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July 2001
S. Halford, et al IntersilSlide 1
doc.: IEEE 802.11-01/436r0
Submission
CCK-OFDM Normative CCK-OFDM Normative Text SummaryText Summary
Steve Halford
Mark Webster
Jim ZyrenIntersil Corporation
July 2001
S. Halford, et al IntersilSlide 2
doc.: IEEE 802.11-01/436r0
Submission
CCK-OFDM Proposal: Modes
• CCK-OFDM at 6,12, & 24 Mbps is required• 802.11a mode is optional • Other CCK-OFDM rates are optional
Mandatory Modes for TGg SystemsMandatory Modes for TGg Systems• Support all 802.11b mandatory functions• Support CCK-OFDM at 6, 12, and 24 Mbps
High Throughput OptionHigh Throughput Option
• Uses 802.11a Preamble• Supports rates from 6 to 54 Mbps• Not backward compatible to 802.11b
802.11a at 2.4 GHzHigh Rate Compatible OptionHigh Rate Compatible OptionCCK-OFDM at optional rates
• Provides backward compatibility to 802.11b systems• Adds support for rates of 9, 18, 36, 48, and 54 Mbps
July 2001
S. Halford, et al IntersilSlide 3
doc.: IEEE 802.11-01/436r0
Submission
TGg Packet Structure: CCK-OFDM Mode
Packets use existing preamble & OFDM modulation Preamble uses barker word modulation
Minor modifications to 802.11b preamble OFDM Modulation used to send data
Based on 802.11a Replaces PSDU of 802.11b Also adds an additional OFDM header & SIFs Pad
SyncField
Header Data Field
Preamble192 useconds (long)96 useconds (short)
6, 9, 12, 18, 24, 36, 48 & 54 MbpsUsing OFDM modulation
OFDMPreamble
OFDMPreamble
Existing IEEE802.11bPreamble
Barker Word Modulation
SIFs Pad
SIFs Pad6 useconds
OFDM Modulation
SFD
July 2001
S. Halford, et al IntersilSlide 4
doc.: IEEE 802.11-01/436r0
Submission
Use of .11b Header for TGg: Overview
Sync Field is unchanged Used for AGC, carrier, & time acquisition Used for channel estimation (if needed)
Sync Field delimiter denotes the end of the sync field Header is used convey parameters about the PSDU
Indicates OFDM modulation used for PSDU Total Length of packet For CCK-OFDM, data rate is not used to determine rate
Set to 2 Mbps so that all legacy equipment will decode as valid
Sync Field Header PSDU (OFDM Modulation)
IEEE802.11b PreambleBarker Word Modulation
used for 802.11gOFDM Modulation
SFD
July 2001
S. Halford, et al IntersilSlide 5
doc.: IEEE 802.11-01/436r0
Submission
Header Field for 802.11g CCK-OFDM
HEADER48 BITS
• Data rate is set to 2 Mbps for all OFDM rates
SIGNAL8 BITS
SERVICE8 BITS
LENGTH16 BITS
CRC16 BITS
• Unchanged
• 1 bit to denote OFDM mode.
• Unchanged in format.• Value used is given on later slide• The Length Field is adequate, since measured in usecs.• OFDM proposal uses PSDU length in an integer number of usecs.
