Maximizing throughput for satellite communication in a hybrid FEC/ARQ scheme using LDPC codes

A hybrid forward error correction (FEC)/automatic repeat request (ARQ) has been considered as a mechanism for providing reliable communication between NASA orbiters and landers at the planet Mars. On such a link it is proposed to use a family of capacity achieving LDPC channel codes for FEC and go-back-N protocols for ARQ. In this paper, we analyze such a system and derive equations for its performance. We then use these equations to optimize the performance of the link in terms of information throughput subject to limitations on maximum channel baud rate and spacecraft power and select the best channel code and packet size accordingly at different link path lengths. In this optimization, first the frame error rate of the channel code is expressed in terms of a simple exponential function of the transmitted bit signal-to-noise ratio (E_b/N₀), obtained through curve-fitting. Then the standard throughput equation for the go-back-N protocol in terms of the channel codes frame error rate is derived. Next, using the equations for the throughput and the frame error rate, the received E_b/N₀ is calculated as a function of transmitted E_b/N₀. By minimizing this function, the throughput of the system is maximized for a given available spacecraft power. In the course of this optimization, we quantify the advantage afforded by a system that supports a set of possible code rates and bandwidths as opposed to a system that uses a single rate at different link path lengths.