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

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

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

Robust 2x2 40 MHz Solution

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}

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

PPDU Format

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

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.

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

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

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

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

Tx Data Path Architecture

Basic Tx Data Path

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

Training Fields

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

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

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

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

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.

HT - Short Training Field Precise specification of STS is TBD

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

40 MHz Channels

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

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

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)

L-SIG & HT-SIG Coding & Modulation

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

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

Beam-Forming Modes (optional)

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

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.

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

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

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

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.

To Be Continued!

Back Up

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

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