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Scrambler Mismatch Correction Using the MAC FEC

Scrambler Mismatch Correction Using the MAC FEC. Mark Webster, Mike Seals, Steve Halford, Paul Chiuchiolo Intersil July 2002. Overview. A forward-error-correction (FEC) option exists in the 802.11e draft which uses an 8-octet correcting Reed-Solomon (255,239) block code.

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Scrambler Mismatch Correction Using the MAC FEC

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  1. Scrambler Mismatch Correction Using the MAC FEC Mark Webster, Mike Seals, Steve Halford, Paul Chiuchiolo Intersil July 2002 Mark Webster, Intersil

  2. Overview • A forward-error-correction (FEC) option exists in the 802.11e draft which uses an 8-octet correcting Reed-Solomon (255,239) block code. • If the 802.11a PHY demodulator makes any bit-errors in the received scrambler seed (state), the FEC blocks will all error. (IEEE 802.11-02/050 and –02/221) • A nice solution to this problem has been proposed (IEEE 802.11-02/325) • Unfortunately, this solution weakens the performance of the Reed-Solomon FEC thru error multiplication • Herein, a simple technique is presented which restores 802.11a performance to 10-9 frame-error-rates • No additional overhead is added to the FEC frames to make this technique work • The power of the FEC itself is used to determine the scrambler mismatch Mark Webster, Intersil

  3. Solution Constraints • The MAC and PHY cannot coordinate. The existing interface must remain as is. • The fix cannot be made in the PHY • Systematic property must remain, so 802.11e radios w/o the FEC option can process PHY-error-free frames • Legacy radio’s (pre-802.11e) must be able to read the MAC header without any knowledge of 802.11e Mark Webster, Intersil

  4. Desirable Solution Features • Works at frame-error-rates to 10-9(IEEE 820.11-00/40 and 802.11-00/377) • Works for short frames and long frames • Introduces no extra frame overhead • Solution works for all 802.11 PHY’s • Prevents MAC from being PHY dependent • Must fix the 802.11a problem (and 802.11g’s OFDM) • Must not introduce a new problem to other PHY’s • The MAC-level fix should be PHY blind • Low complexity, low latency, little buffering Mark Webster, Intersil

  5. 802.11e FEC Frame Format All numbers are in octets Frame Checksum MAC Header Variable Length Frame Body 208 16 208 16 4 to 208 16 4 32 16 FEC Parity Octets MAC Header FEC Block: (48,32) shortened by m = 207 Normal Data FEC Block: (224,208) shortened by m = 31 Mark Webster, Intersil

  6. An Existing Proposed Solution Mark Webster, Intersil

  7. A Proposed Solution • Presented in Sydney, May 2002 • Doc.: IEEE 820.11-02/325r0 • Title: Dual Precoding with FEC Packets • Authors: Chris Heegard, Lior Ophir, Richard Williams and Sid Schrum • Moving average bit-filters g(D) and recursive bit-filters 1/g(D) are used, exploiting the properties of the 802.11a scrambler mismatch sequence Mark Webster, Intersil

  8. Solution Presented in Sydney(doc:IEEE 802.11-02/325r0) Legacy Stations See Systematic Data (MAC Header) Scrambler Mismatch Removed Equivalent 802.11a PHY Self-Sync FIR x(D) FIR IIR IIR x(D) x(D) FEC Encode FEC Decode + g(D) 1/g(D) g(D) 1/g(D) D(D) IIR 1/g(D) Initial state No mismatch: state =0 Mismatch: state ~=0 x2(D)=0 Mark Webster, Intersil

  9. Issues With Sydney’s Solution • Precoding weakens the MAC FEC (IEEE 11-02-394r3-E) • The receive self-synchronizing FIR filter g(D) causes error multiplication • Up to 3 * (Number of bit errors) • Up to 2 * (Number of octet errors) • Instead of the Reed-Solomon (255,239) correcting 8 octets in error, it can be limited by 4 octets in error at the PHY output • The FEC effectiveness is lower-bounded to half the design goal for independent octet errors Mark Webster, Intersil

