Asynchronous Cell-Free Massive MIMO-OFDM: Mixed Coherent and Non-Coherent Transmissions

In this letter, we analyze the performance of asynchronous cell-free multiple-input multiple-output orthogonal frequency division multiplexing (CF mMIMO-OFDM) system with mixed coherent and non-coherent transmission approaches. To this end, we first obtain the achievable downlink sum-rate for the mixed coherent and non-coherent transmissions, and then provide a closed-form expression for the case with the maximum ratio precoding. Subsequently, an efficient access point (AP) clustering algorithm is proposed to group APs into a set of coherent clusters. Numerical results demonstrate that the mixed coherent and non-coherent transmissions can effectively improve the sum-rate of CF mMIMO-OFDM systems under asynchronous reception.