802.11 Sync Field PSDU: OFDM ModulationSFD
July 2001
S. Halford, et al IntersilSlide 6
doc.: IEEE 802.11-01/436r0
Submission
b0 b1 b2 b3 b4 b5 b6 b7
802.11b/802.11g Signal Field
Data Rate Mbps = 0.1 Mbps x ( b7 b6 b5 b4 b3 b2 b1 b0 ) base2
25.5 Mbps maximum
Signal Field Definition
For OFDM mode of TGg -- Data rate is not needed Maximum rate of 25.5 Mbps is inadequate The rate in the Barker modulated signal field is ignored by OFDM
demodulator Data rate is contained in OFDM Signal Field
For compatibility with existing network, rate is arbitrarily set at 2 Mbps
See subclause 18.2.3.3 -- Data rate value is set to X’14’
July 2001
S. Halford, et al IntersilSlide 7
doc.: IEEE 802.11-01/436r0
Submission
Service Field Changes
Reserved
b0
Reserved
b1
LockedClock Bit0 = not1 = Locked
b2
Mod. Selec-tion Bit0 = CCK1 = PBCC
b3
Reserved
b4
Reserved
b5
Reserved
b6
LengthExtensionBit
b7
b0 b1 b2 b3
Reserved
b4 b5
Reserved
b6 b7
Mod. Selec-tion Bit0 = not1 = OFDM
Locked Timing/Carrier Clocks Mandatory
802.11b Service Field
New 802.11g Service Field
LengthExtensionBit
Reserved LockedClock Bit0 = not1 = Locked
Mod. Selec-tion Bit0 = CCK1 = PBCC
Reserved
Identifies modulation type Not used by TGg
July 2001
S. Halford, et al IntersilSlide 8
doc.: IEEE 802.11-01/436r0
Submission
Length Field Calculation for CCK-OFDM
Length(usecs) = OfdmSync + OfdmSigField + 4*Ceiling((16 + 8*LENGTH + 6 )/ N DBPS ) + SifsPad
FEC FlushBits
Scrambler State& Full-RateService Field bits
• OFDMSync = 8 , OfdmSigField = 4, SifsPad = 6• NDBPS : Number of data bits per OFDM Symbol• Ceiling functions rounds up to nearest integer
16 bit unsigned integer -- conveys packet length useconds CCK-OFDM includes SIFS Pad & OFDM overhead Note: Length extension bit in service field is not needed
July 2001
S. Halford, et al IntersilSlide 9
doc.: IEEE 802.11-01/436r0
Submission
OFDM Preamble: Overview
.11a preamble without short sync sequence Short sync purpose fulfilled by .11b preamble
Long sync sequence is used for channel estimation Signal Field Conveys OFDM parameters
Data rate Data Length (OFDM data length)
OFDM Sync12 useconds
Long SyncPreamble OFDM Data
802.11bPreamble High Rate Data
8useconds
OFDM Data SIFS Pad
6 useconds
SIFS Pad6 useconds
6, 9, 12, 18, 24, 36, 48 & 54 MbpsUsing OFDM modulation
192 useconds (long)96 useconds (short)
SignalField
4useconds
July 2001
S. Halford, et al IntersilSlide 10
doc.: IEEE 802.11-01/436r0
Submission
OFDM Long Sync Field
Long sync follows 802.11b symbol field Defined in 802.11a, subclause 17.3.3 Referred to in 802.11a as long training symbol
2 symbols composed from 52 BPSK modulated subcarriers
OFDM Sync12 useconds
Long SyncPreamble OFDM Data
802.11bPreamble High Rate Data
OFDM Data SIFS Pad
SIFS Pad6 useconds
6, 9, 12, 18, 24, 36, 48 & 54 MbpsUsing OFDM modulation
192 useconds (long)96 useconds (short)
SignalField
GuardInterval
Long TrainingSymbol #1
GuardInterval
Long TrainingSymbol #2
Total Duration = 8 useconds
0.4useconds 7.2 useconds
0.4useconds
July 2001
S. Halford, et al IntersilSlide 11
doc.: IEEE 802.11-01/436r0
Submission
OFDM Signal Field
Signal field provides OFDM data length & rate information Defined in 802.11a, subclause 17.3.4 Conveys data rate of the OFDM data (See 17.3.4.1) Conveys length in octets of OFDM data without SIFs Pad
Note: This length is not the length of the packet. Data is coded (rate =1/2) and sent using 6 Mbps mode
Note that the signal field is not scrambled
OFDM Sync12 useconds
Long SyncPreamble OFDM Data
802.11bPreamble High Rate Data
OFDM Data SIFS Pad
SIFS Pad6 useconds
6, 9, 12, 18, 24, 36, 48 & 54 MbpsUsing OFDM modulation
192 useconds (long)96 useconds (short)
SignalField
Rate R Length PSignal
Tail
4Bits
1Bit 12 bits 6 bits
1Bit
Reserved Data Length in octetsAll Zeros
(decoder flush)DataRate
Parity
July 2001
S. Halford, et al IntersilSlide 12
doc.: IEEE 802.11-01/436r0
Submission
OFDM Data Field
Data Field is described in Subclause 17.3.5 Modulation depends on data rate parameter
First 16 bits of data field are used as service field (17.3.5.1) First 6 bits are for scrambler initialization (all zeros) Next 10 bits are reserved (all zeros)
End of data field for TGg consists of 2 parts Tail Bits -- Used for convolutional decoder flush
6 bits set to all zeros Pad Bits -- Used to fill out an OFDM symbol to proper number of bits
All zeros -- number depends on data length & data rate SIFs Pad follows the data field (not the same as pad bits!)
