# Time Domain Analysis of NB-IoT Signals
## 2 NB-IoT: Radio Interface  
NB-IoT inherits most of the functionalities from LTE, but with some simplifications to meet the low cost and low power constraints of NB-IoT modules [13; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. NB-IoT makes use of the same modulation transmission schemes of LTE in both downlink and uplink, i.e., the OFDMA (Orthogonal Frequency-Division Multiple Access) and SC-FDMA (Single-Carrier Frequency-Division Multiple Access) schemes, respectively. NB-IoT has been designed to occupy one or more 180 kHz wide frequency bands, each corresponding in LTE to a Physical Resource Block (PRB). A PRB consists of 12 sub-carriers each having a 15 kHz bandwidth. NB-IoT uses the same frame structure of LTE with differences in channel and signal mapping. The downlink and uplink transmissions are organized in frames each 10 ms long. Each frame is made up of 10 subframes (SF) lasting 1 ms, in turn made up of two time slots each 0.5 ms long (Figure 2). Figure 1: Examples of NB-IoT applications. The 3 GPP standard defines three operating modes for NB-IoT (Figure 3):  
* In-band mode: NB-IoT signal occupies one PRB within the LTE signal bandwidth. * Guard-band mode: the NB-IoT signal occupies one of the PRBs within the unused guard band in the LTE bandwidth. * Stand-alone mode: the NB-IoT signal occupies the spectrum released by the GSM. In this case, the NB-IoT signal still uses 180 kHz of the 200 kHz assigned to the GSM carrier, with 10 kHz of guard band on both sides of the spectrum. In the In-band mode, the six central PRBs of the LTE signal are not usable for NB-IoT to avoid conflicts between NB-IoT transmissions and LTE channels and signals transmitted on these PRBs (PBCH channel and PSS and SSS synchronization signals). The indices of the available PRBs are shown in Table 1 depending on the used LTE bandwidth, while Table 2 shows possible PRB frequency offsets with respect to the center frequency of LTE carrier for Guard-Band mode. Figure 3: NB-IoT operating modes. Figure 2: LTE frame. Table 3 reports the NB-IoT channels and signals while Figure 4 shows the allocation schemes of the even and odd frames, respectively, in the Guard-band and Stand-alone mode [33]. As required by the standard, a maximum of two ports can be used by the transmitter (eNB). In particular:  
* the NPECH channel is transmitted on subframe # 0, typically on symbols # 3 \(\div\) 13
* the NPSS signal is transmitted on subcarriers # 0 \(\div\) 10 of subframe # 5, and typically on symbols # 3 \(\div\) 13
* the NSS signal is transmitted on subframe # 9 of even frames only, typically on symbols # 3 \(\div\) 13 \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**LTE Bandwidth** & **3 MHz** & **5 MHz** & **10 MHz** & **15 MHz** & **20 MHz** \\ \hline \multirow{3}{*}{PRB indices} & \multirow{3}{*}{2, 12} & \multirow{3}{*}{2, 7, 17, 22} & 4, 9, 14, 19, & 27, 7, 12, 27, 32, 42, 47, & 4, 9, 14, 19, 24, 29, \\  & & & 30, 35, 40, 45 & 52, 57, 62, 67, 72 & 34, 39, 44, 55, 60, 65, 70, 75, 80, 85, \\ \cline{1-1}  & & & 30, 35, 40, 45 & 52, 57, 62, 67, 72 & 90, 95 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Indices of available PRBs for NB-IoT in In-band mode [13; 15; 18; 30]. Adapted from ref [13].