Multiple-input multiple-output (MIMO) has been a key technology of wireless communications for decades. A typical MIMO system employs antenna arrays with the inter-antenna spacing being half of the signal wavelength, which we term as compact MIMO. Looking forward towards the future sixth-generation (6G) mobile communication networks, MIMO system will achieve even finer spatial resolution to not only enhance the spectral efficiency of wireless communications, but also enable more accurate wireless sensing. To this end, by removing the restriction of half-wavelength antenna spacing, sparse MIMO has been proposed as a new architecture that is able to significantly enlarge the array aperture as compared to conventional compact MIMO with the same number of array elements. In addition, sparse MIMO leads to a new form of virtual MIMO systems for sensing with their virtual apertures considerably larger than physical apertures. As sparse MIMO is expected to be a viable technology for 6G, we provide in this article a comprehensive overview of it, especially focusing on its appealing advantages for integrated sensing and communication (ISAC) towards 6G. Specifically, assorted sparse MIMO architectures are first introduced, followed by their new benefits as well as challenges. We then discuss the main design issues of sparse MIMO, including beam pattern synthesis, signal processing, grating lobe suppression, beam codebook design, and array geometry optimization. Last, we provide numerical results to evaluate the performance of sparse MIMO for ISAC and point out promising directions for future research.

翻译：多输入多输出（MIMO）技术数十年来一直是无线通信的关键技术。典型的MIMO系统采用天线间距为信号波长一半的天线阵列，我们称之为紧凑型MIMO。展望未来第六代（6G）移动通信网络，MIMO系统将实现更精细的空间分辨率，不仅能提升无线通信的频谱效率，还能支持更精确的无线感知。为此，通过突破半波长天线间距的限制，稀疏MIMO作为一种新型架构被提出，在相同阵元数量的情况下，其阵列孔径相比传统紧凑型MIMO能显著扩大。此外，稀疏MIMO催生了一种新型虚拟MIMO感知系统，其虚拟孔径远超物理孔径。鉴于稀疏MIMO有望成为6G的可行技术，本文对其进行了全面综述，特别聚焦于该技术在面向6G的集成感知与通信（ISAC）中的显著优势。具体而言，本文首先介绍了多种稀疏MIMO架构，随后分析了其新优势与挑战。接着探讨了稀疏MIMO的主要设计问题，包括波束模式合成、信号处理、栅瓣抑制、波束码本设计和阵列几何优化。最后，我们通过数值结果评估了稀疏MIMO在ISAC中的性能，并指出了未来研究的潜在方向。