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CCK-OFDM Summary

CCK-OFDM Summary. Steve Halford Mark Webster Jim Zyren Paul Chiuchiolo Intersil Corporation. Why OFDM for High Rate?. OFDM recognized as best solution for W-LAN Selected by 802.11a & ETSI for W-LAN at 5 GHz OFDM meets current & future needs: Highest rates and backward compatibility

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CCK-OFDM Summary

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  1. CCK-OFDM Summary Steve Halford Mark Webster Jim Zyren Paul Chiuchiolo Intersil Corporation S. Halford, et al Intersil Corporation

  2. Why OFDM for High Rate? • OFDM recognized as best solution for W-LAN • Selected by 802.11a & ETSI for W-LAN at 5 GHz • OFDM meets current & future needs: • Highest rates and backward compatibility • Meets consumer expectations set by 802.11a • High throughput with ultra-short preamble • New deployments & outdoor bridge applications • Share baseband with 802.11a • Dual band radios • Multiple baseband vendors • Best performance for complexity trade • Multipath & Bluetooth S. Halford, et al Intersil Corporation

  3. Overview of Intersil’s Proposal for 802.11g S. Halford, et al Intersil Corporation

  4. OFDM for High Rate Extension • Replace data portion of packet with OFDM modulation • Data rates 6, 9, 12, 18, 24, 36, 48 or 54 Mbpsusing 20 MHz symbol rate • Existing .11b radios will recognize preamble and header • Length field will be correctly decoded • CCA mechanisms maintained • Use reserve bits in 802.11b header for OFDM parameters OFDM Proposal is compatible with 802.11b S. Halford, et al Intersil Corporation

  5. OFDM-Specific Fields • Add OFDM-only sync and SIFs pad • Reduces complexity of receiver & allows for flexible transmit filtering • Short Sync allows time for clock rate change • 4 useconds duration is half duration of 802.11a • Allows time to switch rates • Can also use to refine time & frequency estimates S. Halford, et al Intersil Corporation

  6. OFDM-Specific Fields • Long Sync provides training data for channel estimation • Provide 8 useconds of training data (same as 802.11a) • Do not need to rate-change 802.11b channel estimate • Can switch filters at transmitter for OFDM mode • SIFs pad extends the SIFs time to match 802.11a • 802.11g receivers will see a 16 usec SIFs during OFDM operation • 802.11b receivers will still see a 10 usec SIFs during OFDM operation S. Halford, et al Intersil Corporation

  7. Impact of OFDM-specific Fields • Simplify Radio design & add flexibility with added fields • OFDM Sync: Transition time & channel estimation • SIFs Pad: SIFs time compatibility between 802.11b & 802.11a • What is the impact on throughput? • Add 18 useconds of overhead Reduction in Throughput 100 byte: 310 kbps 1000 byte: 500 kbps 2346 byte: 704 kbps ** Short Preamble option at 24 Mbps, No ACK Throughput impact is negligible S. Halford, et al Intersil Corporation

  8. Ultra-Short Preamble Option • Modulation & preambles identical to 802.11a • Reduces total preamble to 20 usecond • New deployments • Outdoor point-to-point links • Gives a route to 802.11a in the 2.4 GHz band • Compatible with mandatory 802.11g • Single PHY solution to high rate S. Halford, et al Intersil Corporation

  9. Radio Design Issues • Baseband processor change only for 36 Mbps • Current RF supports all rates up to 36 Mbps • OFDM preserves current channelization • 3 channels spaced by 25 MHz (U.S. deployments) • 48 & 54 Mbps supported with new RF • Same baseband • Higher density constellation has more stringent requirements on radio front end • Requirements are well from 802.11a designs • Design issues are well understood S. Halford, et al Intersil Corporation

  10. Forward Compatibility of CCK-OFDM S. Halford, et al Intersil Corporation

  11. Advantages of compatibility • CCK-OFDM provides .11a & .11g compatibility • Could also add HiperLAN2 compatibility • Marketplace will see single waveform as high rate wireless LAN solution • Introduction of a new waveform like PBCC will only fracture the marketplace • Lower cost dual band radios • Dual band 802.11a with PBCC requires one to build two complex basebands (OFDM & PBCC) • Multiple vendors provide basebands & IP for .11g S. Halford, et al Intersil Corporation

  12. Dual Band Radio • Allow seamless transitions for laptop WLANs • Single low-cost card could provide support for .11b, .11a, and .11g • Auto-detect network or best connection type S. Halford, et al Intersil Corporation

