In an apparatus for data transmission in a communication system, a turbo
encoder encodes data bits to generate systematic bits and parity bits,
and a rate matcher matches the systematic bits and parity bits. A first
interleaver writes the rate-matched systematic bits on a row by row
basis, and performs inter-column permutation. A second interleaver writes
the rate-matched parity bits on a row-by-row basis, and performs
inter-column permutation. A modulator alternatively collects the
permutated bits on a column by column basis from the first and second
interleavers, and maps collected bits from the first and second
interleavers onto one modulation symbol, wherein a size of the first
interleaver is equal to a size of the second interleaver.