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Spectral Control Issues for TGg

Spectral Control Issues for TGg. Steve Halford Michael Seals Mark Webster Jim Zyren. Motivation for Spectral Control. Adjacent Channel Interference Number of comments in LB33 on ACI OFDM to OFDM, OFDM to CCK, CCK to OFDM Issue also raised in January 802 meeting

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Spectral Control Issues for TGg

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  1. Spectral Control Issues for TGg Steve Halford Michael Seals Mark Webster Jim Zyren Steve Halford, Intersil

  2. Motivation for Spectral Control • Adjacent Channel Interference • Number of comments in LB33 on ACI • OFDM to OFDM, OFDM to CCK, CCK to OFDM • Issue also raised in January 802 meeting • See IEEE 802.11-02/043 & 044 • FCC Forbidden Band Compliance • A few comments in LB33 question FB compliance • DOC 02/220r0 showed +20 dBm possible for OFDM Steve Halford, Intersil

  3. Adjacent Channel Interference • Interference depends on Tx spectrum & power • Receiver reduces with filtering & processing • 802.11a spacing is 20 MHz • Mask allows for more ACI than 802.11b • Need to ‘skip’ adjacent channels for adjacent AP’s • ACI/ACR issue is not as critical • Physical separation will reduce interference • 802.11b spacing is 25 MHz • Fewer channels so cannot avoid adjacent freq • ACI/ACR is important issue to resolve • Affects 802.11g & legacy 802.11b systems Steve Halford, Intersil

  4. Approaches for ACI • Spectral Mask… • TGg draft uses 802.11b for Barker/CCK & PBCC modes • TGg draft uses 802.11a mask for OFDM • Different spacing & number of channels provides motivation to consider a different mask? • or, Adjacent Channel Rejection Test… • Specifies the level of adjacent channel interference tolerated by receiver • 802.11a also specifies an alternate channel rejection • Spectral Mask Advantages • Approach is implementation independent • Protects legacy systems from interference Steve Halford, Intersil

  5. Spectral Control via Masks • TGg Draft Spec allows for two different mask • Both masks are relative to peak of the main lobe Picture of Masks Steve Halford, Intersil

  6. Simulation Model Used Steve Halford, Intersil

  7. Power Amplifier Model P_sat= 24.2 dBm P_1dB = 21.5 dBm Steve Halford, Intersil

  8. 802.11a Mask Limit for OFDM Measured EVM = -12.55 dB (measured for BPSK modulation) Back-off from 1-dB Compression Point= 1.59 dB Back-off from Saturation Point= 4.29 dB Steve Halford, Intersil

  9. ACI Results for 802.11a Mask Receive Filter Spectral Response Transmit Spectrum PSD of Received Adjacent Channel Signal Main Channel Receives -12.6 dBm from Adjacent Channel Adjacent Channel Rejection = 32.6 dB Steve Halford, Intersil

  10. 802.11b Mask Limit for QPSK Back-off from 1-dB Compression Point= 1.15 dB Back-off from Saturation Point= 3.85 dB Steve Halford, Intersil

  11. ACI Results for QPSK Receive Filter Spectral Response Transmit Spectrum PSD of Received Adjacent Channel Signal Main Channel Receives -19.4 dBm from Adjacent Channel Adjacent Channel Rejection = 39.74 dB Steve Halford, Intersil

  12. Comparison of ACI Results • Output power for OFDM near that of QPSK • About 0.45 dB added back-off • More forgiving mask of OFDM increases ACI • About 6.5 dB more than current 802.11b • Are changes needed to protect 802.11b? Steve Halford, Intersil

  13. Solutions to reduce ACI • Could increase ACR spec for .11g • Helps future 802.11g systems • Doesn’t help current 802.11b radios • Adopt a different spectral mask • Select mask to control amount of ACI • Protects legacy 802.11b networks • Will reduce the total output power Steve Halford, Intersil

  14. Why not use .11b mask? • Suggested by several voters • Shape was selected to fit single carrier • Corners of mask too restrictive for OFDM • Requires a back-off of 6.64 from 1 dB compression • Resulting ACI is -34.72 dBm (15 dB less than QPSK) Meeting mask corners requires large back-off Steve Halford, Intersil

  15. Potential 802.11g OFDM Mask 0 dB for -9 to 9 MHz 20 dB down by 11 MHz offset 50 dB down by 29.5 MHz offset Steve Halford, Intersil

  16. Results for proposed mask Back-off from 1-dB Compression Point= 3.0 dB Back-off relative to .11a mask = 1.4 dB ACI reduced to -19.4 dBm -- Equal to QPSK ACI Steve Halford, Intersil

  17. Karaoguz Mask • Mask proposed in LB 33 comments • Proposed by J. Karaoguz, C. Hansen, & others From Comment #123-127 under Clause 19 Tab IEEE DOC 802.11-02/209r6 “First, the transmit spectral mask of the OFDM signal should be modified such that the transmitted spectrum shall have a 0 dBr bandwidth not exceeding 18 MHz, -30 dBr at 11 MHz frequency offset, -38 dBr at 20 MHz frequency offset, and -47 dBr at 30 MHz frequency offset and above.” Steve Halford, Intersil

  18. Results for Karaoguz Mask Back-off from 1-dB Compression Point= 5.8 dB Back-off relative to .11a mask = 4.2 dB ACI reduced to -28.2 dBm-- 8.8 dB below QPSK ACI Steve Halford, Intersil

  19. ACI with Various Mask • ACI level is same level as legacy system • Mask design fits OFDM better than .11b • Output power is reduced by 1.4 dB • Still have better theoretical range than 802.11a Steve Halford, Intersil

  20. EVM Limits Steve Halford, Intersil

  21. Error Vector Magnitude • Specification on the transmit quality • Mean Square Receiver of the constellation at receiver • Requirement depends on data rate • For many data rates, EVM limits transmit power more than mask Steve Halford, Intersil

  22. EVM Limits • This limit applies to center channel & edge channels Steve Halford, Intersil

  23. Summary of Results Steve Halford, Intersil

  24. Forbidden Band & OFDM Steve Halford, Intersil

  25. Forbidden Band Limits • FCC Limits power into forbidden bands • Average Power must be less than -41.25 dBM/MHz • 47 CFR 15.205 & 15.209 • Two ‘edge’ channels are 22 MHz & 21.5 MHz away from forbidden bands • Channel 1 (2412 MHz) -- 22 MHz away • Channel 11 (2462 MHz) -- 21.5 MHz away Steve Halford, Intersil

  26. 802.11b FB Compliance • .11b mask provides a ‘guide’ to meeting • Mask Compliance = FB Compliance some cases 3-dB of Margin often used for FCC testing -- Allows for process variations Back-off from 1-dB compression point = 2.67 dB Back-off from full saturation = 5.37 dB Steve Halford, Intersil

  27. OFDM & FB Compliance Back-off from 1-dB compression point = 5.57 dB 3-dB of Margin often used for FCC testing -- Allows for process variations Back-off from full saturation = 8.23 dB Additional back-off of 2.86 dB over QPSK -- NOT 10 dB Additional back-off of 3.94 dB over 802.11a mask Steve Halford, Intersil

  28. Conclusions • 802.11a mask not optimal @ 2.4 GHz • 802.11b mask is too restrictive for OFDM • Proposed mask reduces ACI to same level as 802.11b • Requires 1.4 dB back-off over 802.11a mask • EVM requirements already restrictive for some rates • OFDM will satisfy Forbidden Band • Back-off of 2.86 dB relative to QPSK Steve Halford, Intersil

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