doc.: ieee 802.11-01/154 submission march 2001 s. halford, m. webster, & j. zyren, intersil...

March 2001

S. Halford, M. Webster, & J. Zyren, Intersil CorporationSlide 1

doc.: IEEE 802.11-01/154

Submission

OFDM as a High Rate Extension to the CCK-based

802.11b Standard

Steve Halford, Ph.D.

Mark Webster

Jim Zyren

Intersil Corporation

March 2001


doc.: IEEE 802.11-01/154

Submission

Why OFDM for High Rate?• OFDM recognized as best solution for W-LAN

Selected by 802.11a & ETSI for W-LAN at 5 GHz• Intersil’s proposed OFDM waveform offers:

Fully backwards compatible with 802.11bProvides forward compatibility with 802.11aMeets data rate needs & expectations set by

802.11aBest complexity versus performance tradeGood performance in real world W-LAN Well-known & proven technology

March 2001


doc.: IEEE 802.11-01/154

Submission

Overview of Intersil’s Proposal for 802.11g

March 2001


doc.: IEEE 802.11-01/154

Submission

OFDM for 2.4 Ghz band• Use long & short preamble for backward compatibility

– Ultra-short preamble possible for certain CCA modes• Replace current CCK data with OFDM

– Data modulation identical to 802.11a• Maintain the same 2.4 GHz channels

– 25 MHz center frequency spacing (wider than 802.11a)• Use 802.11a clock rates (20 MHz) for OFDM mode

– Data rates identical to 802.11a (6,9,12,18,24,36,48,54 mbps)– Originally proposed using 802.11b clock of 22 MHz– Now feel acceptance would be faster with 802.11a rates

• Technical differences are very small• Rate change circuitry is common in low power IC

– Open to changes from the group

March 2001


doc.: IEEE 802.11-01/154

Submission

Packet Structure: Backwards Compatible

PREAMBLE/HEADER(Barker Words -- 802.11b)

802.11g LONG (Short) Preamble Packets

192 usecs (Long)96 usecs (Short)

12 usecs

PSDU SELECTABLEOFDM Symbols

@ 6, 9, 12, 18, 24, 36, 48 or 54 Mbps

OFDMSYNC

6 usecs

SignalExtension

Existing .11b radios will recognize preamble and header Length field will be correctly decoded Use reserve bits in header to denote switch

Add OFDM Sync after .11b header to simplify design Add signal extension for SIFS compatibility

Uses OFDM Modulation

OFDM Proposal is compatible with 802.11bOFDM Proposal is compatible with 802.11b

March 2001


doc.: IEEE 802.11-01/154

Submission

OFDM Symbol Structure• OFDM uses industry standard R=1/2, K=7 code

– Known performance, complexity, and IP issues

• OFDM symbols are formed by IFFT of symbol block– Maps the coded data onto narrow carriers– IFFT block includes 4 pilot/training signals– Carriers retain orthogonality in multipath

• OFDM symbols include guard intervals for multipath – Provides “buffer” to absorb ISI

16Samples

64Samples

64 pt. IFFT of coded dataGuardInterval

time

4 usecs OFDM Symbol

Preceding Symbol

Multipath will cause preceding symbol to “bleed” into current symbol. Guard

interval absorbs this interference

March 2001


doc.: IEEE 802.11-01/154

Submission

Radio Design Issues• Requires a change in baseband processor only

– Current RF gives adequate performance up to 36 Mbps

• OFDM preserves current channelization– 3 channels spaced by 25 MHz (U.S. deployments)

• Power requirements are same as present products

• For 48 Mbps & 54 Mbps, new RF design is required– Standard would be in place & spur development

– Design issues are well understood

March 2001


doc.: IEEE 802.11-01/154

Submission

Performance of OFDM with Prism II Radio

9 10 11 12 13 14 15 1610

-4

10-3

10-2

10-1

100

PER vs. Eb/No for 39.6 Mbps Prism II Radio

Eb/No (dB)

