eliminates a matrix multiplication prior MIMO

equalizer Minimal Overhead

Num LTF = NSS NTx

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LTF Requirements

Per spatial stream training One LTF per spatial stream

LTF Tx at full Tx power Full Tx power = total transmit power across

all spatial streams during the DATA part of the PPDU

This rule applies even when power settings vary across spatial streams

2 LTS (symbols) per LTF

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Channel Sounding PPDUs

Channel sounding provides estimation of the antenna-to-antenna channel. This is required for SVD calculations.

A channel sounding PPDU is a minor extension of a per antenna MIMO PPDU. MIMO spatial streams are transmitted per antenna, but sometimes there are more Tx antennas than spatial streams. In this case the number of HT-LTF equals the number of Tx antennas. HT-SIG contains a flag to indicate that a PPDU can be used for channel sounding, and the number of Tx antennas. Training data is transmitted on only the first Nss antennas.

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To Be Continued!

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Back Up

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Tentative HT-SIG Contents

Length 18 MCS 6 Adv. Coding 1 Sounding Packet Flag 1 Num. Tx Antenna 2 Legacy LTS Reuse 1 Aggregrate 1 Scrambler Init 2 CRC 8 Conv. Code Tail 6 Total 46 Spares 2

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Why full power LTF?Why 2 symbols per LTF?

Equivalent Noise Variance:

See Wang & Sadowsky for generalizations – ref. TBD

0cov 1 Xe SS

T

N N

N IEE

Energyper spatial streamper OFDM symbol

Energy used totrain a spatial stream

For 11a, and (2 symbols in LTF)

1SSN 2T XE E

12

2X

SS T SS XT

N N E

E EE

For equivalent performance

2 symbols Full power