Successive interference cancellation for LDPC coded nonorthogonal multiple access systems

We design, for the first time, novel successive interference cancellation (SIC) schemes for a two-user nonorthogonal multiple access system using low-density parity-check codes. We first develop a low-complexity SIC scheme treating interference as noise, referred to as the N-SIC scheme, and a SIC with joint detection scheme treating the modulation of interference signal at the near user as side information, referred to as the J-SIC scheme. We then propose a new extrinsic information assisted SIC scheme, referred to as the E-SIC scheme, which exchanges the extrinsic decoding information of the signals for two users and thus incurs an increase complexity. With simulations, we demonstrate that the error performance advantage of the E-SIC scheme over the N-SIC and J-SIC schemes is more significant when the power allocated to the near user is larger, e.g., higher than 45%. When this power becomes smaller, e.g., less than 10%, the low-complexity N-SIC scheme can achieve the best error performance. In addition, we have an important finding that when the two users adopt different order modulation, the decoding convergence speed of the E-SIC scheme improves when the difference between the constellation sizes of two users is larger.