A New Efficient 16-QAM Mapping Approach for Iterative Receiver using Turbo Codes over SISO Channel

Iterative receivers (IR) have a feedback between demapper and channel decoder and iteratively improve estimation of the transmitted information. On the contrary, in conventional receivers, demapper and channel decoder work their functions independently. In such conventional systems, powerful error control codes (turbo codes, for example) usually promise better performance. However, we will show that in IR with multilevel modulation, powerful error control codes do not always guarantee better performance. That is, when we design IR, a careful attention to combining mapping and error control code must be paid. In this paper, we design IR for communication systems with turbo codes and the 16-QAM modulation. In performance comparisons, we will show that in some cases the IR for the system with convolutional code can have better performance than that of the turbo code. Our work answers why the turbo code has poor performance and present a good mapping which gives insights into a general design rule for mapping with powerful error control codes such as turbo and LDPC codes.