Novel network coding strategy for TDD

1. Random Linear Network Coding for Time Division Duplexing (TDD) Lucani, Médard, Stojanovic joint with CBMANET ACHIEVEMENT DESCRIPTION Cluster Network STATUS QUO IMPACT NEW INSIGHTS NEXT-PHASE GOALS • 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 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 • Network coding has studied throughput or delay performance considering minimal feedback • TDD has used ARQ/FEC schemes 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 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 • Extend broadcast: effect of clusters of cooperative nodes • Sensitivity analysis • Extend to general network scenario 3. Transmission time depends on ACK and channel conditions: Exists optimal choice ASSUMPTIONS AND LIMITATIONS: Random linear coding, prior knowledge/estimate of propagation delay and errors Feedback, coding and optimal choice of transmission time minimizes delay, while keeping throughput performance similar or better than typical TDD ARQ schemes

2. 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?

3. M data packets . . . . . . Description of Scheme Erasure Channel Tx Rx ACK i Receives (M – i) random linear coded packets Acknowledges i missing dofs Generates N random linear coded packets M ACK degrees of freedom required to decode (dofs req’d): Not particular data packet Received degrees of freedom When to stop talking and start listening Degrees of freedom needed by Rx: Degrees of freedom needed to decode: M M i Choice of Ni, i determines performance of scheme

4. Tx Rx Channel ACK ACK Minimizing Completion Time or Energy Full Duplex: Minimum Completion TDD constraint • Ni, i chosen to minimize • Mean completion time • Mean completion energy e.g: M = 10, round trip time = 250 ms, Rate = 1,5 Mbps Time Pe = 0.8 1.1dB

5. Tx Rx Channel ACK ACK Minimizing Completion Time or Energy Full Duplex: Minimum Completion TDD constraint • Ni, i chosen to minimize • Mean completion time • Mean completion energy e.g: M = 10, round trip time = 250 ms, Rate = 1,5 Mbps Energy Pe = 0.8 1.15dB

6. 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

7. Cluster 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: Extension to general networks Extension to broadcast with cooperative nodes