Wireless Network CodingWireless Network Coding

Martin Xu

OutlineOutline

• Introduction• New Solutions– COPE– ANC

• Conclusions

IntroductionIntroduction

• Wireless: mobility• Problem: severe throughput limitation• Solutions– COPE– ANC–…

• Let’s take a look at traditional wireless…

Traditional WirelessTraditional Wireless

Router

Router

4 transmission required

Traditional WirelessTraditional Wireless

Router

4 time slots required

Traditional WirelessTraditional Wireless

COPE

• Katti, S., Rahul, H., Hu, W., Katabi, D., Médard, M., and Crowcroft, J. 2006. XORs in the air: practical wireless network coding. In Proceedings of the 2006 Conference on Applications, Technologies, Architectures, and Protocols For Computer Communications (Pisa, Italy, September 11 - 15, 2006). SIGCOMM '06. ACM Press, New York, NY, 243-254.

• Forward multiple packets in a single transmission

• Let’s take a look at how COPE deals with the aforementioned example

XOR

=

Router

COPE

Router

COPE

XOR

= XOR

=

Router

Higher throughput (3 transmissions required)

COPE

COPE

• Transparent coding layer between IP and MAC

• Forward multiple packets in a single transmission

• Never delay packets• HOW?– Opportunistic Listening– Opportunistic Coding– Learning neighbor state

COPE - Listening

• “Broadcast” in a small neighbor• Each node stores overheard packets

for a limited time• Pseudo broadcast– Broadcast results in poor reliability and

lack of backoff– Pseudo broadcast unicast packets that

are meant for broadcast

COPE - Coding

• To transmit n packets, p1, ..., pn, to n nexthops, r1, ..., rn, a node can XOR the n packets together only if each next-hop ri has all n − 1 packets pj for j = i.

COPE - data structure

• Reception report– Reports are piggybacked on packets– If no packets to send, periodically send

reports• Output queue (FIFO)• Two per-neighbor virtual queue– packet-size distribution in the Internet is

bimodal with peaks at 40 and 1500 bytes• Packet info

COPE - Performance

• With no hidden terminals, TCP’s throughput can increase by 38%

• flows, COPE increases UDP throughput by 3-4x

ANC

• Katti, S., Gollakota, S., and Katabi, D. 2007. Embracing wireless interference: analog network coding. In Proceedings of the 2007 Conference on Applications, Technologies, Architectures, and Protocols For Computer Communications (Kyoto, Japan, August 27 - 31, 2007). SIGCOMM '07. ACM Press, New York, NY, 397-408.

• Strategically exploit interference instead of avoiding it– Interfered signal is not exactly the sum– Channel distorts signals– Two signals are never synchronized

ANC - Algorithm

• Decode small part of uninterfered signal (MSK)

• Decode interfered signal– Decomposition using amplitudes of the

original ones and the interfered– Four possible angles– Choose the 90 degree one

• Decode the rest of the uninterfered signal

ANC - Performance

• For the example used at the beginning, median throughput gain of ANC over routing 70%, COPE 30%

• For X-topology, median throughput gain over routing is 65%, over COPE 28%

• For chain topology, median throughput gain over routing is 37%

Conclusion

• Both implementation that yields large throughput gains

• COPE– Simple and practical

• ANC– Embrace Broadcast and Interference