Uplink Performance of Cell-Free Massive MIMO with Access Point Selections

Cell-free massive multiple-input multiple-output (MIMO), in which a massive number of access points (APs) distributed over a large area serve a smaller number of users in the same time and frequency resources, inherits advantages from conventional massive MIMO (i.e. favourable propagation and channel hardening), and distributed system (i.e. macro diversity gain). As a result, cell-free massive MIMO can provide a great spectral efficiency, high capacity and offer uniformly great service for all users. To contribute to this great concept, an uplink and downlink performance of cell-free massive MIMO are investigated in this work. Novel access point selection and signal detection schemes are proposed to reduce the requirements of backhaul links connecting the APs and the central processing unit, and to improve the system performance in terms of the achievable rate. Note that most of signal detection schemes for cell-free massive MIMO in the literature rely on the channel hardening property, with results in less accuracy for small and moderate number of APs. Firstly, closed-form expressions for the achievable rate of the downlink and uplink are derived. Then, performance comparisons between the proposed signal detection scheme and the conventional scheme are exploited. The result shows that the proposed scheme (with the novel AP selection and signal detection) outperforms the conventional scheme in terms of the achievable rate and the amount of data load exchanging over the backhaul links.