1. Vivek Raghunathan (joint work with Min Cao, P. R. Kumar) Coordinated Science Laboratory University of Illinois, Urbana-Champaign Exploiting MAC layer diversity in wireless networks

2. Coordinated Science Laboratory, UIUC Motivation Two fundamental issues in wireless networks Fading small scale signal variation due to multi-path reflections. Interference management which combination of users can transmit simultaneously? Traditional approach: layered separation of functionality Fading mitigated in PHY using diversity coding. Interference management solved in MAC using random access or scheduling. PHY MAC Interference Management Fading Mitigation txrx

3. Coordinated Science Laboratory, UIUC Experimental testbed on a tabletop using 3M 1181 Cu tape to reduce range. Measurement methodology Broadcast ICMP ECHO REQUEST and log received sequence using tcpdump. Channel in one of two states: During time interval when an in-order sequence of packets is received, channel is “good”. Else, channel is “bad”. Measure durations of “good” and “bad” states. Fading effects at the MAC layer: an experimental study … Tx Rx ICMP ECHO REQUEST broadcast good state (G) bad state (B) b g

4. Coordinated Science Laboratory, UIUC Fast fading relative to 802.11Slow fading relative to 802.11 Timescale X (ms) Prob(Fading Timescale < X) Timescale X (ms) Prob(Fading Timescale < X) 1 101001 10100 Fading is seen at the MAC layer … Fading may even interact with 802.11 handshaking timescale (1500 bytes at 1 Mbps = 12 ms).

5. Coordinated Science Laboratory, UIUC Tx 1, Tx 2 collocated; Rx 1, Rx 2 collocated; Tx 1 and Tx 2 broadcast 29 byte ICMP ECHO REQUEST on Channel 1 and 10 respectively. Let X(t), Y(t) be channel state (bad = 0/good = 1) across (Tx 1, Rx 1) and (Tx 2, Rx 2). Cross-correlation of a run: In all runs, in (-0.004, 0.2718). Mean of | | = 0.05548. There is very little statistical correlation between the fading across orthogonal channels. Tx 1Rx 1 Tx 2Rx 2 Channel 1 Channel 10 Fading is independent across channels …

6. Coordinated Science Laboratory, UIUC In practice, link conditions to different wireless receivers vary in time, space and frequency. We need to exploit this! Both fading and interference provide different forms of MAC diversity. Treat fading and interference in a unified manner at the MAC layer. Exploit MAC diversity and preferentially transmit on each frequency to the receiver with the “best” fading and interference conditions. Main Idea: opportunistic harnessing of diversity MAC Diversity Pick the best link conditions

7. Coordinated Science Laboratory, UIUC Varying link conditions provide different forms of diversity Multi-receiver fading diversity Multi-receiver interference diversity Multi-channel fading diversity Multi-channel interference diversity A B C Ch 2 Ch 1 AB Ch 2 Ch 1 ABCDA B C Ch 2 Ch 1 AB Ch 2 Ch 1 ABCD A B CE D F G A B CE D F G

8. Coordinated Science Laboratory, UIUC A subtle form of fading diversity … Fading that occurs at 802.11 timescale provides a new form of diversity across transmission attempts. On lossy fading links, exploiting this diversity provides huge reduction in exponential backoff cost. Ch 2 Ch 1 AB RTS CTS DATA ACK Packet 1 goes through on attempt 1 on ch 2 Ch 2 Ch 1 AB RTS CTS DATA ACK Packet 2 succeds on attempt 2 on ch 1 Ch 2 Ch 1 AB RTS CTS Packet 2 fails on attempt 1 on ch 2

9. Coordinated Science Laboratory, UIUC DB-MCMAC: Architecture Dynamic binding per neighbor queuing Scheduler fading + interference aware floor acquisition fading + interference aware floor acquisition ch 1ch 11 Network layer Link layer MAC PHY Link condition estimation Exploit multi- channel and multi- receiver diversity from fading and interference Exploit transmit attempt diversity Assumptions 1) Multiple channels and interfaces 2) Single rate network.

