Interference Suppression for Memoryless Nonlinear Multiuser Systems Using Constellation Structure

A generalized concept of interference suppression is introduced for multiuser systems with a known memoryless transmit non-linearity, such as in the downlink amplifier of a satellite communication system. By considering the operation of commonly used multiuser detection techniques from a viewpoint of constellation structure, we extend these notions to nonlinear multiuser systems. We also analyze the performance of such multiuser detectors in terms of their asymptotic multiuser error exponents, which reduce to the asymptotic multiuser effective energies for the respective detectors in the absence of nonlinearity. The asymptotic conditional error exponent is introduced as a quantitative measure for evaluating nonlinear multiuser detectors based on the conditional probability of error at high signal-to-noise ratio (SNR). The optimal affine detector that maximizes the asymptotic error exponent for a given user is derived along with a successive interference annulment (SIA) scheme that maximizes the asymptotic conditional error exponent at each stage of successive detection. We have shown that the optimal affine detector (and hence the SIA detector) is a generalization of existing linear (and successive interference canceller) detectors to include the possibility of nonlinearity and therefore, nonadditive multiuser interference. We introduce the concept of joint successive interference annulment (JSIA) for nonlinear systems, which uses the joint structural relationship between two or more users in an ordered set to achieve higher performance gains in terms of the asymptotic conditional error exponent. We generalize the joint successive interference annulment detector to include more than two users at each stage of JSIA detection to scale the tradeoff between detection complexity and performance gain. We also introduce the decorrelating detector for nonlinear systems which essentially employs a successive interference canceller with a decorrelation kernel in an equivalent- - but higher-order linear system. Finally, we analyze the optimal affine detector and hence, the family of proposed joint successive interference annulment detectors when the multiuser system is perturbed with noise inside the nonlinearity. Simulation results demonstrate that the proposed detectors based on successive interference annulment consistently outperform conventional multiuser detectors based on successive interference cancellation.