Massive multiple input multiple output (M-MIMO) technology plays a pivotal role in fifth-generation (5G) and beyond communication systems, offering a wide range of benefits, from increased spectral efficiency (SE) to enhanced energy efficiency and higher reliability. However, these advantages are contingent upon precise channel state information (CSI) availability at the base station (BS). Ensuring precise CSI is challenging due to the constrained size of the coherence interval and the resulting limitations on pilot sequence length. Therefore, reusing pilot sequences in adjacent cells introduces pilot contamination, hindering SE enhancement. This paper reviews recent advancements and addresses research challenges in mitigating pilot contamination and improving channel estimation, categorizing the existing research into three broader categories: pilot assignment schemes, advanced signal processing methods, and advanced channel estimation techniques. Salient representative pilot mitigation/assignment techniques are analyzed and compared in each category. Lastly, possible future research directions are discussed.

翻译：大规模多输入多输出（M-MIMO）技术在第五代（5G）及未来通信系统中发挥着关键作用，带来了从频谱效率提升到能效增强及更高可靠性等一系列优势。然而，这些优势依赖于基站端精确的信道状态信息。由于相干间隔尺寸受限，进而导致导频序列长度受限，获取精确的信道状态信息颇具挑战性。因此，在相邻小区中复用导频序列引入了导频污染，阻碍了频谱效率的提升。本文综述了近期在缓解导频污染和改善信道估计方面的研究进展及面临的挑战，将现有研究分为三大类：导频分配方案、先进信号处理方法以及先进信道估计技术。对每类中具有代表性的导频缓解/分配技术进行了分析与比较。最后，探讨了未来可能的研究方向。