Iterative Receiver Design for FTN Signaling Aided Sparse Code Multiple Access
IEEE Transactions on Wireless Communications
The sparse code multiple access (SCMA) which is a promising candidate for the next generation wireless communications can support more users using the same resource elements. On the other hand, faster than Nyquist (FTN) signaling can also be used to improve the spectral efficiency by transmitting more data information in the same time period. In this paper, we consider a combined uplink FTN-SCMA system that the data symbols corresponding to a user are further packed using FTN signaling. As a
... ult, a higher spectral efficiency is achieved at the cost of introducing intentional inter-symbol interference (ISI). To perform the joint channel estimation and decoding, we design a low complexity iterative receiver based on the factor graph framework. In addition, to reduce the signaling overhead and transmission latency of SCMA system, we further consider a grant free scheme. Consequently, the active and inactive users should be distinguished. To address the problem, we extend the aforementioned receiver and develop a new algorithm that jointly estimates the channel state, detects the user activity and performs decoding. In order to further reduce the complexity, an energy minimization based approximation is employed to restrict the user state to Gaussian and hybrid message passing algorithm is performed. Simulation results show the considered FTN-SCMA system with the proposed receiver design is capable of transmitting more data bits than conventional SCMA scheme with negligible performance loss.