  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.
