implementation and complexity issues for ofdm

November 2000

S. Halford, P. Chiuchiolo, G. Dooley, and M. WebsterSlide 1

doc.: IEEE 802.11-00/396

Submission

Implementation and Complexity Implementation and Complexity Issues for OFDMIssues for OFDM

Steve Halford

Paul Chiuchiolo

Glenn Dooley

Mark Webster

Intersil Corporation

Palm Bay, FL

November 2000


doc.: IEEE 802.11-00/396

Submission

Outline of Proposal PresentationsOutline of Proposal Presentations TGg Regulatory Approval Plan Speaker: Jim Zyren Overview of OFDM for High Rate Speaker: Steve Halford Reuse of 802.11b Preambles with OFDM Speaker: Mark Webster Ultra-short Preamble with HRb OFDM Speaker: Mark Webster OFDM System Performance Speaker: Steve Halford Power Am Effects for HRb OFDM Speaker: Mark Webster Channelization for HRb OFDM Speaker: Mark Webster Phase Noise Sensitivity for HRb OFDM Speaker: Jim Zyren Implementation and Complexity Issues for OFDM Speaker: Steve Halford Why OFDM for the High Rate 802.11b Extension? Speaker: Jim Zyren

November 2000


doc.: IEEE 802.11-00/396

Submission

Outline of Implementation PresentationOutline of Implementation Presentation

9.1 Main Issue for Complexity: Equalization9.1 Main Issue for Complexity: Equalization

9.2 Baseband Complexity9.2 Baseband Complexity

9.3 Power Consumption9.3 Power Consumption

9.4 RF/IF Complexity9.4 RF/IF Complexity

9.5 Time to Market9.5 Time to Market

November 2000


doc.: IEEE 802.11-00/396

Submission

9.1 Main Issue for Complexity: Equalization9.1 Main Issue for Complexity: Equalization

• Main issue is complexity of Equalizer vs. FFT

“One of the main reasons to use OFDM is its ability to deal with large delay spreads with a reasonable implementation complexity. In a single-carrier system, the implementation complexity is dominated by equalization, which is necessary when the delay spread is larger than about 10% of the symbol duration. OFDM does not require an equalizer. Instead, the complexity of an OFDM system is largely determined by the FFT, which is used to demodulate the various subcarriers.”

Quote from pg. 48 of R. Van Nee & R. Prasad, OFDM for Wireless Multimedia Communications, Artech House Publishers, Boston, MA, 2000.

November 2000


doc.: IEEE 802.11-00/396

Submission

9.1.1 Equalizer and FFT Complexity9.1.1 Equalizer and FFT Complexity

** Based on R. Van Nee & R. Prasad, OFDM for Wireless Multimedia Communications, Artech House Publishers, Boston, MA, 2000.

• 64 point FFT using radix-4 requires 96 complex multiplies• Equalizer then requires 48 complex multiplies

• Could simplify since all that is really needed is a phase rotation & soft-decision scale• Perform once every 80*(1/22 x 106) = 3.63 x 10 -6 seconds• Equivalent to (4 x 144)/(3.63 x 10 -6 ) = 158.4 x 106 real multiplies per second

• Linear Equalizer of length L requires 4*L complex multiplies per symbol• Number of real multiplies = (4*L*11 x 106 ) = L * (44 x 106 )• Length L must be less than (158.4/44) = 3.6 to match complexity of FFT• Using pulse shaping makes this worse due to matched filter! • Doesn’t include the complexity of estimating the equalizer types

• Matrix inverse proportional to L• Alternative is a full Viterbi Equalizer with channel matched filter

Single Carrier Linear Equalizer ComplexitySingle Carrier Linear Equalizer Complexity

FFT for OFDM EqualizationFFT for OFDM Equalization

November 2000


doc.: IEEE 802.11-00/396

Submission

9.2 Baseband Complexity Summary9.2 Baseband Complexity Summary

Basic CCKDemod

CCK Demod+ Equalizer

CCK + CodedMulti-code

(e.g., Walsh Seq.)

CCK + CodedPSK/QAM

CCK + OFDM

CCK Demodulator Carrier/Timing Track CCA Mechanism Viterbi Decoder FFT/IFFT

(fast walsh)

Interleave/De-interleave

Puncture/De-Puncture

Soft-DecisionGenerator

Scrambler/De-scrambler

Equalizer

November 2000


doc.: IEEE 802.11-00/396

Submission

9.2 Relative Complexity Estimate9.2 Relative Complexity Estimate

BASIC CCKDEMODULATOR

BASIC CCKDEMOD WITHEQUALIZER

BASIC CCKDEMODULATOR

WITH CODEDPSK/QAM

BASIC CCKDEMODULATORWITH OFDM

CCK Demodulator 1.0 1.0 1.0 1.0Viterbi Decoder 0.3 0.3FFT/IFFT 0.15Equalizer 0.25 0.25Other (Multi-code) 0.40Other (Symbol) 0.20

TOTAL 1.0 1.25 1.75 1.85

NOTE 1. Estimates for the Basic CCK Demodulator & Basic CCK Demodulator with Equalizer are based on Intersil Baseband processors 3860B and 3863

Complexity (gate count) relative to a Basic CCK DemodulatorComplexity (gate count) relative to a Basic CCK Demodulator

November 2000


doc.: IEEE 802.11-00/396

Submission

9.3 Power Consumption for OFDM9.3 Power Consumption for OFDM

MODE Current(0.35 m)

Power Current(0.18 m)

Power

Transmit 15 mA 46.8 mW 13 mA 32.8 mWReceive Packet 60 mA 187.2 mW 44 mA 95 mWReceive (noise only) 45 mA 140.4 mW 35 mA 75.6 mWSleep 1 mA ~2 mW 1 mA ~ 2 mW

Assumptions & Notes about Power EstimatesAssumptions & Notes about Power Estimates

• 0.35 m current estimates based on Intersil 3863 baseband processor• 0.18 m current estimates based on 40% reduction from 0.35 m for digital functions• CCK functions can be powered down during OFDM operation• 60% of current during transmit & 30% of current during receive is in analog

• This will not change for OFDM• This will not change at 0.18 m

• Does not include power for MAC functions

Power Estimates for Baseband Processor with CCK & OFDMPower Estimates for Baseband Processor with CCK & OFDM

November 2000


doc.: IEEE 802.11-00/396

Submission

9.4 RF/IF Design Issues for OFDM9.4 RF/IF Design Issues for OFDM

• OFDM has different spectrum than CCK

• Higher order modulations (e.g., 64-QAM) will require “cleaner” RF front end

Can we re-use current 802.11b RF front-ends?

Yes!

Detailed Simulations of Intersil’s Prism II indicate that 26.4 Mbps & 39.6 Mbps operate within 802.11a requirements for both transmitter & receiver

performance

November 2000


doc.: IEEE 802.11-00/396

Submission

9.5 Time to Market Issues9.5 Time to Market Issues

• OFDM is well established as a viable waveform for W-LAN applications– Mature technology

– Proven to be practical for ASIC implementation

– RF technology exists to support at 2.4 GHz

• Standards process can be accelerated by adopting large portions of existing 802.11a standard

• FCC issue will drive the time to market

implementation and complexity issues for ofdm

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