# Capacity Bounds for Dense Massive MIMO in a Line-of-Sight Propagation Environment
## 1 Introduction  
During the past years, we have been witnessing massive multiple-input multiple-output (MIMO) becoming an efficient and indispensable sub-6 GHz physical-layer technology for wireless and mobile networks. The embodiment of such technology was vital for the current 5G new radio (NR) interface [1]. The central concept behind massive MIMO is the use of a large antenna array deployed at base stations (BS) to simultaneously serve a large number of devices over the same time-frequency resources. In this way, the technique allows exploiting differences among the propagation signatures of the devices in order to perform spatial multiplexing [1]. Even though the massive MIMO technology looks quite mature, being adopted by new standards, it does not mean an end for research in this subject but just the beginning of unforeseen possibilities [2]. Current requirements for next-generation networks such as high bit rates, very low latency, high energy efficiency, and link robustness are not wholly met even by the current 5G solutions [3]. Thus, there are still several open challenges that need to be addressed by researchers. One of the approaches used to increase throughput is by increasing the network density, that is, decreasing the size and increasing the number of cells in the same coverage area. As a result, the size ofthe cells is becoming smaller and smaller, and consequently, it is quite probable that wireless channels will be predominantly line-of-sight (LoS) [4; 5]. Furthermore, the LoS characteristic becomes even more apparent as technology moves to millimeter-wave (mmWave) bands in order to fulfill the requirements for wider bandwidths (i.e., micro and femtocells) [6; 7]. The mmWave bands have the potential to significantly raise user throughput, enhance spectral and energy efficiencies, and increase the capacity of mobile and wireless networks using the joint capabilities of the huge available bandwidth paired with the high multiplexing gains achievable with massive antenna arrays [8; 9]. Above all, as the wavelengths of mmWave bands are extremely short, it becomes possible to add a huge number of antenna elements to a small area, which consequently helps in achieving the high multiplexing gains offered by the massive MIMO technology at both the BS and devices [10]. Additionally, the huge deployment of small cells, i.e., densification, renders the short-range mmWave technologies very useful. Therefore, the mmWave bands can be considered as one of the potential technologies to meet the requirements of 5G networks [11]. Densification is a natural process for wireless communication networks as the demand for connectivity increases. Thanks to smartphones, tablets, and the Internet of Things, wireless subscribers are using more network resources with no sign of a decrease in the demand rate. Operators need to add more capacity to their networks to continue handling all the traffic while providing the network speeds those users expect. An arbitrary coverage region can be expected to follow three different degrees of BS densification [2; 12]: low-density, dense, and ultra-dense. Starting from low density, the smart farming and rural broadband services provision (i.e., distant areas with low population density) poses a significant gap in the research body of massive MIMO nowadays [13; 14]. In the sense of wireless services, it is essential to observe that most economically viable areas for agriculture are the plain terrains with few obstacles [15]. Thus, since the multi-path channel statistics of rural and distant areas are different from those found in urban centers, the wireless channel will likely be predominantly LoS in such areas [16; 17]. For regions where high value goods are constantly under surveillance, a dense network is required to communicate with unmanned aerial vehicles (UAVs), also known as drones [18]. As with the other examples, these communications will probably be LoS-based as well.