With the increasing demand for ultra-reliable and low-latency communication (URLLC), spatiotemporal two-dimensional (2-D) channel coding has received growing interest. By leveraging the spatial degrees of freedom in massive multiple-input multiple-output (MIMO) systems, it shortens the time-domain blocklength, thereby reducing latency and enhancing reliability. However, existing spatiotemporal coding schemes typically assume uniform reliability across spatial streams. This assumption does not hold in practical MIMO channels, where the underlying propagation environment generally leads to unequal spatial-eigenmode gains and reliabilities, making the conventional Gaussian-approximation-based construction for 2-D polar codes less effective. This paper investigates spatiotemporal 2-D polar coding over non-uniform MIMO channels, where the spatial domain exhibits inherently heterogeneous signal-to-noise ratios (SNRs). We propose a reciprocal channel approximation (RCA)-based reliability-aware 2-D polar coding framework that accurately characterizes such heterogeneous SNRs without relying on log-likelihood-ratio distribution assumptions. Simulation results demonstrate that the proposed RCA-based spatiotemporal 2-D polar coding scheme achieves clear performance gains and strong robustness, confirming its effectiveness in jointly exploiting temporal and spatial polarization for URLLC in practical MIMO systems.

翻译：随着超可靠低延迟通信（URLLC）需求的日益增长，时空二维（2-D）信道编码受到广泛关注。通过利用大规模多输入多输出（MIMO）系统中的空间自由度，该方法缩短了时域块长度，从而降低延迟并提升可靠性。然而，现有的时空编码方案通常假设空间流之间的可靠性是均匀的。这一假设在实际MIMO信道中并不成立——由于底层传播环境通常导致空间特征模增益和可靠性不均匀，使得传统基于高斯近似的二维极化码构造方法效果欠佳。本文研究了非均匀MIMO信道下的时空二维极化编码，其中空间域呈现固有的异构信噪比（SNR）。我们提出一种基于互易信道近似（RCA）的可靠性感知二维极化编码框架，该框架无需依赖对数似然比分布假设即可精确刻画此类异构SNR。仿真结果表明，所提出的基于RCA的时空二维极化编码方案实现了显著的性能增益和强鲁棒性，验证了其在实际MIMO系统中联合利用时间和空间极化特性支持URLLC的有效性。