Cooperative techniques at the network level
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COOPERATIVE TECHNIQUES AT THE NETWORK LEVEL. Anthony Ephremides University of Maryland Colorado State University January 29, 2008. Based on joint work with: R. Liu (UMD), S. Misra (ARL/Cornell U.), A. Sadek (UMD), Y. Sung (Qualcomm), L. Tong (Cornell U.), and L. Yu (UMD). Preview.

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COOPERATIVE TECHNIQUES AT THE NETWORK LEVEL

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Cooperative techniques at the network level

COOPERATIVE TECHNIQUES AT THE NETWORK LEVEL

Anthony Ephremides

University of Maryland

Colorado State University

January 29, 2008

Based on joint work with:

R. Liu (UMD), S. Misra (ARL/Cornell U.), A. Sadek (UMD),

Y. Sung (Qualcomm), L. Tong (Cornell U.), and L. Yu (UMD)


Preview

Preview

  • Cooperation in a Network can occur at different levels

  • One example at the MAC layer

  • One example at the Routing layer

  • Unless and until we break the barriers among layers, considering cooperation in a broader sense is useful


A little but important background

A little (but important) Background

  • Much touted question of ‘‘capacity‘‘

    • Maximum Throughput Region (packets/slot) (saturated queues)-TR

    • Maximum Stable Throughput Region (packets/slot) (finite delays)-STR

    • Capacity Region (bits/s) (reliable communication limits)-C

  • TR, STR, C : need not coincide

Ad Hoc Wireless Network


Example 2 users in random access collision channel

Example: 2-users in random access collision channel

2

1

1

R

TR

2

TR=STR=C

Error-free when no collision

  • “Thorny” model to extend

  • Interacting Queues-Dominant Systems

  • No-Cooperation yet!

1

1

STR


More general network

More General Network

  • Similar goal= Find TR, STR, C

    • Gupta & Kumar (2000)=TR (asymptotically)

    • Tassiulas and Ephremides (1992) (STR)

    • Tassiulas & Neely & Geo (2006) (STR)

  • Point-to-point

  • Scheduled access (mostly)

D

Based on “back-pressure” algorithm

Equalizes queue loads

Delay can be substantial (not “routing-savvy”)

1

1


1 st example cooperation at the mac level

1

2

M

i

1st Example-Cooperation at the MAC level

  • Network view of the relay channel

  • TDMA underlying structure (i): ith-user’s portion (interference-free)

  • Success if SNR>

  • Channel sensing is possible

  • Feedback ACK is perfect

M Source Terminals

Relay

hld

hil

Destination (d)

hid

Objective: Exploit the capabilities of the relay


Cooperation method 1

Cooperation Method 1

  • Each terminal transmits HOL packet in its assigned slot (if empty, slot is free)

  • If D receives successfully, it sends ACK (heard by both the relay and the user)

  • If D does not succeed but R does: at first sensed empty slot R transmits to D the failed packet

  • If neither D nor R succeed, packet gets retransmitted by the terminal in next frame

  • Relay does not keep packets after the end of the frame

Idle slots are utilized!

Stable throughput for the M terminals

Remarks:

  • Relay has always a finite queue (M packets Max)

  • Individual terminals interact

  • Successful service of a packet in a frame depends on whether the other terminals are idle or not


Cooperation method 2

Cooperation Method 2

  • Each terminal transmits HOL packet in its assigned slot (if empty, slot is free)

  • If D receives successfully, it sends ACK (heard by both the relay and the user)

  • If D does not succeed but R does: at first sensed empty slot R transmits to D the failed packet

  • If neither D nor R succeed, packet gets retransmitted by the relay at next opportunity

  • Relay keeps all packets it receives correctly

Remarks:

Again: Idle slots are utilized!

Stable throughput for the M terminals and the Relay

  • Relay has a possibly growing queue

  • Individual terminals do not interact

  • They release the unsuccessful packets to the relay

  • Enhanced version: Relay retransmits only packets of terminals with inferior channels


Relationship to other schemes

Relationship to Other Schemes

  • Plain TDMA (no relay help)

    • Note that at saturation both schemes reduce to TDMA (no idle slots)

    • But in 2nd method with losses (what does throughput mean?)

  • Random Access (no relay help)

  • Selective Decode-and-Forward (interpreted at Network Level)

    • Relay forwards if it decodes correctly (in next slot)

    • Must keep “apples and apples”-hence no saturation

      • Idle slots not utilized

      • Gets two chances at twice the rate against a simple chance at the lower rate (per fixed packet)

  • Others in similar vein


Cooperative techniques at the network level

Results for 2-users

2

R1(S1)

2f2d+21(1-f1d)f1lfld

2f2d

R2(S1)

1

1f1d

1f1d+21(1-f2d)f2lfld

Method 1 at a specific resource sharing vector (1,2)

Comparison


Cooperative techniques at the network level

Coop-DF: Relay transmits at twice the rate and utilizes one time slots. (Rate and SNR-threshold are related through the Gaussian mutual information formula

Coop-DF: Relay transmits at the same rate and utilizes two time slots.


Delay

Delay

  • Notoriously difficult for interacting queues

  • Symmetric System: 2-users


Questions

Questions

  • Other possible uses of the Relay

  • Fundamental Relationship of Information-theoretic view of cooperation to Network-level view

  • More complex networks possibly tractable (max throughput result and methods can be used)


2 nd example cooperation at the routing level

2nd Example-Cooperation at the Routing Level

  • Detection of target signal

  • Objective: maximize PD=prob. of correct detection

  • Determine routes (not a priori fixed)

  • Need mapping of PD on link metrics

  • Sensors cooperate as follows

    • Every node receives a sufficient statistic from “upstream”

    • It makes its own measurement as well

    • Transmits its best sufficient statistic downstream


Tandem case

Tandem Case

Fusion center

  • Markovian signal in space (good physical model)

  • Detection performance is well approximated by sum of terms along the links (one for each link)

  • Each link’s terms is monotonically related to its length (assume polynomial attenuation and independent noises)

  • The longer the link the better (the less correlation the better)

  • Breakthrough of costs, … except…

Target X


Use in blind routing

Use in “Blind Routing”

  • First of all who starts?

  • Chasing farthest nodes

  • No guaranteed convergence

  • Poor performance (correlation along path is not monotonic anymore)

Target X

Fusion center


Possible fixes

Possible “Fixes”

  • Add energy component to metric (moderate the bias toward longer links)

  • Add exclusion region around visited nodes to enforce directivity

  • Repeat analysis in 2-dimensions

  • Remark:

    • At the heart of the calculation there is an interesting coupling of mutual information and detection performance


Summary

Summary

  • Two examples of “Network-level” cooperation

  • Stable Throughput Region: Fundamental

  • Are the differences from the Information Theoretic approach leading to interesting new views of cooperation?


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