1. MAC based FEC – improvement for 802.11a
Month 2000
doc.: IEEE /xxx
January 2001
MAC based FEC – improvement for a
Nir Metzer
hLAN inc.
Israel
Nir Metzer, hLAN
John Doe, His Company

2. Scope FEC general requirements. MAC-based FEC specific requirements.
January 2001
Scope
FEC general requirements.
MAC-based FEC specific requirements.
MAC-based FEC weak points.
Proposed solution.
Conclusion.
Nir Metzer, hLAN

3. FEC General Requirements
January 2001
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
January 2001
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
January 2001
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 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
January 2001
Weak point in MAC FEC solution
However, PLCP header in a 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.)
January 2001
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. January 2001
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 a compliant PHY’s to be interoperable with PHY’s modified with a SERVICE field FEC.
Nir Metzer, hLAN

9. January 2001
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 a PHY’s
Nir Metzer, hLAN