###### Abstract  
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 (Nrep) 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 Nrep, 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 Nrep and MCSs for a vast scope of their values. Future investigations will concern the impact of multipath channels, typical in real-life deployment scenarios. maximum coupling loss; low throughput network; LTN; low power wide area network; LPWAN, LoRa; LoRaWAN; Weightless; SigFox; ultranarrowband; UNB; narrowband Internet of Things; repetitions +
Footnote †: journal: Medical  
application group, besides the energy-saving operation and remarkable sensitivity. The large number mentioned earlier is estimated to be between 20 (by Business Insider, 2017) and 200 (IDC, Intel) billion devices by the year 2020;
* Ultra-reliable low latency communications (URLLC) [4] feature low losses of information, very high responsivity to triggering incentives, and high availability and security, and is usually required in industrial control systems or medical applications. For better interaction with the world of things, improved radio and haptic interfaces are necessary, leading to a paradigm of the tactile Internet;  
The other categories, besides the enhancement of vehicle-to-everything (V2X), include the following:  
* Enhanced mobile broadband (eMBB) [5] is a continuation of the initial Release 8 long-term evolution (LTE) development, aimed at achieving ultra-high capacities (on the order of several Gb/s in the radio interface), rendering services to densely populated users, handling the traffic of high throughput dynamics, and providing convergence between stationary and mobile services. Peak capacities are expected to be available in extremely small capacity cells (with coverage \(<\)50 m from the base station) with the use of so-called Frequency Range 2 (or new radio (NR)) frequency ranges between 24 GHz and 100 GHz, as opposed to range cells operating in Frequency Range 1 bands (below 6 GHz), providing wider-area, lower-throughput coverage and connection setup services;
* Network operations (NO) [6] cater to such functional requirements as elastic functionalities, creating new values, migration and internetworking, optimization, enhancements, and security. In response to the growing demand for systems surpassing the sensitivity performance of traditional general packet radio system (GPRS) for handling sensor and meter traffic, a range of communication technologies have been spawned. These, termed as IoT systems, can be further subdivided into non-cellular low-throughput networks (LTNs) and cellular IoT (CIoT) systems (see Figure 1). The former is divided further into low-power wide-area network (LPWAN) [7; 8; 9] and ultra-narrowband (UNB) [10] solutions; however, both are beyond the scope of this article.