Orbital angular momentum (OAM) and rate splitting (RS) are the potential key techniques for the future wireless communications. As a new orthogonal resource, OAM can achieve the multifold increase of spectrum efficiency to relieve the scarcity of the spectrum resource, but how to enhance the privacy performance imposes crucial challenge for OAM communications. RS technique divides the information into private and common parts, which can guarantee the privacies for all users. In this paper, we integrate the RS technique into downlink OAM-MIMO communications, and study the precoding optimization to maximize the sum capacity. First, the concentric uniform circular arrays (UCAs) are utilized to construct the downlink transmission framework of OAM-MIMO communications with RS. Particularly, users in the same user pair utilize RS technique to obtain the information and different user pairs use different OAM modes. Then, we derive the OAM-MIMO channel model, and formulate the sum capacity maximization problem. Finally, based on the fractional programming, the optimal precoding matrix is obtained to maximize the sum capacity by using quadratic transformation. Extensive simulation results show that by using the proposed precoding optimization algorithm, OAM-MIMO communications with RS can achieve higher sum capacity than the traditional communication schemes.

翻译：轨道角动量（OAM）与速率分割（RS）是未来无线通信的潜在关键技术。作为一种新的正交资源，OAM能够实现频谱效率的倍增，从而缓解频谱资源稀缺的问题，但如何提升其隐私性能对OAM通信构成了关键挑战。RS技术将信息划分为私有部分与公共部分，能够为所有用户保障隐私性。本文将RS技术集成到下行链路OAM-MIMO通信中，并研究以最大化总容量为目标的预编码优化问题。首先，利用同心均匀圆阵构建了结合RS的OAM-MIMO通信下行链路传输框架。特别地，同一用户对内的用户采用RS技术获取信息，而不同用户对之间使用不同的OAM模式。随后，推导了OAM-MIMO信道模型，并建立了总容量最大化问题的数学模型。最后，基于分式规划理论，通过二次变换得到最优预编码矩阵以实现总容量最大化。大量仿真结果表明，采用所提出的预编码优化算法，结合RS的OAM-MIMO通信能够获得比传统通信方案更高的总容量。