# Channel Capacity

Definitions :: Symmetric Channels :: Closer Look :: Typical Sequences :: Theorem

## The channel coding theorem and its proof

We are now ready for the big theorem of the semester.

Note that by the statement of the theorem, we only state that "there exists'' a sequence of codes. The proof of the theorem is
*
not
*
constructive, and hence does not tell us how to find the codes.

Also note that the theorem is asymptotic. To obtain an arbitrarily small probability of error, the block length may have to be infinitely long.

Being the big proof of the semester, it will take some time. As we grind through, don't forget to admire the genius of the guy who was able to conceive of such an intellectual
*
tour de force
*
. Aren't you glad this wasn't your homework assignment to prove this!

**
Whew!
**

The proof used a random-coding argument to make the math tractable. In practice, random codes are
**
never
**
used. Good, useful, codes are a matter of considerable interest and practicality.