## 2 State-of-the-Art NB-IoT Performance Research and the Paper's Novelty  
Although lots of research on NB-IoT performance has been documented, including measurement-based investigations on the energy and power consumption of NB-IoT UEs [13], boundary pathloss conditions are most often investigated in theory and by means of simulations, due to the severe difficulty associated with obtaining stable extreme attenuations in lab facilities. Accordingly, in [14], the coverage was theoretically found to improve by 3 dB per each doubling of the number of repetitions, leading to a 172.7 dB MCL for 2048 repetitions. Valuable takeaways, closely related to the findings presented in this paper, can be found in [15], where an in-depth analysis of the signal-to-noise ratio (SNR) and the resulting throughput was performed for various multipath models, focusing on two uplink channels, the narrowband physical uplink shared channel (NPUSCH) and the narrowband physical random access channel (NPRACH), and concluding with a proposal of a modified receiver structure. Though mainly theoretical, the investigations were then validated with the use of an emulated NB-IoT Qualcomm FSM chipset-based LTE base station setup. The emphasis in [15] was placed on changing SNR values by generating a high-power noise signal, rather than by forcing NB-IoT devices to operate at extremely low received signal powers, nearing an MCL of 164 dB (as was done in the current paper). A proposal for a link adaptation scheme integrated with a proper selection mechanism of the repetitions number and the modulation coding scheme (MCS) for achieving maximized throughput and coverage is included in [16]. Figure 1: A classification of Internet of Things systems. Measurement-based investigations found in the literature, in turn, focus on site-specific campaigns that aim at determining the NB-IoT coverage obtained at particular locations. For instance, in [17], a 20 dB coverage enhancement, with respect to LTE, was demonstrated indirectly by observing the measured received power cut-off of \(-130\) dBm during indoor and outdoor measurements in Poland. In [18], a method for extending NB-IoT cell coverage up to 45 km (confirmed in an experiment in the West Sea of South Korea), based on adding some extra traffic delay, was proposed. The authors of [19], in turn, performed measurements in Belgium, allowing a terminal to perform automatic adaptations of MCSs and repetitions to a current link quality while measuring the throughput and the delay. The resultant sensitivity (\(-141\) dBm) served as a confirmation of the expected MCL of 164 dB. A similar approach can be found in a plethora of other studies that either concern specific networks or refer to estimating NB-IoT applicability to specific scenarios, such as healthcare or transportation [20; 21; 22; 23]. The novelty of this paper consists of providing real-life evidence of the performance of NB-IoT uplink channels under propagation conditions near the maximum coupling loss of 164 dB. For this purpose, the authors constructed an instrumentation setup for emulating a free-space propagation environment, in which the link attenuation could be precisely controlled. The setup was assembled in the form of combined high-quality cabling, electromagnetically isolated chambers, and high-rank attenuators. The achieved ability to control the pathloss at a precision of \(<\)1 dB enabled observation of the performance of the two uplink NB-IoT channels, namely NPRACH and NPUSCH (see Section 3.2 for details) in reaction to the application of various combinations of {MCS;Nrep}. The outcomes allow one to estimate the factual measured NB-IoT response to an MCS and Nrep, and thus they may serve as benchmark figures for validating theoretical derivations found throughout the literature. According to the authors' best knowledge, these investigations are the first contribution performed in a systematic, thorough, and fully measurement-based manner, regarding the actual NB-IoT uplink performance.