A hybrid small-cell and clustered cell-free massive MIMO system under mobility scenarios

Recently, cell-free (CF) massive multiple-input multiple-output (MIMO) becomes a promising architecture for the next generation wireless communication system, where a large number of distributed access points (APs) are deployed to simultaneously serve multiple user equipments (UEs) for improved performance. Meanwhile, a clustered CF system is considered to tackle the backhaul overhead issue in the huge connection network. In this paper, taking into account the more realistic mobility scenarios, we propose a hybrid small-cell (SC) and clustered CF massive MIMO system through classifications of the UEs and APs, and constructing the corresponding pairs to run in SC or CF mode. A joint initial AP selection of this paradigm for all the UEs is firstly proposed, which is based on the statistics of estimated channel. Then, closed-form expressions of the downlink achievable rates for both the static and moving UEs are provided under Ricean fading channel and Doppler shift effect. We also develop a semi-heuristic search algorithm to deal with the AP selection for the moving UEs by maximizing the weight average achievable rate. Numerical results demonstrate the performance gains and effective rates balancing of the proposed system.