Iterative decoding of turbo codes, as well as other concatenated codin
g schemes of similar nature, requires knowledge of the signal-to-noise
ratio (SNR) of the channel so that proper blending of the a posterior
i information of the separate decoders is achieved. In this paper we s
tudy the sensitivity of decoder performance to misestimation of the SN
R, and propose a simple online scheme that estimates the unknown SNR f
rom each code block, prior to decoding, We show that this scheme is su
fficiently adequate in accuracy to not appreciably degrade performance
.