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MAC based FEC – improvement for 802.11a

MAC based FEC – improvement for 802.11a. Nir Metzer hLAN inc. Israel nirm@hlan.com. Scope. FEC general requirements. MAC-based FEC specific requirements. MAC-based FEC weak points. Proposed solution. Conclusion. FEC General Requirements.

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MAC based FEC – improvement for 802.11a

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  1. MAC based FEC – improvement for 802.11a Nir Metzer hLAN inc. Israel nirm@hlan.com Nir Metzer, hLAN

  2. Scope • FEC general requirements. • MAC-based FEC specific requirements. • MAC-based FEC weak points. • Proposed solution. • Conclusion. Nir Metzer, hLAN

  3. FEC General Requirements • Provide PER less than <1.15x10-9 for 1000 octet packets. • Do not change the PHY specification => implies MAC-based FEC . Nir Metzer, hLAN

  4. MAC based FEC description PLCP Header RATE 4 bits Reserved 1 bits LENGTH 12 bits Parity 1 bits Tail 6 bits SERVICE 16 bits PSDU Tail 6 bits Pad bits Coded OFDM (BPSK, r=1/2) Coded OFDM (Rate is indicated in SIGNAL) PLCP Preamble 12 Symbols SIGNAL One OFDM Symbol DATA Variable number of OFDM symbols MAC Header With FEC MSDU or MMPDU With FEC Nir Metzer, hLAN

  5. MAC-based FEC specific Requirements • PLCP header should not be less protected than the PSDU. (PLCP header failure =3.15 x 10 –4 at 6 Mbps (BER=1.3x10–5 @ sensitivity level) ) • Requirement satisfied for High Data rates (e.g. QAM 16 modulation of each sub-carrier) where required SNR for QAM implies very high robustness of part of PLCP which is BPSK. Nir Metzer, hLAN

  6. Weak point in MAC FEC solution • However, PLCP header in 802.11a includes a SERVICE field located at the second symbol of the PSDU. This symbol is transmitted at the rate of data payload (can be high data bit rate) and therefore is not robust compared to BPSK. • SERVICE field determines the scrambler initialization, and any error in this field will drop the entire packet. The SERVICE is pseudo random and not predefined. Nir Metzer, hLAN

  7. Weak point in MAC FEC solution (cont.) • SERVICE field has 7 meaningful bits long. • SERVICE failure rate = 9x10-5 at sensitivity level. • This means that PSDU robustness (10-9 PER) does not help because packet robustness is basically limited by Service failure rate, i.e. 9x10-5 . Nir Metzer, hLAN

  8. Proposed solution • SERVICE field is 16 bits. 7 bits are used and there are 9 additional reserved bits. • The 9 bits can be used to protect the SERVICE field. • A systematic code can be used. A simple shortened (15,11) Hamming code gives a failure rate of 6x10-9. Better schemes can be considered • This is a change in the PHY but this addendum does not prevent 802.11a compliant PHY’s to be interoperable with PHY’s modified with a SERVICE field FEC. Nir Metzer, hLAN

  9. Conclusion • Adding FEC to PSDU without full protection of the PLCP will not improve performance. • Additional code is suggested for the SERVICE field in order to solve the problem. • The suggested change affects the PHY specification but does not affect interoperability of modified PHY’s with standard 802.11a PHY’s Nir Metzer, hLAN

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