## 4 The Measurement Setup and Scenarios  
### The Infrastructure for Generating Ultra-high Attenuation  
Credible measurements of the system performance under boundary MCL conditions (i.e., close to 164 dB) were performed experimentally under laboratory conditions. For this purpose, a measurement setup was built based on cable tracks in facilities of the specialized Laboratory of Electromagnetic Compatibility (EMC), located at Wroclaw University of Science and Technology. In order to provide a repeatable, fully controlled, interference-free setup for bidirectional, extremely high pathloss measurements, a cable track was assembled as shown in Figure 2. Variable attenuation of the cable tracks was used to simulate the air interface with an ultra-high MCL, as is required for testing NB-IoT systems. Building the test setup in limited space with the use of imperfect elements (i.e., with parasitic leakage of the signal and limited shielding effectiveness), as well as the use of attenuators with high attenuation (40-50 dB) in the cable track, yielded possible unwanted couplings between the signal track segments, leading to the lack of a fully controlled and repeatable test setup. To avoid these unwanted couplings, each attenuation stage was located in one of the three separate chambers, electromagnetically isolated from the others. The specific chambers are identified in the figure as CR3 (for the control room), SAC (for the semi-anechoic chamber) and SB (for the shielded box), each providing isolation exceeding 80 dB. During the measurements, both the NB-IoT core segment and the eNB were located in the CR3 chamber, whereas the test UE was locked in the SB placed in the SAC. As shown in the schematic (Figure 2), the end-to-end connection was provided by means of cable transmission lines and attached attenuators. From the eNB side, to implement transmission diversity, radio-frequency (RF) signals with 40 dBm of power at each of the two outputs were passed separately to the UE through 50 dB fixed attenuators, which were connected directly to each eNB output.  
The signals were then passed simultaneously along each line through respective grommets on the penetration panel separating the CR3 and SAC, on both sides of which 40 dB fixed attenuators were attached. Thereupon, by using the RF cabling laid down beneath the reference ground plane (RGP). In fact, for the SAC's floor, the signal was eventually guided to the turn table located at the opposite part of the SAC and to the last stage of attenuation. The final adjustment (0-30 dB) of the pathloss attenuation was attained by means of manually controlled attenuators, as opposed to the former use of remote-controlled attenuators, which could not provide stable and reproducible attenuation conditions. The outputs of the regulated attenuators were fed into the signal combiner and passed on to the SB inlet, leading directly to the NB-IoT UE placed inside. Considering the combined attenuation of the three fixed attenuators (totaling 130 dB), the total cabling loss (approximately 13 dB, varying by the track), and the intrinsic attenuation of the regulated attenuators (0.2 dB and 0.7 dB), the desired pathloss of about 164 dB was achievable by adjusting them within a range from 10 dB to 30 dB. Measurements of each section attenuation and the received signal level confirmed that the developed test set-up could emulate high pathloss values, guaranteeing stable conditions required for testing the NB-IoTperformance. Moreover, based on the measured eNB's transmitter output power and the test set-up attenuation, values of the signal power at the input of the UE could be determined with high precision. These were then compared to the indications of the UE. For better validation of the MCL achieved during the performance tests, the above procedure was carried out for two UEs from different vendors.