1. 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 IMPACT NEXT-PHASE GOALS ACHIEVEMENT DESCRIPTION STATUS QUO NEW INSIGHTS 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 3. Transmission time depends on ACK and channel conditions: Exists optimal choice Cluster Network

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

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

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

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. 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