Spatial Division Assisted Precoding for OTFS-NOMA System with Multiple Antennas

In the next generation mobile network, high-mobility is an important issue with the development of autonomous vehicles, high speed train, etc. The orthogonal time frequency space (OTFS) has been proven more suitable than the traditional orthogonal frequency division multiplexing (OFDM) in a high-mobility environment, especially for multiple input multiple output (MIMO) communication systems. However, it is challenging to achieve high spectrum efficiency in OTFS system with large number of high-mobility users. To tackle this issue, this paper studies a MIMO-OTFS system combined with nonorthogonal multiple access (NOMA) technique. In specific, a user grouping scheme is first proposed by using the spatial information. Then a power allocation scheme for OTFS modulation is introduced. Furthermore, we propose to use a spatial division based precoding scheme, which is inspired by the assumption of the OTFS channel sparsity along angle domain and orthogonality, to reduce the inter-group-interference. Therefore, the spectrum efficiency of the studied system can be improved. The simulation results demonstrate that our proposed system can achieve higher spectrum efficiency compared with the traditional MIMO system and an existing OTFS-NOMA system.