  13. Further Advantages of Compatibility • Consider the cost of compatibility Optional Mandatory .11a (OFDM-only) + .11b(CCK) + .11g (PBCC) TI Compromise Proposal .11b (CCK) + .11g(OFDM) Intersil Proposal inlcudes .11a(OFDM) S. Halford, et al Intersil Corporation

  14. Adding CCK Baseband to OFDM • Adding CCK support is much easier than alternative • PBCC requires number of complex design efforts • PBCC-22 requires 30x ops/bit over CCK (00/384r1) • Must still support CCK • CCK receivers based on rake receiver • Implement with a channel matched filter & correlator S. Halford, et al Intersil Corporation

  15. Complexity Comparison between OFDM and PBCC S. Halford, et al Intersil Corporation

  16. PBCC:Reduced State Approach • Suggested approach by PBCC proponents • Still higher complexity than OFDM • Minimum MF length will be 10 taps • Retain only 64 states out of 216 states at each update • 8 symbols form the channel state • Each update, surviving state generate 4 new candidates • 64 leads to 256 states • Retain the most likely 64 (requires sort) • Updates occur at symbol rate for this approach S. Halford, et al Intersil Corporation

  17. Compare PBCC & OFDM Compare Complexity of WMF with FFT & FEQ Compare Complexity of the two 64-state decoders S. Halford, et al Intersil Corporation

  18. Compare WMF w/ FFT & FEQ • PBCC: WMF Complexity is driven by length • Ignore the estimation problem • Needs 10 taps to handle 5 multipath rays • PBCC-22: Requires 880 x 106 real multiplies per second • PBCC-33: Requires 1980 x 106 real multiplies per second • Increase length to 15 to cover same delay spread • OFDM: Consider the FEQ & FFT • FEQ: 52 multiplies/symbol = 52 x 106 real multiplies per second • FFT: Radix 4, 96 multiplies/symbol = 96 x 106 real mps • Total: 148 x 106 real multiplies per second • Remains fixed for all data rates S. Halford, et al Intersil Corporation

  19. Compare 64-state Decoders • Compare Trellis search approaches in terms of: • branch metrics calculations • path metric updates • compare selects • PBCC: 64 States  256 states  64-states for each symbol • 256 branch metric calculations • 256 path metric updates • Select 64 best of 256 states -- Variety of approaches • OFDM: 64 states 128 paths  64 states for each information bit • 128 branch metric calculations • 128 path metric updates • Compare each pair & select best -- 64 compare-selects S. Halford, et al Intersil Corporation

  20. Compare Complexity: Summary • PBCC has much higher complexity due to matched filter • Equivalent OFDM operation (FFT & FEQ) have fixed complexity • OFDM & PBCC have nearly same complexity in trellis search (?) • PM & BM difference is proportional to data rate difference S. Halford, et al Intersil Corporation

  21. Conclusions for 802.11g S. Halford, et al Intersil Corporation

  22. Conclusions PBCC relies on coding & sophisticated receiver • Non-standard code matched to 8-PSK signal • Different code than used for optional PBCC-11 • No interleaver to help spread burst errors • Sensitive to burst errors like generated by Bluetooth • Cover code benefit never demonstrated • Why add unnecessary elements? • Requires complex decoder design to get adequate performance • 3 years in development with no product or public demo • Single company provider ? • Already lags OFDM systems for data rate S. Halford, et al Intersil Corporation

  23. Conclusions • OFDM is forward & backwards compatible • Uses existing long & short preamble for compatibility • 802.11a modulation in place of CCK • OFDM-specific training data added for reduced complexity • Dual band radios possible • Offers 802.11a in the 2.4 GHz band through ultra-short preamble • OFDM offers the highest rates of all proposals • 36 Mpbs with current radio (baseband only change) • 48 & 54 Mbps possible with new radio design • Only proposal that meets consumer expectations S. Halford, et al Intersil Corporation

  24. Conclusions OFDM is ideal for W-LAN environment • Equalization split between transmitter & receiver for lower overall complexity • Guard Interval -- absorbs multipath without complexity • FFT & FEQ -- Low complexity & fixed for any rate • Nearly MLSE without complexity of PBCC • See Documents IEEE Submissions 01/153 & 01/060 • Lower complexity error correction code • 64 state code • Single code used for all code rates via puncturing • More robust to narrowband interference • Simple to remove known interference S. Halford, et al Intersil Corporation

  25. Conclusions OFDM now meets regulatory approval (5/10/01) • OFDM is now in the 2.4 GHz band • Higher rates than PBCC already being offered • IEEE should embrace & ensure network compatibility • OFDM (802.11a) development was collaborative • Multiple companies contributed ideas • Complexity & design is well known & proven • Many companies will offer products S. Halford, et al Intersil Corporation

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