OTFS Signaling for SCMA With Coordinated Multi-Point Vehicle Communications

Abstract

This paper investigates an uplink coordinated multipoint (CoMP) coverage scenario, in which multiple mobile users are grouped for sparse code multiple access (SCMA), and served by the remote radio head (RRH) in front of them and the RRH behind them simultaneously. We apply orthogonal time frequency space (OTFS) modulation for each user to exploit the degrees of freedom arising from both the delay and Doppler domains. As the signals received by the RRHs in front of and behind the users experience respectively positive and negative Doppler frequency shifts, our proposed OTFS-based SCMA (OBSCMA) with CoMP system can effectively harvest extra Doppler and spatial diversity for better performance. Based on maximum likelihood (ML) detector, we analyze the single-user average bit error rate (ABER) bound as the benchmark of the ABER performance for our proposed OBSCMA with CoMP system. We also develop a customized Gaussian approximation with expectation propagation (GAEP) algorithm for multi-user detection and propose efficient algorithm structures for centralized and decentralized detectors. Our proposed OBSCMA with CoMP system leads to stronger performance than the existing solutions. The proposed centralized and decentralized detectors exhibit effective reception and robustness under channel state information uncertainty.

Low-Complexity Memory AMP Detector for High-Mobility MIMO-OTFS SCMA Systems

“…Therefore, the overall computational complexity of Memory AMP is O U M N (S B + 3S H + 1) + MN JD(2+3Q) K T . To summarize, a detailed complexity comparison of GMP [10], [11], EP [8], [12], OAMP/VAMP [7] and our Memory AMP is illustrated in TABLE I. It is obvious that our proposed Memory AMP can achieve comparable complexity with GMP and EP, and has relatively low complexity than OAMP/VAMP.…”

“…Here, we can use a factor graph to represent the system model of (8), where a factor node y is connected to multiple variable nodes x c , c = 1, 2, • • • , M N J/K. We approximate the messages updated and passed between the factor node y and variable nodes x c , c = 1, 2, • • • , M N J/K on the factor graph as Gaussian.…”

“…In addition, multiple-input multiple-output (MIMO) can be combined with OTFS to further increase spectral efficiency and transmission reliability for high mobility scenarios [4]- [6]. Meanwhile, non-orthogonal multiple access (NOMA) has been integrated with OTFS to improve spectrum utilization and massive user connectivity [7], [8]. In OTFS-NOMA, multiple users are allowed to access the same delay-Doppler resources simultaneously, and distinguished by different power levels [7] or via sparse code multiple access (SCMA) [8].…”

“…Meanwhile, non-orthogonal multiple access (NOMA) has been integrated with OTFS to improve spectrum utilization and massive user connectivity [7], [8]. In OTFS-NOMA, multiple users are allowed to access the same delay-Doppler resources simultaneously, and distinguished by different power levels [7] or via sparse code multiple access (SCMA) [8]. However, a major challenge for MIMO-OTFS multi-user systems is the data detection at the receiver due to the substantial increase of system dimension and the additional inter-user interference, which requires complicated processing with high complexity.…”

“…Unfortunately, the typical OTFS channel matrix structures do not always hold and the interference cancellation may cause error propagation. By leveraging the sparsity of OTFS channel matrix, the efficient Gaussian message passing (GMP) [10], [11] and expectation propagation (EP) [8], [12] detectors are developed with lowcomplexity. However, the GMP and EP detectors may suffer from performance loss if so many short girths (i.e., girth-4) exist in the factor graph.…”