PE

R

baseline backoff=6dBbackoff=7dBbackoff=8dBbackoff=9dB

• 10% PER -- 2.7 dB• 1% PER -- 4.1 dB

36 Mbps Mode**

24 Mbps Mode**• 10% PER -- 1.5 dB• 1% PER -- 2.1 dB

Current radio provides Current radio provides sufficient quality to operate at sufficient quality to operate at

rates up to 36 Mbpsrates up to 36 Mbps** see notes pages for more details

Implementation loss due to radio(Loss relative to ideal OFDM performance)

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doc.: IEEE 802.11-01/154

Submission

Preambles and Throughput

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doc.: IEEE 802.11-01/154

Submission

0 500 1000 1500 2000 25000

5

10

15

20

25

30

35

40

45

Data Length in Bytes

Max

imum

Eff

ectiv

e T

hrou

ghpu

t (M

bps)

Throughput without ACK using short preamble

OFDM-12 SP OFDM-24 SP OFDM-36 SP OFDM-54 SP CCK-11 SP OFDM-24 w/o OS

Throughput with added OFDM sync

Througput without added OFDM sync

Throughput Impact of OFDM Sync

• Proposed OFDM Sync in addition to 802.11b & SIFS pad– Up to 18 usecs of additional overhead

– Simplifies the receiver design

– Allows future flexibility

• What is the impact on throughput?

Decrease in throughputDecrease in throughput100 byte: 310 kbits/sec

1000 byte: 500 kbits/sec2346 byte: 704 kbits/sec

Throughput impact is negligible

March 2001


doc.: IEEE 802.11-01/154

Submission

Complexity and Performance for 802.11g

March 2001


doc.: IEEE 802.11-01/154

Submission

OFDM Transmitter• OFDM distributes “equalization” between the transmitter &

receiver– Single carrier proposals relies on receiver for multipath protection– W-LAN systems are in receive mode 90% of time so reducing receive

complexity is critical for power savings

• OFDM adds IFFT and cyclic extension operations to transmitter– Simplifies the equalizer in the receiver

Data Scrambler

Data Scrambler

ConvolutionalEncoder

ConvolutionalEncoder PuncturePuncture InterleaveInterleave

ConstellationMapping(bits to

symbols)

ConstellationMapping(bits to

symbols)

64-ptInverse

FFT

64-ptInverse

FFT

UncodedInformation

Bits

CyclicExtension

CyclicExtension

To RF TransmitterTransmitter

Only added items compared to single carrier system like PBCC

March 2001


doc.: IEEE 802.11-01/154

Submission

TimingAdjust

TimingAdjust

CNCOCNCO

Frequency Correction

Carrier/Timing Correction

Carrier/Timing Correction

TrimGuard Interval

TrimGuard Interval

FFT &FEQ52 tones

FFT &FEQ52 tones

Frequency Domain Equalizer:Multiply each tone by inverse gain &

phase of the channel

Soft-Decisionson Bits

(symbol to bits)

Soft-Decisionson Bits

(symbol to bits)

De-interleave &De-puncture

De-interleave &De-puncture

Extract 4 PilotTones

Extract 4 PilotTones

Viterbi Decoder

Viterbi Decoder

Compute BranchMatrix

Compute BranchMatrix

De-ScramblerDe-ScramblerTo MAC

From A-to-D

Receiver

• Major difference is use of FFT to simplify equalizer• Reduce tracking complexity with pilot tones

OFDM Receiver Structure

March 2001


doc.: IEEE 802.11-01/154

Submission

Error Correction Coding for High Rate

•Encoding process is relatively low complexity•Decoding complexity depends on code properties

•Decoders are based on Viterbi algorithm•VA searches trellis at each step for most likely state sequence

•Complexity depends on the number of states in decoder•Number of states determines size of the trellis searched by VA

•PBCC-11 & OFDM use a 64-state decoder•PBCC-22 uses a 256-state decoder

•Trellis size is 4x the equivalent all OFDM decoders•Trace-back depth is larger than OFDM-24

OFDM has a less complex error correction code

March 2001


doc.: IEEE 802.11-01/154

Submission

0.8 dB Advantage for PBCC-22 at 1% PER1.0 dB Advantage for PBCC-22 at 10% PER

0.8-1.0 dBCoding Gain

relative to punctured industry

standard code. Requires Trellix 4x

larger...