  10. Bit-Error Multiplication Problem FIR Bit-Filter, g(D) RX PHY OUTPUT TO FEC DECODER + + Input Bit Error Pattern 00100000000 Output Bit Error Pattern 00100010010 D4 D3 One input error 3 output errors For every input bit error, up to 3 output bits errors can occur. Mark Webster, Intersil

  11. Octet-Error Multiplication Problem FIR Bit-Filter, g(D) RX PHY OUTPUT TO FEC DECODE + + Input Octet Error Pattern 01000000 00000000 Output Bit Error Pattern 01000100 10000000 D4 D3 One octet in error 2 octets in error For every input octet in error, up to 2 output octets can be in error. Mark Webster, Intersil

  12. PHY Error Events • 1 and 2 Mbps DSSS & FH PHY’s experience random bit errors • 5.5 Mbps CCK experiences nibble errors (4 bit chunks) • 11 Mbps CCK experiences octet errors • OFDM experiences some form of error clustering due to Viterbi trace-back re-sync behavior (see 802.11-02-414r0-E) Mark Webster, Intersil

  13. A New Idea Mark Webster, Intersil

  14. A New Idea • MAC header is short compared to full FEC blocks (32 vs. 208 data octets), so the MAC header is much more robust. • Therefore, use dual-precoding on the MAC header only. • Dual-precoding enables FEC decoding in the face of scrambler mismatch • The power of the FEC can then be used to compute the scrambler mismatch reliably Mark Webster, Intersil

  15. New Solution’s Frame Format • Use dual precoding only on the MAC header • Little performance loss because MAC header is so short • No additional overhead added to frame Frame Checksum MAC Header Variable Length Frame Body 208 16 208 16 4 to 208 16 4 32 16 • Use dual precoding • MAC computes scrambler mismatch • Correct scrambler mismatch • Switch to pure FEC • Do not use precoding here Mark Webster, Intersil

  16. New Solution’s Block Diagram Compute Scrambler Mismatch Self-Sync FIR FIR IIR IIR FEC Encode FEC Decode g(D) 1/g(D) g(D) 1/g(D) x(D) P H Y x(D) M U X M U X M U X M U X FEC Encode FEC Decode + Scrambler Correct Jam State Tx MUX Control MAC HDR Blk: Upper Path Data Blk: Lower Path Rx MUX Control MAC HDR Blk: Upper Path Data Blk: Lower Path Mark Webster, Intersil

  17. Computing Scrambler Mismatch: Basic Concept Only exists for 802.11a OFDM And 802.11g OFDM Scrambler Mismatch Noise Bit Errors Desired Data Strip Errors Strip Data Any 7 error-free bits specify state. Location specifies state phase offset. x(D)+D(D)+E(D) D(D) D(D)+E(D) PHY + + x(D) E(D) Error Pattern Recovered Data Provided by FEC Decoder Mark Webster, Intersil

  18. Compute Scrambler Mismatch Using MAC Header Scrambler Mismatch Strip Errors Desired Data Strip Data Bit Errors Scrmblr Mismatch State D(D) • Unique State • Any 7 error-free • bits (address) • Plus, location in frame (index) x(D)+D(D)+E(D) D(D)+E(D) + + Recovered Data g(D)x(D) Self-Sync FIR Rx Data x(D) IIR FEC Decode PHY g(D) 1/g(D) g(D)x(D) +d(D) +g(D)E(D) Error Pattern d(D)+g(D)E(D) Trimmed E(D) Trimmed g(D)E(D) Trim Input To Start at Zero State 1/g(D) IIR d(D): Self-Sync start-up errors. Occur only in first 7 bits. (first octet). Jam Error Pattern State=0 Mark Webster, Intersil

  19. 802.11a/802.11g OFDM Performance Short frame Long frame • Performance reaches 10-9 frame error rates • No degradation with 7 FEC payload blocks • Small degradation with 1 FEC payload block • All curves were generated using • the technique described in • 802.11-02-414r0-E • Red and black (w/o MA filter) • Blue (w/ MA filter) Error Rate vs. Bit SNR Mark Webster, Intersil

