The capacity of coded systems

There are many situations in which a coding scheme is fixed for a given channel, The data processing theorem implies that the capacity of the composite channel cannot be higher than that of the original channel; usually, the capacity will be strictly lower, This paper examines the problem of how much capacity must be lost in the most common such situations, The various combinations of presence and absence of encoder and decoder, along with choice of encoder and decoder, are considered, The degree to which coding schemes can both deliver low error probability and maintain high capacity in the composite channel Is examined.