# Path Loss Measurements of Indoor LTE System for the Internet of Things
## 1 Introduction  
The Internet of Things (IoT) for the production operations is one of the hottest topics in Industrie 4.0 and Smart Manufacturing [1]. The system architecture of the IoT consists of four layers, including the sensing layer, the network layer, the service layer, and the interface layer [2]. The IoT provides important service for future advanced manufacturing which relies on the concept of connectivity. For connecting machines, devices, sensors, and people with each other in a factory, reliable and scalable communication networks used in the IoT are of great importance. There are several significant relevant research works in the area, such as those reported in References [3; 4; 5]. Among the various technologies in the system architecture of the IoT, the narrow-band IoT (NB-IoT) and the small cell are considered very promising technologies.  
The Third-Generation Partnership Project (3 GPP) already standardized the NB-IoT, a new cellular radio technology that meets the requirements of the IoT, in 2016 [6; 7]. The design goal of the NB-IoT is to achieve low-cost devices, high coverage, long battery lifetime, and massive capacity [8; 9; 10; 11].  
Industrial and academic research has anticipated the increasing use of small cells to provide the indoor data transmission required to meet the emerging communication and networking needs of future IoT applications [12]. A small cell is a low-power and low-cost radio base station, which can provide suitable cellular coverage in factory environments [13]. Therefore, the small cell is considered one of the key technologies for future IoT.  
While performing the system planning and deployment of small cell base stations for the IoT applied in a factory, it is critical to understand the wireless channel characteristics in the indoor environment. [PERSON], [PERSON], and [PERSON] proposed connectivity modeling and layout design methods to support a plant designer during wireless coverage prediction, virtual network deployment, and post-layout verification [14]. Their proposed methods could predict the radio signal coverage in typical industrial environments. However, they only considered the IEEE 802.15.4 channel. Whether their work could be applied to an NB-IoT is still unknown.  
This paper aims at channel parameter measurements of indoor Long Term Evolution (LTE) systems for the deployment of base stations to have good coverage and service reliability (SR). First, we built a software-defined radio (SDR)-based small cell experimental platform, which included a set of LTE evolved node B (eNB) and user equipment (UE). By using the SDR techniques, we could easily modify important system parameters by software programming. Based on this experimental platform, we performed indoor channel parameter measurements to determine the path loss (PL) exponent and the standard deviation of the received shadow fading signal. Finally, based on the experimental results, the precise formula of the PL model and the calculated fade margin (FM) for 90% SR were exploited to predict the coverage range of the small cell base station optimally deployed in the factory. The measured PL channel models were compared with the (International Telecommunication Union ) ITU PL channel model.  
The major contributions of this paper are twofold:  
* An experimental method was designed to perform the wireless channel parameter measurements of indoor LTE systems for the IoT. This method could be applied to factorial environments.
* Based on the experimental results, we provided the indoor channel model with modified parameter values. This facilitates the system planning and deployment of small cell base stations with 90% SR for the IoT.  
The rest of this paper is organized as follows. The main principles of the path loss model and the coverage range with 90% SR that we adopt in this paper are presented in Section 2. The experimental methods and evaluation results are provided in Sections 3 and 4, respectively. Finally, Section 5 concludes this paper.