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in brief shanon put the the boundary for error free comunication ,and all research try to come closer to it ,till FEC codes fiend that long constraint lenth can get better code gain but it unfeasable till "forny" partition the long constraint length to two portion to make turbo code which abart 0.5 db from shannon limit
* What is specail about turbo code?
in my view, there is no special about turbo code, since all the "novel" ideas have appeared before its birth. However, turbo code is just born at the right time, and then it succeeded. As for these "novel" ideas, they are
1) long block lengths, which has already been pointed by Shannon;
2) use short codes to build long codes, which was firstly invested by P. Elias, Gallager, and Forney;
3) relieve the decoding complexity in a divide and conquer fashion, more clearly using iterative decoding, which was firstly used by Elias and Gallager.
However, in the early of 90's of last century, hardware tecnnologies have reached a rather high level. Then, turbo codes can be realized in practice. Finally, turbo codes made a revolution to the coding theory.
* Why do we need coded modulation?
bandwidth efficiency is the answer. bandwidth efficiency is becoming more and more important factor due to the scarce bandwidth. However, we can not improve bandwidth efficiency only by coding. Thus, coded modulation comes.