| Summary: | This paper presents experimental results of investigations on narrowband Internet of Things (NB-IoT) uplink channel performance under extreme pathloss conditions, approaching the boundary maximum coupling loss (MCL) defined for cellular IoT systems. The system performance was systematically tested against several modulation coding schemes (MCSs) and a set of repetition numbers (N<sub>rep</sub>) in electromagnetically isolated facilities, guaranteeing proper environment separation from external factors that usually affect measurements in less controlled setups. Some of the major takeaways include the following observations. Firstly, every increase in the number of repetitions by a factor of four caused the performance to improve by about 3 dB. Secondly, the switchover between extreme MCSs (i.e., MCS0 and MCS10) caused the performance to shift by about 6 dB. Thirdly, chipsets from different vendors performed similarly at small values of N<sub>rep</sub>, but tended to diverge for larger numbers of repetitions. These findings may serve as benchmark figures for other theoretical and simulation-based studies by demonstrating performance in response to N<sub>rep</sub> and MCSs for a vast scope of their values. Future investigations will concern the impact of multipath channels, typical in real-life deployment scenarios.
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