# Software-Defined NB-IoT Uplink Framework--The Design, Implementation and Use Cases
## 2 The NB-IoT Interface  
The NB-IoT standard [2] is directly connected with 4G technology, especially the physical layer. The number of signal processing operations of the uplink and downlink Orthogonal Frequency Division Multiple Access/Single Carrier Frequency Division Multiple Access (OFDMA/SC-FDMA) techniques is definitely challenging, bearing in mind the signal formation computational cost and the low complexity of the nodes. The NB-IoT device lifespan should reach at least 10 years [3], and because of that, the software design, especially considering repetitive operations, should be computationally effective. Unfortunately, the obligatory compliance with the 3 GPP standard dismisses the ability to omit the most complicated operations (by simplifying the radio interface) as per in the typical industrial, scientific, and medical (ISM) sensor networks. The NB-IoT is a cellular network standard developed by the 3 rd Generation Partnership Project (3 GPP) organization in 2016 [4]. The physical layer mostly inherits from the long-term evolution (LTE), e.g., the OFDMA technique for downlink, the SC-FDMA technique for uplink, the channel coding, the interleaving operation, and functioning in the licensed spectrum [2,3]. All the assumptions regarding the NB-IoT interface were published for the first time in Release 13 of the 3 GPP standard. The fact that the NB-IoT interface is based on LTE allows reducing the final cost of the device as well as the system deployment time [5]. The main goal of the NB-IoT is to provide the communication between devices in harsh radio conditions, e.g., underground or in basement environments [6, 7, 8].  
Coverage enhancement defined in the NB-IoT specification gives 20 dB higher maximal coupling loss with respect to the Global System for Mobile Communication (GSM) packet data service [9,10]. Nevertheless, apart from abovementioned features, there are other advantages of the NB-IoT usage, e.g., energy efficiency or an easy integration with the preexisting cellular networks. NB-IoT provides two types of energy saving modes, i.e., power-saving mode (PSM) and extended/enhanced discontinuous reception mode (eDRX) [11], that may increase user equipment (UE) battery lifetime up to 10 years. Considering coexistence with other cellular systems, it is worth mentioning that the NB-IoT system may operate in three different modes, determining the NB-IoT physical resource blocks (PRBs) deployment within the current spectrum, i.e., standalone, in-band, and guard-band [12]. According to the in-band and guard-band modes, the NB-IoT PRBs (each occupying 180 kHz of spectrum--12 subcarriers with 15 kHz spacing or 48 subcarriers with 3.75 kHz spacing) are allocated inside the LTE resource grid. To be more precise, the NB-IoT signal may occupy the unused PRB of LTE bandwidth (in-band) or PRB of LTE guard bands (guard-band). Alternatively, regarding standalone mode, NB-IoT PRBs may utilize the GSM channel as well [13]. Considering its bandwidth of 200 kHz, it is possible to allocate 180 kHz NB-IoT PRB with two additional guard buffers, 10 kHz each. The key differences of the NB-IoT interface with respect to LTE may be identified in the protocol stack, like the Hybrid Automatic Repeat Request (HARQ) and terminal modes of operation. By definition, the NB-IoT devices are dedicated for controlling or monitoring applications. Therefore, due to the assumed lack of nodes mobility, it not only reduces the amount of radio resources utilized in control channels but also eliminates the problem of handover between the evolved NodeBs (eNBs). It is worth mentioning that the maximal throughput reaches 200 kbps or 160-250 kbps [14; 15] for uplink and downlink, respectively, and it is allocated only for the data transfer services.