1. Improving Wireless Link Throughput via Interleaved FEC
Ling-Jyh Chen, Tony Sun, M. Y. Sanadidi, Mario Gerla
Computer Science Department, University of California at Los Angeles

2. Background Dealing with packet losses: End-to-end Link Layer Level
Improving Wireless Link Throughput via Interleaved FEC
Background
Dealing with packet losses:
End-to-end
ARQ: Retransmission in TCP
FEC: Robust Audio Tool (RAT) [14]
Partial Correctness: Interleaving as in Video transfer protocols [RFC 2354]
Link Layer Level
ARQ: as in HDLC, or Bluetooth link layer protocol
FEC: as in a and Bluetooth
RAT:
June 2004
ISCC 2004

3. Background Problems: ARQ does not perform well for high error rates
Improving Wireless Link Throughput via Interleaved FEC
Background
Problems:
ARQ does not perform well for high error rates
FEC works if errors are not bursty
Interleaving introduces latency
Our Proposal: I-FEC
combines FEC and Interleaving at Bluetooth link layer for high-rate bursty errors
June 2004
ISCC 2004

4. Background In Bluetooth DM mode, FEC uses a (15, 10) Hamming code
Improving Wireless Link Throughput via Interleaved FEC
Background
In Bluetooth DM mode, FEC uses a (15, 10) Hamming code
Each block of 10 information bits is encoded into a 15 bit codeword
Capable of correcting single bit error in each block
Mode
FEC
Packet
Symmetric Throughput (kbps)
Asymmetric Throughput (kbps)
Size (bytes)
Length (slots)
DM1
Yes 17 1
108.8
DM3
121 3
258.1
387.2
54.4
DM5
227 5
286.7
477.8
36.3
DH1 No 27
172.8
DH3
183
390.4
585.6
86.4
DH5
339
433.9
723.2
57.6
DM: has FEC coding; DH: has no FEC coding
Time slots accommodate one packet, different packet lengths… feedback (occupy one time slot) at end of the number of slots == and of a packet
Symmetric means data in two directions
June 2004
ISCC 2004

5. Gilbert-Elliott Burst Error Model
Improving Wireless Link Throughput via Interleaved FEC
Gilbert-Elliott Burst Error Model
June 2004
ISCC 2004

6. Improving Wireless Link Throughput via Interleaved FEC
Burst Error Length
At high Pbb, burst length are too long; do not expect FEC to work
June 2004
ISCC 2004

7. Packet Error Rate after FEC
Improving Wireless Link Throughput via Interleaved FEC
Packet Error Rate after FEC
Pgb =
Packet Size
FEC Level
Pbb = 0.1
Pbb = 0.3
Pbb = 0.5
Pbb = 0.7
Pbb = 0.9
100 bytes
(15,10)
3.25 %
9.42 %
15.44 %
21.47 %
28.61 %
(7,4)
3.39 %
10.24 %
16.92 %
23.45 %
30.18 %
None
32.92 %
32.93 %
32.94 %
32.98 %
33.23 %
200 bytes
7.40 %
20.48 %
31.84 %
42.00 %
51.16 %
6.76 %
19.39 %
30.79 %
41.28 %
50.91 %
55.09 %
55.10 %
55.11 %
55.13 %
55.29 %
500 bytes
16.86 %
42.59 %
60.57 %
73.33 %
82.68 %
16.09 %
41.90 %
60.51 %
73.76 %
83.15 %
86.49 %
86.50 %
86.55 %
Simulation results; Monte Carlo, multiple runs, average the results obtained
When Pbb is high, FEC does not help much!
June 2004
ISCC 2004

8. Proposed Approach: I-FEC
Improving Wireless Link Throughput via Interleaved FEC
Proposed Approach: I-FEC
Inherits both the robustness to random errors from FEC and the survivability to burst errors from Interleaving
Simulated I-FEC in Bluetooth, and to Bluetooth DH (no FEC coding) and DM (FEC coding) modes
June 2004
ISCC 2004

9. Proposed Approach: I-FEC
Improving Wireless Link Throughput via Interleaved FEC
Proposed Approach: I-FEC
Bluetooth DM mode (FECed already!)
(b) I-FEC
Interleaving by transposition
The FEC overhead is the same as DM mode
The latency caused by interleaving is negligible, since I-FEC interleaves data in bit level within one link layer packet, instead of packet level cross different packets
June 2004
ISCC 2004

10. Improving Wireless Link Throughput via Interleaved FEC
Evaluation
Comparison of I-FEC, DH, and DM mode packets using 5-timeslot Bluetooth packets.
PER with different Pgb and Pbb (using Monte Carlo Simulation)
TCP Throughput with different Pgb and Pbb (using NS2 simulator)
Simulation Topology: (a) one hop (b) two hops
I-FEC is implemented here on each link in (b)
TCP is end-to-end
June 2004
ISCC 2004

11. Packet Error Rates (1 hop)
Improving Wireless Link Throughput via Interleaved FEC
Packet Error Rates (1 hop)
Pgb =
Pbb = 0.2
June 2004
ISCC 2004

12. TCP Performance Improving Wireless Link Throughput via Interleaved FEC
Pgb not Pab
Pbb = 0.2
Pbb = 0.2
June 2004
ISCC 2004

13. TCP Performance Improving Wireless Link Throughput via Interleaved FEC
FEC overhead will cause its performance to be worse than no FEC after some Pbb, that is after some average burst length is exceeded; in that range application throughput will drop sue to useless use of FEC redundancy bits
Pgb =
Pgb =
June 2004
ISCC 2004

14. Improving Wireless Link Throughput via Interleaved FEC
Conclusion
I-FEC, a hybrid approach incorporating the robustness of FEC coding to random errors and the survivability of interleaving to burst errors
I-FEC is simple and applicable to other link technologies, as long as FEC is already there
June 2004
ISCC 2004

15. T h a n k s Improving Wireless Link Throughput via Interleaved FEC
June 2004
ISCC 2004