1. Scrambler Mismatch and MAC FEC: Is there a problem?
September 2002
doc.: IEEE /636r0
September 2002
Scrambler Mismatch and MAC FEC: Is there a problem?
Monisha Ghosh, Vasanth Gaddam
Philips Research.
Monisha Gosh et. al. Philips
Monisha Gosh et. al. Philips

2. September 2002
Problem Statement
The first 7 bits of the service field carry the scrambler initialization information.
This field is not RS encoded in the MAC.
A single bit error in this 7-bit sequence can cause the entire packet to be in error, since it is not RS encoded.
Hypothesis: this will limit the effectiveness of the MAC FEC.
Monisha Gosh et. al. Philips

3. Past Results/Solutions
September 2002
Past Results/Solutions
Assuming the bit error rate (BER) to be uniform over the bit stream, it has been shown by analysis that a scrambler error can limit the MAC FEC performance.
The analysis assumed AWGN only. Multipath can cause error bursts that will limit the efficacy of the MAC FEC independent of scrambler errors.
Solutions proposed: dual precoding (can cause error propagation), extra bytes (increased overhead).
Monisha Gosh et. al. Philips

4. September 2002
BER of a trellis code
The 7-bit sequence containing the scrambler initialization is at the start of the bit sequence to be encoded.
The trellis at that point is not “fully connected”, i.e. there is only a single path entering each state for the first 6 trellis transitions.
The analytical expression for BER of a trellis code considers a fully connected trellis.
Monisha Gosh et. al. Philips

5. September 2002
Hypothesis
The BER for the first 7 bits of the trellis is lower than elsewhere in the trellis.
The analytical results presented to date do not take this into effect.
Simulation results indicate that indeed there is about an order of magnitude difference between the error rate of the scrambler initialization (7 bits) and bytes elsewhere in the data stream.
Monisha Gosh et. al. Philips

6. September 2002
Simulation Results: 54 Mbps, 1616 bytes/packet, 5000 packets, AWGN Channel
Monisha Gosh et. al. Philips

7. September 2002
Simulation Results: 54 Mbps, 496 bytes/packet, packets, AWGN + Multipath Channel
Even though the byte error rate is higher when there is a scrambler initialization error, the packet error rate is not.
This means that when there is an error in the scrambler initialization field, there are also errors in the rest of the data packet
Monisha Gosh et. al. Philips

8. September 2002
Simulation Results Comparing Scrambler Initialization Error Rate with Byte Error Rate
54 Mbps, 50 byte packets, Multipath + AWGN channel, 1 million packets, SNR = 28 dB.
The size of packets was reduced to make the simulation run faster.
Byte Error Rate, assuming perfect initialization: 2.12e-05
Scrambler Error rate: 2.0e-06
25 byte packets, 4 million packets, SNR = 29 dB, Byte Error Rate = 5.38e-06, Scrambler error rate = 7.5e-07.
Monisha Gosh et. al. Philips

9. September 2002
Conclusions
The position of the 7-bit initialization sequence at the beginning of the trellis gives it added protection not accounted for in the analysis.
Simulation results corroborate the above hypothesis: no discernable effect of scrambler error on packet error rate was noticed especially at higher SNRs.
Simulation results indicate approximately an order of magnitude difference in the error rates of the scrambler initialization field versus the byte error rate.
The MAC FEC gives a gain of about 2 dB. The limitation seems to be more from the lack of interleaving than from scrambler initialization errors.
Monisha Gosh et. al. Philips