If AWGN performance is needed, better codes could be developed for

OFDMIs 1 dB worth greatly increased complexity?

PBCC 256 state code vs. Industry Standard

March 2001


doc.: IEEE 802.11-01/154

Submission

Multipath & Equalization for 802.11g

March 2001


doc.: IEEE 802.11-01/154

Submission

Performance & Complexity Trades

• W-LAN performance is dominated by multipath• OFDM is designed for both AWGN and multipath

– Error correcting code to provide AWGN– Use guard interval to absorb ISI (0.96 dB AWGN loss)– Use pilot tones for improved tracking (0.34 dB AWGN loss)

• PBCC is optimized for AWGN only– Error correcting code for AWGN – Multipath performance depends entirely on receiver– Tracking depends entirely on receiver implementation

OFDM is less complex than PBCC OFDM is less complex than PBCC for W-LAN environment for W-LAN environment

March 2001


doc.: IEEE 802.11-01/154

Submission

• Linear Equalizer -- Invert the channel with linear filter– Length of filter depends on number of multipath rays (15- 20 taps)– Matrix Inverse required for each packet – More complex than FFT based equalizer for OFDM

• Decision Feedback Equalizer (DFE) -- Subtracts interference – Uses hard decisions on received symbols prior to error correction – May need a whitened matched filter (matrix inverse to compute)

• Viterbi Equalizer – Maximum likelihood sequence estimate or MLSE– Performance depends on number of paths “tracked”– May require whitened matched filter (# of taps ?)– Finds the most likely sequence of transmitted symbol based on channel

• Similar complexity & implementation to decoding a convolutional code

Neither Linear nor DFE equalizers make sense for PBCC

Equalizers for PBCC

March 2001


doc.: IEEE 802.11-01/154

Submission

• Equalizer estimates the most likely sequence based on knowledge of the channel and the received data– Viterbi itself requires only a channel estimate– Matrix inverse may be required for WMF

• Can include in Viterbi -- affects the observed channel

• Similar to decoding a convolutional code– Searches a trellis for best path between states

• MLSE is the likely equalizer for PBCC-11 & 22– Need to track 4 or more paths for adequate performance

Whitened Matched

Filter

Whitened Matched

FilterViterbi

Equalizer

Viterbi Equalizer

Equalized Symbols

Received Data

MLSE/Viterbi Equalizer

March 2001


doc.: IEEE 802.11-01/154

Submission

MLSE: Complexity Considerations• Complexity is similar to convolutional decoder• Number of states depends on constellation size

and number of multipath rays being tracked

1Number of states

: Constellation Size

: Number of rays tracked by equalizer

LM

M

L

Number of States in Joint DecoderNumber of States in Joint Decoder

ExampleExample

Track 4 rays for 8-level PSK (PBCC-22)Number of states = 83 = 512 states

Eight times as complex as Eight times as complex as the 64 state the 64 state

PBCC-11/OFDM PBCC-11/OFDM decoder& only 4 rays are decoder& only 4 rays are

being tracked!being tracked!** See pg. 590, J. G. Proakis, Digital Communication, 3rd Ed., McGraw-Hill, 1995.

