1. Geographic Random Forwarding with Hybrid-ARQ for Ad Hoc Networks with Rapid Sleep Cycles Rohit Iyer Seshadri and Matthew C. Valenti West Virginia University Morgantown, WV 26506-6109 {Iyerr,mvalenti}@wvu.edu Bin Zhao Efficient Channel Coding Inc. Brooklyn Heights, OH 44131 {bzhao}@eccincorp.com This work was supported by the Office of Naval Research under grant N00014-00-0655

2. 12/01/20042/19 Communications in Ad Hoc Networks Limitations in conventional multihop –OSI model ignores interdependency among layers Suboptimal in energy efficiency –Routing relies on cascade of point-point links Not robust to variations in topology and channel –Messaging Ignores the wireless broadcast nature Spatial (MIMO) diversity not exploited Vanishing average transport capacity in large scale networks (Gupta/Kumar’00) A rediscovered solution: relaying –Exploits distributed spatial diversity via virtual antenna array –Additional macrodiversity (non-linear path loss) benefit –Cross-layer design involves layer coupling and device cooperation

3. 12/01/20043/19 Wireless Relaying and Distributed Array Spatial diversity through antenna array Distributed spatial diversity through virtual antenna array Receiver Transmitter Receiver #2 Transmitter Receiver #1 Wireless relay link Virtual transmit array

4. 12/01/20044/19 Previous Work on Relaying Unconstrained relay –Relay nodes receives and transmits simultaneously –Source/relays transmit coherently: Beamforming –Classic relay channel (Cover/ El Gammal ’79) –Network with multi-user relaying (Gupta / Kumar’03) Constrained relay: cheap relay (H Ø st Madsen; Khojastepour/Aazhang) –Relay transmits and receives in orthogonal channels, i.e. TDD –Source and relay still achieve beamforming effect when relay transmits Orthogonal relay networks: cheaper relay –Relays operate in TDD mode –Source and relays transmit in orthogonal channels –Cooperative diversity (Sendonaris) Two users pair-up and relay for each other Distributed space-time codes (Laneman) Cooperative coding (Hunter) SourceDestination Relay Source #2 Source #1 Destination #2 Destination #1

5. 12/01/20045/19 Practical Relaying Scalable to Large Networks Practical concerns of relaying –Different frequency offset and propagation delay in distributed arrays –Coherent transmission requires CSI in fading channels –Network need to schedule relaying in a distributed fashion Position-based orthogonal relaying: generalized hybrid-ARQ –Hybrid-ARQ Encode data into a low-rate R M mother code Break the mother codeword into M distinct blocks with rate-compatible puncturing; each block has rate R = R M /M Each block is sent successively until destination signals with an ACK –After mth transmission, effective rate is R m = R/m –Generalized hybrid-ARQ allows retransmission from any node that could decode the message Use geographic information to guide ARQ retransmission (orthogonal relaying)

6. 12/01/20046/19 Info Theory of Hybrid-ARQ Throughput analysis of hybrid-ARQ in block fading channel (Caire/Tuninetti 2001) –Let  m denote the received SNR during the m th transmission –The instantaneous capacity is –The cumulative capacity is –An outage occurs if

7. 12/01/20047/19 Position Based Multihop: GeRaF Geographic Random Forwarding (Zorzi/Rao’03) – Node activity follows a random sleep schedule to conserve energy –Source broadcasts over an AWGN channel If one node is within range it becomes the designated relay for the next hop If multiple nodes, pick the node closest to destination Otherwise, source reattempts later to see if any node awakes –Cross-layer design combining media access control and routing Reactive routing based on geographic information –Single transmission per hop (no ARQ) Flush memory at the beginning of each transmission Time and spatial diversity is not exploited –Areas with low (active) node density become bottleneck Source may reattempt many times before any node within range awakes

8. 12/01/20048/19 Position Based Relaying: HARBINGER Hybrid ARq-Based INtercluster GEographic Relaying –Source broadcasts first packet, m=1 –Relays that can decode are added to the decoding set D The source is always in D –The next packet is sent by a node in D Position-based Relaying: Pick the node closest to destination –The process continues until the destination can decode Generalized hybrid-ARQ exploits time and spatial diversity –Effectively expands radio coverage region –Increases active node population, thus relieves the bottleneck Energy-latency tradeoff can be analyzed by generalizing Caire and Tuninetti’s analysis

