This paper addresses the performance improvement that structured pilot assignment policies can bring about, relative to a random pilot assignment, in cell-free massive MIMO wireless networks. It is shown that structured policies can deliver a multiple-fold reduction in the pilot overhead required to keep pilot contamination at some acceptably low level. While the implementation of the structured policies considered in the paper might require some degree of centralized control, their performance also captures what distributed nonrandom pilot assignment schemes (e.g., greedy or collision-detecting algorithms) can hope to approach.

The work of A. Lozano was supported by Project TEC2015-66228-P (MINECO/FEDER, UE) and by the European Research Council under the H2020 Framework Programme/ERC grant agreement 694974.