The Design and Performance of Distributed LT Codes

This paper describes techniques to decompose LT codes (a class of rateless erasure-correcting codes) into distributed LT (DLT) codes. DLT codes can be used to independently encode data from multiple sources in a network in such a way that, when the DLT-encoded packets are combined at a common relay, the resulting bit stream (called a modified LT (MLT) code) has a degree distribution approximating that of an LT code, with simulations indicating comparable performance. In essence, DLT codes are designed so that the final stage of encoding for erasure correction can be carried out by a low-complexity relay that selectively xors the bit streams generated at each source and transmits the result to the sink. This paper presents results for two-source and four-source networks. It is shown that, when the relay-to-sink link is the bottleneck, the DLT/MLT approach can yield substantial performance benefits compared with a competing strategy wherein each of the sources uses its own independent LT encoder and the resulting bit streams are time-multiplexed through the relay.