  We propose a strategy for orthogonal downlink pilot assignment in cell-free
massive MIMO (multiple-input multiple-output) that exploits knowledge of the
channel state information, the channel hardening degree at each user, and the
mobility conditions for the users. These elements, properly combined together,
are used to define a user pilot utility metric, which measures the user's real
need of a downlink pilot for efficient data decoding. The proposed strategy
consists in assigning orthogonal downlink pilots only to the users having a
pilot utility metric exceeding a predetermined threshold. Instead, users that
are not assigned with an orthogonal downlink pilot decode the data by using the
statistical channel state information. The utility-based approach guarantees
higher downlink net sum throughput, better support both for high-speed users
and shorter coherent intervals than prior art approaches.