March 2001


doc.: IEEE 802.11-01/154

Submission

Joint Decoder MLSE: Complexity Considerations

• Possible to use single “super-trellis” for decoder• Includes both multipath and FEC memory

Number of States in SuperTrellisNumber of States in SuperTrellis

ExampleExample

Track 4 rays for 8-level PSK with 256 stateConv. Code (PBCC-22 or 33)

Number of states = 256 x 83 = 217 = 131072

Over 2000 times as Over 2000 times as complex as the 64 state complex as the 64 state

PBCC-11/OFDM decoderPBCC-11/OFDM decoder

Number of States in SuperTrellis = S ML-1

M: Constellation Size, L: Number of paths tracked, S: number states in FEC

March 2001


doc.: IEEE 802.11-01/154

Submission

OFDM gives MLSE type performance

• OFDM uses a guard interval to absorb multipath interference• Outside the guard interval, signal is multipath free

– Multipath causes individual tones to fade• After FFT, each tone is multipath free

– Relative fade is known from channel estimation

• Viterbi Decoder of error correction code gives MLSE in multipath– Reliability of each soft-decision is weighted by known fade– Optimum receiver is realized with only a FFT– True provided multipath is entirely inside guard interval

• Path delay less than 800 nSecs

March 2001


doc.: IEEE 802.11-01/154

Submission

• OFDM proposal includes 800 nSecs Guard Interval• Equivalent to 800e-9 x 11e6 = 8.8 paths at PBCC symbol rate• Multipath tolerance equivalent to tracking 8 paths• FFT complexity is approximately twice the complexity of a 64 state decoder

OFDM Multipath ToleranceOFDM Multipath Tolerance

Equivalent SC MLSE ComplexityEquivalent SC MLSE Complexity1

8-1

Recall Number of States = where is the constellation size &

is number of paths

For PBCC-22, 8 Number of states = 8 2,097,152 states

LM M

L

M

This is 215 (over 32,000) times the complexity of the 64 state decoder!

OFDM: MLSE performance w/o complexity

March 2001


doc.: IEEE 802.11-01/154

Submission

Impact of interference on 802.11g

March 2001


doc.: IEEE 802.11-01/154

Submission

Interference in 2.4 GHz band• 2.4 GHz spectrum is a shared resource

– BlueTooth & other FH systems generate in-band interference on 802.11b & 802.11g radios

– Other sources of interference include microwave ovens

• Higher data rates specified by 802.11g will be more sensitive to interference– Errors generated by presence of interference source can

greatly influence the throughput

• BlueTooth enabled devices will proliferate at same time as 802.11g

PBCC is sensitive to real world interference sources

March 2001


doc.: IEEE 802.11-01/154

Submission

PBCC performance is sensitive to BlueTooth

0

2

4

6

8

10

12

1 10 100 1000

WLAN Receiver to Transmiter distance (M)

WL

AN

th

rou

gh

pu

t (M

bp

s)

8PSK with Random Tone Jammer

OFDM with Random Tone Jammer

8PSK without Interference

OFDM without Interference

E. Zehavi, et al (IEEE documents IEEE802.11-

01/061r0 & IEEE802.15-01/066r0)

showed that the throughput of coded 8-PSK w/o an interleaver

was very sensitive to the presence of a

BlueTooth- like interferer.

March 2001


doc.: IEEE 802.11-01/154

Submission

Extending PBCC to higher rates (>22 Mbps)

March 2001


doc.: IEEE 802.11-01/154

Submission

Approaches to Higher Data Rates

• OFDM provides a known path to higher rates• Higher data rates can be achieved by:

– Increasing the constellation size and/or decrease code rate• Used by OFDM to give rates of 6 Mbps to 54 Mbps• PBCC-22 uses 8-psk with rate 2/3 code to go from 11 Mbps (QPSK with rate 1/2)

to 22 Mbps

– Increasing symbol rate• PBCC-33 uses 1.5x clock speed to go from 22 Mbps to 33 Mbps

• Increasing the data rate increases the required SNR

OFDM equalizer complexity is same for all rates -- OFDM equalizer complexity is same for all rates -- What is the impact on the PBCC receiver? What is the impact on the PBCC receiver?

