  In this paper q-ary Raptor codes under ML decoding are considered. An upper
bound on the probability of decoding failure is derived using the weight
enumerator of the outer code, or its expected weight enumerator if the outer
code is drawn randomly from some ensemble of codes. The bound is shown to be
tight by means of simulations. This bound provides a new insight into Raptor
codes since it shows how Raptor codes can be analyzed similarly to a classical
fixed-rate serial concatenation.
