View
215
Download
0
Category
Preview:
Citation preview
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
2
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
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
11 11b 11a 243 Mbps
TGn Sync 2x2-40 MHz
4
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
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 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 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
10
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
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
STF LTFL-
SIGHT-SIG
STF LTFLegacy
Data
or
Legacy Preamble
L-SIG
13
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
14
Tx Data Path Architecture
15
Basic Tx Data Path
channelencoder
frequencyinterleaver
QAMmap
IFFT
spatialstreamparser
GI &RF chain
puncture
frequencyinterleaver
QAMmap
IFFTGI &
RF chain
16
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
17
Training Fields
18
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
19
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.
20
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
21
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
22
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
23
HT - Short Training Field
Precise specification of STS is TBD
24
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
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
0 32 64
16 30 442 58-44 -30 -16-58 -2
-32-64
Note: Actual pilot locations are TBD
27
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
28
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
29
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
30
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
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 20-40 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
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.
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
steeringmatrix
V
spatial streamQAM symbol
vectorX
ant.-to-ant.channel
H
receiverobservable
vectorY
combined channel H
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 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 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
42
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
Recommended