OFDM Sync12 useconds
Long SyncPreamble OFDM Data
802.11bPreamble High Rate Data
OFDM Data SIFS Pad
SIFS Pad6 useconds
6, 9, 12, 18, 24, 36, 48 & 54 MbpsUsing OFDM modulation
192 useconds (long)96 useconds (short)
SignalField
July 2001
S. Halford, et al IntersilSlide 13
doc.: IEEE 802.11-01/436r0
Submission
SIFs Pad
SIFs pad matches SIFs intervals between .11a & .11b 802.11g receivers will see a 16 usec SIFs during OFDM operation
Begin processing at end of OFDM data field 802.11b receivers will still see a 10 usec SIFs during OFDM operation
SIFs pad is cyclic extension of last data symbol
OFDM Sync12 useconds
Long SyncPreamble OFDM Data
802.11bPreamble High Rate Data
OFDM Data SIFS Pad
SIFS Pad6 useconds
6, 9, 12, 18, 24, 36, 48 & 54 MbpsUsing OFDM modulation
192 useconds (long)96 useconds (short)
SignalField
: Samples of the last data symbol for 0,1, ...,79Ds n n
for 0,1,...,79
80 for 80,81,...,119D
PadD
s m ms m
s m m
July 2001
S. Halford, et al IntersilSlide 14
doc.: IEEE 802.11-01/436r0
Submission
Transmit Frequencies
Frequency range is defined in Subclause 18.4.6.1 US & Europe: 2.4 GHz to 2.4835 GHz Japan: 2.471 to 2.479 These are the same as 802.11b
Channel numbering and definition in Subclause 18.4.6.2 Channel spacing is 5 MHz 14 channels identified in 18.4.6.2
Must comply with all regulatory restrictions Out-of-band emissions Power Levels
July 2001
S. Halford, et al IntersilSlide 15
doc.: IEEE 802.11-01/436r0
Submission
Transmit Spectral Mask
Spectral mask is defined in Subclause 17.3.9.2. Spectral mask same as 802.11a
Spectral Flatness – Subclause 17.3.9.6.2 Average energy of spectral lines +/-16 to +/-1 will deviate no
more than +/-2 dB from the average Average energy for +/-26 to +/-17 will deviate no more than
+2/-4 dB from the average of the +/-16 to +/-1
July 2001
S. Halford, et al IntersilSlide 16
doc.: IEEE 802.11-01/436r0
Submission
Accuracy Requirements Error Vector Magnitude specifies the transmit modulation
accuracy for data Measure of MSE normalized by average power See Subclause 17.3.9.6.3 Data rate dependent
Higher rates need more accuracy For TGg, EVM applies to both single carrier portion as well
as the OFDM portion of the packet When the 802.11b accuracy spec is more stringent, it shall be
used for the single-carrier portion Transmit center frequency tolerance: +/- 20 ppm max Symbol clock frequency tolerance: +/- 20 ppm max
July 2001
S. Halford, et al IntersilSlide 17
doc.: IEEE 802.11-01/436r0
Submission
EVM Spec from 802.11a
Data Rate Mbps EVM Spec6 -59 -812 -1018 -1324 -1636 -1948 -2254 -25 Very high fidelity.
Distortion is 25 dB down.
