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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

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
slide10

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

slide11

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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