10. Coordinated Science Laboratory, UIUC Link condition estimation Use a set of contention windows and backoff timers on a per channel, per receiver basis. CW j k is a unified measure of transmitter-side channel fading state information (CSI) and receiver interference conditions to receiver j on channel k. Four-way handshake acts like a probe and is used to adapt CW j k to track the link conditions. T XY CW X ch1 CW Y ch1 CW Y ch2 CW X ch 2 Ch 2 Ch 1 AB RTS CTS DATA ACK Packet 1 succeeds on attempt 1 on ch 2 CW B 2 /d CW B 2 probe success Ch 2 Ch 1 AB RTS CTS Packet 2 fails on attempt 1 on ch 2 CW B 2 /d CW B 2.u/d probe failure

11. Coordinated Science Laboratory, UIUC Floor acquisition Fading and interference-aware floor acquisition Preferentially select receivers with better fading and interference conditions on each channel. T XY CW X ch1 CW Y ch1 CW Y ch2 CW X ch 2 Time Ch 1 idle BV X ch1 := uni(0, CW X ch1 ) BV Y ch1 := uni(0, CW Y ch1 ) BV X ch1 fires; Freeze BV Y ch1 pull a pkt P from X’s queue bind to ch 1 start four-phase handshake RTS X ch1 DATA X ch1 CTS X ch1 ACK X ch1 Unfreeze BV Y ch1 BV X ch1 := uni(0, CW X ch1 /d)

12. Coordinated Science Laboratory, UIUC Dynamic binding Keep attempting across channels. Never statically bind a packet to a channel. Time Ch 1 idle BV X ch1 := uni(0, CW X ch1 ) BV Y ch1 := uni(0, CW Y ch1 ) BV X ch1 fires; Freeze BV Y ch1 pull a pkt P from X’s queue Bind P to ch 1 start 4-phase handshake RTS X ch1 DATA X ch1 CTS X ch1 Handshaking Timeout! Unfreeze BV Y ch1 BV X ch1 := uni(0, u.CW X ch1 ) Unbind P from ch 1 and place in X’s queue T XY CW X ch1 CW Y ch1 CW Y ch2 CW X ch 2 Ch 2 idle Time BV X ch2 fires; Freeze BV Y ch2 pull pkt P from X’s queue Bind P to ch 2 BV Y ch2 := uni(0, CW Y ch2 ) BV X ch2 := uni(0, CW X ch2 )

13. Coordinated Science Laboratory, UIUC Multi-receiver diversity 200-350% gains with 3 receivers. Multi-channel diversity 15-150% gains with 3 channels. ns-2 performance evaluation: fading diversity 01 ch 1 ch k UDP tx UDP rx 0 123 SB: static binding DB: dynamic binding

14. Coordinated Science Laboratory, UIUC Distributed Opportunistic Scheduling (DOS)* Multi-user/time diversity Fading causes quality of links to fluctuate Some links are better than others at a given time Let the good links transmit, and at high data rate Distributed Opportunistic Scheduling for Ad-hoc Communications: An Optimal Stopping Approach, D. Zheng, W. Ge, J. Zhang, MobiHoc'07

15. Coordinated Science Laboratory, UIUC DOS Mechanism Transmitter contends for the medium, e.g. RTS Receiver measures channel quality If channel is good Receiver replies CTS with desired data rate Data transmission follows Else receiver does not reply; contention continues Successful probing Idle Collision Data transmission time Poor channelGood channel

16. Coordinated Science Laboratory, UIUC Tradeoff in DOS Tradeoff More channel probing, better channel condition, higher data rate Less channel probing, more useful time for data What is the optimal stopping time? Time to think the channel is good and transmit To maximize network throughput Optimal stopping rule for throughput maximization is presented