The upcoming 6G wireless networks have to provide reliable communications in high-mobility scenarios at high carrier frequencies. However, high-mobility or high carrier frequencies will bring severe inter-carrier interference (ICI) to conventional orthogonal frequency-division multiplexing (OFDM) modulation. Orthogonal time frequency space (OTFS) modulation is a recently developing multi-carrier transmission scheme for wireless communications in high-mobility environments. This paper evaluates the performance of coded OTFS systems. In particular, we consider 5G low density parity check (LDPC) codes for OTFS systems based on 5G OFDM frame structures over high mobility channels. We show the performance of the OTFS systems with 5G LDPC codes when sum-product detection algorithm and iterative detection and decoding are employed. We also illustrate the effect of channel estimation error on the performance of the LDPC coded OTFS systems.

Non-orthogonal multiple access (NOMA) has been recognized as a promising multiple access technique for the next generation cellular communication networks. In this paper, we first discuss a simple NOMA model with two users served by a single-carrier simultaneously to illustrate its basic principles. Then, a more general model with multicarrier serving an arbitrary number of users on each subcarrier is also discussed. An overview of existing works on performance analysis, resource allocation, and multiple-input multiple-output NOMA are summarized and discussed. Furthermore, we discuss the key features of NOMA and its potential research challenges.