March 2001


doc.: IEEE 802.11-01/154

Submission

Are Higher Data Rates Possible?• OFDM Equalizer has fixed complexity for all proposed rates

– Higher rates does impact performance due to fading of tones• Guard interval however reduces the impact independent of rate

• MLSE complexity will grow exponentially when constellation size increases– Higher rates will impact performance

• No guard interval to protect from increased ISI sensitivity– Example: Track 4 paths -- Number of states = (constellation size)4-1

• 22 Mbps (8-PSK) requires 83 = 512 states (8x the PBCC-11 decoder)• 33 Mbps (16-QAM) will require 163 = 4096 states (64x the PBCC-11 decoder)• 44 Mbps (64-QAM) will require 643 = 262144 states (4096x the PBCC-11

decoder)

Extending PBCC to higher rates by increasing constellationis not practical

March 2001


doc.: IEEE 802.11-01/154

Submission

Are Higher Data Rates Possible?• OFDM uses a fixed symbol rate for all data rates

– Guard interval protection is same for all rates

• PBCC-33 is PBCC-22 at a higher symbol rate– Pulse shaping used to keep same spectral width

• Increasing symbol rate impacts performance– Increasing timing accuracy requirements

• Increasing rate increase number of equalizer paths – Example:Example: 8-PSK -- Number of states = 8(number of paths -1)

• 22 Mbps (11 Mhz, 4 paths) -- 84-1 = 512 states (8x the PBCC-11 decoder)• 33 Mbps (16.5 Mhz, 6 paths) -- 86-1 = 32,768 states (512x PBCC-11 decoder)• 44 Mbps (22 Mhz, 8 paths) -- 88-1 = 2,097,152 states (32,768x PBCC-11 decoder)

Extending PBCC to higher rates by increasing symbol rateis not practical

March 2001


doc.: IEEE 802.11-01/154

Submission

Conclusions on OFDM for 802.11g

March 2001


doc.: IEEE 802.11-01/154

Submission

Summary of Data Rates & Summary of Data Rates & ParametersParameters

Data Rate (Mbps) Code Rate Constellation Bits/Symbol6.0 1/2 BPSK 249.0 3/4 BPSK 36

12.0 1/2 QPSK 4818.0 3/4 QPSK 7224.0 1/2 16-QAM 9636.0 3/4 16-QAM 14448.0 2/3 64-QAM 19254.0 3/4 64-QAM 216

Modulation OFDMFundamental Sample Rate 20 Mhz

Data Symbol Length 4.0 Sec (80 samples @ 20 Mhz)Guard Interval Length 800 nSec (16 samples @ 20 Mhz)FFT Window Length 3.2 Sec (64 samples @ 20 Mhz)

Number of Data Tones 48Number of Pilot Tones 4

Frequency of Operation 2.4 GHz ISM band

March 2001


doc.: IEEE 802.11-01/154

Submission

Conclusions• OFDM is forward & backwards compatible

– Uses existing long & short preamble for compatibility’

– 802.11a signaling used in place of CCK

– Minor impact on throughput of added headers

• OFDM offers the highest rates of all proposals– 36 Mpbs with current radio (baseband only change)

– 48 & 54 Mbps possible with new radio design

– PBCC complexity grows exponentially

March 2001


doc.: IEEE 802.11-01/154

Submission

ConclusionsOFDM is ideal for W-LAN environment

– Equalization split between transmitter & receiver for lower overall complexity

– Lower complexity error correction code

– Nearly MLSE without complexity

– PBCC Joint Decoder approach requires RSSE• Complexity vs. Performance ?

OFDM is robust to narrowband interference– PBCC seems to have an inherent problem with BT

March 2001


doc.: IEEE 802.11-01/154

Submission

ConclusionsOFDM will meet regulatory approval

– All high rate waveforms possible under new rules (?)

– OFDM will be in this band -- IEEE should ensure network compatibility

• OFDM has been developed in an open process– No hidden details

– Complexity of PBCC never adequately described

– Complexity and design is well known & proven

doc.: ieee 802.11-01/154 submission march 2001 s. halford, m. webster, & j. zyren, intersil...

Documents

ofdm transmitter ofdm

performance of ofdm

b slide

ofdm symbol structure

throughput slide

ideal ofdm performance

ofdm modulation ofdm

g slide