Abstract
In this thesis, we propose an adaptive quantization to reduce the number of quantization bits of extrinsic values. We can use different quantization schemes at different iteration numbers in order to achieve better error performance. In addition, we can reduce the storage of the second minimum of extrinsic message in an LDPC decoder using OMS algorithm, since this second minimum has little effect on the error performance of high-rate LDPC decoders. Finally, we also try to reduce transmitted codeword energy for DVB-S2 system by selective mapping without degradation in error performance.