Beam Focusing for Near-Field Cell-Free Massive MIMO

Beam focusing is one of the key points to distinguish near-field from far-field, which is widely studied in extremely large-scale multiple-input-multiple-output (MIMO) systems. In this paper, we investigate the beam focusing effect in a near-field cell-free massive MIMO (CF mMIMO) system with a limited number of antenna elements at each access point. Based on the orthogonal frequency division multiplexing (OFDM) modulation, we propose a delay-based beam focusing scheme. Furthermore, we introduce the sparse array which contributes to another angle-based beam focusing scheme. By combining these two schemes, we develop the joint-delay-angle-based beam focusing which can generate a quite explicit focal region. Numerical results validate the theoretical analysis and show that the beam focusing effect can be significantly enhanced by increasing the number of the subcarriers and expanding the array aperture.