COOPERATION INSTEAD OF CONTENTION! THE NEBULOUS CONCEPT OF WIRELESS - - PowerPoint PPT Presentation

COOPERATION INSTEAD OF CONTENTION!

Anna Scaglione -- Cornell University

THE NEBULOUS CONCEPT OF WIRELESS LINK….

Network architecture & process of abstraction go hand in hand Wired nets “link” is a useful abstraction directly tied to the

physical medium

Wireless nets the “link” notion is nebulous Assumptions in current multi-hop networks (Constraint I)

(Constraint I) A functional physical layer communication link can originate from only one transmitter;

(Constraint II)

(Constraint II) Concurrent transmissions of multiple transmitters result in interference that, if not sufficiently attenuated by spatial or channel multiplexing, produces a collision distortion irreversible at the ultimate receiver

(INCONVENIENT?) TRUTHS AND COMMON MISCOCEPTIONS

Tr Truths

There are always unintended recipients Signals do not collide, they add up

Misconcep Misconceptions ions

• (A)

(A) Cooperation is practically more complex than the collision model

• (B)

(B) Because the information theoretic capacity of networks is unknown, the physical layer is going to get us into truly messy models that will not scale and will be not lead to a decentralized protocol

FOUNDATIONS OF MISCONCEPTION (A)

You have to select one cooperative node at a

time or you need to use space-time coding which is more complicated than doing multiple access

Distributed Space Time coding: [Laneman’03]

RANDOMIZED COOPERATION SHOWS THAT (A) IS A MISCOCEPTION

You can cooperate without having any idea of who else is doing it The receiver does not need to know how many nodes actually

cooperate

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y x w G s r r r r h w G s h w

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In case y In case you care: u care: L=code diversity N=number of nodes You get full diversity if L=N-1 The loss in BER is very modest compared to a centralized assignment

THIS ALLOWS TO ESTABLISH MISCONCEPTION (B)

Flooding is the perfect example where the collision

model is an oxymoron

Cooperation alleviates the collision resolution

problem and routing problems

The analysis is not only possible, it is fascinating…. CONTENTION Source Traditional network flooding COOPERATION Source Cooperative broadcast

DENSE COOPERATIVE NETWORKS

Model

very dense cooperative networks, the source is in the middle

and there is a continuum of nodes each relaying with infinitesimal power

Each node only cooperates once and does so if its received

SNR is above a certain threshold (cooperation in successive levels/groups)

Signal received: Random signal to noise ratio Random locations of nodes in level k

A COUNTERINTUITIVE TRUTH: DUMB CODING WORKS BEST!

Probability of receiving

at the K-th level A fraction of node experiences excellent fades (opportunistic beam-forming) You cannot get that if you do not add on the channel

Finite network simulation

CONCLUSIONS

What is the cost/overhead of cooperation??

*Vir Virtually none* ually none*

Multicast case just shut the MAC down and let them

transmit

Using RTS-CTS you can apply this also to unicast…

Message for the “systems” group:

We can (and I my view should) consider a broader

concept of link even if we do not know the information theoretic capacity of a network

Lesson I learned:

Even when looking at PHY, do not expect always to read

what to do from setting up an optimization problem

APOLOGIES AND ACKNOWLEDGMENTS

I apologize for discussing my group’s work let me acknowledge the thesis work Birsen Sirkeci Mergen

(faculty at San Jose’ State since 2006)