Abstract: This paper presents a novel lower boundary for the coherence block (ChB) length in timevariant wireless channels. A rigorous estimation of the ChB length is important for the proper design of systems based on time division duplex-orthogonal frequency division multiplexing (TDD-OFDM). ChB length is especially relevant in the case of massive multiple input-multiple output (m-MIMO) systems, as it determines the overhead due to the massive channel estimation and, consequently, the spectral efficiency that can be achieved. The proposed boundary is based on a tractable propagation model, is related to easily obtainable channel parameters, and applicable to radio channels with temporal variation due to both the movement of the users and the movement of objects that surround them; including vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I) and industrial Machine-to-Machine (M2M) communications.

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