To improve spectral efficiency (SE), how can we reduce pilot contamination in massive MIMO by making the pilot reuse factor proportional to the frame length symbols and suppressing inter-cell interference?
The answer is basically in your text. If the same pilots are only used in every 3rd, 4th, or 7th cell (assuming a hexagonal cell geometry), the distance between the users that interfere with each other can be made negligible.
Chapter 6 in the textbook "Fundamentals of Massive MIMO" describes this in detail.