1. A method to transmit a signal in a signal transmission apparatus, the method comprising: generating a final codeword vector by encoding an information vector including A (Aâ‰§1) information bits using one of a child parity check matrix and a parent parity check matrix; and transmitting the final codeword vector, wherein, if a child Low Density Parity Check (LDPC) codeword vector as the final codeword vector is generated by encoding the information vector using the child parity check matrix supporting a first coding rate, a child bipartite graph corresponding to the child parity check matrix is generated by using a parent bipartite graph corresponding to the parent parity check matrix supporting a second coding rate according to an edge merge scheme, and if a parent LDPC codeword vector is generated by encoding the information vector using the parent parity check matrix and the final codeword vector is generated by using the parent LDPC codeword vector, the parent bipartite graph is generated by using the child bipartite graph according to an edge split scheme.