In this paper, we consider a downlink (DL) massive multiple-input multiple-output (MIMO) system, where different users have different mobility profiles. To support this system, we propose to use a hybrid orthogonal time frequency space (OTFS)/orthogonal frequency division multiplexing (OFDM) modulation scheme, where OTFS is applied for high-mobility users and OFDM is used for low-mobility users. Two precoding designs, namely full zero-forcing (FZF) precoding and partial zero-forcing (PZF) precoding, are considered and analyzed in terms of per-user spectral efficiency (SE). With FZF, interference among users is totally eliminated at the cost of high computational complexity, while PZF can be used to provide a trade-off between complexity and performance. To apply PZF precoding, users are grouped into two disjoint groups according to their mobility profile or channel gain. Then, zero-forcing (ZF) is utilized for high-mobility or strong channel gain users to completely cancel the inter-group interference, while maximum ratio transmission (MRT) is applied for low-mobility users or users with weak channel gain. To shed light on the system performance, the SE for high-mobility and low-mobility users with a minimum-mean-square-error (MMSE)-successive interference cancellation (SIC) detector is investigated. Our numerical results reveal that the PZF precoding with channel gain grouping can guarantee a similar quality of service for all users. In addition, with mobility-based grouping, the hybrid OTFS/OFDM modulation outperforms the conventional OFDM modulation for high-mobility users.

翻译：本文研究了下行大规模多输入多输出（Massive MIMO）系统中不同用户具有不同移动性特征的问题。为此，我们提出采用混合正交时频空间（OTFS）/正交频分复用（OFDM）调制方案：对高移动性用户应用OTFS，对低移动性用户应用OFDM。考虑两种预编码设计——全迫零（FZF）预编码与部分迫零（PZF）预编码，并从每用户频谱效率（SE）角度进行分析。FZF以高计算复杂度为代价完全消除用户间干扰，而PZF可在复杂度与性能间实现折中。应用PZF预编码时，根据用户移动性特征或信道增益将用户划分为两个不相交的组别。随后对高移动性组或强信道增益组用户采用迫零（ZF）技术以完全消除组间干扰，对低移动性组或弱信道增益组用户采用最大比传输（MRT）。为揭示系统性能，本文研究了采用最小均方误差-串行干扰消除（MMSE-SIC）检测器时高移动性用户与低移动性用户的频谱效率。数值结果表明，基于信道增益分组的PZF预编码能为所有用户保障相似的服务质量。此外，采用移动性分组时，混合OTFS/OFDM调制在高移动性用户场景下的性能优于传统OFDM调制。