Multi Carrier Operation
NB-IoT can operate in multi carrier mode. In this case, a specific carrier is configured for initial connection setup + data transfer and other carriers are configured only for data communication. The carrier for the initial connection setup is called Anchor carrier and the other carriers are called Non-Anchor carrier.
When NB-IoT is operating in 'inband with the same PCI' mode, only a specific PRB within the legacy LTE System Band can be configured as Anchor carrier. The PRB index within LTE System BW are as follows.
|
LTE System BW |
LTE PRB indices for NB-IoT synchronization |
|
3 Mhz |
2, 12 |
|
5 Mhz |
2, 7, 17, 22 |
|
10 Mhz |
4, 9, 14, 19, 30, 35, 40, 45 |
|
15 Mhz |
2, 7, 12, 17, 22, 27, 32, 42, 47, 52, 57, 62, 67, 72 |
|
20 Mhz |
4, 9, 14, 19, 24, 29, 34, 39, 44, 55, 60, 65, 70, 75, 80, 85, 90, 95 |
The above table is derived from following equation and table described in 36.213-16.8. The values in above table is nPRB in the following equation. 
is the max number of RB for a specific channel bandwidth. eutra-CRS-SequenceInfo is the parameter in NPBCH (NR MIB).
For example, if eutra-CRS-SequenceInfo = 7 and LTE channel Bandwidth is 5 Mhz. The nPRB can be calculated as follows.
n'PRB = 5 (for 5Mhz, = 25 which is odd number)
5 = nPRB - floor(25/2) = nPRB - 12
Therefore, nPRB = 17
< 36.213 - Table 16.8-1: Definition of eutra-CRS-SequenceInfo >
In terms of physical channel composition, there are some differences between Anchor and Non-Anchor carrier.
- Anchor Carrier : As shown in Figure 1, Anchor Carrier carries NRS, NPSS, NSS, NPBCH, NPDSCH, NPDCCH
- NonAnchor Carrier : As shown in Figure 2, NonAnchor Carrier carries only NRS, NPDCCH, NPDSCH
NOTE : I created Figue 1 and 2 using Matlab LTE Toolbox based on Matlab 2018b
< Figure 1 : Physical Channel Composition - Anchor Carrier >
< Figure 2 : Physical Channel Composition - NonAnchor Carrier >
Reference :
[1] Narrowband Internet of Things (R&S Whitepaper)