c. Implementation of Raptor code: To encode the input
symbols using a traditional erasure correcting code,
and then apply suitable LT-code to the new set of
symbols in a way that the traditional code is capable
of recovering all the input symbols even in face of a
fixed fraction of erasures. To deal with the first issue
[6], need to design the traditional code and the LTcode
appropriately. Let Ω(x) be a linear code of block
length and dimension, and let be a degree
distribution. A Raptor code with parameters (k, C,
Ω(x)) is an LT-code with distributioΩn (x) on
symbols which are the coordinates of code words in
C. The code C is called the pre-code of the Raptor
code. The input symbols of a Raptor code are the
symbols used to construct the code word in C
consisting of n intermediate symbols. LT-code are
generated the output symbols from the n intermediate
symbols.Typically, to assume that is equipped with a
systematic encoding, though this is not necessary.
The definition of the encoding cost of a Raptor code
differs slightly from that of a Fountain code. This is
because the encoding cost of the pre-code has to be
taken into account. We define the encoding cost of a
Raptor code as E(c)/k+ Ω’(1), where E(c) is the
number of arithmetic operations sufficient for
generating a code word in from the input code
symbols. The encoding cost equals the per-symbol
cost of generating k output symbols.
The decoding cost of a decoding algorithm for a Raptor
code is the expected number of arithmetic operations
sufficient to recover the k input symbols, divided by k. As
with the Fountain codes, this cost counts the expected
number of arithmetic operations per input symbol.