Next came the letter , so it should be recoded as 10, and so on. This kind of scheme is Huffman coding, an algorithm for the construction of efficient signalling alphabets. So in many applications the data is interleaved just prior to transmission. 89,99, and re-ordered upon reception. So a burst of channel errors is spread out in the re- ordered block, making the decoder's job less arduous.
More advanced systems use a mixture of a block codes, a convolutional code and interleaving.
This represents a coding gain of 8 db, a very substantial figure. It is natural to ask if these improvements have a limit. Nevertheless recent schemes have been proposed which approach the Shannon limit within 1-2 db or so. Well, we can start by looking at the performance limits of our current phase 3 type digital links. Assume the spacecraft is limited to 1 watt e.i.r.p, and that the range is 42000 km.