Computer Networks GroupUniversität Paderborn

TANDEM project meeting Protocols, oversimplification, and cooperationor: Putting wireless back into WSNs

Holger Karl

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Overview

Protocols in WSN and oversimplified models

Cooperation diversity – motivation & approach

Cooperation diversity in wireless sensor networks Relationship to 802.15.4 Relationship to TANDEM candidate routing scheme

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Routing and oversimplified models

To a computer scientist, a network is a graph Well-behaved, stable Resulting model: Unit disc graph

To an electrical engineer, a network is a collection of channels

Wireless channels: fickle, changing, hard to predict

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Protocols in unit disk graphs

Under unit disk graph assumptions (and similar): Protocol design is easy…

Neighbors stay neighbors Error control is easy Dynamics in the network are slow

… but futile Networks do not behave like this Typical problem: Short- and long-term fading

! Variations in network on short time-scales than

! Need to design protocols accordingly

But not all is hopeless!

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Cooperation diversity – motivation

Fading is a source of serious problems But also an opportunity

Transmission A ! C fails (fading) Transmission A ! B works (wireless multicast advantage!) Transmission B ! C would work – but should B cooperate?

A possible gains from cooperation in presence of channel diversity ! cooperation diversity

A

B

C

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Store-and-forward vs. cooperative relaying

Store-and-forward:

TDMA example:

User cooperative diversity:

TDMA example:

AB

D

A

B

D

A

B

A

B

A

B

Time

A

B

Phase1 Phase2 Time

A

B

Phase1 Phase2

Each relaying scheme employs: Two phases, two types of channels Phases: Reception/retransmission Channels: Inter-user/uplink

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Cooperation diversity – known properties

Cooperation diversity substantially improves outage probability

Specifically advantageous if “inter-partner” channel is good Has many variations

How does “partner” behave when an incorrect packet arrives? How to forward a packet? Completely? Or partial redundancy?

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Schemes: Regeneration vs. non-regeneration

Objectives:1. Do not transmit redundancy if user does not need help

Requires a priori knowledge of uplink channel state

2. Do not relay erroneous data in phase 2 Requires regeneration and error-checking partner’s data

Non-regenerative schemes: e.g. Amplify-and-Forward, saves energy but relays possibly

erroneous data ) Useful for “good” inter-user channels

Regenerative schemes: Enable error-checking Decode-and-Forward: Repeat partner’s data

Correct reception in phase 1: Partner’s data is repeated in phase 2

Wrong reception in phase 1: Own data is transmittedA B A B

Phase 1 Phase 2

B!A, A‘s decoding fails:

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Coded cooperative schemes, Cooperation level

Regeneration further enables: More efficient coding Instead of repeating partner’s data:

Relay only parts (redundancy) in phase 2 Use efficient FEC and/or space-time codes

Many approaches for these coded cooperative schemes Whitepaper provides detailed discussion

With coded cooperation redundancy can be adjusted: Cooperation level controls proportion of time used for diversity Example: Long phase 1 (less red.) if partner’s uplink channel good

) Channel-aware div/mux optimization: If we can estimate uplink channel state we can adjust phase

lengths, i.e. user cooperative diversity accordingly

A B A B

Phase 1 Phase 2

1/2: A B A B

Phase 1 Phase 2

3/4:

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Cooperation diversity and wireless sensor networks?

Applicable to wireless sensor networks movement exists Nodes themselves Or object in the environment!

Concrete problems High density of WSNs – how to handle possibly many partners? How to balance energy constraints and willingness to cooperate

Similar to multi-hop forwarding, but possibly with different trade-offs if nodes are allowed to forward own data along with cooperation data

How to handle energy consumption, tradeoffs vs. improved error probabilities?

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Cooperation in TANDEM: MAC protocol

TANDEM assumes a commodity-off-the-shelf MAC/link layer to improve compatibility – IEEE 802.15.4

Possible to Integrate cooperation into existing 802.15.4? Extend 802.15.4 by cooperation? With minimal impact on compatibility?

Integrate partner selection into a wakeup mechanism?

Consequences for energy efficiency, implementation complexity, … ?

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Cooperation in TANDEM: Routing protocol

TANDEM proposed routing mechanism: Use routes with sparse neighborhood

To circumvent false wakeup/idle listening

This trade offs against the smaller number of possible partners

What are the trade-offs? Possible to characterize when to avoid dense regions?

When the channel conditions in the sparse region are approximately AWGN?

Integrate such knowledge into cooperation scheme?

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Conclusion

Cooperation diversity is a promising mechanisms to handle wireless channel imperfections in networks

Specifically, lowers error probability

Application to wireless sensor networks little researched

Promising research questions in TANDEM Integration into MAC, routing – geared towards WSN

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The coded cooperation approach

Additional coding of data across spatial separated users

A simple cooperative coding scheme:1. Select partner to cooperate

and cooperation level 2. Both partner transmit first

subframe using code rate R1=R/

3. Each node decodes data; if no errors occur cooperation is possible

4. With cooperation: Each node transmits redundancy for its partner to ensure that full code-rate R is reached Without cooperation: Node

transmits redundancy for its own data

Gain: If N user cooperate scheme achieves full diversity order N, at high SNR this means up to 270% spectral efficiency

Similar gains are provided by comparable approaches, e.g. decode-and-forward, space-time cooperation

Data B

Data A

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Red. B

Red. A

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