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Exploiting MAC layer diversity in wireless networks

Exploiting MAC layer diversity in wireless networks. Vivek Raghunathan (joint work with Min Cao, P. R. Kumar) Coordinated Science Laboratory University of Illinois, Urbana-Champaign. Interference Management. MAC. Fading Mitigation. PHY. tx. rx. Motivation.

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Exploiting MAC layer diversity in wireless networks

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

  2. Interference Management MAC Fading Mitigation PHY tx rx 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.

  3. g Rx Tx good state (G) bad state (B) ICMP ECHO REQUEST broadcast b Fading effects at the MAC layer: an experimental study … • 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.

  4. Prob(Fading Timescale < X) Prob(Fading Timescale < X) Timescale X (ms) Timescale X (ms) 10 100 10 100 1 1 Fast fading relative to 802.11 Slow fading relative to 802.11 Fading is seen at the MAC layer … • Fading may even interact with 802.11 handshaking timescale (1500 bytes at 1 Mbps = 12 ms).

  5. Channel 1 Channel 10 Tx 1 Rx 1 Tx 2 Rx 2 Fading is independent across channels … • 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.

  6. Main Idea: opportunistic harnessing of diversity • 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. MAC Diversity Pick the best link conditions

  7. B A C F B F B D B D A A A C C E C E G G Ch 1 A B Ch 2 Ch 1 A B Ch 2 Ch 1 A B C D Ch 1 A B C D Ch 2 Ch 2 Varying link conditions provide different forms of diversity • Multi-receiver fading diversity • Multi-receiver interference diversity • Multi-channel fading diversity • Multi-channel interference diversity

  8. RTS RTS RTS CTS CTS CTS DATA DATA ACK ACK Ch 1 Ch 1 A B Ch 1 Ch 2 A B A B Ch 2 Ch 2 Packet 1 goes through on attempt 1 on ch 2 Packet 2 fails on attempt 1 on ch 2 Packet 2 succeds on attempt 2 on ch 1 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.

  9. Network layer per neighbor queuing Link layer Exploit multi-channel and multi-receiver diversity from fading and interference Scheduler MAC Exploit transmit attempt diversity ch 1 fading + interference aware floor acquisition fading + interference aware floor acquisition ch 11 Dynamic binding Link condition estimation PHY DB-MCMAC: Architecture • Assumptions1) Multiple channels and interfaces • 2) Single rate network.

  10. T CWXch1 CWYch1 CWXch2 CWYch2 X Y RTS RTS CTS CTS DATA ACK probe failure CWB2.u/d Ch 1 A B Ch 1 CWB2/d Ch 2 A B CWB2 Ch 2 probe success CWB2/d Packet 1 succeeds on attempt 1 on ch 2 Packet 2 fails on attempt 1 on ch 2 Link condition estimation • Use a set of contention windows and backoff timers on a per channel, per receiver basis. • CWjk 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 CWjk to track the link conditions.

  11. T CWXch1 CWYch1 CWXch2 CWYch2 X Y Ch 1 idle Time Floor acquisition • Fading and interference-aware floor acquisition • Preferentially select receivers with better fading and interference conditions on each channel. DATAXch1 RTSXch1 BVXch1 fires; Freeze BVYch1 CTSXch1 ACKXch1 Unfreeze BVYch1 BVXch1 := uni(0, CWXch1/d) BVYch1 := uni(0, CWYch1) BVXch1 := uni(0, CWXch1) • pull a pkt P from X’s queue • bind to ch 1 • start four-phase handshake

  12. T CWXch1 CWYch1 CWXch2 CWYch2 X Y Ch 1 idle Ch 2 idle Time Time Dynamic binding • Keep attempting across channels. • Never statically bind a packet to a channel. • pull a pkt P • from X’s queue • Bind P to ch 1 • start 4-phase • handshake Handshaking Timeout! Unfreeze BVYch1 BVXch1 := uni(0, u.CWXch1) Unbind P from ch 1 and place in X’s queue DATAXch1 RTSXch1 CTSXch1 BVXch1 fires; Freeze BVYch1 BVYch1 := uni(0, CWYch1) BVXch1 := uni(0, CWXch1) BVXch2 fires; Freeze BVYch2 • pull pkt P from X’s queue • Bind P to ch 2 BVYch2 := uni(0, CWYch2) BVXch2 := uni(0, CWXch2)

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

  14. 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. 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 Poor channel Good channel time Collision Successful probing Data transmission Idle

  16. 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

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