•Novel network coding strategy for TDD

•Use of feedback (ACK) improves delay/energy/ throughput performance, especially for high latency- high errors scenarios

•Random linear coding allows extension to networks

Random Linear Network Coding for Time Division Duplexing (TDD) Lucani, Médard, Stojanovic joint with CBMANET

•Extend broadcast: effect of clusters of cooperative nodes

•Sensitivity analysis

•Extend to general network scenario

Feedback, coding and optimal choice of transmission time minimizes delay, while keeping throughput performance similar or better than typical TDD ARQ schemes

MAIN RESULTS: Novel network coding scheme for TDD channels

1. Delay and Energy Analysis for Link and Broadcast cases

2. Exists optimal transmission time in terms of minimizing block delay, with close-to-optimal energy performance.

3. Outperforms Selective Repeat schemes in high latency- high error

scenarios. Similar performance otherwise.

4. Delay/throughput is close to

full duplex network coding, requiring much less energy

IMPA

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ACHIEVEMENT DESCRIPTION

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•Network coding has studied throughput or delay performance considering minimal feedback

•TDD has used ARQ/FEC schemes

ASSUMPTIONS AND LIMITATIONS: Random linear coding, prior knowledge/estimate of propagation delay and errors

HOW IT WORKS: 1. Transmission time computed to minimize delay in data block transmissions, using ACK and channel conditions 2. Stop transmission to wait for ACK from receiver (s). ACK used to update transmission time

1. Use feedback to improve delay performance: ACK states required number of coded packets to decode data 2. Transmit coded packets for some time, stop to wait for ACK

Transmit Time Wait Time ACK Transmit TimeTransmit Time Wait Time ACK Transmit Time

3. Transmission time depends on ACK and channel conditions: Exists optimal choice

Cluster

Network

10-5

10-4

10-3

10-2

10-1

100

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

5

Packet Erasure Probability

(b

ps)

GBN Window = 10

SR Window = 10Network Coding TDD Optimal M = 10

R = 10 Mbps

R = 1 Mbps

R = 0.1 Mbps

R = 10 Mbps

Introduction

• Reliable communication for time division duplexing (TDD) channels, i.e. when a node can transmit and receive, but not at the same time, has been commonly achieved using– ARQ schemes: focuses of retransmission. Typically no coding– FEC schemes: focuses on coding. No retransmissions.

• Most network coding results focus on throughput or delay performance assuming minimal feedback

• Objective: Minimize completion time of blocks of packets in TDD channels, especially in long latency channels

• Key Question: How much should we talk before stopping to listen?

Description of Scheme

...M data packets

Degrees of freedomneeded to decode:

M

Degrees of freedom needed by Rx:

M

Generates N random linear coded packets

M

i

Tx Rx

...

ACK iErasure Channel

Choice of Ni, i determines performance of scheme

ACK degrees of freedomrequired to decode (dofs req’d):

Not particular data packet

Receives (M – i) random linear coded packets

Acknowledges i missing dofs

When to stop talkingand start listening

Receiveddegreesof freedom

10-4

10-3

10-2

10-1

100

0

0.2

0.4

0.6

0.8

Packet Erasure Probability

Mea

n C

ompl

etio

n E

nerg

y

10-4

10-3

10-2

10-1

100

1

2

3

0.4

0.8

Packet Erasure Probability

Mea

n C

ompl

etio

n T

ime

(s)

Full Duplex

TDD Opt. Time

TDD Opt. Energy

Full Duplex

TDD Opt. Time

TDD Opt. Energy

Minimizing Completion Time or Energy

TDD constraint• Ni, i chosen to minimize

– Mean completion time– Mean completion energy

Tx Rx

ACK ACKChannel

Full Duplex: Minimum Completion

Pe = 0.8

1.1dB

e.g: M = 10, round trip time = 250 ms, Rate = 1,5 Mbps

Time

Minimizing Completion Time or Energy

TDD constraint• Ni, i chosen to minimize

– Mean completion time– Mean completion energy

Tx Rx

ACK ACKChannel

Full Duplex: Minimum Completion

e.g: M = 10, round trip time = 250 ms, Rate = 1,5 Mbps

10-4

10-3

10-2

10-1

100

0

0.2

0.4

0.6

0.8

Packet Erasure Probability

Mea

n C

ompl

etio

n E

nerg

y

10-4

10-3

10-2

10-1

100

1

2

3

0.4

0.8

Packet Erasure Probability

Mea

n C

ompl

etio

n T

ime

(s)

Full Duplex

TDD Opt. Time

TDD Opt. Energy

Full Duplex

TDD Opt. Time

TDD Opt. Energy

Pe = 0.8

1.15dB

Energy

Throughput

Throughput Metric

= #bits / E[Time]

• Increasing latency, favors network coding TDD scheme

• Better performance than Go-back-N (GBN) and Selective Repeat (SR) for TDD

Conclusions

• We have developed a novel network coding strategy for TDD channels– Link– Broadcast

• Coupling coding and feedback (ACK) improves delay/throughput performance, especially for high latency- high erasures scenarios

• New Goals:

Cluster

Extension to broadcast with cooperative nodes

Extension to general networks