Lower fidelity.Distortion is 5 dB down.
This same fidelity is required of the 802.11g systems for1. The single carrier portion (unless more stringent by 802.11b)2. The OFDM portion
July 2001
S. Halford, et al IntersilSlide 18
doc.: IEEE 802.11-01/436r0
Submission
Transition Behavior• To provide for the most flexibility in implementation, standard
should define the waveform behavior as it transitions from single carrier to multi-carrier– Previously we only had single carrier (Barker) to single carrier (CCK) – Previously we had the same symbol rate– Standardizing the transition will allow TGg receivers to make use of the
existing .11b preamble • Specific areas that will be specified are:
– Transmit Spectrum for Barker words– Linear transmit distortions– Power Matching between OFDM and Barker word modulation– Time alignment of the differing clocks– Termination behavior of the single carrier– Carrier Frequency and Phase
July 2001
S. Halford, et al IntersilSlide 19
doc.: IEEE 802.11-01/436r0
Submission
Transmit Spectrum for Single CarrierKey Ideas
1. Design a spectrum/time shaping pulse which makes the single-carrier portion of the signal look like OFDM. Specified pulse will meet all 802.11b requirements.
2. Make this pulse known so that the receiver can compensate the channel impulse response obtained on the single-carrier preamble for use by the OFDM portion of the packet.
3. Specify this pulse in continuous time, so that it is implementation independent.
4. For digital implementations, the pulse can be sampled at the user’s preferred implementation rate.
July 2001
S. Halford, et al IntersilSlide 20
doc.: IEEE 802.11-01/436r0
Submission
Transmit Spectrum for Single Carrier
• Create a single-carrier spectrum that looks like OFDM’s. It should provide a nearly flat spectrum withsufficient steep roll-off on the edges.
• Transmit pulse must be easily handled by 802.11b receiver.– Hence, it must have a dominate peak in the impulse response
with a small amount of spread. This allows the 802.11b to lock on to this impulse response component.
• Want a short duration pulse to minimize implementation complexity in the transmitter
Desired Characteristics
July 2001
S. Halford, et al IntersilSlide 21
doc.: IEEE 802.11-01/436r0
Submission
Transmit Pulse Design Steps
1. Choose the target spectrum. The target spectrum is a a brick wall approximation to the desired OFDM spectrum.
2. Since a brick wall spectrum has an infinite impulse response in the time domain, truncate this pulse using a continuous-time window.
3. Choose a long enough window to give the desired spectral characteristics. (Engineering judgement)
4. Choose a short enough window to minimize complexity. (Engineering judgement)
July 2001
S. Halford, et al IntersilSlide 22
doc.: IEEE 802.11-01/436r0
Submission
Target Brickwall Spectrum Limit Frequency = 27 * (20 MHz / 64 ) = 8.4375 MHzAbout the same as 802.11a OFDM.
Associated Infinite-Duration Time ResponseBrickwall Spectrum
sinsinc , where 52 20 / 64 MHzW
IdealBW W W W WW
f th t f f f t f
f t
July 2001
S. Halford, et al IntersilSlide 23
doc.: IEEE 802.11-01/436r0
Submission
Truncate Impulse Response with Window
Continuous Time VersionOf Hanning Window
Overlay of Pulse and Window
0.5 1 cos 2 , where 0.8 usecsWindow SPANSPAN
th t T
T
July 2001
S. Halford, et al IntersilSlide 24
doc.: IEEE 802.11-01/436r0
Submission
Impulse Response After Windowing
• Same duration as 802.11a Short Sync (0.8 usecs)• At 22 MHz, this can be represented with an 18 tap filter• Short duration provides low complexity
Desired Pulse
Window IdealBWp t h t h t
July 2001
S. Halford, et al IntersilSlide 25
doc.: IEEE 802.11-01/436r0
Submission
Transmit Pulse for Barker Preamble Transmit Pulse shape filter defined by the following
equation must be used:
sinsinc , where 52 20 / 64 MHzW
IdealBW W W W WW
f th t f f f t f
f t
0.5 1 cos 2 , where 0.8 usecsWindow SPANSPAN
th t T
T
Window IdealBWp t h t h t
Digital implementations must satisfy Nyquist criterion Digital implementation must meet EVM requirement
for the selected data rate Error is defined as deviation from the continuous time pulse defined
above over the window
July 2001
S. Halford, et al IntersilSlide 26
doc.: IEEE 802.11-01/436r0
Submission
Linear Distortion Requirement
• In addition to pulse shape, there are linear distortions in the transmit and receive chain which influence the received spectrum.– For example -- SAW filter
• In order to re-use the channel information from the Barker preamble, these linear distortions must be the same for the barker preamble and the OFDM data.
