1. Hybrid-ARQ Based Intra-Cluster Geographic Relaying
Matthew Valenti, Ph.D.
Assistant Professor
West Virginia University
Morgantown, WV
This work was supported by the Office of Naval Research
under grant N
Bin Zhao, Ph.D.
Efficient Channel Coding
Brooklyn Heights, OH

2. Problem Statement Consider the following ad hoc network: Questions:
How can the message be efficiently routed to the destination?
What is the tradeoff between latency and energy efficiency?
How to jointly implement error control, routing, and access control?
Joint (cross-layer) solution is emphasized.
Source
Destination
Relays
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HARBINGER

3. Conventional Hybrid ARQ
Consider a point-to-point link:
Hybrid-ARQ using incremental redundancy
The data is encoded into a rate RM mother code
Implemented using rate-compatible puncturing.
Break the codeword into M distinct blocks
Each block has rate R = MRM
Source begins by sending the first block.
If destination does not signal with an ACK, the next block is sent.
Process continues until source receives an ACK or all blocks sent.
After mth transmission, effective rate is Rm = R/m
Source
Destination
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4. Info Theory of Hybrid-ARQ
Throughput of hybrid-ARQ over block fading channels has been studied by Caire and Tuninetti (IT 2001).
Let m denote the received SNR during the mth transmission
The instantaneous capacity (mutual information) is:
The cumulative capacity is:
An outage occurs if
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5. Conventional Approach: Multihop
Multihop picks from among several possible routes:
Creates the route from a cascade of point-to-point links
Each point-to-point link could use hybrid-ARQ
Drawbacks
Routing tables need to be created and maintained.
Not robust to changes in topology, interference, or channel.
Routing ultimately relies on cascade of point-point links.
Need to keep retrying over bad links.
Spatial (MIMO) diversity not exploited.
Wireless is broadcast-oriented, not link-oriented!
The network could instead be interpreted as a large distributed array.
Source
Destination
Relays
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6. Generalized Hybrid-ARQ
Now consider a multi-terminal network:
Suppose the source attempts to communicate with the destination using hybrid-ARQ.
After each ARQ transmission, some of the intermediate nodes could “overhear” the transmissions.
Overhearing nodes that correctly decode could serve as relays.
The ARQ retransmission could come from a relay instead of the source.
“Decode and forward” relaying.
Source
Destination
Relays
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7. HARBINGER Source broadcasts first packet, m=1.
Relays that can decode are added to the decoding set D.
The source is also in D
The next packet is sent by a node in D.
The choice of which node depends on the protocol.
Geographic-Relaying: Pick the node in D closest to destination.
The process continues until the destination can decode.
We term this protocol “HARBINGER”
Hybrid ARq-Based INtercluster GEographic Relaying.
Energy-latency tradeoff can be analyzed by generalizing Caire and Tuninetti’s analysis.
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8. HARBINGER: Initialization
Source
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.
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9. HARBINGER: First Hop hop I Solid circles are in the decoding set D.
Source
Destination
hop I
Solid circles are in the decoding set D.
Amount of fill is proportional to the accumulated entropy.
Keep transmitting until Destination is in D.

10. HARBINGER: Selecting the Relay for the Second Hop
Source
Destination
hop I
ACK
/CTS
contention period

11. HARBINGER: Second Hop hop II Solid circles are in the decoding set D.
Source
Destination
Relay
hop II
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 Solid circles are in the decoding set D.
Relay
Source
Destination
hop IV
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 Solid circles are in the decoding set D.
Relay
Source
Destination
hop III
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
Rayleigh block fading
d0 = 1 m reference dist
Monte Carlo Integration
B. Zhao and M. C. Valenti. “A block-fading perspective on energy efficient random access relay networks”, to appear in JSAC special issue on Wireless Ad Hoc Networks.

15. Discussion Advantages. Disadvantages.
Better energy-latency tradeoff than multihop.
Nodes can transmit with significantly lower energy.
System exploits momentarily good links to reduce delay.
No need to maintain routing tables (reactive).
Disadvantages.
More receivers must listen to each broadcast.
Reception consumes energy.
Nodes within a cluster must remain quiet.
Longer contention period in the MAC protocol.
Results are intractable, must resort to simulation.
Requires position estimates.
These tradeoffs can be balanced by properly selecting the number of relays in a cluster.
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16. Simplifying Assumptions
Closed-form analysis is not tractable.
Statistically variable channels.
Nodes have memory for entire source-destination transaction.
Possible changes in topology.
Nodes could cycle on-and-off according to a sleep schedule.
Analysis is possible under simplifying assumptions:
Channels are non-faded (AWGN).
Nodes flush memory once a new relay is selected.
Still maintain memory of ARQ packets from current transmitter.
Topology is 2-D Poisson.
Nodes cycle on-and-off according to a sleep schedule.
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17. Versions of HARBINGER
Consider a network with nodes that cycle on and off.
Network coherence time = time nodes are awake.
Two main versions
“Fast HARBINGER”
After each ARQ transmission, nodes cycle in/out of sleep state.
Coherence time  1 block
“Slow HARBINGER”
Nodes only cycle in/out sleep state after entire codeword transmitted.
Coherence time  M blocks
Slow HARBINGER-A
Tries to minimize latency
Slow HARBINGER-B
Tries to minimize energy consumption
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18. GeRaF Geographic Random Forwarding (GeRaF)
Zorzi and Rao (Trans Mobile Computing 2003)
Node activity follows a sleep schedule.
Common strategy for sensor networks.
Source broadcasts over an AWGN channel.
If one node is within range it becomes the designated relay.
If multiple nodes, the one closest to destination becomes relay.
Otherwise, source tries again later to see if a relay awoke.
No ARQ or diversity combining effect.
This is precisely HARBINGER with the simplifying assumptions and M=1 (no ARQ)
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19. Slow HARBINGER-A 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
d0 = 1 m reference dist
Protocol picks the
node that is closest to
the destination.

20. Slow HARBINGER-B 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
d0 = 1 m reference dist
Protocol picks the best
relay that can be reached
with fewest ARQ
transmissions

21. Conclusions Wireless is a broadcast-oriented medium
Link-oriented protocols do not exploit this.
Ad hoc network can be viewed as a distributed MIMO system.
Cooperative diversity (orthogonal relaying) can give a better tradeoff between energy and latency than traditional multihop.
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
Analytical results possible under simplified conditions.
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