earer have to be reestablished. 6.2.2 Modes of Operation NB-IoT may be deployed as a standalone system using any available spectrum exceeding 180 kHz. It may also be deployed within an LTE spectrum allocation, either inside an LTE Internet of Things 753 channel or in the guard-band. These different deployment scenarios are illustrated in Fig. 6.4. The deployment scenario, standalone, in-band, or guard-band should be transparent to a UE when it is first turned on and scans for an NB-IoT carrier. Similar to existing LTE UEs, an NB-IoT UE is only required to search for a carrier on a 100 kHz raster. An NB-IoT carrier that is intended for facilitating UE initial access and synchronization is referred to as an anchor carrier. The 100 kHz UE search raster implies that for in-band deployments, an anchor carrier can only be placed in certain physical resource blocks (PRBs). For example, in a 10 MHz LTE carrier, the indices of the PRBs that are best aligned with the 100 kHz grid and can be used as an NB-IoT anchor carrier are 4, 9, 14, 19, 30, 35, 40, and 45. The PRB indexing starts from index 0 for the PRB occupying the lowest frequency within the LTE channel bandwidth. In the in-band operation, the assignment of resources between LTE and NB-IoT is not fixed. However, not all frequencies; that is, resource blocks (RBs) within the LTE carrier are allowed to be used for cell connection, rather they are restricted to the values shown in Table 6.1. Fig. 6.5 illustrates the deployment options of NB-IoT in conjunction with a 10 MHz LTE system. The PRB immediately after the DC subcarrier, that is, PRB_25, is centered at 97.5 kHz; that is, at a distance of 6.5 subcarriers from the center of the band. Since the LTE DC subcarrier is placed on the 100 kHz raster, the center of PRB_25 is 2.5 kHz from the nearest 100 kHz grid. The spacing between the centers of two neighboring PRBs above the In-band operation Guard-band operation Standalone operation Multicarrier in-band operation LTE carrier LTE carrier GSM carrier LTE carrier Figur