Orthogonal time frequency space (OTFS) is a recently introduced modulation technique that multiplexes information symbols in the delay-Doppler (DD) domain rather than the time frequency (TF) domain as in traditional multi-carrier modulation such as orthogonal frequency division multiplexing (OFDM). It has been shown that OTFS outperforms OFDM in a high Doppler environment. OTFS can be utilized in the multi-user (MU) multi-input multi-output (MIMO) scenario, exactly as OFDM. In this paper, we focus on low complexity transmitter precoding for MU MIMO-OTFS. The computational complexity of traditional transmitter precoding is too high in the MU MIMO-OTFS system. We propose two low complexity precoding algorithms for MU MIMO-OTFS. The proposed low complexity precoding utilizes the specific property of the OTFS channel in the DD domain; meanwhile, the inversion of a large dimensional matrix is achieved by employing the matrix inversion lemma. The simulation results illustrate that our proposed precoding algorithms work well in the MU MIMO-OTFS system, MU MIMO-OTFS outperforms MU MIMO-OFDM and the performance of MU MIMO-OTFS is robust to different user speeds. Meanwhile, compared with block diagonalization (BD) precoding, our proposed precoding significantly reduces the computational complexity with a small performance loss.

正交时频空间（OTFS）是最近引入的一种调制技术，该技术在延迟多普勒（DD）域而不是在时频（TF）域中复用信息符号，如在传统的多载波调制（如正交频分复用（OFDM））中那样）。已经表明，在高多普勒环境中，OTFS优于OFDM。可以像在OFDM中一样，在多用户（MU）多输入多输出（MIMO）场景中使用OTFS。在本文中，我们专注于MU MIMO-OTFS的低复杂度发射机预编码。在MU MIMO-OTFS系统中，传统发射机预编码的计算复杂度过高。我们为MU MIMO-OTFS提出了两种低复杂度的预编码算法。所提出的低复杂度预编码利用了DD域中OTFS信道的特定属性。同时，大维矩阵的求逆是通过采用矩阵求逆引理实现的。仿真结果表明，我们提出的预编码算法在MU MIMO-OTFS系统中运行良好，MU MIMO-OTFS的性能优于MU MIMO-OFDM，并且MU MIMO-OTFS的性能对于不同的用户速度具有鲁棒性。同时，与块对角化（BD）预编码相比，我们提出的预编码显着降低了计算复杂度，而性能损失却很小。