Title: Asynchronous Cell-Free Massive MIMO With Rate-Splitting Show Chinese title

Title: 异步无基站大规模MIMO与速率分割

Abstract: In practical cell-free (CF) massive multiple-input multiple-output (MIMO) networks with distributed and low-cost access points, the asynchronous arrival of signals at the user equipments increases multi-user interference that degrades the system performance. Meanwhile, rate-splitting (RS), exploiting the transmission of both common and private messages, has demonstrated to offer considerable spectral efficiency (SE) improvements and its robustness against channel state information (CSI) imperfection. The signal performance of a CF massive MIMO system is first analyzed for asynchronous reception capturing the joint effects of propagation delays and oscillator phases of transceivers. Taking into account the imperfect CSI caused by asynchronous phases and pilot contamination, we derive novel and closed-form downlink SE expressions for characterizing the performance of both the RS-assisted and conventional non-RS-based systems adopting coherent and non-coherent data transmission schemes, respectively. Moreover, we formulate the design of robust precoding for the common messages as an optimization problem that maximizes the minimum individual SE of the common message. To address the non-convexity of the design problem, a bisection method is proposed to solve the problem optimally. Simulation results show that asynchronous reception indeed destroys both the orthogonality of the pilots and the coherent data transmission resulting in poor system performance. Besides, thanks to the uniform coverage properties of CF massive MIMO systems, RS with a simple low-complexity precoding for the common message obtained by the equal ratio sum of the private precoding is able to achieve substantial downlink sum SE gains, while the application of robust precoding to the common message is shown to be useful in some extreme cases, e.g., serious oscillator mismatch and unknown delay phase. Show Chinese abstract

Abstract: 在具有分布式和低成本接入点的实际无蜂窝（CF）大规模多输入多输出（MIMO）网络中，用户设备处信号的异步到达增加了多用户干扰，从而降低了系统性能。 同时，速率分割（RS）通过传输公共消息和私有消息，已被证明可以显著提高频谱效率（SE），并且对信道状态信息（CSI）不完善具有鲁棒性。 首先分析了CF大规模MIMO系统的信号性能，以捕捉传播延迟和收发器振荡器相位的联合影响。 考虑到由异步相位和导频污染引起的不完善的CSI，我们推导出了新颖且闭合形式的下行链路SE表达式，分别用于表征采用相干和非相干数据传输方案的RS辅助系统和传统非RS系统性能。 此外，我们将公共消息的鲁棒预编码设计 formulated 为一个优化问题，该问题最大化公共消息的最小个体SE。 为了解决设计问题的非凸性，提出了一种二分法来最优地解决该问题。 仿真结果表明，异步接收确实破坏了导频的正交性和相干数据传输，导致系统性能较差。 此外，由于CF大规模MIMO系统的均匀覆盖特性，使用通过私有预编码的等比和获得的简单低复杂度预编码的RS能够实现显著的下行链路总SE增益，而将鲁棒预编码应用于公共消息在某些极端情况下被证明是有用的，例如严重的振荡器失配和未知的延迟相位。