1. TGn Sync An IEEE 802.11n Protocol Standard Proposal Alliance PHY Overview
Agere Systems, Inc.
Atheros Communications, Inc.
Cisco Systems, Inc.
Intel Corporation
Nokia Corporation
Royal Philips Electronics
Sony Corporation
Toshiba Corporation
June 2004

2. Fundamental Philosophies
MIMO Evolutions of 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 n MAC features
Support for Advanced Features
SVD MIMO, Advanced coding

3. 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 b a
 243 Mbps TGn Sync 2x2-40 MHz

4. Robust 2x2 40 MHz Solution

5. 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}

6. 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

7. PPDU Format

8. PPDU Fields Legacy Compatible Preamble HT Signal Field
STF, L-LTF, L-SIG
This is the key to PHY support for seamless legacy interoperability
HT Signal Field
MIMO Training Fields
MIMO Data

9. PPDU Format
“Legacy compatible” means that a legacy a/g device can acquire, demodulate and decode through the legacy Signal Field (L-SIG).
HT-SIG onward is NOT legacy compatible.

10. Spoofing RATE and LENGTH  PPDU length in OFDM symbols RATE Spoofing
 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

11. 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

12. 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

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

14. Tx Data Path Architecture

15. Basic Tx Data Path

16. Basic Tx Data Path FEC Coding Spatial stream parsing
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

17. Training Fields

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

19. 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

20. HT Training Fields HT-STS HT-LTF Tone interleaving of spatial streams
Used for 2nd AGC
HT-LTF
Used for channel estimation
Additional frequency or time alignment
Tone interleaving of spatial streams

21. 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
=> Distinct symbol designs for different Nss

22. Why Tone Interleaving?
Clipping in Rx ADC?
No tone interleaving
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.

23. HT - Short Training Field
Precise specification of STS is TBD

24. Long Training Fields Guard Interval Other Details TBD
1.6  sec for L-LTF
0.8  sec for HT-LTF
Other Details TBD

25. 40 MHz Channels

26. 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

27. Duplicate Format for 40 MHz Legacy 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

28. 40 MHz PPDU Format Duplicate Format Preamble HT Pare
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)

29. L-SIG & HT-SIG Coding & Modulation

30. Duplicate Receiver Combining Equalizer: Simple MRC combining
Note: If upper sub-channel is not present, combining weights are zero.

31. 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 inter-op

32. Beam-Forming Modes (optional)

33. 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

34. SVD-MIMO Data Path Per Spatial Stream Puncturing Per Spatial Stream
Power Settings
Per Subcarrier
Beam Steering
Transformation
Per spatial stream puncturing & power setting are used only for SVD-MIMO.

35. 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

36. 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

37. LTF Requirements Per spatial stream training LTF Tx at full Tx power
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

38. 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.

39. To Be Continued!

40. Back Up

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

42. Why full power LTF? Why 2 symbols per LTF?
Energy
per spatial stream
per OFDM symbol
Equivalent Noise Variance:
Energy used to train a spatial stream
For 11a, and (2 symbols in LTF)
For equivalent performance
Full power
2 symbols
See Wang & Sadowsky for generalizations – ref. TBD