TGn Sync An IEEE 802'11n Protocol Standard Proposal Alliance PHY Overview

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TGn SyncAn IEEE 802.11n Protocol Standard
Proposal AlliancePHY Overview

• Agere Systems, Inc.
• Atheros Communications, Inc.
• Cisco Systems, Inc.
• Intel Corporation
• Nokia Corporation
• Royal Philips Electronics
• Sony 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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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 ? minNTx,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.
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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

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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

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HT PPDU Detection

• Solution HT-SIG is modulated using
  Q-BPSK
• Detection algorithm

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Tx Data Path Architecture
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Basic Tx Data Path
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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.
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Legacy Compatible Preamble
CDD
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.
or single antenna
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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

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Spatial Stream Tone Interleaving

• 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
• gt Distinct symbol designs for different Nss

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Why Tone Interleaving?
Clipping in Rx ADC?
No toneinterleaving
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.
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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

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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

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
  forL-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)

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L-SIG HT-SIG Coding Modulation
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Duplicate Receiver
Combining Equalizer Simple MRC combining Note
If upper sub-channel is not present, combining
weights are zero.
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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 lt NTx ? Array Gain
• Tx Diversity via CDD

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SVD-MIMO Data Path
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

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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