  20. Performance with 802.11b (mandatory modes) Short frame Long frame • Uncorrelated (i.i.d.) octet errors • Some performance loss exists if used on low-error-rate 802.11b packets • Some MAC may choose to not be PHY blind? Black and red curves were generated using the technique described in 802.11-02-414r0-E. Blue curve was simulation generated. (Scrambler mismatch, precoding, FEC) Error Rate vs. Byte Error Rate Mark Webster, Intersil

  21. Additional Detail Mark Webster, Intersil

  22. FEC Decoder Detail • Error pattern is easily output from FEC decoder Error Pattern d(D)+g(D)E(D) Mark Webster, Intersil

  23. Using an FEC Codec Core QUESTION: I have a Reed-Solomon IP core where I do not have access to the error pattern. How can I generate the error pattern? CONVENTIONAL FEC CORE Receive Bits Corrected Bits FEC DECODER CORE GENERATING ERROR PATTERN Corrected Bits Receive Bits FEC DECODER CORE - + + Error Pattern Mark Webster, Intersil

  24. FIR Bit-Filter, g(D) + + D4 D3 Corrected Bits Receive Bits FEC DECODER CORE - + + Error Pattern Simple Components, Low Latency, Little Buffering IIR Bit-Filter, 1/g(D) + + D4 D3 Mark Webster, Intersil

  25. Recovering Bit Errors E(D):Linear Superposition • In practice, • Provided by FEC decoder • 1st octet is discarded to eliminate self-sync start-up errors d(D) Bit Errors Self-Sync FIR IIR E(D) g(D)E(D) E(D) 1/g(D) PHY g(D) • IIR state must match FIR state • IIR is not self-synchronizing • IIR state must not contain false errors • IIR has infinite false-error propagation • It is shown how to do this on next 2 pages Mark Webster, Intersil

  26. Example: Trim Error Pattern and Jam IIR State Error Pattern provided by FEC Decoder 10001001 00000000 00000000 00000000 00000000 00000000 1101010 00101101 … At this point trim-off precursor. g(D) is error-flushed with probability = 1 1101010 00101101 … Self-Sync FIR IIR Trim Input To Start at Zero State Trimmed E(D) PHY 1/g(D) g(D) Jam Error Pattern State=0 Mark Webster, Intersil

  27. Trim Error Pattern and Jam IIR State to Zero • 8 octets in error maximum at FEC input, so 48 – 8 = 40 octets in MAC header are not in error • Min-Max separation between FEC-input error octets is 48/8 = 6 octets. Therefore, >= 5 sequential octets are not in error in any given MAC header. • Therefore, FEC-input error pattern always has a straight of 5*8 = 40 bit-zeros or longer • At the end of 40 bits of zeros at FEC input, the self-sync FIR has been flushed (state=0) with essentially probability = 1 • Jam IIR to same state (state=0) and start injecting error pattern into IIR filter at this point • Only a PHY-output bit-error pattern which matches the scrambling sequence can spoof this. But, if this occurs the bit-error-rate is nearly 50%, and the octet error rate is 100%. The spoof can only occur on frames impossible to FEC decode. Spoof rate rate << 2-40 Mark Webster, Intersil

  28. Example Where g(D) Flush Event is Spoofed (<< 2-40 event) Thermal bit-error-rate is 50% Thermal octet-error-rate is 100% (Matches scrambling pattern) PHY Output Thermal Bit-Error-Pattern 00001110 11110010 11001001 00000010 00100110 00101110 10110110 00001100 11010100 11100111 10110100 00101010 11111010 01010001 10111000 1111111 (Provided by FEC Decoder) FEC-Input Bit-Error-Pattern 00001110 11110000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 0000000 It appears g(D) FIR Is flushed of errors Self-Sync FIR E(D) g(D)E(D) FEC Decode PHY g(D) But, G(d) is never flushed Of errors Mark Webster, Intersil

  29. Conclusion • This submission has described a technique for restoring the full performance of the 802.11e FEC in the face of 802.11a scrambler mismatch • The power of both dual precoding and FEC correction is used to compute mismatch • No additional overhead is used • The technique is simple to implement Mark Webster, Intersil

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