In non-orthogonal multiple access (NOMA), signals for both users are set to transmit simultaneously, so the bandwidth-limited user can get more spectrum resources, while the interference-limited user can obtain a larger portion of power. Code division multiple access shares some common features with NOMA. This chapter proposes a general error propagation model in a downlink MIMO NOMA system, where decoding errors are modeled as residual interference. It formulates sum rate optimization problem with error propagation in SIC. The chapter introduces the concept of equivalent channel and propose a sequential solution which solves for precoding matrix by applying an iterative algorithm first, in order to solve the problem. It investigates the impact of power allocation by two cases: small error propagation factor and large error propagation factor. The chapter presents simulations to verify the superiority of proposed precoding design and our analyses on power allocation with residual interference.