ansmission is based on single-carrier frequency division multiple access (SC-FDMA) with the same 15 kHz subcarrier spacing, slot, and subframe length as LTE. Single-tone transmission supports two numerologies: 15 and 3.75 kHz. The 15 kHz subcarrier spacing is identical to that of LTE, ensuring coexistence with LTE in the uplink. The 3.75 kHz single-tone numerology uses 2 ms slot duration. Similar to the downlink, an uplink NB-IoT carrier uses a total system bandwidth of 180 kHz [2,11]. In addition to the system frames, the concept of hyper frames is further defined, which counts the number of system frame periods; that is, it is incremented each time the system frame number (SFN) wraps. Similar to the SFN, the hyper frame number (HFN) is a 10-bit counter; thus the hyper frame period spans 1024 system frame periods, corresponding to a time interval of approximately 3 hours. Because NB-IoT only uses a single PRB in the downlink, the physi- cal channels are only multiplexed in the time domain, as shown in Fig. 6.8. 6.3.2 Narrowband Primary Synchronization Signal Unlike LTE, the NB-IoT physical channels and signals are primarily multiplexed across time. Fig. 6.8 illustrates how NB-IoT subframes are allocated to different physical channels and signals. The first three OFDM symbols are not used for NB-IoT, because they carry the Even-numbered frame (10 ms) Odd-numbered frame (10 ms) 12 subcarriers (180 kHz) Figure 6.8 NB-loT frame structure [14,25]. 758 Chapter 6 physical control format indicator channel (PCFICH) and physical downlink control channel (PDCCH) in LTE when NB-IoT is operated in the in-band mode. Note that during the time when the UE synchronizes to the narrowband primary synchronization signal (NPSS) and narrowband secondary synchronization signal (NSSS), it may not know the operation mode; consequently, this guard time applies to all modes. In addition, both synchronization signals are punctured by the LTE cell-specific reference signals. It is not specified which of the antenna ports is used for the syn