Selection of Bit-Rate for Wireless Network Coding

Network coding is known to improve throughput by mixing information from different flows and conveying more information in each transmission. Recently some proposals have demonstrated the benefits of applying network coding to wireless networks with broadcast transmissions. It is expected that the opportunities for coding and the corresponding gains depend on the bit-rate chosen for determining routes and transmitting packets. However, the previous work on wireless network coding assumed a fixed rate and did not explicitly account for the interaction between rate selection and coding gain. In this paper, we define a new metric, expected coded time (ECT), that measures the total time needed by a node to deliver two packets to their receivers given the bit-rate for transmitting coded packets. We then investigate how the optimal bit-rate for coded packets differs from that for transmission of native packets individually. We also study the performance of network coding under different fixed bit-rates for the whole network. Our evaluation shows that 11 Mbps is the best default fixed rate for MIT Roofnet and 5.5 Mbps is mostly the optimal rate to transmit coded packets when the ideal individual bit-rate for each receiver is different.