9. 12/01/20049/19 HARBINGER: Initialization Solid circles are in the decoding set D. Amount of fill is proportional to the accumulated entropy. Keep transmitting until Destination is in D. SourceDestination

10. hop I SourceDestination Contention window ACK window 0 window 1 window 2 Priority Zone 2 Priority Zone 1 Priority Zone 0 Coverage circle t HARBINGER: First Hop Solid circles are in the decoding set D. Amount of fill is proportional to the accumulated entropy. Keep transmitting until Destination is in D.

11. HARBINGER: Second Hop hop II SourceDestination Relay Solid circles are in the decoding set D. Amount of fill is proportional to the accumulated entropy. Keep transmitting until Destination is in D.

12. HARBINGER: Third Hop hop IV Source Relay Destination Solid circles are in the decoding set D. Amount of fill is proportional to the accumulated entropy. Keep transmitting until Destination is in D.

13. HARBINGER: Fourth Hop hop III Source Relay Destination Solid circles are in the decoding set D. Amount of fill is proportional to the accumulated entropy. Keep transmitting until Destination is in D.

14. HARBINGER: Results Topology: Relays on straight line S-D separated by 10 m Coding parameters: Per-block rate R=1 No limit on M Code Combining Channel parameters: n = 3 path loss exponent 2.4 GHz d 0 = 1 m reference dist Renewal-reward theorem with Monte Carlo integration B. Zhao and M. C. Valenti. “Practical relay networks: A generalization of hybrid-ARQ”, JSAC special issue on Wireless Ad Hoc Networks 2005.

15. 12/01/200415/19 HARBINGER: Discussion Advantages –Generalized hybrid-ARQ exploits time and spatial diversity to achieve better energy-latency tradeoff than multihop Disadvantages –More receivers listen to each packet (receiver energy dissipation) –Longer contention period in the protocol –Results are intractable, must resort to simulation Variations on HARBINGER –Fast Harbinger Network topology changes after every ARQ retransmission –Slow Harbinger Topology remains fixed for multiple ARQ retransmissions

16. 12/01/2004 16/19 Simplifying Assumptions Closed-form analysis is not tractable –Statistically variable channels –Nodes have memory for entire source-destination transaction –Possible changes in topology Simplifying assumptions –Packet transmits in AWGN channel –Topology follows 2-D Poisson process –Each node follows a random sleep schedule –Nodes flush memory once a new relay is selected Still maintain memory of ARQ packets from current transmitter –GeRaF is HARBINGER under the simplifying assumptions and without ARQ (rate constraint M=1)

17. 12/01/200417/19 Fast-HARBINGER Analysis Hybrid-ARQ expands radio coverage Bound on message delay (lower) code combining diversity combining Pr{ hop from band j to band (j-k) takes b delay} Delay at band j Delay at band (j-k) R1R1 Source Destination Band j Band (j-k) R3R3 R2R2 Band (j-R M ν ) Total number of bands Rate constraint Pr{bands j~(j-R m ν ) are empty }

18. Fast-HARBINGER: Analytical Result Topology: 2-D Poisson S-D separated by 10 m Coding parameters: Per-block rate R=1 Code Combining Normalized power (Initial TX range is 1 m) Channel parameters: n = 3 path loss exponent 2.4 GHz d 0 = 1 m reference dist Only requires calculating areas of the geographically advantaged regions B. Zhao and M. C. Valenti. “Position-based relaying with hybrid-ARQ for efficient ad hoc networking,” submitted to EURASIP issue on Ad Hoc Networks: Cross-Layer Issues.

19. 12/01/200419/19 Conclusions Relaying exploits broadcast-nature of radio device HARBINGER implements “distributed” spatial diversity by generalized hybrid-ARQ in wireless networks Cooperative diversity (orthogonal relaying) can give a better tradeoff between energy and latency –The number of participating relays should be carefully chosen A cross-layer approach can yield significant gains –Error control using hybrid-ARQ –CSMA-style medium access control –Position-based relaying