doc.: ieee 802.11-01/436r0 submission july 2001 s. halford, et al intersilslide 1 cck-ofdm normative...

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


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


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


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


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


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


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


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


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


192 useconds (long)96 useconds (short)

SignalField

4useconds

July 2001


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 Data SIFS Pad

SIFS Pad6 useconds



SignalField

GuardInterval

Long TrainingSymbol #1

GuardInterval

Long TrainingSymbol #2

Total Duration = 8 useconds

0.4useconds 7.2 useconds

0.4useconds

July 2001


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 Data SIFS Pad

SIFS Pad6 useconds



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


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 Data SIFS Pad

SIFS Pad6 useconds



SignalField

July 2001


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 Data SIFS Pad

SIFS Pad6 useconds



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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


doc.: IEEE 802.11-01/436r0

Submission

Signal Power Matching Requirement

802.11bPreamble/HDR

Kernel

OFDMKernel

SOFTSWITCH DAC


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


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


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


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


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


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


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


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


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


doc.: IEEE 802.11-01/436r0

Submission

Transition Carrier-Frequency Requirement

802.11bPreamble/HDR

Kernel

OFDMKernel

SOFTSWITCH DAC


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


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


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


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


Multiply OFDMsymbols by -j

45

real

imag


Multiply OFDMsymbols by -1

Phase of last Barker word (not last chip!) determines the phase reference for the OFDM symbols

doc.: ieee 802.11-01/436r0 submission july 2001 s. halford, et al intersilslide 1 cck-ofdm normative...

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