  For the benefit of improved intermediate performance, recently online
fountain codes attract much research attention. However, there is a trade-off
between the intermediate performance and the full recovery overhead for online
fountain codes, which prevents them to be improved simultaneously. We analyze
this trade-off, and propose to improve both of these two performance. We first
propose a method called Online Fountain Codes without Build-up phase (OFCNB)
where the degree-1 coded symbols are transmitted at first and the build-up
phase is removed to improve the intermediate performance. Then we analyze the
performance of OFCNB theoretically. Motivated by the analysis results, we
propose Systematic Online Fountain Codes (SOFC) to further reduce the full
recovery overhead. Theoretical analysis shows that SOFC has better intermediate
performance, and it also requires lower full recovery overhead when the channel
erasure rate is lower than a constant. Simulation results verify the analyses
and demonstrate the superior performance of OFCNB and SOFC in comparison to
other online fountain codes.
