Characterization of Indoor Small Cells Propagation Rooderson M. Andrade, Rui R. Paulo, Salom˜ ao M. Francisco, Emanuel Teixeira and Fernando J. Velez Instituto de Telecomunicac ¸˜ oes and Universidade da Beira Interior Faculdade de Engenharia, Departamento de Engenharia Electromecˆ anica Covilh˜ a, Portugal 0000-0001-6070-1595, 0000-0002-8541-6675, 0000-0002-5431-3163, 0000-0002-6546-5277, 0000-0001-9680-123X Abstract—The characterization of the wireless medium in indoor small cell networks is essential to obtain appropriate mod- eling of the propagation environment. Universal Software Radio Peripherals (USRPs) and simple dipole antennas can emulate LTE-Advanced networks. In this work, we verify WINNER II propagation modeling for the indoor femtocell environment by considering different classrooms of 7.32 × 7.32 square meters near a common University Department corridor while measuring the power received in UEs placed in a grid of 49 points (radiated by the small eNodeB in the centre of the classroom of the own cell). These measurements have been carried out either by using the Software Radio Systems LTE that emulates the LTE- Advanced network and its UEs, or by measuring the received power in the UES with a Rohde & Schwarz FSH8 spectrum analyzer. In room 1, by varying the UE position, the highest values of the received power have occurred close to the central BS, and then in the opposite wall, further away from the interferer. Nevertheless, it was verified that the received power does not decrease suddenly because of the effect of the radiation pattern of the BS and UE antennas for large angles of apertures, as well as due to the non-omnidirectional horizontal antenna pattern. In addition, it was demonstrated that there is an effect of “wall loss” proven by the fact that path loss increases between room 2 and room 1 (or between room 3 and 2). If we consider an attenuation for each wall of circa 7-9 dB the behavior of the WINNER II model at 2.625 GHz for the interference coming across different walls is verified. Index Terms—Propagation, OFDM, small cell networks, LTE- Advanced, srsLTE, WINNER II model. I. I NTRODUCTION To answer to the increasing user demand, mobile networks toward 5G has evolving heterogeneous networks, with differ- entiation of Evolved NodeBs (eNBs) coverage, as established by 3GPP Release 16 [1]. Small cells, overlaid by larger cells, can be deployed either indoor or outdoor, to provide indoor coverage, outdoor coverage, or even to provide indoor coverage from transmitter nodes placed outdoor. It is important to explore strategies to increase LTE- Advanced (LTE-A) end-to-end service reliability of mobile multimedia exchanges, e.g., video streaming. One option is to use Universal Software Radio Peripherals (USRPs) that emulate 4G network [2], [3] in indoor or outdoor deployment scenarios, in conjunction with Software Radio Systems LTE This work is funded by Marie Sklodowska-Curie Actions (MSCA) ITN TeamUp5G (813391), ORCIP, and by UIDB/50008/2020. TeamUp5G project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie project number 813391. (srsLTE), an open source library for the PHY layer of LTE- A written in ANSI C. It includes a complete software radio [4] and provides interfaces for the Universal Hardware Driver (UHD) that supports USRPs [5]. Research on the characterization of these femtocell envi- ronments at 2.6, 3.5 or 5.62 GHz is still of great interest. Authors from [6], [7], have made their experiments in linear- shaped paths within indoor scenarios where the user equipment (UE) and transmitter (TX) eNodeBs are few meters apart. The path loss is affected not only by factors such as architectural structures, including the material and the thickness of the walls, diffraction, reflection and interference [8] but also by the distance between the transmitter and receiver (Tx) and receiver (Rx). The objective of this paper is to investigate the coverage and co-channel interference trade-off in OFDM-based indoor small cells with a given reuse pattern (k). A small cell LTE- A University environment is considered. Although the LTE-A downlink (DL) is assumed, results can easily be generalized to 5G New Radio cellular networks. First, we consider an analytical approach to compute the signal-to-interference plus noise ratio (SINR) by considering the WINNER II propagation model [9]. Second, an experimental field is performed test in an indoor scenario using srsLTE with a set of USRPs B210 to measure the quality of the received signal as a function of the distance, either by determining the received power (P r ) or computing the carrier-to-interference ratio, C/I , either with the USRP B210 used as a UE (and measurement device) or by using a Rohde & Schwarz FSH8 spectrum analyzer. UE receives data packets from the own cell (eNodeB of interest), while the other eNodeBs only cause interference. In our experiments, we have measured the received power in different class rooms from our Department while verifying the Winner II propagation model. The USRP B210 has a high- speed serial interface for integration with other boards, al- lowing the improvement of SISO systems [10]. Its transmitter power is in between 50 and 200 mW. A temporary license was assigned to us by ICP-ANACOM, the Portuguese regulator, to perform field tests in the 2625 MHz (DL) and 2505 (uplink) MHz frequency bands, each with bandwidth of 10 MHz. The rest of the paper is organized as follows. In Sec. II we present a theoretical study on the average SINR. Sec. III presents the experimental setup with the USRPs that emulate 4G networks as well results for the C/I (not SINR, as the wireless system is interference-limited) obtained the USRPs