1. Cross-layer Two-stage FEC Scheme for Scalable Video Transmission over Wireless Networks
Yufeng Shan, Su Yi, Shivkumar Kalyanaraman
and John W. Woods
Rensselaer Polytechnic Institute
Google “Shiv RPI”

2. Outline Problems Our proposed schemes Simulations Conclusions
Enhanced wireless MAC/PHY layer
Two-stage FEC
Simulations
Conclusions
First, I will talk about the problems why current wireless LANs are not efficient on multimedia data transmission.

3. Problems – 802.11 LANs No cooperation between layers
Any bit error can cause a whole packet being dropped
No cooperation between layers
wireless LANs are designed for reliable data transmission. One bit error can cause the whole packet being discarded, even though the packet can be used by error resilience CODEC in the receiver or recovered by application layer error control codes.
(2) Each layer has it own protection mechanisms.
(3) (a) packets are dropped due to routing disruption and congestion in the intermediate nodes, and (b) packets are discarded in the MAC/PHY layers due to internal bit errors.
MAC ARQ is not efficient for
packet bit error

4. Our proposed two schemes
Enhanced MAC/PHY layer using Header CRC/FEC
helps to pass packet with errors to application and to forward more packets to next node
Two-stage FEC scheme at application layer
to cooperative with enhanced MAC/PHY layer for error recovery, both packet drop and bit error.
To solve the above mentioned problems, we propose two scheme.
Since packet with errors are passed to application layer, we need to have a scheme to solve both bit level and packet drops
Cyclical Redundancy Check/Forward Error Correction

5. Enhanced Protocol Stack
System Diagram
Video Encoder
Stage1 FEC Encoder
Stage2 FEC Encoder
Application
UDP-lite
Enhanced Protocol Stack IP
MAC
PHY
Video Decoder
Stage1 FEC Decoder
Stage2 FEC Decoder
This is the system diagram show how the two scheme works together to improve the efficiency for multimedia over wireless networks
We slight modified the protocol stack and packets with errors are passed to application layer. Further, our enhanced MAC/PHY layer using header CRC/FEC increases application layer throughput
Enhanced MAC/PHY layer using header CRC/FEC increases application layer throughput
Stage 1: Packet level
Stage 2: Bit level

6. Enhanced MAC/PHY layer
Only header CRC/FEC
Header FEC
Headers
FEC
Packet payload
APP
UDP IP
MAC
Enhance MAC/PHY layer for multimedia:
Header CRC: only CRC the header part
Header FEC: add small bit level FEC to protect header part from bit errors
We use two methods to enhance the MAC/PHY layer for multimedia data transmission, header CRC and header FEC.

7. Enhanced MAC/PHY layer
physical bandwidth 2 Mbps
Packet payload 1000 bytes
Packet CRC checks whole packet
Header CRC checks only headers
Header FEC protect headers with BCH(511,502,1)
Here we show the analytical results using matlab of our enhanced MAC/PHY layer compared with conventional
Single hop application throughput
Packet may have errors inside

8. Two-stage FEC FECs are processed only at application layer
Can recover both packet losses and bit errors inside packet
Cooperate with enhanced MAC/PHY layer to improve the application layer throughput.
To combat with this errors, we proposed two stage FEC scheme
Packets with errors are passed to the application layer, to effectively recover the two kind of packets loss, we proposed two stage FEC scheme.
Packets first are encoded with packet level FEC across packets, then each packet including FEC packet are further protected by bit level codes within each packet.

9. Two-stage FEC Error free throughput
Protections
FEC codes
Code Rate
802.11
SW-ARQ
Retransmit one time
RS only
RS(255,239)
239/255
Two-stage FEC + HCRC
RS(255,245) +
Header CRC
239/245
Two-stage FEC + HFEC
BCH(511,502,1)
Error free throughput
In this simulation
Effective application throughput increased
Two-stage FEC: payload is protected by BCH(8191,8000,14)

10. ns-2 simulations
ns b wireless module, PHY layer bandwidth is set to 2Mbps;
MAC layer retransmission 2; Channel model: Gilbert (burst length: 2)
Two-stage FEC: RS code + BCH code
Two scenarios:
Single hop: from sender to receiver1, CBR traffic is set to 2Mbps
Multi-hop: from sender to recever2, receiver 1 is relay node, CBR traffic is set to 1.2Mbps

11. ns-2 simulations - throughput
Single hop
Multi-hop
Header FEC: BCH(8191,8000,14) for payload, BCH(510,480,3) for header
Packet CRC: b
Header CRC: BCH(8191,9000,14) for payload

12. ns-2 simulations – video
Monochrome Foreman, CIF, 30fps, 16 frame/GOP, source coder: MC-EZBC
Bitstream is pre-encoded with MD-FEC at 1 Mbps, 10% loss rate
Two kinds of simulations:
Single hop fixed FEC, CBR 1Mbps
Multi-hop w/ adaptive FEC, bitstream and FEC protection adapts based on network conditions

13. ns-2 simulations - video
Video PSNR of the first 7 GOPs
PSNR = 0
if no enough bandwidth for base layer
Single hop
Multi-hop

14. Conclusions
Two-stage FEC with enhanced MAC/PHY layer using header CRC/FEC increases application layer throughput
Efficient in wireless multimedia error protection.
Cooperation between layers can increase performance for multimedia over wireless networks.

15. Thanks