# On the Evaluation of the NB-IoT Random Access Procedure in Monitoring Infrastructures
## 2 State of the Art  
### NarrowBand--Internet of Things  
NB-IoT is a cellular radio access technology that requires a smaller bandwidth than Long Term Evolution (LTE), that is, 180 kHz for both downlink and uplink [13]. A single carrier can be configured according to the chosen operation mode, whereas multiple carriers can also be used to supply a higher bandwidth. The three possible operation modes include:  
* **Stand-Alone** operation: An operator can replace one GSM carrier of 200 kHz with NB-IoT, leaving a guard interval of 10 kHz on both sides of the spectrum. * **In-Band** operation: One or more NB-IoT carriers are deployed inside a larger LTE channel. * **Guard-Band** operation: One or more carriers are allocated within the guard-band of LTE bandwidth to NB-IoT. All three deployment scenarios are transparent to non-NB-IoT devices. Consequently, LTE devices that do not implement NB-IoT functionality simply do not see the NB-IoT channel inside the main LTE bandwidth or in the guard band. At the same time, legacy GSM devices will not see an NB-IoT carrier if used alongside 180 kHz GSM carriers. Such devices will only see noise where NB-IoT is active [26]. At the physical layer, NB-IoT fully inherits from LTE in the downlink. The transmission scheme is based on conventional Orthogonal Frequency Division Multiplexing (OFDM) using normal Cyclic Prefix (CP). An end user operates in the downlink using seven consecutive symbols on 12 subcarriers (also referred to as tones) grouped into a Resource Block (RB), with a subcarrier spacing of 15 kHz [13]. The duration of a slot, subframe, and frame is analogous to LTE, as well. Therefore, in a single NB-IoT carrier, there is only one RB and only one user can receive data at a time using all 12 subcarriers for each subframe. As regards modulation, only Quadrature Phase-Shift Keying (QPSK) is supported. Instead, the uplink supports not only the standard LTE subcarrier spacing of 15 kHz but also a subcarrier spacing of 3.75 kHz. The elementary NB-IoT radio resource, termed Resource Unit (RU), is the smallest unit to map a transport block [27]. It is assigned to a single user only. Unlike the well-known resource block of LTE, an RU is dynamically defined as shown in Table 1[28]. Specifically, the uplink supports two different configurations: _Single-Tone_ and _Multi-Tone_. _Single-Tone_ uses either 3.75 or 15 kHz subcarrier spacing and each subcarrier represents an RU. Consequently, a 180 kHz carrier is divided into either 48 or 12 RUs, respectively, thus resulting in two different RU length values: 32 and 8 ms. In the case of _Multi-Tone_ configuration, the subcarrier spacing is set to 15 kHz only. Nevertheless, 3, 6 or 12 adjacent subcarriers may shape a single RU. Again, depending on the number of tones per RU, its length changes accordingly (as illustrated in Table 1). The modulation schemes available in the uplink are restricted to Binary Phase-Shift Keying (BPSK) and QPSK. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Transmission Mode** & **Subcarriers** & **Subcarrier Spacing [kHz]** & **RU Duration [ms]** \\ \hline Single-Tone & 1 & 3.75 & 32 \\  & 1 & 15 & 8 \\ \hline  & 3 & 15 & 4 \\ Multi-Tone & 6 & 15 & 2 \\  & 12 & 15 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Uplink RUs in NB-IoT. One of the main objectives defined in NB-IoT study item description [29] is to achieve 20 dB coverage extension compared with legacy General Packet Radio Service (GPRS) while limiting the device maximum transmit power to 23 dBm (200 mW), which is a factor of ten lower than the maximum output power of GPRS devices. Coverage extension is achieved by increasing the number of repetitions at the expense of higher data rates. Moreover, NB-IoT supports up to three coverage classes, namely _Normal_, _Extended_, and _Extreme_, to serve devices experiencing different ranges of path loss. The _Extreme_ class corresponds to incredibly low received power levels. The _Normal_ class refers to considerably higher received power levels.