1. A turbo decoding apparatus for performing iterative decoding on a received signal, the apparatus comprising: a decoder for receiving a signal and an (N−1) th decoding result, performing N th decoding on the received signal and the (N−1) th decoding result, and outputting first extrinsic information; a transformer for reducing a bit width of the first extrinsic information output from the decoder, and outputting second extrinsic information; a memory for storing the second extrinsic information output from the transformer; and a de-transformer for de-transforming the second extrinsic information stored in the memory, to third extrinsic information having a same bit width as the first extrinsic information, and inputting the third extrinsic information to the decoder.