July 2001
S. Halford, et al IntersilSlide 27
doc.: IEEE 802.11-01/436r0
Submission
Example Common Linear Distortions
802.11bPreamble/HDR
Kernel
OFDMKernel
SOFTSWITCH DAC
DigitalTo AnalogConverter
LPFSAWFilter
LowPassFilter
Up to this point the waveformBehavior is Defined by the 802.11gStandard
Linear Distortions InducedOn Both Signal Segments
It is easy to design a transmitter to meet this requirement.
July 2001
S. Halford, et al IntersilSlide 28
doc.: IEEE 802.11-01/436r0
Submission
Requirement for Legacy Standards
Barker Preamble1 Mbps
Barker Header1 or 2 Mbps
CCK5.5 or 11 Mbps
PSDUSELECTABLE
@ 6, 9, 12, 18, 24, 36, 48 or 54 Mbps
SIGNALSYMBOLSSYNC
16 usecs 4 usecs
LSYNC
802.11b
802.11a
Linear DistortionsAssumed Common
Linear DistortionsAssumed Common
This is not a new concept or requirement.
July 2001
S. Halford, et al IntersilSlide 29
doc.: IEEE 802.11-01/436r0
Submission
Requirement for 802.11g Standard
Barker Preamble1 Mbps
Barker Header1 or 2 Mbps
OFDM
802.11g
Linear distortions must be the same
July 2001
S. Halford, et al IntersilSlide 30
doc.: IEEE 802.11-01/436r0
Submission
Signal Power Matching Requirement
802.11bPreamble/HDR
Kernel
OFDMKernel
SOFTSWITCH DAC
DigitalTo AnalogConverter
LPFSAWFilter
LowPassFilter
Average Signal Power Must be Equal
• In order to maintain AGC settings, the average power seen during the Barker preamble and during the OFDM data portion must be same.
July 2001
S. Halford, et al IntersilSlide 31
doc.: IEEE 802.11-01/436r0
Submission
Transition Time Alignment• The 802.11b uses a chip rate of 11 MHz. With 11 chip Barker
words, the Barker words are sent at a 1 MHz rate• The 802.11a OFDM uses 20 MHz sample rate.• To maintain time synchronized from the Barker preambles into the
OFDM data, we need to the time relationship between the end of the Barker word preamble and the beginning of the OFDM data.
• The 802.11b 11 MHz and 802.11a 20 MHz clocks shall be aligned on the 1 MHz boundary (e.g, each 1 useconds).
• The first chip of each Barker words will be centered on this 1 usec alignment.
• The first full 20 MHz sample of the OFDM will occur 1 usec after the zero-phase peak of 1st chip of the last Barker word in the header. Effectively, one half-scale OFDM sample will occur before the full scale sample(for smoothing).
July 2001
S. Halford, et al IntersilSlide 32
doc.: IEEE 802.11-01/436r0
Submission
Clock Alignment on 1 usecond
AlignmentEpoch Alignment
Epoch
Every 1 usec the 802.11b Clock and 802.11a
Clock Realigns
July 2001
S. Halford, et al IntersilSlide 33
doc.: IEEE 802.11-01/436r0
Submission
Transition Time Alignment Requirement
1 2 3 4 5 6 7 8 9 10 11Barker Chip #
1 usec
Single-Carrier:Last BarkerWordOf Header
Pulses AlignedOn Zero-PhasePeaks
Multi-Carrier:OFDM RampUp
time
time
20 MHz Samples of OFDM Long Sync as described in Annex G of the 802.11a standard
11 MHz ChipRate
July 2001
S. Halford, et al IntersilSlide 34
doc.: IEEE 802.11-01/436r0
Submission
Single-Carrier Termination Requirement
• When transitioning from single-carrier to multi-carrier, the single-carrier will be terminated in a controlled fashion.
