A method for transmitting data in a communication system using a Low Density Parity Check, LDPC, matrix, the method comprising: generating LDPC codeword bits by encoding information data bits, wherein the number of the LDPC codeword bits is N; ordering the LDPC codeword bits in descending order of degree; interleaving the ordered LDPC codeword bits, [c i], i=1, 2,..., N; dividing the interleaved LDPC codeword bits into 8 groups, [G j], j=0, 1,..., 7, wherein group G k comprises LDPC codeword bits [c kN/8+m], m=1, 2,..., N/8; generating a mapped signal by mapping the interleaved LDPC codeword bits separately to bits, y 0, y 2, y 4, y 6, corresponding to a real part of a modulation symbol and bits, y 1, y 3, y 5, y 7, corresponding to an imaginary part of the modulation symbol, wherein the modulation symbol is represented as having the real part and the imaginary part, wherein 256-ary Quadrature Amplitude Modulation, 256-QAM, is used; generating a modulation signal by high-order-modulating the mapped signal; Radio Frequency, RF,-processing the modulation signal, and transmitting the RF-processed signal via a transmission antenna, wherein generating the mapped signal comprises: mapping interleaved LDPC codeword bits belonging to a first group, G 0, to highest-reliability bits, y 0 or y 1, corresponding to one of an imaginary part or a real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a second group, G 1, to medium- reliability bits, y 5 or y 4, corresponding to one of the imaginary part or the real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a third group, G 2, to lowest- reliability bits, y 6 or y 7, corresponding to one of the imaginary part or the real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a fourth group, G 3, to the remaining bits, y 7 or y 6, having the same reliability as the bits to which the third group, G 2, is mapped; and mapping interleaved LDPC codeword bits belonging to fifth to eighth groups, G 4, G 5, G 6, G 7, to the remaining bits which are not mapped to the first to fourth groups, G 0, G 1, G 2, G 3. generating LDPC codeword bits by encoding information data bits, wherein the number of the LDPC codeword bits is N; ordering the LDPC codeword bits in descending order of degree; interleaving the ordered LDPC codeword bits, [c i], i=1, 2,..., N; dividing the interleaved LDPC codeword bits into 8 groups, [G j], j=0, 1,..., 7, wherein group G k comprises LDPC codeword bits [c kN/8+m], m=1, 2,..., N/8; generating a mapped signal by mapping the interleaved LDPC codeword bits separately to bits, y 0, y 2, y 4, y 6, corresponding to a real part of a modulation symbol and bits, y 1, y 3, y 5, y 7, corresponding to an imaginary part of the modulation symbol, wherein the modulation symbol is represented as having the real part and the imaginary part, wherein 256-ary Quadrature Amplitude Modulation, 256-QAM, is used; generating a modulation signal by high-order-modulating the mapped signal; Radio Frequency, RF,-processing the modulation signal, and transmitting the RF-processed signal via a transmission antenna, wherein generating the mapped signal comprises: mapping interleaved LDPC codeword bits belonging to a first group, G 0, to highest-reliability bits, y 0 or y 1, corresponding to one of an imaginary part or a real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a second group, G 1, to medium- reliability bits, y 5 or y 4, corresponding to one of the imaginary part or the real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a third group, G 2, to lowest- reliability bits, y 6 or y 7, corresponding to one of the imaginary part or the real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a fourth group, G 3, to the remaining bits, y 7 or y 6, having the same reliability as the bits to which the third group, G 2, is mapped; and mapping interleaved LDPC codeword bits belonging to fifth to eighth groups, G 4, G 5, G 6, G 7, to the remaining bits which are not mapped to the first to fourth groups, G 0, G 1, G 2, G 3. mapping interleaved LDPC codeword bits belonging to a first group, G 0, to highest-reliability bits, y 0 or y 1, corresponding to one of an imaginary part or a real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a second group, G 1, to medium- reliability bits, y 5 or y 4, corresponding to one of the imaginary part or the real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a third group, G 2, to lowest- reliability bits, y 6 or y 7, corresponding to one of the imaginary part or the real part of the modulation symbol; mapping interleaved LDPC codeword bits belonging to a fourth group, G 3, to the remaining bits, y 7 or y 6, having the same reliability as the bits to which the third group, G 2, is mapped; and mapping interleaved LDPC codeword bits belonging to fifth to eighth groups, G 4, G 5, G 6, G 7, to the remaining bits which are not mapped to the first to fourth groups, G 0, G 1, G 2, G 3.