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DAC: Distributed Asynchronous Cooperation for Wireless Relay Networks

Xinyu Zhang, Kang G. Shin

University of Michigan

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Outline

Introduction Design Conclusion

DAC(routing)

PHY layer

MAC layerCSMA/CR

Implementation & evaluation

GNURadio/USRP

analysis

simulation

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Motivation: sync problem in cooperative relaying

Non-orthogonal cooperative relaying

Multiple transmitters sending the same packet (V-MISO)

A

B

CS

Major obstacle towards practical use:

Sync among distributed relays

In theory, realized via STBC or beamforming

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DAC: asynchronous non-orthogonal relaying

(b) DAC, asynchronous non-orthogonal relaying

(a) Synchronous non-orthogonal relaying protocol

A

B

CS

A

B

CS

Preserve transmit diversity

Circumvent the sync problem

DAC:

Via a new MAC/PHY: CSMA/CR (CSMA with collision resolution)

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The DAC network stack

PHY

MAC

Resolve collisions via signal processing

Encourage resolvable collisions via intelligent sensing and scheduling

CSMA/CR

Cooperative relaying

Asynchronous, non-orthogonal relaying protocol

DAC

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CSMA/CR: PHY layer

Resolve the collided packet by iterative decoding

S --- the received symbol.

A’ --- estimated based on A

C = S – A’

P1

P1

A

A'

B

B'

C

C'

S=A' + C

D E

D' E' Y' Z'

Y Z

Key problem: how to reconstruct A’ based on A?

A

B

C

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Challenges and solutions:

Channel estimation (phase and amplitude): correlation

Frequency offset estimation: Costas loop

Symbol and sample level timing offset: MM circuit

Identify exact start time of packets: sample level correlation

Transmitter distortion: reverse engineering tx filter

A

A'

B

B'

C

C'

S=A' + C

D E

D' E' Y' Z'

Y Z

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Implementation on GNURadio and verification on an SDR network:

A

D

B

A, B transmit the same packets to D

PER performance of forward-direction collision resolution

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CSMA/CR: MAC layer

Key rule: If the channel is busy, and the packet on the air is the one to transmit, then start the transmission.

Sensing and scheduling:

--- encourage resolvable collision

Otherwise, degenerate to CSMA/CA

P1P1

P1

P1

A

B

C

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DAC: CSMA/CR-based cooperative relaying

Objective:

Improve throughput performance of cooperative relaying using collision resolution

Basic idea:

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5

6

S

8

9

D

3

4

1

2

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DAC (Distributed Asynchronous Cooperation)

Add secondary relays to primary relays

11How to select relays?

Establish a primary path

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DAC: relay selection

Select secondary relays:

Optimal relay selection:

primary relay

'iR

1iR1iR iRS D

secondary relay

Select resulting in minimum delay from to 'iR 1iR 1iR

A model-driven approach, based on average link quality

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DAC: Diversity-multiplexing tradeoff

Q: Does DAC improve total network throughput?

1S2S 2D

1D

Interference range

2R

1R

3R Throughput ++

Secondary relay provides diversity gain for the primary path, but may reduce the multiplexing opportunity of other flows.

Throughput −−

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Network model:

Homogeneous erasure network with reception probability

Grid network:

Arbitrary network topology:

p

86.0p

64.0p

Wireless LAN: 1p

:d throughput of DAC:c throughput of the single-path routing protocol

Analytical results:

Sufficient conditions for :cd

A: DAC improves the throughput of lossy networks (e.g. Roofnet)

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Implement DAC in ns-2

Benchmark protocol: ETX routing

* D. Couto, D. Aguayo, J. Bicket and R. Morris, A High-throughput Path Metric for Multi-hop Wireless Routing, In Proc. of ACM MobiCom, 2003

DAC: Simulation experiments

• Routing metric: expected transmission count

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Single-unicast scenario:

DAC throughput gain ranges from 1.1 to 2.9, avg 1.7

Throughput gain is higher for low-throughput paths

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Multiple-unicast scenario:

DAC results in higher network throughput

DAC shows a higher level of fairness

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Multiple-unicast scenario, non-lossy networks:

DAC may have lower network throughput

DAC still maintains a higher level of fairness

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Related work:

Iterative cancellation:

* S. Gollakotam, D. Katabi. ZigZag Decoding: Combating Hidden Terminals in Wireless Networks, in Proc. of ACM SIGCOMM, 2008.

Cooperative relaying:

* R. Mudumbai, et al. On the Feasibility of Distributed Beamforming in Wireless Networks, in IEEE Trans. On Wireless Communications. Vol. 6, No. 5, May 2007.

* J. Zhang, J. Jia, Q. Zhang and E. M. K. Lo, Implementation and Evaluation of Cooperative Communication Schemes in Software-Defined Radio Testbed. In Proc. of IEEE INFOCOM, 2010

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Conclusion

Circumvent sync problem in cooperative relaying via PHY layer signal processing

DAC (Distributed Asynchronous Cooperation):

Diversity-multiplexing tradeoff

Collision tolerant scheduling & relay selection

DAC: asynchronous cooperative relaying, based on a SDR PHY

Thank you!