To enable realistic studies of massive multiple-input multiple-output systems, the COST 2100 channel model is extended based on measurements. First, the concept of a base station-side visibility region (BS-VR) is proposed to model the appearance and disappearance of clusters when using a physically-large array. We find that BS-VR lifetimes are exponentially distributed, and that the number of BS-VRs is Poisson distributed with intensity proportional to the sum of the array length and the mean lifetime. Simulations suggest that under certain conditions longer lifetimes can help decorrelating closely-located users. Second, the concept of a multipath component visibility region (MPC-VR) is proposed to model birth-death processes of individual MPCs at the mobile station side. We find that both MPC lifetimes and MPC-VR radii are lognormally distributed. Simulations suggest that unless MPC-VRs are applied the channel condition number is overestimated. Key statistical properties of the proposed extensions, e.g., autocorrelation functions, maximum likelihood estimators, and Cramer-Rao bounds, are derived and analyzed. ). 2 I. INTRODUCTION Massive multiple-input multiple-output (MIMO), MaMi for short, has since its inception [3] attracted considerable attention in the wireless communication community [4]-[6]. Within the last few years, an abundant body of theoretical [5], [6] and experimental [7], [8] research has shown that MaMi systems can improve the energy and spectral efficiencies of today's wireless communication systems by one to two orders of magnitude. For this reason, MaMi is considered a crucial component of the new radio (NR) air interface of the fifth generation (5G) wireless communication standard [9].As is well known, the radio propagation channel ultimately dictates the performance of any wireless communication system. The availability of sufficiently accurate propagation channel models is therefore of critical importance to the design and evaluation of new wireless systems. The 3GPP SCM-3D [10], the WINNER II/WINNER+ [11], [12], and the COST 2100 [13], [14] are examples of channel models customarily used in the development and validation of 5G networks. Among them, the Quasi Deterministic Radio Channel Generator (QuaDRiGa) [15], [16], an extension of the 3GPP-3D and WINNER II/WINNER+ channel models, is especially popular because of its enhanced spatiotemporal consistency. Moreover, it possesses certain features that enable MaMi simulations, namely support for spherical wavefronts, dispersion of clusters in elevation, and independence of arrival/departure angles [16]. Experiment shows [17]-[19] that at least two additional aspects of the MaMi propagation channel need to be considered. The first of them [17], [18] is the presence at the base station (BS) of non-stationarities caused by the appearance and disappearance of clusters along physicallylarge arrays (PLAs). Such BS-side non-stationarities have been addressed in theoretical MaMi channel models [20], [21] in which the appearances and disappearances o...