• This termination is similar to that used for 802.11a OFDM shaping.• The single-carrier signal will be terminated in nominally 100 nsecs.
– Note: it is not necessary to completely flush the single-carrier pulse shaping filter.
• The resulting distortion to the last Barker word in the header is trivial compared to the 11 chips processing gain, thermal noise and multipath distortion.
• The termination can be accomplished either in the digital signal processing or by analog filtering.
July 2001
S. Halford, et al IntersilSlide 35
doc.: IEEE 802.11-01/436r0
Submission
802.11a OFDM Symbol Concatenation: Overlap and Onset/Termination
This example will be used to determine the single carrier termination.
July 2001
S. Halford, et al IntersilSlide 36
doc.: IEEE 802.11-01/436r0
Submission
802.11a OFDM Symbol Onset and Termination: Mathematical Description
2sin 0.5 for 2 2 2
TR TRT
TR
t T Tw t t
T
1 for 2 2TR TR
T
T Tw t t T
2sin 0.5 for 2 2 2
TR TRT
TR
t T T Tw t T t T
T
TTR is the transitionDuration.
July 2001
S. Halford, et al IntersilSlide 37
doc.: IEEE 802.11-01/436r0
Submission
Zoomed 802.11a Symbol Onset and Termination Characteristic
Zoomed OFDM Symbol Onset Zoomed OFDM Symbol Termination
July 2001
S. Halford, et al IntersilSlide 38
doc.: IEEE 802.11-01/436r0
Submission
Single-Carrier Termination and OFDM Onset Requirement
Single CarrierBPSK/QPSKBarker Codes
Multi-CarrierOFDM
time
ShapedIdentical to 802.11a
Shaped Consistent With 802.11a
~100nsecs
July 2001
S. Halford, et al IntersilSlide 39
doc.: IEEE 802.11-01/436r0
Submission
Transition Carrier-Frequency Requirement
802.11bPreamble/HDR
Kernel
OFDMKernel
SOFTSWITCH DAC
DigitalTo AnalogConverter
LPFSAWFilter
LowPassFilter
Carrier Frequency is coherentFor both waveform segments
x
LocalOscillator
To maintain channel information, carrier frequency must remain coherent across the single carrier to OFDM transition
Receiver can maintain carrier frequency lock with PLL
July 2001
S. Halford, et al IntersilSlide 40
doc.: IEEE 802.11-01/436r0
Submission
Transition Carrier Phase Alignment
• Phase coherency is needed between the single-carrier and multi-carrier signal segments in order to use the channel estimate from the Barker words
• The receiver can exploit knowledge about the phase coherency to maintain carrier phase lock across the transition using a PLL
July 2001
S. Halford, et al IntersilSlide 41
doc.: IEEE 802.11-01/436r0
Submission
802.11b Header Modulation
real real
imagimag
BPSK QPSK
Recall that the .11b header uses modulated Barker words Modulation is either BPSK (long preamble) or QPSK (short preamble)
This will also be true for TGg systems Use the modulating phase of the last barker word to establish a phase
reference for the OFDM Data
July 2001
S. Halford, et al IntersilSlide 42
doc.: IEEE 802.11-01/436r0
Submission
Phase Reference Requirement
45
real
imagPhase of lastBarker Word
Multiply OFDMsymbols by 1
45
real
imag
Phase of lastBarker Word
Multiply OFDMsymbols by j
45
real
imag
Phase of lastBarker Word
Multiply OFDMsymbols by -j
45
real
imag
Phase of lastBarker Word
Multiply OFDMsymbols by -1
Phase of last Barker word (not last chip!) determines the phase reference for the OFDM symbols