1. CCH: Cognitive Channel Hopping in Vehicular Ad Hoc Networks Brian Sung Chul Choi, Hyungjune Im, Kevin C. Lee, and Mario Gerla UCLA Computer Science Department VTC 2011 Fall Presentation 1

2. Motivation  Wi-Fi based vehicular networks  Each vehicle equipped with a Wi-Fi interface.  Multi-hop communication among vehicular nodes. 2

3. Motivation  Problem: The Wi-Fi band is shared with other devices.  Residential access points, Bluetooth, etc. 3 13 2 1 4 2

4. Channel Sensing  Each node periodically monitors the wireless medium. 4 13 2 1 4 2 channel quality vectors

5. Channel Assignment  Nodes exchange sensing results and channels are assigned.  Exchange of channel quality vectors incurs overhead.  Optimal channel assignment is difficult. 5 13 2 1 4 2 channel quality vectors 3 4 4 22

6. CCH: Cognitive Channel Hopping  Each node generates a channel hopping sequence based on channel qualities, and independently channel-hops according to the sequence. 6 13 2 1 4 2

7. CCH Protocol Operation  A node x periodically triggers Channel Quality Assessment (CQA).  A channel quality vector a = {a 1, …, a |C| } is produced.  Based on the channel qualities, x picks a channel set Q = {q 1, …, q k } from a predefined list, which is a quorum system. 7 Example list of channel sets.  It picks the channel set with the highest combined channel quality, defined as:

8. CCH Protocol Operation  Given Q, x generates two hopping sequences, u tx and u rx, such that it makes a channel rendezvous with any arbitrary neighbor in a finite duration. 8

9. CCH Protocol Operation  When there is no packet to transmit, follow u rx.  When there are packets to transmit, follow u tx to locate the neighbor.  A channel rendezvous is guaranteed within the length of u rx. 9

10. CCH Protocol Details  RTS/CTS-based link establishment.  Retransmissions occur within a slot (random errors), and over slots (channel discordance).  u ry of neighbor nodes are cached, such that when x has packets destined to a previously encountered neighbor, it can find that neighbor immediately, instead of scanning along u tx.  Broadcast packets are transmitted in the beginning of each slot, for several slots. 10

11. Evaluation  Simulated in QualNet 4.5.  13 orthogonal channels in the 5-GHz band.  Data rate fixed at 54Mbps, Tx power at 21dBm.  Channel set size: 5.  Channel switching delay: 80µs.  Slot size: 10ms.  Period: 3 seconds (this is how often CQA is performed). 11

12. Evaluation (static)  Linear topology 12 123489... (without interfering sources)(4 hops, random interfering sources, each of which is active 20% of the time) (4 hops, random interfering sources, each of which is active 60% of the time)

13. Evaluation (static)  Large scale  200m x 200m region, 100 nodes, 10 random streams, 300 interfering sources. 13 static case (throughput)static case (e2e delay)

14. Evaluation (mobile)  Large scale  200m x 200m region, 100 nodes, 10 random streams, 300 interfering sources. 14 mobile case

15. Evaluation (vehicular)  Vehicular Scenario  200m x 200m region, 100 vehicular nodes, 10 random streams, 300 interfering sources.  Vehicular mobility generated using VanetMobiSim. 15 200m

16. Conclusion  Channel hopping is an effective way of utilizing multiple channels, improving spatial reuse and coping with node mobility.  Cognitive techniques can be useful in an unlicensed band-based network, demonstrated by CCH’s ability to avoid channels that are busy.  Future Work  Can channel quality be accurately characterized during a short sensing duration?  How can this scheme be incorporated into VANETs that use the roadside Wi-Fi infrastructure? 16

17. Thank you. Questions? 17

18. Backup slides 18

19. Channel Rendezvous “Recovery” Mechanism  Channel rendezvous property breaks if:  Node x wants to send a frame to y, so switches to x ’s transmitting channel, while y is also trying to transmit to another channel, thus not in its receiving channel, or  Node x wants to send a frame to y, so switches to y ’s receiving channel (because it has u(y) in its cache), while y is also trying to transmit to another channel, thus not in its receiving channel. 19 tx x y y ’s rx channel. tx rx x has a packet for y. where is y? y is involved in a transmission. tx

20. Channel Rendezvous “Recovery” Mechanism  We mitigate this effect by forcing each node “yield” for a small amount of time after each frame transmission.  During a “yield” period, the node waits in its receiving channel.  This “yield” duration is dynamically adjusted based on the level of congestion the node sees.  This little trick turns out to be very effective. 20 txrxtxrx x y y ’s rx channel. tx rx yield y ’s rx rx x has a packet for y. x finds y, and starts transmitting to y. y is involved in a transmission.

21. Channel Rendezvous Example  Consider the case where x is trying to send a frame to y.  Assume x ’s channel set is {0, 1, 2}, and y ’s channel set is {2, 3, 4}. There is one common channel: 2.  The following matrices are used to generate hopping sequences. 21 012012012234342423 M tx (x)M rx (y)

22. Channel Rendezvous Example  u tx (x)  u rx (y) 22 012012012234342423 M tx (x)M rx (y) 012012012234342423

23. Channel Rendezvous Example  u tx (x)  u rx (y) 23 012012012012234342423234 … …

24. Channel Rendezvous Example  u tx (x)  u rx (y) 24 012012012012 … … 343424232343